Pultrusion Process - Composite Manufacturing 2g3r1k
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SEMINAR REPORT ON PULTRUSION PROCESS
MANIPAL INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL AND MANUFATURING
SUBMITTED BY CHETAN P BHAT 080922016
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Pultrusion Process
Introduction Pultrusion is a continuous molding process using fiber reinforcement in polyester or other thermosetting resin matrices. The process is similar to the metal extrusion process, with the difference being that instead of material being pushed through the die in the extrusion process, it is pulled through the die in a pultrusion process. Pultrusion creates parts of constant cross-section and continuous length. Pultrusion is a simple, low-cost, continuous, and automatic process. Pultrusion is a continuous, automated closed-moulding process that is cost effective for high volume production of constant cross section parts. Due to uniformity of cross-section, resin dispersion, fibre distribution & alignment, excellent composite structural materials can be fabricated by pultrusion. The basic process usually involves pulling of continuous fibres through a bath of resin, blended with a catalyst and then into pre-forming fixtures where the section is partially pre-shaped & excess resin is removed. It is then ed through a heated die, which determines the sectional geometry and finish of the final product. The profiles produced with this process can compete with traditional metal profiles made of steel & aluminium for strength & weight. The pultrusion process has developed slowly compared to other composite fabrication processes. The initial pultrusion patent in the United States was issued in 1951. In the early 1950s pultrusion machines for the production of simple solid rod stock were in operation at several plants. Most of these machines were the intermittent pull type. In the mid-1950s, continuous pull machines were available. The late 1950s were producing pultruded structural shapes and by 1970, there has been a dramatic increase in market acceptance, technology development, and pultrusion industry sophistication. The process provides maximum flexibility in the design of pultruded FRP profiles. Currently, profiles up to 72 inches wide and 21 inches high are possible. Since the process is continuous, length variations are limited to shipping capabilities. Specific strength characteristics can be designed into the composite, optimizing laminate performance for a particular application by strategic placement of high performance reinforcements. Color is uniform throughout the cross section of the profile, eliminating the need for many painting requirements. Processing capabilities include the production of both simple and complex profiles, eliminating the need for much postproduction assembly of components.
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The tooling required to start producing pultruded parts is fairly inexpensive and straightforward compared to the complex and sometimes very costly molds that are necessary for other plastics molding processes. It is worth noting though that open profiles are generally less expensive to produce than hollow ones. As a rule of thumb, parts with a small cross section can be manufactured at a speed of roughly one meter per minute, whereas larger profiles will require up to ten times longer. Pultrusion is a high-volume manufacturing process and most manufacturers will ask for a minimum order of 500 meters to start production.
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Basic Raw Materials One can use a wide variety of fibrous reinforcement and resin system to get a composite material with a broad spectrum of properties by pultrusion process. Since each fibre and resin material brings its own contribution to the composite, knowledge of raw material properties is the first step in deg a satisfactory composite product. The reinforcement provides mechanical properties such as stiffness, tension and impact strength and the resin system (matrix) provides physical properties including resistance to fire, weather, ultraviolet light and corrosive chemicals.
Reinforcement Types Reinforcements serve as the primary load bearing entity in the part; reinforcements can enhance functional performances such as electrical conductivity, radar cross section, and thermal performance. In the process, the reinforcement allows the part to be pulled through the die acting as both a load transfer media as well as the source of bulk, which allows the die to be continuously, uniformly filled. Three characteristics must be considered when choosing reinforcements: first the fibre type (glass fibre, aramid and carbon); second the form (roving strands, mat & fabrics) and third the orientation.
Based on Fiber Type The glass fibre continues to be the most widely used reinforcement, because they are readily available and comparatively cheaper. Electrical grade E-glass fibres, the most common, exhibits a tensile strength of approximately 3450 MPa and a tensile modulus of 70 GPa, but they have relatively low elongation of 3 to 4%. A variety of fibre diameters and yields are available for specific applications. Surface sizing of glass fibres provides optimum impregnation and chemical bonding between the fibres and matrix resins, thus ensuring maximum strength development and retention. S-glass fibre exhibits high tensile strength (4600 Mpa) & tensile modulus (85 Gpa) and is used for high-performance applications. The Carbon fibre exhibits tensile strength from 2050 to 5500 MPa and tensile modulus from 210 to 830 GPa with elongation of 0.5 to 1.5%. Carbon fibre has various unique properties like electrical conductivity, high lubricity and low specific gravity (1.8 versus 2.60 for E-glass). Very tough composites having good flexural and impact strength can be fabricated by using Organic fibres such as aramids, having high tensile strength (2750 MPa) and modulus (130 GPa) along with elongations of up to 4%. Polyester fibres with appropriate binders have been used as a replacement for glass in applications that would benefit from increased toughness and impact resistance but where tensile and flexural strengths can be sacrificed.
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Based on Form Rovings Rovings are continuous fibers, which are one of the primary reinforcements used in pultrusion. Rovings come in three main forms; Conventional rovings, Single-End Continuous Rovings or direct rovings, and bulky or texturized rovings. Conventional Rovings Conventional or multi-end roving is assembled from a number of forming packages into the desired final yield or Tex. Conventional rovings are most commonly used in applications containing large thickness of unidirectional reinforcement. Conventional rovings tend to fill space at lower glass levels, giving a more resin rich cross-section. Single-End Continuous Rovings or direct rovings: Single-End Continuous Rovings or direct rovings are the most commonly used reinforcements in the pultrusion process. Single- End Continuous Rovings combine ease of handling due to low catenary and fuzz, with highly reproducible mechanical properties in both its standard unidirectional usage and when used in stitched and woven fabrics. Single-End Continuous Rovings are widely used due their excellent processing, and laminate performance. Considerably higher shear strengths are achieved with single-end rovings compared to conventional rovings Bulky or Texturized Roving Bulky, texturized, or fluffy rovings are specialty rovings designed to fill corners in complex shapes, "clean" the die, preventing formation of resin rich areas, which could cause local spaulling. Bulky rovings are intended to act as local filler, though they do provide some reinforcement.
Mats, Complexes, and Veils Mats, fabrics, and veils are used in pultrusion processes to give properties to the part not achievable using roving reinforcement. Mats give the ability to develop off axis structural performance, create a higher resin content part, and develop unique surface qualities for both visual and non-visual attributes, such as corrosion resistance.
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Continuous Filament Mat Continuous filament mat or swirl mat is the most common mat used in pultrusion. Continuous filament mats (CFM) provide strength and stiffness in the transverse or nonpulling direction of the pultruded part. They provide a degree of bulk, which improves processing and limits resin rich sections of the part. This bulk also reduces the glass fraction required for the processing of a specific cross section. CFM mat also improves the shear strength of the laminate produced. CFM used in pultrusion contains a fastwetting non-soluble binder that maintains mat integrity through the preforming operations. It is less prone to skewing (misalignment) common in fabrics. Mats are generally composed of coarse glass strands, which are highly porous thus ensuring complete wet out of each individual filament. The roll is slit to the appropriate width of the part. The variation in slitting widths will cause some variation in the localized reinforcement contents within the part; hence there will be an impact on the mechanical strength. Weight variation (as well as resin variation) and shrinkage will also induce variation in product characteristics. Continuous strand mat provides the most economical method of obtaining a high degree of transverse physical properties. The mats are layered with roving; this process forms the basic composition found in most pultruded products. The ratio of mat to roving determines the relationship of transverse to longitudinal physical properties.
Fabrics and Stitched Complexes Fabrics and stitched complexes are the newest generation reinforcements for the pultrusion process. The construction of fabrics can be tailored to give specific reinforcing properties to the part in order to achieve the needed strength in parts with demanding design requirements. When the mix of required physical properties is not satisfied by conventional mat roving construction, selected fabrics can be used to meet the end use requirements. Varieties of these products can be used by themselves or in conjunction with the standard mat roving construction to obtain the necessary results. The fiberglass fabrics are available in balanced, high longitudinal, high transverse or ± 45° multi-ply construction. Since these materials are more costly, the composites using these reinforcements are more expensive than standard construction pultrusion. Manipal Institute of Technology Department of Mechanical and Manufacturing
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Chopped Strand Mats The use of chopped strand mats in pultrusion is normally related to specific needs for improved surface or corrosion resistance in flat or gently curved laminates. Care must be taken in selecting a chopped strand mat for pultrusion, as most existing products are not designed to handle the tension associated with the pulling of the product through the process.
Veils Veils in the pultrusion process are designed to establish a high quality surface layer on the pultruded part and protect the die from scarring by the other reinforcements during the pulling process. Veils can be materials such as spun polyester, glass veil, and for special requirements carbon veils have been used. In many cases the veils can have pre-printed designs and logos, which become the part surface finish aiding the part appearance. Since pultrusion is a low-pressure process, fiberglass reinforcements normally appear close to the surface of the product. These can affect the appearance, corrosion resistance or handling of the products. The two most commonly used veils are A-glass and polyester .
Matrix Choice The resin matrix has several functions in a pultruded composite. The resin's basic functions are to fill the space between filaments, to fix the strand alignment, and to distribute the bonding and shearing stresses. Due to the much higher modulus of the glass, and its normally high percentage of volume in the composite, the strength effect of the resin is usually quite small. As in the case with all FRP/GRP material systems, the resin plays an important role in determining the chemical and environmental durability of the total system. It also controls the thermal, electrical, and visual The composite properties such as high-temperature performance, corrosion resistance, dielectric properties, flammability and thermal conductivity are determined exclusively by the properties of resin matrix.
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Polyester Resins Unsaturated Polyester resins are most commonly used in pultrusion. Orthophthalic, isophthalic acids or anhydrides, in combination with maleic anhydride and various glycols, are the basic elements. Pultrusion polyester has the ability to gel and cure rapidly to form the strong gel structure required for release at the die wall. Generally resins with the viscosities of 500 are used for pultrusion. Higher viscosity low-reactive monomer versions can be blended with additional styrene to suit the processing need. The styrene level must be properly maintained to achieve good cross-link structure without having residual (unreacted) styrene in the finished composite. Polyester resins exhibit good corrosion resistance to aliphatic hydrocarbons, water, dilute acidic & alkaline environments. They do not perform well when exposed to aromatic hydrocarbons, ketons, and concentrated acids. A high degree of unsaturation in polyester chain exibits shrinkage up to 7% on curing. This level can be reduced using fillers and lowprofile additives. Composite based on polyesters retains high percentage of their electrical insulation properties even if used continuously at temperature up to 200oC. Though polyester s combustion without modification, hence backbone bromination or the use of additives greatly improves its flammability and smoke generation properties. The electrical properties of polyesters make them suitable for use as primary insulators in many high-voltage applications. Retention of electrical properties even at elevated temperatures has made polyester insulators the materials of choice in many applications. The weatherability of polyester is fair to good. Additional protection is usually through a variety of ultraviolet absorption additives or using polyester surface veils and even painting (done after pultrusion) Vinyl Ester Resins These resins are used when additional performance is sought. Vinyl esters (VE) offer better corrosion resistance, higher mechanical properties at elevated temperatures, and improved toughness properties such as impact and shear. They provide very efficient wet out and they have higher temperature capability with improved flexibility compared to polyester resins. The chemical structure of vinyl ester resins is such that the reaction sites are at the end of each polymer chain rather than along the chain resulting in rigid segments along the polymer backbone. This leads to lower-link density and high-temperature capability of these materials. VE resins are superior to polyesters, but this advantage comes at a cost in two ways: 1. VE resins can be as high as double the cost of polyester resins 2. VE resins usually run at speeds about 2/3 the speed of polyester due to their lower crosslink density. Many VE have a narrow temperature window. A 10°C temperature change can cause blistering in pultruded profiles. Operators should be aware of the small processing window which VE resins have. Manipal Institute of Technology Department of Mechanical and Manufacturing
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Epoxy Resins Epoxy resins typically offer the highest physical property performance as well as the best strength retention at high temperatures of all the resins used in pultrusion. Epoxies are frequently used for primary electrical insulation, aircraft, aerospace, and defence applications. Epoxies can be used in continuous use applications at 300°F (150°C) and epoxies provide increased flexural strengths and shear strengths when compared to polyesters and VE. Epoxies have excellent corrosion resistance and electrical properties. The disadvantages of epoxy resins can be: • Poor toughness as a result of their rigid structure • Can be more expensive to purchase • Slower processing speeds vs Polyester and VE resins • Lower pot life • Mold sticking considerations • More difficult to clean up They require a higher reinforcement content than either polyester or vinyl ester Other Resins A variety of resin alternatives is also available for specific applications. The resins based on Methacrylate Vinyl Ester Resins although expensive than polyesters but could be used for their special properties viz. improved physical properties, very low viscosity which allows them to be highly filled, rapid processing speeds, smooth profile surfaces and improved flame retardancy and weathering. One concern with MVE is odor which plant personnel may find objectionable. Phynolic resins are also used in pultrusion owing to their high heat resistance and flameretardancy/low-smoke characteristic. Phenolic resins are suitable typically for pultruding natural fibres such as jute. A desire to improve toughness and post processing formability has lead to the use of thermoplastic resins. The engineering thermoplastic resins provide excellent heat distortion properties. The technology for impregnating fibres with thermoplastic resins includes hotmelt application and solvent solution impregnation.
Filler and additives Filler and additives are used to enhance specific performance, reduce cost, influence viscosity, or improve processability of resin systems. Fillers can be incorporated into the resins in quantities up to 50% of the total resin formulation by weight. ). The usual volume limitation is based on the development of usable viscosity, which depends on the particle size and the characteristics of the resin. There are three fillers frequently chosen for use in pultrusion.
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These fillers are: Calcium carbonate is the most popular and is used as a volume extender. Calcium carbonate is generally used where performance is not critical. Clay (alumina silicate) fillers are used for their corrosion resistance and in profiles requiring electrical insulation. They can provide very good surface finish. Alumina trihydrate is used when flame or smoke suppression is desired. Calcium carbonate is primarily used as a volume extender to provide the lowest-cost-resin formulation in areas in which performance is not critical. Special purpose additives include ultraviolet radiation screens for improved weatherability, antimony oxide for flame retardance, pigments for coloration, and low-profile agents for surface smoothness and crack suppression characteristics. Mould release agents (metallic sterates or organic phosphate esters) are important for adequate release from the die wall to provide smooth surfaces and low processing friction. Pigments may be used to impart color, weatherability, or flame retardency to the finished part.
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Components of Pultrusion Process There are four components required in any pultrusion processing operation: 1. Creels 2. Forming or preforming guides 3. Resin impregnation systems 4. Primary die 5. Puller/clamping pads 6. Cut off saws
Creels The creel should provide a position from which the roving can be fed to the pultrusion process under controlled and uniform tension. It also provides a location for the transfer of the roving strand from the running package to a second back-up package for continuous uninterrupted production. There may even be room for extra roving packages for replacement or maintenance as required. The size, shape and type of creel will normally be determined by space considerations such as roving package dimensions, the distance the strand must be conveyed and the number of packages to be handled. The amount of glass being used on a continuous basis must also be considered. The two common types of creels used are shown. Each creel arrangement has a range of possible number of packages for the best process efficiency: Table creel - up to 50 packages Bookshelf creel - 20 to several hundred packages The bookshelf-type creel is the most common and usually provides the best balance of accessibility and maximum utilization of floor space. The size will vary widely, but the creels shown provide a standard module concept for creeling. Shown are creels for handling 32 packages (16 active-16 transfer) in both a side pull and end pull configuration. To handle different numbers of packages, the creels can be increased or decreased in length or multiple creels can be used.
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In the end pull creel, commonly used in pultrusion, it may be necessary to guide each strand within the creel using steel rods or ceramic guide eyes. This can prevent sagging and whipping could tangle two or more strands on the same shelf. Most pultrusion processes use stationary roving packages on bookshelf or table creels, pulling the roving strand from the inside of the roving package. For some applications requiring minimal strand twist for better wet-out and a flatter and wider strand profile, pulling the roving strand from the outside of the roving package may be an option.
Forming guides It is important to determine how the reinforcement is organized, aligned, and fed into the primary die. Besides formulation and heating control, preforming system is also critical for successful and constancy pultrusion. Before entering die, impregnated fibreglass rovings and mats must be properly arranged and placed. Un-proper preforming system causes failure of pultrusion, bad quality, and other problems. Please note, preforming system is far from easy as most people think of. Firstly, it must be designed based on profile design to meet the physical requirements. Secondly, it must let all reinforcements running freely and smoothly, to avoid any breaking of rovings, mats or cloth. Thirdly, for dies with mandrels, the preforming system is the only device to keep uniform thickness of pultruded profiles. In-proper preforming may cause eccentric and even breaking of pultrusion. Fourthly, for complex profiles, preforming system is a large challenge. A preforming die gently shapes the material and removes all but about 10% of the excess resin prior to entry into the pultrusion die. There are two primary materials used in the forming guide tooling: steel and ultra high molecular weight polyethylene (UHMWPE). The advantage of steel is that it is less expensive (if carbon steel is used vs. stainless) and standard plate, sheet, bar, and rod can all be used. A disadvantage is corrosion of non-stainless steel and the difficulty to machine. The advantage of UHMWPE is that it is lighter, resistant to chemical attack, less damaging to the glass reinforcement, and therefore, easier Manipal Institute of Technology Department of Mechanical and Manufacturing
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to clean. A major advantage is that it is easy to fabricate, on-line, when modifications are required. UHMWPE is easy to drill and machine slots for mat. A disadvantage is that UHMWPE wears faster than steel. Except for the primary die the forming guides and the guiding of the reinforcement into the die is the most important aspect in pultrusion technology
Resin Impregnation System The dimensions of resin baths are restricted to minimize the volume of catalyzed resin and can be heated to control the resin viscosity to promote fiber wetting, although this will reduce the working life of the bath. To facilitate lacing up, the roller assembly in the resin bath is manufactured in two parts—a lower fixed set of rollers submerged in the resin bath and a moveable upper set, under which the fiber is positioned. The assembly is then pressed down to push the fiber into the bath to with the lower set of rollers. This system facilitates an easy lace-up procedure and ensures good compaction to expel all air and promote fiber wetting. Alternatively, the fiber can be ed over a drum upon which the correct amount of resin has been metered and adjusted by a doctor blade.It is extremely important to allow the resin and reinforcement enough time to fully wet-out. The impregnation or bath system directly impacts wetout. There are three resin impregnation systems available today. They are: 1. Dip bath 2. Straight through bath 3. Resin Injection systems Dip Bath The dip bath, or open bath, has the reinforcements travelling from the creels down into the bath where the rovings go through an "S" bar guide, which breaks apart the roving bundle, allowing better coating of the filaments by the resin. The bath system is typically used for all roving reinforcements, as well as for simple mat and roving profiles where the mat can be handled horizontally, or where taking the mat out of the horizontal plane will not induce a bow into the finished profile. This system exposes a large amount of resin to the air and permits styrene evaporation into the plant environment. Styrene emission is a environmental and health consideration,which needs to be assessed with this impregnation system.
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Straight Through Bath A straight through resin bath is a trough with forming cards on each end. These cards can also be used to begin the forming process. The cards allow resin to leak from the open areas. The excess resin is then collected and pumped back into the bath trough. The advantage of this design is the reinforcements are taken from the horizontal plane, allowing the profiles to be made with fewer tendencies to warp or bow, and this design also reduces the amount of styrene released into the plant environment. Resin Injection System The resin injection system is the newest process for impregnation. With resin injection, a steel chamber is attached to the front of die. The chamber contains port(s), which allow resin to be injected into the cavity. The combination of cavity design, resin pressure, and movement of the reinforcement being pulled into the die generate hydraulic pressure forcing resin to penetrate the reinforcement bundle, resulting in impregnation. With resin injection the resin is not open to the plant environment, reducing the amount of styrene released into the workplace. Resin injection systems require reduced clean up time, due to resin with fewer components. The disadvantage of resin injection is the potential for incomplete impregnation of profiles with thick walls, or incomplete impregnation in resin systems with high filler loading incorporating a high number of mat or veil reinforcements.
Primary Die The die is the heart of the pultrusion system and is the limiting step in production rate since the part is both shaped and, usually, cured in the die. The processes of shaping and curing along with the correspondent line speed are dependent upon the shape of the part, the type of resin, the internal friction in the die, the heat expansion of the resin, the contraction of the resin, and mechanical warpage which may occur in the part because of nonsymmetries in the fiber orientations.
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The opening of the die is usually somewhat larger than the final shape, permitting easy collection of the fibers bundle, and then the die interior dimensions gradually reduce in size until the final shape is achieved. During this shaping process, the part is cured. Cure is accomplished either by thermally heating the die (usually with common electrical heaters) or by subjecting the material to rf frequencies. Both of these systems have their advantages. The thermal heating is simple and can be used with metal dies, thus limiting die wear. However, the poor heat transfer of the resin means that as the thickness of parts increases, the speed of the pultrusion line must slow. Studies have shown that thicknesses of about .5 inches can be thermally cured at 2 feet per minute but that parts thicker than 3 inches cannot be cured at all using just thermal energies, regardless of the line speed. If rf curing is used, the thickness of the parts which can be cured and the line speeds possible are both improved — about 3 times faster with parts that are .5 inches thick. However, rf curing does not work well for metallic dies nor for conductive parts. Hence with rf curing, non-metallic dies are generally used and these are prone to rapid erosion and poor dimensional control. Parts with conductive components (such as carbon fibers) cannot be effectively cured using rf radiation. These materials are thermally cured. Recent machines which combine both thermally heated metal dies and rf heating after exiting the die have proven to give much better performance than either of the methods alone. Parts must be quite hard (essentially cured) when they exit the die so that they will not be deformed by the pulling mechanism, although some curing after exiting the die is possible if done before the pullers. Post-die curing can be done with a tunnel heater, although this adds considerable length to the line and is notoriously inefficient in heat use. Another method of post die curing is to use heated, moving C-shaped dies (also called split dies) that have cavities in the shape of the finished part and close on the part as it exists the die. These dies Manipal Institute of Technology Department of Mechanical and Manufacturing
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are mounted on a moving belt or chain and stay in with the part long enough to insure that sufficient cure is achieved to withstand the forces of the puller. Off-line curing could then be used, as required.
The following are some considerations, which need to be taken into when purchasing primary dies: Material selection The die material should be capable of withstanding continual heating and cooling without deformation. Usually this means high-grade tool steel. The material should be capable of resisting wear from abrasion, and be damage tolerant for repeated assembly and disassembly. The material should be capable of receiving chrome plating of 0.001-0.002 inches (0.025-0.0508 mm) of thickness for wear resistance. Two widely used steels for die manufacture are A-2 hardened to 55-60 Rockwell hardness and P-20 prehardened to 28-30 Rockwell hardness. Shrinkage Factors It is the nature of most resins to shrink after reaching peak exotherm, and during the cooling process. Because of this a shrinkage factor must be calculated into the die design. This will enable the die to form the part to the proper dimensions after the part is completely cooled. A shrinkage factor cannot be unilaterally determined, as each resin system and reinforcement lay-up is different, however recommended shrinkage factors are: Thickness dimensions: 1% shrinkage All other dimensions 0.3% shrinkage Die Opening Design The die opening design must accommodate the smooth entry of reinforcements into the proper position. Generally a symmetrically shaped die is made to utilize either end of the die as entry or exit, if possible.This enables longer die life between re-chroming. At the opening of a die a minimum radius of 0.250 inches should be used. The die inlet is tapered at 7–100, with well rounded edges to prevent fiber fracture. One Piece or Split Cavity Dies The advantage of a one-piece die is that the finished part will not have a parting line.A onepiece gun barrel drilled die is usually less expensive to manufacture, however it may be more expensive in the long run. If a part seizes up inside a one-piece die during processing, the die may be impossible to repair.
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Multiple piece dies have the advantage of being easily opened to allow cleaning and maintenance, however care needs to be given in deg the die so the parting line between the mold halves does not cause a problem with the part being molded. Die Surface Treatments Due to the abrasive nature of fiberglass reinforcements, a protective surface treatment is required on the die cavity. The most commonly used treatment is hard chrome plating at 0.001-.002 inches (25-50 microns) thick. For dies expected to perform in long term service, nitriding may be considered. Die Maintenance Die maintenance is one of the most important factors in extending die life. Opening the die after each production run and recording the number of feet of production and the wear conditions is important.Testing the die surface with copper sulfate for wear spots is critical.The sooner die wear can be detected and treated the longer the die will last before major rework.The best time to perform die inspection and maintenance is just after the die has been pulled from production, prior to being stored. Inspection just prior to installation is not recommended, as production pressures may make repairs, if needed, difficult, shorting the life of the die and potentially compromising part quality. Acidic mold release agents are often used to ease the separation of the part from the metal die. Steel dies exposed to acidic conditions must be cleaned thoroughly between uses to maximize the lifetime of the die. Puller Clamp System The pultruded product is cooled prior to the traction unit, which can be a counter rotating caterpillar unit, or preferably, a hand-over-hand reciprocating clamp type unit, since the caterpillar unit requires the tracks to be fitted with machined rubber pads to accommodate each pultruded profile. The hand-over-hand unit grips above and below and while one unit is pulling, the other unit returns to position, ready to take over the role of pulling. Typical line speeds vary in the range 1.5–100 mh1 , depending on the section(s) being produced. The pulling forces depend on the type of machine which are available upwards to some 30 MT. There are two common puller systems 1. Caterpillar belt 2. Reciprocating clamp puller
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In both systems, pads, typically made of urethane, must be shaped to match the part profile in order to apply a uniform clamp load, which will not cause damage to the pulled part. The advantage of the caterpillar belt system is the capacity to provide large pulling force, spread over a larger part surface area. The advantage of the reciprocating clamp puller system is cost, as it requires only 2 puller pads per clamp vs 10-100 depending on the caterpillar belt size.
Cut off Saws Most pultruders utilize what are known as flying cut off saws. A flying cut off saw moves at the same speed as the moving part, so the cut edge of the part is square and straight. Using a nonflying saw results in cut edges that are not square and straight. Flying cut off saws are recommended for part quality. There are two basic types of flying cut-off saws: 1. Wet saw – A wet saw uses water during the cutting cycle to cool and lubricate the blade and flush the fiberglass particulate to a filter. This effectively eliminates dust and airborne particulate. 2. Dry saw – A dry-cut saw uses a continuous rim diamond blade that does not require any fluid during the cutting cycle, but which gives a good clean cut. A dry-cut saw requires a dust collection system to capture the fiberglass dust, both to address operator health and comfort and from a housekeeping standpoint. Using a dry-cut saw without a properly designed dust collection system is not recommended, due to the dust generated during cutting, both from an operator health/comfort and a housekeeping standpoint. In most cases pultruders will utilize an automatic cut-off saw, which automatically cuts the part into the proper lengths. This enables employees to carry out other duties in order to enhance productivity.
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Basic Processing Steps The basic pultrusion process can be divided into the following operations: 1. Reinforcement handling A suitable creel positions the requisite number of tows, with minimal damage, prior to entry into the resin bath. If tows are supplied from containers, a christmas tree with ceramic eyelets will be required to direct the tows to the resin bath. In some pultruded sections, smaller tows are used in parts where the profile shape does not permit the use of larger tows. Higher size contents (2–5% w/w) will permit easier handling and minimize fiber damage. Tows can be ed by knotting, but knots must be staggered to ease their age through the die. Hybrid constructions can be used (e.g. with glass and aramid) and transverse properties can be introduced with an In-feed system using woven cloth, knitted fabrics, braid, or mat 2. Resin impregnation The dimensions of resin baths are restricted to minimize the volume of catalyzed resin and can be heated to control the resin viscosity to promote fiber wetting, although this will reduce the working life of the bath. To facilitate lacing up, the roller assembly in the resin bath is manufactured in two parts—a lower fixed set of rollers submerged in the resin bath and a moveable upper set, under which the fiber is positioned. The assembly is then pressed down to push the fiber into the bath to with the lower set of rollers. This system facilitates an easy lace-up procedure and ensures good compaction to expel all air and promote fiber wetting. Alternatively, the fiber can be ed over a drum upon which the correct amount of resin has been metered and adjusted by a doctor blade. 3. Pre-die forming A preforming die gently shapes the material and removes all but about 10% of the excess resin prior to entry into the pultrusion die. 4. Heated die to shape and cure the resin The pultrusion die can be made from polished chromium plated tool steels, or when pultruding epoxies, a high chromium content tool steel. The die must be accurately lined up and its length typically 300–1000 mm, which is governed by the size of the section being pulled, the pulling speed and the resin system. Longer dies require greater pulling forces due to the increased frictional drag and a die lubricant, such as zinc stearate, can be added to the resin mix to help reduce frictional resistance, but which may interfere with any subsequent composite bonding process. The die inlet is tapered at 7–100, with well rounded edges to prevent fiber fracture. The excess resin exudes from the inlet end of the die,
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causing the entering fiber bundle to swell, eventually attaining equilibrium with the process conditions. Adding this exuded resin to the resin bath will curtail the life of the bath. Cartridge or plate heaters are preferred for heating the die to a uniform temperature within ±10C and maintaining a temperature gradient along the die to avoid premature gelation, while taking into any exotherm. An RF (Radio Frequency wave generator) unit can be used to either heat the fiber entering the die or the die/resin. The die must be preheated prior to commencement of pultrusion. Shrinkage during polymerization reduces die forces and should always be arranged to be greater than the thermal expansion caused by the temperature rise. 5. Pulling unit to provide traction The pultruded product is cooled prior to the traction unit, which can be a counter rotating caterpillar unit, or preferably, a hand-over-hand reciprocating clamp type unit, since the caterpillar unit requires the tracks to be fitted with machined rubber pads to accommodate each pultruded profile. The hand-over-hand unit grips above and below and while one unit is pulling, the other unit returns to position, ready to take over the role of pulling. Typical line speeds vary in the range 1.5–100 mh_1, depending on the section(s) being produced. The pulling forces depend on the type of machine which are available upwards to some 30 MT. 6. Cut off saw Once the pultruded section has left the die and cooled sufficiently, it is clamped and a flying saw moves along with the clamped section to cut off required lengths. Extra long lengths can be accommodated by feeding the pultrusion out through a door, window or hatch at the end of the building. 7. Post cure oven For optimum properties, all pultruded sections will require post curing and care must be taken to ensure adequate along the entire pultruded length to prevent deformation occurring in the post cure oven.
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Pultrusion Process
Following are some of the considerations while manufacturing and deg pultruded parts. Wall Thickness Wherever possible, select uniform thickness in the cross-section because it provides uniform cooling and curing, and thus avoids the potential of residual stress and distortions in the part. Moreover, uniform thickness will provide uniform shrinkage in the part and thus will limit the warpage in the product. Typically, 2 to 3% shrinkage occurs in the pultruded part. Also, maintain symmetry in the cross section for minimal distortion. For high-volume production, the thickness of the part is critical because the curing time and therefore the rate of pull depend on the thickness of the part. For example, a 0.75-in. thick cross-section can be produced at a rate of approximately 9 in./min, whereas a 0.125-in. thick cross-section can provide a production rate of 3 to 4 ft/min. Therefore, if a design requires high rigidity in the part, then it can be achieved by creating deeper sections with thinner wall or by including ribs in the cross-section. Similarly, if there is a choice between selecting a thick rod or tube, select the tube because it offers a higher production rate, lower cost, and higher specific strength. Corner Design In a pultruded part, avoid sharp corners and provide generous radii at those corners. Generous radii offer better material flow at corners as well as improve the strength by distributing stress uniformly around the corner. A minimum of 0.0625-in. radius is recommended at corners. Another important consideration in the design of corners is to maintain uniform thickness around the corner. This will avoid the build-up of resin rich areas, which can crack or flake off during use. Moreover, uniform thickness will provide uniformity in fiber volume fraction and thus will help in obtaining consistent part properties. Tolerances, Flatness, and Straightness Dimensional tolerances, flatness, and straightness obtained in pultruded parts should be discussed with the supplier. Standard tolerances on fibreglass pultruded profiles have been established by industry and ASTM committees. Refer to ASTM 3647-78, ASTM D 3917-80, and ASTM D 3918-80 for standard specifications on dimensional tolerances and definitions of various relating to pultruded products. Pultrusion is a low-pressure process and therefore does not offer tight tolerances in the part. Shrinkage is another contributing factor that affects tolerances, flatness, and straightness. The cost of a product is significantly affected by tolerance requirements. Tight tolerance implies higher product cost. Therefore, whenever possible, provide generous tolerances on the part as long as the functionality of the product is not affected. Surface Texture Pultrusion is a low-pressure process and typically provides a fiber-rich surface. This can cause pattern-through of reinforcing materials or fibers getting easily exposed under wear or weathering conditions. Surfacing veils or finer fiber mats are used as an outer layer to minimize this problem. To create good UV and outdoor exposure resistance, a 0.001- to 0.0015-in. thick layer of polyurethane coating is applied as a secondary operation. Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Advantages of using pultrusion Pultrusion has a number of benefits over other composite processing systems. Some of the lowest cost, highest quality composite profiles are created by this process. This is because it is automated and has very little manual interface. A manufacturer can be assured the 1st ten-meters of pultrusion will have the same quality and consistency as the 100th ten-meters of pultrusion. Human interface is eliminated, as required in most other processes, such as molding and hand-lay-up. Quality is not a function of motivation of factory technicians. Another distinct advantage of the pultrusion process is cost. It is not unusual to find 80-90% of the cost of pultrusion profiles are the raw material costs. The amortized machine costs and the labor to run pultrusion machines is a small portion of the total factory costs. This has been a primary driver for pultrusion being one of the fastest growing and accepted manufacturing processes in the composites industry. Features
Strong
Light Weight
Corrosion Resistant
Electrical Insulation
Description
Benefits
Applications
Unit strength in tension & compression is approx. 20 x that of steel when these properties are combined on the basis of unit density
Optional strength as desired. Exceptionally high impact strength reduces damage potential
Structural process equipment . Tank s. Cooling tower ancillaries. Flooring s. Trusses & ts.
Density of pultruded components is about 20% of steel and 60% of aluminium
Higher performance at less weight. Lower shipping, handling & installation costs. Less operational energy demand.
Automotive leaf springs & bumpers. Prefabricated walkways & platforms. Bus components.
Chemical plant hand Unaffected by exposure to Minimum maintenance railings, gratings, a great variety of corrosive costs. Long term safety. walkways & bridges. environment & chemicals. Longer life. Cable trays. Pipe s.
Provides strength & rigidity with dielectric properties.
Lesser no. of components. Nonmagnetic & safe. Predictable insulation values for wide range of frequencies.
Manipal Institute of Technology Department of Mechanical and Manufacturing
Ladders, Cable trays. Switch gear components. Mounting braces and backboards.
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Pultrusion Process
Thermal Insulation
Consolidation
Dimensional Stability
Safety
Pultruded components have a low thermal conductivity, 1/250 of aluminium & 1/60 of steel.
Reduces installation thickness. Eliminates condensation problems. Reduces energy operation requirements.
Many individual components can be combined into a large profile.
Reduced assembly cost. Window latch s. Reduced inventory. Roll up door Fewer parts improve reliability.
Pultruded components are highly resistant to warping stretch/swelling over a wide range of temperature & stresses.
No permanent Spring bumpers. deformation under high Crossing gate arms. stress. Close Scrubber components. tolerances.
The pultruded components are very strong & safe to work with. They are microbes and insect proof.
Many gratings suffer from the problem of microbes etc. due to wet or unhygienic working conditions.
Manipal Institute of Technology Department of Mechanical and Manufacturing
Bulk head frames. Walk in refrigerator door jams. Window frames. Insulated roll up doors.
This property makes them ideal choice for pharmaceutical & food industries.
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DISADVANTAGES OF PULTRUSION 1. It is suitable for parts that have constant cross-sections along their length. Tapered and complex shapes cannot be produced. 2. Very high-tolerance parts on the inside and outside dimensions cannot be produced using the pultrusion process. 3. Thin wall parts cannot be produced. 4. Fiber angles on pultruded parts are limited to 0°. Fabrics are used to get bidirectional properties. 5. Structures requiring complex loading cannot be produced using this process because the properties are mostly limited to the axial direction. 6. Voids may result in parts if excessive opening given at die entrance 7. Standards play an important role in acceptance of new materials. The lack of design standards is a significant constraint to the use and growth of composites in structural applications. 8. Shrinkage (commonly 2% - 3%)
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References Pultrusion of Glass Fiber Composites - A Technical Manual (Owens Corning) PULTRUSION — HIGH PRODUCTIVITY NOW, GETTING EVEN BETTER, By A. Brent Strong/Brigham Young University Manufacture by Pultrusion - Dr J M Methven, MACE Pultrusion of Composites - An Overview, Atul Mittal & Soumitra Biswas Carbon Fibers and Their Composites - Peter Morgan COMPOSITES MANUFACTURING - Materials, Product, and Process Engineering, Sanjay.K.Mazumdar, Ph.D.
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MANIPAL INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL AND MANUFATURING
SUBMITTED BY CHETAN P BHAT 080922016
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Pultrusion Process
Introduction Pultrusion is a continuous molding process using fiber reinforcement in polyester or other thermosetting resin matrices. The process is similar to the metal extrusion process, with the difference being that instead of material being pushed through the die in the extrusion process, it is pulled through the die in a pultrusion process. Pultrusion creates parts of constant cross-section and continuous length. Pultrusion is a simple, low-cost, continuous, and automatic process. Pultrusion is a continuous, automated closed-moulding process that is cost effective for high volume production of constant cross section parts. Due to uniformity of cross-section, resin dispersion, fibre distribution & alignment, excellent composite structural materials can be fabricated by pultrusion. The basic process usually involves pulling of continuous fibres through a bath of resin, blended with a catalyst and then into pre-forming fixtures where the section is partially pre-shaped & excess resin is removed. It is then ed through a heated die, which determines the sectional geometry and finish of the final product. The profiles produced with this process can compete with traditional metal profiles made of steel & aluminium for strength & weight. The pultrusion process has developed slowly compared to other composite fabrication processes. The initial pultrusion patent in the United States was issued in 1951. In the early 1950s pultrusion machines for the production of simple solid rod stock were in operation at several plants. Most of these machines were the intermittent pull type. In the mid-1950s, continuous pull machines were available. The late 1950s were producing pultruded structural shapes and by 1970, there has been a dramatic increase in market acceptance, technology development, and pultrusion industry sophistication. The process provides maximum flexibility in the design of pultruded FRP profiles. Currently, profiles up to 72 inches wide and 21 inches high are possible. Since the process is continuous, length variations are limited to shipping capabilities. Specific strength characteristics can be designed into the composite, optimizing laminate performance for a particular application by strategic placement of high performance reinforcements. Color is uniform throughout the cross section of the profile, eliminating the need for many painting requirements. Processing capabilities include the production of both simple and complex profiles, eliminating the need for much postproduction assembly of components.
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Pultrusion Process
The tooling required to start producing pultruded parts is fairly inexpensive and straightforward compared to the complex and sometimes very costly molds that are necessary for other plastics molding processes. It is worth noting though that open profiles are generally less expensive to produce than hollow ones. As a rule of thumb, parts with a small cross section can be manufactured at a speed of roughly one meter per minute, whereas larger profiles will require up to ten times longer. Pultrusion is a high-volume manufacturing process and most manufacturers will ask for a minimum order of 500 meters to start production.
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Pultrusion Process
Basic Raw Materials One can use a wide variety of fibrous reinforcement and resin system to get a composite material with a broad spectrum of properties by pultrusion process. Since each fibre and resin material brings its own contribution to the composite, knowledge of raw material properties is the first step in deg a satisfactory composite product. The reinforcement provides mechanical properties such as stiffness, tension and impact strength and the resin system (matrix) provides physical properties including resistance to fire, weather, ultraviolet light and corrosive chemicals.
Reinforcement Types Reinforcements serve as the primary load bearing entity in the part; reinforcements can enhance functional performances such as electrical conductivity, radar cross section, and thermal performance. In the process, the reinforcement allows the part to be pulled through the die acting as both a load transfer media as well as the source of bulk, which allows the die to be continuously, uniformly filled. Three characteristics must be considered when choosing reinforcements: first the fibre type (glass fibre, aramid and carbon); second the form (roving strands, mat & fabrics) and third the orientation.
Based on Fiber Type The glass fibre continues to be the most widely used reinforcement, because they are readily available and comparatively cheaper. Electrical grade E-glass fibres, the most common, exhibits a tensile strength of approximately 3450 MPa and a tensile modulus of 70 GPa, but they have relatively low elongation of 3 to 4%. A variety of fibre diameters and yields are available for specific applications. Surface sizing of glass fibres provides optimum impregnation and chemical bonding between the fibres and matrix resins, thus ensuring maximum strength development and retention. S-glass fibre exhibits high tensile strength (4600 Mpa) & tensile modulus (85 Gpa) and is used for high-performance applications. The Carbon fibre exhibits tensile strength from 2050 to 5500 MPa and tensile modulus from 210 to 830 GPa with elongation of 0.5 to 1.5%. Carbon fibre has various unique properties like electrical conductivity, high lubricity and low specific gravity (1.8 versus 2.60 for E-glass). Very tough composites having good flexural and impact strength can be fabricated by using Organic fibres such as aramids, having high tensile strength (2750 MPa) and modulus (130 GPa) along with elongations of up to 4%. Polyester fibres with appropriate binders have been used as a replacement for glass in applications that would benefit from increased toughness and impact resistance but where tensile and flexural strengths can be sacrificed.
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Pultrusion Process
Based on Form Rovings Rovings are continuous fibers, which are one of the primary reinforcements used in pultrusion. Rovings come in three main forms; Conventional rovings, Single-End Continuous Rovings or direct rovings, and bulky or texturized rovings. Conventional Rovings Conventional or multi-end roving is assembled from a number of forming packages into the desired final yield or Tex. Conventional rovings are most commonly used in applications containing large thickness of unidirectional reinforcement. Conventional rovings tend to fill space at lower glass levels, giving a more resin rich cross-section. Single-End Continuous Rovings or direct rovings: Single-End Continuous Rovings or direct rovings are the most commonly used reinforcements in the pultrusion process. Single- End Continuous Rovings combine ease of handling due to low catenary and fuzz, with highly reproducible mechanical properties in both its standard unidirectional usage and when used in stitched and woven fabrics. Single-End Continuous Rovings are widely used due their excellent processing, and laminate performance. Considerably higher shear strengths are achieved with single-end rovings compared to conventional rovings Bulky or Texturized Roving Bulky, texturized, or fluffy rovings are specialty rovings designed to fill corners in complex shapes, "clean" the die, preventing formation of resin rich areas, which could cause local spaulling. Bulky rovings are intended to act as local filler, though they do provide some reinforcement.
Mats, Complexes, and Veils Mats, fabrics, and veils are used in pultrusion processes to give properties to the part not achievable using roving reinforcement. Mats give the ability to develop off axis structural performance, create a higher resin content part, and develop unique surface qualities for both visual and non-visual attributes, such as corrosion resistance.
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Pultrusion Process
Continuous Filament Mat Continuous filament mat or swirl mat is the most common mat used in pultrusion. Continuous filament mats (CFM) provide strength and stiffness in the transverse or nonpulling direction of the pultruded part. They provide a degree of bulk, which improves processing and limits resin rich sections of the part. This bulk also reduces the glass fraction required for the processing of a specific cross section. CFM mat also improves the shear strength of the laminate produced. CFM used in pultrusion contains a fastwetting non-soluble binder that maintains mat integrity through the preforming operations. It is less prone to skewing (misalignment) common in fabrics. Mats are generally composed of coarse glass strands, which are highly porous thus ensuring complete wet out of each individual filament. The roll is slit to the appropriate width of the part. The variation in slitting widths will cause some variation in the localized reinforcement contents within the part; hence there will be an impact on the mechanical strength. Weight variation (as well as resin variation) and shrinkage will also induce variation in product characteristics. Continuous strand mat provides the most economical method of obtaining a high degree of transverse physical properties. The mats are layered with roving; this process forms the basic composition found in most pultruded products. The ratio of mat to roving determines the relationship of transverse to longitudinal physical properties.
Fabrics and Stitched Complexes Fabrics and stitched complexes are the newest generation reinforcements for the pultrusion process. The construction of fabrics can be tailored to give specific reinforcing properties to the part in order to achieve the needed strength in parts with demanding design requirements. When the mix of required physical properties is not satisfied by conventional mat roving construction, selected fabrics can be used to meet the end use requirements. Varieties of these products can be used by themselves or in conjunction with the standard mat roving construction to obtain the necessary results. The fiberglass fabrics are available in balanced, high longitudinal, high transverse or ± 45° multi-ply construction. Since these materials are more costly, the composites using these reinforcements are more expensive than standard construction pultrusion. Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Chopped Strand Mats The use of chopped strand mats in pultrusion is normally related to specific needs for improved surface or corrosion resistance in flat or gently curved laminates. Care must be taken in selecting a chopped strand mat for pultrusion, as most existing products are not designed to handle the tension associated with the pulling of the product through the process.
Veils Veils in the pultrusion process are designed to establish a high quality surface layer on the pultruded part and protect the die from scarring by the other reinforcements during the pulling process. Veils can be materials such as spun polyester, glass veil, and for special requirements carbon veils have been used. In many cases the veils can have pre-printed designs and logos, which become the part surface finish aiding the part appearance. Since pultrusion is a low-pressure process, fiberglass reinforcements normally appear close to the surface of the product. These can affect the appearance, corrosion resistance or handling of the products. The two most commonly used veils are A-glass and polyester .
Matrix Choice The resin matrix has several functions in a pultruded composite. The resin's basic functions are to fill the space between filaments, to fix the strand alignment, and to distribute the bonding and shearing stresses. Due to the much higher modulus of the glass, and its normally high percentage of volume in the composite, the strength effect of the resin is usually quite small. As in the case with all FRP/GRP material systems, the resin plays an important role in determining the chemical and environmental durability of the total system. It also controls the thermal, electrical, and visual The composite properties such as high-temperature performance, corrosion resistance, dielectric properties, flammability and thermal conductivity are determined exclusively by the properties of resin matrix.
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Pultrusion Process
Polyester Resins Unsaturated Polyester resins are most commonly used in pultrusion. Orthophthalic, isophthalic acids or anhydrides, in combination with maleic anhydride and various glycols, are the basic elements. Pultrusion polyester has the ability to gel and cure rapidly to form the strong gel structure required for release at the die wall. Generally resins with the viscosities of 500 are used for pultrusion. Higher viscosity low-reactive monomer versions can be blended with additional styrene to suit the processing need. The styrene level must be properly maintained to achieve good cross-link structure without having residual (unreacted) styrene in the finished composite. Polyester resins exhibit good corrosion resistance to aliphatic hydrocarbons, water, dilute acidic & alkaline environments. They do not perform well when exposed to aromatic hydrocarbons, ketons, and concentrated acids. A high degree of unsaturation in polyester chain exibits shrinkage up to 7% on curing. This level can be reduced using fillers and lowprofile additives. Composite based on polyesters retains high percentage of their electrical insulation properties even if used continuously at temperature up to 200oC. Though polyester s combustion without modification, hence backbone bromination or the use of additives greatly improves its flammability and smoke generation properties. The electrical properties of polyesters make them suitable for use as primary insulators in many high-voltage applications. Retention of electrical properties even at elevated temperatures has made polyester insulators the materials of choice in many applications. The weatherability of polyester is fair to good. Additional protection is usually through a variety of ultraviolet absorption additives or using polyester surface veils and even painting (done after pultrusion) Vinyl Ester Resins These resins are used when additional performance is sought. Vinyl esters (VE) offer better corrosion resistance, higher mechanical properties at elevated temperatures, and improved toughness properties such as impact and shear. They provide very efficient wet out and they have higher temperature capability with improved flexibility compared to polyester resins. The chemical structure of vinyl ester resins is such that the reaction sites are at the end of each polymer chain rather than along the chain resulting in rigid segments along the polymer backbone. This leads to lower-link density and high-temperature capability of these materials. VE resins are superior to polyesters, but this advantage comes at a cost in two ways: 1. VE resins can be as high as double the cost of polyester resins 2. VE resins usually run at speeds about 2/3 the speed of polyester due to their lower crosslink density. Many VE have a narrow temperature window. A 10°C temperature change can cause blistering in pultruded profiles. Operators should be aware of the small processing window which VE resins have. Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Epoxy Resins Epoxy resins typically offer the highest physical property performance as well as the best strength retention at high temperatures of all the resins used in pultrusion. Epoxies are frequently used for primary electrical insulation, aircraft, aerospace, and defence applications. Epoxies can be used in continuous use applications at 300°F (150°C) and epoxies provide increased flexural strengths and shear strengths when compared to polyesters and VE. Epoxies have excellent corrosion resistance and electrical properties. The disadvantages of epoxy resins can be: • Poor toughness as a result of their rigid structure • Can be more expensive to purchase • Slower processing speeds vs Polyester and VE resins • Lower pot life • Mold sticking considerations • More difficult to clean up They require a higher reinforcement content than either polyester or vinyl ester Other Resins A variety of resin alternatives is also available for specific applications. The resins based on Methacrylate Vinyl Ester Resins although expensive than polyesters but could be used for their special properties viz. improved physical properties, very low viscosity which allows them to be highly filled, rapid processing speeds, smooth profile surfaces and improved flame retardancy and weathering. One concern with MVE is odor which plant personnel may find objectionable. Phynolic resins are also used in pultrusion owing to their high heat resistance and flameretardancy/low-smoke characteristic. Phenolic resins are suitable typically for pultruding natural fibres such as jute. A desire to improve toughness and post processing formability has lead to the use of thermoplastic resins. The engineering thermoplastic resins provide excellent heat distortion properties. The technology for impregnating fibres with thermoplastic resins includes hotmelt application and solvent solution impregnation.
Filler and additives Filler and additives are used to enhance specific performance, reduce cost, influence viscosity, or improve processability of resin systems. Fillers can be incorporated into the resins in quantities up to 50% of the total resin formulation by weight. ). The usual volume limitation is based on the development of usable viscosity, which depends on the particle size and the characteristics of the resin. There are three fillers frequently chosen for use in pultrusion.
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Pultrusion Process
These fillers are: Calcium carbonate is the most popular and is used as a volume extender. Calcium carbonate is generally used where performance is not critical. Clay (alumina silicate) fillers are used for their corrosion resistance and in profiles requiring electrical insulation. They can provide very good surface finish. Alumina trihydrate is used when flame or smoke suppression is desired. Calcium carbonate is primarily used as a volume extender to provide the lowest-cost-resin formulation in areas in which performance is not critical. Special purpose additives include ultraviolet radiation screens for improved weatherability, antimony oxide for flame retardance, pigments for coloration, and low-profile agents for surface smoothness and crack suppression characteristics. Mould release agents (metallic sterates or organic phosphate esters) are important for adequate release from the die wall to provide smooth surfaces and low processing friction. Pigments may be used to impart color, weatherability, or flame retardency to the finished part.
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Pultrusion Process
Components of Pultrusion Process There are four components required in any pultrusion processing operation: 1. Creels 2. Forming or preforming guides 3. Resin impregnation systems 4. Primary die 5. Puller/clamping pads 6. Cut off saws
Creels The creel should provide a position from which the roving can be fed to the pultrusion process under controlled and uniform tension. It also provides a location for the transfer of the roving strand from the running package to a second back-up package for continuous uninterrupted production. There may even be room for extra roving packages for replacement or maintenance as required. The size, shape and type of creel will normally be determined by space considerations such as roving package dimensions, the distance the strand must be conveyed and the number of packages to be handled. The amount of glass being used on a continuous basis must also be considered. The two common types of creels used are shown. Each creel arrangement has a range of possible number of packages for the best process efficiency: Table creel - up to 50 packages Bookshelf creel - 20 to several hundred packages The bookshelf-type creel is the most common and usually provides the best balance of accessibility and maximum utilization of floor space. The size will vary widely, but the creels shown provide a standard module concept for creeling. Shown are creels for handling 32 packages (16 active-16 transfer) in both a side pull and end pull configuration. To handle different numbers of packages, the creels can be increased or decreased in length or multiple creels can be used.
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Pultrusion Process
In the end pull creel, commonly used in pultrusion, it may be necessary to guide each strand within the creel using steel rods or ceramic guide eyes. This can prevent sagging and whipping could tangle two or more strands on the same shelf. Most pultrusion processes use stationary roving packages on bookshelf or table creels, pulling the roving strand from the inside of the roving package. For some applications requiring minimal strand twist for better wet-out and a flatter and wider strand profile, pulling the roving strand from the outside of the roving package may be an option.
Forming guides It is important to determine how the reinforcement is organized, aligned, and fed into the primary die. Besides formulation and heating control, preforming system is also critical for successful and constancy pultrusion. Before entering die, impregnated fibreglass rovings and mats must be properly arranged and placed. Un-proper preforming system causes failure of pultrusion, bad quality, and other problems. Please note, preforming system is far from easy as most people think of. Firstly, it must be designed based on profile design to meet the physical requirements. Secondly, it must let all reinforcements running freely and smoothly, to avoid any breaking of rovings, mats or cloth. Thirdly, for dies with mandrels, the preforming system is the only device to keep uniform thickness of pultruded profiles. In-proper preforming may cause eccentric and even breaking of pultrusion. Fourthly, for complex profiles, preforming system is a large challenge. A preforming die gently shapes the material and removes all but about 10% of the excess resin prior to entry into the pultrusion die. There are two primary materials used in the forming guide tooling: steel and ultra high molecular weight polyethylene (UHMWPE). The advantage of steel is that it is less expensive (if carbon steel is used vs. stainless) and standard plate, sheet, bar, and rod can all be used. A disadvantage is corrosion of non-stainless steel and the difficulty to machine. The advantage of UHMWPE is that it is lighter, resistant to chemical attack, less damaging to the glass reinforcement, and therefore, easier Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
to clean. A major advantage is that it is easy to fabricate, on-line, when modifications are required. UHMWPE is easy to drill and machine slots for mat. A disadvantage is that UHMWPE wears faster than steel. Except for the primary die the forming guides and the guiding of the reinforcement into the die is the most important aspect in pultrusion technology
Resin Impregnation System The dimensions of resin baths are restricted to minimize the volume of catalyzed resin and can be heated to control the resin viscosity to promote fiber wetting, although this will reduce the working life of the bath. To facilitate lacing up, the roller assembly in the resin bath is manufactured in two parts—a lower fixed set of rollers submerged in the resin bath and a moveable upper set, under which the fiber is positioned. The assembly is then pressed down to push the fiber into the bath to with the lower set of rollers. This system facilitates an easy lace-up procedure and ensures good compaction to expel all air and promote fiber wetting. Alternatively, the fiber can be ed over a drum upon which the correct amount of resin has been metered and adjusted by a doctor blade.It is extremely important to allow the resin and reinforcement enough time to fully wet-out. The impregnation or bath system directly impacts wetout. There are three resin impregnation systems available today. They are: 1. Dip bath 2. Straight through bath 3. Resin Injection systems Dip Bath The dip bath, or open bath, has the reinforcements travelling from the creels down into the bath where the rovings go through an "S" bar guide, which breaks apart the roving bundle, allowing better coating of the filaments by the resin. The bath system is typically used for all roving reinforcements, as well as for simple mat and roving profiles where the mat can be handled horizontally, or where taking the mat out of the horizontal plane will not induce a bow into the finished profile. This system exposes a large amount of resin to the air and permits styrene evaporation into the plant environment. Styrene emission is a environmental and health consideration,which needs to be assessed with this impregnation system.
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Straight Through Bath A straight through resin bath is a trough with forming cards on each end. These cards can also be used to begin the forming process. The cards allow resin to leak from the open areas. The excess resin is then collected and pumped back into the bath trough. The advantage of this design is the reinforcements are taken from the horizontal plane, allowing the profiles to be made with fewer tendencies to warp or bow, and this design also reduces the amount of styrene released into the plant environment. Resin Injection System The resin injection system is the newest process for impregnation. With resin injection, a steel chamber is attached to the front of die. The chamber contains port(s), which allow resin to be injected into the cavity. The combination of cavity design, resin pressure, and movement of the reinforcement being pulled into the die generate hydraulic pressure forcing resin to penetrate the reinforcement bundle, resulting in impregnation. With resin injection the resin is not open to the plant environment, reducing the amount of styrene released into the workplace. Resin injection systems require reduced clean up time, due to resin with fewer components. The disadvantage of resin injection is the potential for incomplete impregnation of profiles with thick walls, or incomplete impregnation in resin systems with high filler loading incorporating a high number of mat or veil reinforcements.
Primary Die The die is the heart of the pultrusion system and is the limiting step in production rate since the part is both shaped and, usually, cured in the die. The processes of shaping and curing along with the correspondent line speed are dependent upon the shape of the part, the type of resin, the internal friction in the die, the heat expansion of the resin, the contraction of the resin, and mechanical warpage which may occur in the part because of nonsymmetries in the fiber orientations.
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Pultrusion Process
The opening of the die is usually somewhat larger than the final shape, permitting easy collection of the fibers bundle, and then the die interior dimensions gradually reduce in size until the final shape is achieved. During this shaping process, the part is cured. Cure is accomplished either by thermally heating the die (usually with common electrical heaters) or by subjecting the material to rf frequencies. Both of these systems have their advantages. The thermal heating is simple and can be used with metal dies, thus limiting die wear. However, the poor heat transfer of the resin means that as the thickness of parts increases, the speed of the pultrusion line must slow. Studies have shown that thicknesses of about .5 inches can be thermally cured at 2 feet per minute but that parts thicker than 3 inches cannot be cured at all using just thermal energies, regardless of the line speed. If rf curing is used, the thickness of the parts which can be cured and the line speeds possible are both improved — about 3 times faster with parts that are .5 inches thick. However, rf curing does not work well for metallic dies nor for conductive parts. Hence with rf curing, non-metallic dies are generally used and these are prone to rapid erosion and poor dimensional control. Parts with conductive components (such as carbon fibers) cannot be effectively cured using rf radiation. These materials are thermally cured. Recent machines which combine both thermally heated metal dies and rf heating after exiting the die have proven to give much better performance than either of the methods alone. Parts must be quite hard (essentially cured) when they exit the die so that they will not be deformed by the pulling mechanism, although some curing after exiting the die is possible if done before the pullers. Post-die curing can be done with a tunnel heater, although this adds considerable length to the line and is notoriously inefficient in heat use. Another method of post die curing is to use heated, moving C-shaped dies (also called split dies) that have cavities in the shape of the finished part and close on the part as it exists the die. These dies Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
are mounted on a moving belt or chain and stay in with the part long enough to insure that sufficient cure is achieved to withstand the forces of the puller. Off-line curing could then be used, as required.
The following are some considerations, which need to be taken into when purchasing primary dies: Material selection The die material should be capable of withstanding continual heating and cooling without deformation. Usually this means high-grade tool steel. The material should be capable of resisting wear from abrasion, and be damage tolerant for repeated assembly and disassembly. The material should be capable of receiving chrome plating of 0.001-0.002 inches (0.025-0.0508 mm) of thickness for wear resistance. Two widely used steels for die manufacture are A-2 hardened to 55-60 Rockwell hardness and P-20 prehardened to 28-30 Rockwell hardness. Shrinkage Factors It is the nature of most resins to shrink after reaching peak exotherm, and during the cooling process. Because of this a shrinkage factor must be calculated into the die design. This will enable the die to form the part to the proper dimensions after the part is completely cooled. A shrinkage factor cannot be unilaterally determined, as each resin system and reinforcement lay-up is different, however recommended shrinkage factors are: Thickness dimensions: 1% shrinkage All other dimensions 0.3% shrinkage Die Opening Design The die opening design must accommodate the smooth entry of reinforcements into the proper position. Generally a symmetrically shaped die is made to utilize either end of the die as entry or exit, if possible.This enables longer die life between re-chroming. At the opening of a die a minimum radius of 0.250 inches should be used. The die inlet is tapered at 7–100, with well rounded edges to prevent fiber fracture. One Piece or Split Cavity Dies The advantage of a one-piece die is that the finished part will not have a parting line.A onepiece gun barrel drilled die is usually less expensive to manufacture, however it may be more expensive in the long run. If a part seizes up inside a one-piece die during processing, the die may be impossible to repair.
Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Multiple piece dies have the advantage of being easily opened to allow cleaning and maintenance, however care needs to be given in deg the die so the parting line between the mold halves does not cause a problem with the part being molded. Die Surface Treatments Due to the abrasive nature of fiberglass reinforcements, a protective surface treatment is required on the die cavity. The most commonly used treatment is hard chrome plating at 0.001-.002 inches (25-50 microns) thick. For dies expected to perform in long term service, nitriding may be considered. Die Maintenance Die maintenance is one of the most important factors in extending die life. Opening the die after each production run and recording the number of feet of production and the wear conditions is important.Testing the die surface with copper sulfate for wear spots is critical.The sooner die wear can be detected and treated the longer the die will last before major rework.The best time to perform die inspection and maintenance is just after the die has been pulled from production, prior to being stored. Inspection just prior to installation is not recommended, as production pressures may make repairs, if needed, difficult, shorting the life of the die and potentially compromising part quality. Acidic mold release agents are often used to ease the separation of the part from the metal die. Steel dies exposed to acidic conditions must be cleaned thoroughly between uses to maximize the lifetime of the die. Puller Clamp System The pultruded product is cooled prior to the traction unit, which can be a counter rotating caterpillar unit, or preferably, a hand-over-hand reciprocating clamp type unit, since the caterpillar unit requires the tracks to be fitted with machined rubber pads to accommodate each pultruded profile. The hand-over-hand unit grips above and below and while one unit is pulling, the other unit returns to position, ready to take over the role of pulling. Typical line speeds vary in the range 1.5–100 mh1 , depending on the section(s) being produced. The pulling forces depend on the type of machine which are available upwards to some 30 MT. There are two common puller systems 1. Caterpillar belt 2. Reciprocating clamp puller
Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
In both systems, pads, typically made of urethane, must be shaped to match the part profile in order to apply a uniform clamp load, which will not cause damage to the pulled part. The advantage of the caterpillar belt system is the capacity to provide large pulling force, spread over a larger part surface area. The advantage of the reciprocating clamp puller system is cost, as it requires only 2 puller pads per clamp vs 10-100 depending on the caterpillar belt size.
Cut off Saws Most pultruders utilize what are known as flying cut off saws. A flying cut off saw moves at the same speed as the moving part, so the cut edge of the part is square and straight. Using a nonflying saw results in cut edges that are not square and straight. Flying cut off saws are recommended for part quality. There are two basic types of flying cut-off saws: 1. Wet saw – A wet saw uses water during the cutting cycle to cool and lubricate the blade and flush the fiberglass particulate to a filter. This effectively eliminates dust and airborne particulate. 2. Dry saw – A dry-cut saw uses a continuous rim diamond blade that does not require any fluid during the cutting cycle, but which gives a good clean cut. A dry-cut saw requires a dust collection system to capture the fiberglass dust, both to address operator health and comfort and from a housekeeping standpoint. Using a dry-cut saw without a properly designed dust collection system is not recommended, due to the dust generated during cutting, both from an operator health/comfort and a housekeeping standpoint. In most cases pultruders will utilize an automatic cut-off saw, which automatically cuts the part into the proper lengths. This enables employees to carry out other duties in order to enhance productivity.
Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Basic Processing Steps The basic pultrusion process can be divided into the following operations: 1. Reinforcement handling A suitable creel positions the requisite number of tows, with minimal damage, prior to entry into the resin bath. If tows are supplied from containers, a christmas tree with ceramic eyelets will be required to direct the tows to the resin bath. In some pultruded sections, smaller tows are used in parts where the profile shape does not permit the use of larger tows. Higher size contents (2–5% w/w) will permit easier handling and minimize fiber damage. Tows can be ed by knotting, but knots must be staggered to ease their age through the die. Hybrid constructions can be used (e.g. with glass and aramid) and transverse properties can be introduced with an In-feed system using woven cloth, knitted fabrics, braid, or mat 2. Resin impregnation The dimensions of resin baths are restricted to minimize the volume of catalyzed resin and can be heated to control the resin viscosity to promote fiber wetting, although this will reduce the working life of the bath. To facilitate lacing up, the roller assembly in the resin bath is manufactured in two parts—a lower fixed set of rollers submerged in the resin bath and a moveable upper set, under which the fiber is positioned. The assembly is then pressed down to push the fiber into the bath to with the lower set of rollers. This system facilitates an easy lace-up procedure and ensures good compaction to expel all air and promote fiber wetting. Alternatively, the fiber can be ed over a drum upon which the correct amount of resin has been metered and adjusted by a doctor blade. 3. Pre-die forming A preforming die gently shapes the material and removes all but about 10% of the excess resin prior to entry into the pultrusion die. 4. Heated die to shape and cure the resin The pultrusion die can be made from polished chromium plated tool steels, or when pultruding epoxies, a high chromium content tool steel. The die must be accurately lined up and its length typically 300–1000 mm, which is governed by the size of the section being pulled, the pulling speed and the resin system. Longer dies require greater pulling forces due to the increased frictional drag and a die lubricant, such as zinc stearate, can be added to the resin mix to help reduce frictional resistance, but which may interfere with any subsequent composite bonding process. The die inlet is tapered at 7–100, with well rounded edges to prevent fiber fracture. The excess resin exudes from the inlet end of the die,
Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
causing the entering fiber bundle to swell, eventually attaining equilibrium with the process conditions. Adding this exuded resin to the resin bath will curtail the life of the bath. Cartridge or plate heaters are preferred for heating the die to a uniform temperature within ±10C and maintaining a temperature gradient along the die to avoid premature gelation, while taking into any exotherm. An RF (Radio Frequency wave generator) unit can be used to either heat the fiber entering the die or the die/resin. The die must be preheated prior to commencement of pultrusion. Shrinkage during polymerization reduces die forces and should always be arranged to be greater than the thermal expansion caused by the temperature rise. 5. Pulling unit to provide traction The pultruded product is cooled prior to the traction unit, which can be a counter rotating caterpillar unit, or preferably, a hand-over-hand reciprocating clamp type unit, since the caterpillar unit requires the tracks to be fitted with machined rubber pads to accommodate each pultruded profile. The hand-over-hand unit grips above and below and while one unit is pulling, the other unit returns to position, ready to take over the role of pulling. Typical line speeds vary in the range 1.5–100 mh_1, depending on the section(s) being produced. The pulling forces depend on the type of machine which are available upwards to some 30 MT. 6. Cut off saw Once the pultruded section has left the die and cooled sufficiently, it is clamped and a flying saw moves along with the clamped section to cut off required lengths. Extra long lengths can be accommodated by feeding the pultrusion out through a door, window or hatch at the end of the building. 7. Post cure oven For optimum properties, all pultruded sections will require post curing and care must be taken to ensure adequate along the entire pultruded length to prevent deformation occurring in the post cure oven.
Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Following are some of the considerations while manufacturing and deg pultruded parts. Wall Thickness Wherever possible, select uniform thickness in the cross-section because it provides uniform cooling and curing, and thus avoids the potential of residual stress and distortions in the part. Moreover, uniform thickness will provide uniform shrinkage in the part and thus will limit the warpage in the product. Typically, 2 to 3% shrinkage occurs in the pultruded part. Also, maintain symmetry in the cross section for minimal distortion. For high-volume production, the thickness of the part is critical because the curing time and therefore the rate of pull depend on the thickness of the part. For example, a 0.75-in. thick cross-section can be produced at a rate of approximately 9 in./min, whereas a 0.125-in. thick cross-section can provide a production rate of 3 to 4 ft/min. Therefore, if a design requires high rigidity in the part, then it can be achieved by creating deeper sections with thinner wall or by including ribs in the cross-section. Similarly, if there is a choice between selecting a thick rod or tube, select the tube because it offers a higher production rate, lower cost, and higher specific strength. Corner Design In a pultruded part, avoid sharp corners and provide generous radii at those corners. Generous radii offer better material flow at corners as well as improve the strength by distributing stress uniformly around the corner. A minimum of 0.0625-in. radius is recommended at corners. Another important consideration in the design of corners is to maintain uniform thickness around the corner. This will avoid the build-up of resin rich areas, which can crack or flake off during use. Moreover, uniform thickness will provide uniformity in fiber volume fraction and thus will help in obtaining consistent part properties. Tolerances, Flatness, and Straightness Dimensional tolerances, flatness, and straightness obtained in pultruded parts should be discussed with the supplier. Standard tolerances on fibreglass pultruded profiles have been established by industry and ASTM committees. Refer to ASTM 3647-78, ASTM D 3917-80, and ASTM D 3918-80 for standard specifications on dimensional tolerances and definitions of various relating to pultruded products. Pultrusion is a low-pressure process and therefore does not offer tight tolerances in the part. Shrinkage is another contributing factor that affects tolerances, flatness, and straightness. The cost of a product is significantly affected by tolerance requirements. Tight tolerance implies higher product cost. Therefore, whenever possible, provide generous tolerances on the part as long as the functionality of the product is not affected. Surface Texture Pultrusion is a low-pressure process and typically provides a fiber-rich surface. This can cause pattern-through of reinforcing materials or fibers getting easily exposed under wear or weathering conditions. Surfacing veils or finer fiber mats are used as an outer layer to minimize this problem. To create good UV and outdoor exposure resistance, a 0.001- to 0.0015-in. thick layer of polyurethane coating is applied as a secondary operation. Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
Advantages of using pultrusion Pultrusion has a number of benefits over other composite processing systems. Some of the lowest cost, highest quality composite profiles are created by this process. This is because it is automated and has very little manual interface. A manufacturer can be assured the 1st ten-meters of pultrusion will have the same quality and consistency as the 100th ten-meters of pultrusion. Human interface is eliminated, as required in most other processes, such as molding and hand-lay-up. Quality is not a function of motivation of factory technicians. Another distinct advantage of the pultrusion process is cost. It is not unusual to find 80-90% of the cost of pultrusion profiles are the raw material costs. The amortized machine costs and the labor to run pultrusion machines is a small portion of the total factory costs. This has been a primary driver for pultrusion being one of the fastest growing and accepted manufacturing processes in the composites industry. Features
Strong
Light Weight
Corrosion Resistant
Electrical Insulation
Description
Benefits
Applications
Unit strength in tension & compression is approx. 20 x that of steel when these properties are combined on the basis of unit density
Optional strength as desired. Exceptionally high impact strength reduces damage potential
Structural process equipment . Tank s. Cooling tower ancillaries. Flooring s. Trusses & ts.
Density of pultruded components is about 20% of steel and 60% of aluminium
Higher performance at less weight. Lower shipping, handling & installation costs. Less operational energy demand.
Automotive leaf springs & bumpers. Prefabricated walkways & platforms. Bus components.
Chemical plant hand Unaffected by exposure to Minimum maintenance railings, gratings, a great variety of corrosive costs. Long term safety. walkways & bridges. environment & chemicals. Longer life. Cable trays. Pipe s.
Provides strength & rigidity with dielectric properties.
Lesser no. of components. Nonmagnetic & safe. Predictable insulation values for wide range of frequencies.
Manipal Institute of Technology Department of Mechanical and Manufacturing
Ladders, Cable trays. Switch gear components. Mounting braces and backboards.
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Pultrusion Process
Thermal Insulation
Consolidation
Dimensional Stability
Safety
Pultruded components have a low thermal conductivity, 1/250 of aluminium & 1/60 of steel.
Reduces installation thickness. Eliminates condensation problems. Reduces energy operation requirements.
Many individual components can be combined into a large profile.
Reduced assembly cost. Window latch s. Reduced inventory. Roll up door Fewer parts improve reliability.
Pultruded components are highly resistant to warping stretch/swelling over a wide range of temperature & stresses.
No permanent Spring bumpers. deformation under high Crossing gate arms. stress. Close Scrubber components. tolerances.
The pultruded components are very strong & safe to work with. They are microbes and insect proof.
Many gratings suffer from the problem of microbes etc. due to wet or unhygienic working conditions.
Manipal Institute of Technology Department of Mechanical and Manufacturing
Bulk head frames. Walk in refrigerator door jams. Window frames. Insulated roll up doors.
This property makes them ideal choice for pharmaceutical & food industries.
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Pultrusion Process
DISADVANTAGES OF PULTRUSION 1. It is suitable for parts that have constant cross-sections along their length. Tapered and complex shapes cannot be produced. 2. Very high-tolerance parts on the inside and outside dimensions cannot be produced using the pultrusion process. 3. Thin wall parts cannot be produced. 4. Fiber angles on pultruded parts are limited to 0°. Fabrics are used to get bidirectional properties. 5. Structures requiring complex loading cannot be produced using this process because the properties are mostly limited to the axial direction. 6. Voids may result in parts if excessive opening given at die entrance 7. Standards play an important role in acceptance of new materials. The lack of design standards is a significant constraint to the use and growth of composites in structural applications. 8. Shrinkage (commonly 2% - 3%)
Manipal Institute of Technology Department of Mechanical and Manufacturing
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Pultrusion Process
References Pultrusion of Glass Fiber Composites - A Technical Manual (Owens Corning) PULTRUSION — HIGH PRODUCTIVITY NOW, GETTING EVEN BETTER, By A. Brent Strong/Brigham Young University Manufacture by Pultrusion - Dr J M Methven, MACE Pultrusion of Composites - An Overview, Atul Mittal & Soumitra Biswas Carbon Fibers and Their Composites - Peter Morgan COMPOSITES MANUFACTURING - Materials, Product, and Process Engineering, Sanjay.K.Mazumdar, Ph.D.
Manipal Institute of Technology Department of Mechanical and Manufacturing
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