Part C: Manufacturing

Transcription

1 Part C: A key aspect of composite materials technology is successful manufacturing of the material and component (in many cases carried out at the same time). Reliable and costeffective manufacturing methods leading to high quality products need to be used. Cost effectiveness depends largely on high rates of production, and reliability requires a uniform high quality. This can be a challenging task and often manufacturing becomes a critical issue in composite material projects. Mainly, manufacturing routes for polymer matrix composites have existed for many years, investigations into the basic characteristics and process optimization started in the mid-70s, and are technically mature now. Also, novel routes are developed. Processing of both, thermosetting and thermoplastic matrix composites is discussed. Fundamentals There are three elementary tasks carried out during processing of a PMC: fibres must be wetted out, forming (shaping) must take place, and depending on the manufacturing route, drainage of resin must take place. Frequently, motivated by practicality, all the tasks (stages) need to be carried out in a single operation. As mentioned in Part B, most of polymer composites are based on thermosets, and we start by discussing the curing and other aspects strictly related to thermosetting matrices. 1

2 Transformation of uncured or partially cured fibre-reinforced thermosetting polymers into composite parts or structures involves curing the material. Frequently, elevated temperatures and pressures are involved. The magnitude of these two important process parameters, as well as their duration, frequently enables achieving the manufacturing goal, and significantly affects the service performance of moulded products. A number of crucial processes and factors can be involved, and we will briefly discuss selected ones below. Degree of cure The heat evolved in a curing reaction can be related to the degree of cure achieved at any time during the curing process. For this purpose, differential scanning calorimetery (DSC) is used, where specimen is subject to an isothermal and non-isothermal heating. In Fig. C.1, the heat flow (or rate of heat generation) vs. time, during non-isothermal and isothermal heating is shown Figure C.1 Schematic representation of the rate of heat generation in: (a) nonisothermal and, (b) isothermal, heating of a thermoset polymer in a DSC measurement. (From Mallick, [ref. 1], Fig. 5.1) The total heat generated needed to complete a curing reaction (i.e., 100% degree of cure) is denoted H R, heat of reaction (shown in Fig. C.1a). This heat can be measured only during an increasing temperature experiment (non-isothermal, Fig. C.1a). The amount of heat released in time t, at a given constant curing temperature is denoted H (isothermal, Fig. C.1b). Degree of cure, α c, at any time is then defined as 2

3 α c = H/H R. (B.1) α c increases with both time and temperature, however, the rate of cure, dα c /dt, is decreased as the degree of cure attains asymptotically a maximum level. If the cure temperature is too low, the degree of cure may not reach the 100% level for any reasonable length of time. er temperature increases the rate of cure and produces the maximum degree in shorter periods of time. Viscosity The two most important factors determining viscosity are the temperature and shear rate; the latter one only for high-molecular weight fluids. Viscosity tends to either increase (shear thickening) or decrease (shear thinning), for high molecular weight fluids. Polymer melts, in general, are shear thinning fluids. The starting material (uncured liquid) for a thermosetting resin is a low-viscosity fluid. Its viscosity increases with cure time and temperature and approaches large values as the material is transformed into a solid state. Rate of viscosity increase is low at early stages of curing, and next increases at a rapid rate. The time at which this occurs is called the gel time. Flow of resin in a mould, impregnation, becomes increasingly difficult at the end of this stage. A number of important observations can be made from the viscosity data published in the literature, that is B-staged resin has a much increased viscosity addition of filler, such as calcium carbonate, increases the viscosity as well as the rate of viscosity increase. On the other hand, addition of thermoplastic additives, tends to reduce the rate of viscosity increase during curing increase in viscosity with cure time is less if the shear rate is increased (shear thinning) for B-staged or thickened resins than for neat resins. Fillers and thermoplastic additives tend to increase the shear.thinning behaviour at a constant shear rate and for the same degree of cure, viscosity vs. 1/T plot (T = temperature in K) is linear. This suggests that viscous flow of a thermoset is an energy-activated process. The activation energy for viscous flow increases with the degree of cure, and approaches high value near the gel point. Gel time test Curing characteristics of a resin-catalyst combination are frequently determined by the gel time test. In this standardized test, a small amount (10 g) of a thoroughly mixed resincatalyst combination is poured into a test tube, and the temperature rise is monitored as a function of time by means of a thermocouple, while the test tube is suspended in a 82 C water bath. In this test, the gel time is determined from the temperature vs. time plot. It is sometimes measured by probing the surface of the reacting mass with a clean wooden 3

4 applicator stick every 15 sec until the reacting material no longer adheres to the applicator. Shrinkage Shrinkage is the reduction in volume or linear dimensions caused by curing as well as thermal contraction. Curing shrinkage occurs because of the re-arrangement of polymer molecules into a more compact mass, as the curing proceeds. Addition of fibres or fillers reduces the volumetric shrinkage of a resin. In the case of UD fibres, reduction in shrinkage in the longitudinal direction is higher than in the transverse direction. Shrinkage in UP or VE resins can be reduced significantly by the addition of thermoplastic polymers, such as PE, PMMA, PVAc, and polycaprolactone. resin shrinkage can contribute to moulding defects, such as internal stress, warpage and sink marks, however, can be desirable for easy mould release. Heat transfer Heat transfer is also of great importance in composite manufacturing, since it can govern crosslinking, melting, solidification and crystallization, as well as the efficiency of the manufacturing process. Voids Presence of voids is considered the most critical defect in influencing the mechanical properties, most often in a negative way. Voids are formed during processing of the nonsolidified matrix, and originate from incomplete impregnation, wetting-out of fibres resin cavitation during deformation air or vapours entrapment (from resin viscosity control, relative values of fibre and resin surface energies, moisture, chemical contaminants in the resin, styrene monomer, they may remain dissolved in the resin mix and volatilize during elevated temperature curing) too high viscosity emulsification, when fibres are pulled through the liquid resin. Much of the air or volatiles entrapped at the pre-moulding stages can be removed by degassing the liquid resin 4

5 applying vacuum during the moulding process allowing the resin mix to flow freely in the mould, which helps in carrying the air and volatiles out through the vents in the mould. Volume fraction of voids > 2 % indicates deterioration in quality (0.5 % in the aerospace industry). Mechanical properties strongly deteriorate with increasing volume fraction of voids, roughly by 7% per 1% voids, in the range up to 4% voids. Also, presence of voids generally increases the rate and amount of moisture intake in a humid environment, which in turn increases product dimensions, and reduces its matrix-controlled properties. Voids can be formed in resin rich pockets fibre-free regions between laminae on individual fibres (5-20 µm). There are four main methods of measuring the void content examination of polished sections gravimetric method C-scans, water absorption. Mould One of the most critical aspects in manufacturing of composites is the mould itself (also the term tool, mandrel and die are applicable). Preparing a well-functioning mould can be a great challenge. Details of mould design and fabrication is beyond the scope of this course, and only basic issues will be discussed. Typically moulds are manufacturing technique specific, though in some cases they can be used in more than one technique. In all cases, the material the mould is made of is very important. Materials that have been successfully used for moulds include metals, composites, polymer foams, neat polymers, rubber, wood, plywood, chipboard, ceramics, concrete, monolithic carbon, plaster of Paris, sands and salts. Particularly salts and sands are used as disposable moulds, for example in filament winding. Clearly, mould material must withstand moulding conditions, particularly the pressure, temperature and series length. Cost of the mould depends on the material, and is an important consideration; for example steel moulds can be very expensive. An important 5

6 criterion for mould material selection is thermal expansion. To avoid inaccuracies in composite component dimensions and inclusion of residual stresses, the coefficient of thermal expansion (CTE) of the mould should be closely matched to that of the component. CTE is an important consideration also towards de-moulding, that is removal of the ready part from the mould. There are three conceptually different ways in which a mould of a given shape can be fabricated: direct manual fabrication (common in boat and shipbuilding; low cost, economically feasible for manufacturing prototypes, one-of-a-kind components, short series; disadvantages are rather low geometric precision and low durability) use of a master model (common in aerospace applications; high degree of precision may be maintained and several identical moulds can be produced, however fabrication can be time consuming and cost can be high) direct machining from a block of material (materials used with this approach include wood, expanded polymer foams, syntactic foams, high grade tool steels, monolithic carbon and others; where steel moulds are used there is usually a need for closed moulds; metal moulds are very expensive, they are durable and tolerate high pressures and temperatures, and thus are the norm in compression moulding, injection moulding and pultrusion). In some manufacturing techniques employing open moulds, vacuum is used to apply consolidation pressure onto the composite part as it solidifies. This is achieved by applying a vacuum bag over the part placed in a rigid mould, sealing the bag around the perimeter, and evacuating the air from under the bag (flexible mould). Shaping caul plates, pressure pads and dams are used to facilitate the compaction, as shown in Fig. C.2 6

7 Figure C.2 Schematic of vacuum bagging, and use of caul plate, pressure pad, and dams. (From Åström, ref. [2], Fig. 4-5) Caul plates and pressure pads may be rigid or flexible (elastomeric). Another type of flexible mould is where an elastomer tube or bladder is used. In this case pressurized air or water can be used to compact the moulded composite part. This type of moulding is frequently used to consolidate hollow components against rigid external mould. Overview of manufacturing routes Although some manufacturing routes can be used both, for thermoset and thermoplastic composites, it can be helpful to divide them, conventionally, into these two main groups. Where applicable, the recent modification of these traditionally thermosetting techniques towards thermoplastic composites, will be additionally explained. Thus, there are three broad main approaches to thermoset composites processing liquid resin impregnation - (wet) hand lay-up and spray lay-up (also referred to as contact moulding) - filament winding - pultrusion - Resin Transfer Moulding (RTM) - Vacuum Infusion Process (VIP) (also referred to as Vacuum Injection Moulding) pressurized consolidation of prepregs - directional pressing in a hydraulic press - autoclave moulding - vacuum bagging - prepreg lay-up pressurized consolidation of other moulding compounds - compression moulding (SMC, BMC) - Reaction Injection Moulding (RIM), and two broad main approaches to thermoplastic composites processing injection moulding hot and cold pressing 7

8 - compression moulding (GMT) - prepreg lay-up. routes can also be divided into open-mould processes and closed-mould processes, continual process (practically only pultrusion) and non-continual processes (remaining ones). In the following, main manufacturing routes are briefly outlined both, for thermoset and thermoplastic composites, where applicable. Hand lay-up and spray-up For thermosetting composites, CSMs, woven rovings and other fabrics are placed on the mould and impregnated with resin by painting and rolling. Layers are built up until design thickness is achieved. The mould, free from surface defects and clean, is treated with a release agent (waxes, film formers, lubricating agents or anti-adhesive films). Next, a gel coat (for example isophthalic polyester resin) is applied to a thickness of mm. Gel coat will produce a smooth, cosmetically appealing surface. It is often pigmented and hides the reinforcement structure which otherwise may be visible, unattractive, on the composite surface. After the gel coat becomes tack-free, a coat of matrix is brushed (or sprayed over). Next, woven fabric or CSM is applied, and the procedure is continued until the required thickness is reached. Stacked layers are compacted with rollers. Now curing takes place. Spray-up is less labour and time consuming, and in this case short reinforcing fibres and lower viscosity polyester grades are used. The lay-up technique is now used also for thermoplastic composites. Thermoplastic plies are cut to shape and applied ply-by-ply. The first ply is kept in place, say by adhesive tape. Each subsequent ply is heat tacked to the one below it, for example using a blunt shaped soldering iron. This causes local melting and fusion of the resin to the resin of the adjacent ply. Lay-up is the oldest composite manufacturing method, nevertheless a great deal of components and structures are made in this way, such as automobile, truck, and bus components; boat hulls; windmill blades, storage tanks and vats, bathroom interiors and pools. An obvious advantage is that very large parts can be manufactured, and the disadvantage is that the method is labour consuming. Further disadvantages are difficulty in obtaining high, reproducible quality and high fibre volume fraction. 8

9 Filament winding In a filament winding process, resin (thermosetting or thermoplastic) impregnated fibres (sometimes prepreg sheets) are wrapped around a rotating mandrel to produce axisymmetric or non-axisymmetric hollow parts. In Fig. C.3, a schematic of the process is shown Figure C.3 Schematic of filament winding.(from Mallick, [ref. 1], Fig. 5.23) A large number of rovings are pulled from a series of creels into a liquid resin bath containing the catalyst and other ingredients. Next, the impregnated fibres are pulled through a wiping device to remove excess resin and to control the thickness of coating. Now the fibres are placed on the mandrel. The winding speeds are typically 90 to 110 m/min (at higher speeds non-desirable emulsifying of the resin can takes place and less precise fibre placement can take place). The speed of the resin bath is synchronized with the winding speeds to create the desired helix line at wanted helix angle. In another type of filament winding the mandrel is stationary and whirling arm winds the roving. Polar winding is also used. After winding a number of layers to generate the desired thickness, the filament-wound part is generally cured on the mandrel. The mandrel is then removed from the cured part 9

10 (collapsable, segmented or inflatable mandrels are used). In some cases the mandrel can stay inside the manufactured part. Above described is so called wet winding (low viscosity resin is applied to tows or rovings and wound). A certain modification is a process called tape placement (for example, ATLAS- automated-lay-up system, used in production of helicopter blades), where prepregs are wound. The common defects of filament winding are voids, delaminations and fibre wrinkles. Voids may appear because of poor fibre wet-out, emulsifying of the resin bath, excessive resin squeeze-out from inner layers by too high fibre tension, excessive time lapse between two consecutive layers of winding. Wrinkles result from improper winding tension and misaligned rovings. The main advantage is a high performance, reproducible product. This is supported by computer control of fibre tension and accurate winding angles. Among applications are pipes, drive shafts, pressure vessels, helicopter blades, military launching tubes, masts. Process and component characteristics are summarized in Tab. C.1 Table C.1 Filament winding process and component characteristics (From Åström, ref. [2], pp ) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns Short to intermediate Long Component Geometry Convex and closed Size Any Holes and inserts Holes difficult, inserts possible Bosses and ribs Not possible Undercuts Not possible Surfaces Outer surface poor, inner surface good Fibre arrangement Oriented and continuous Typical fibre volume fraction Void fraction 10

11 Mechanical properties Quality consistency Pultrusion Pultrusion is a continual process where impregnated fibres are passed through a heated mould/die for producing long, mainly prismatic components like solid rods, hollow tubs, flat sheets, various beams (angles, hat-sectioned etc.). In more recent years, pultrusion processes for producing variable (non-prismatic) cross-sections and curved members have also been developed. Thus the major constituent is UD continuous strand roving. Layers of CSMs or woven rovings can be added at or near outer surface. Thermoplastic polyester surfacing veils can be added to fibre-resin stream just outside resin bath to improve surface smoothness. In total, the fibre content may be as high as 70 wt.%. Instead of open-bath impregnation, as represented in fig. C.4, injection can be used. Figure C.4 Schematic of an open-bath pultrusion process. (From Mallick, ref. [1], 5.23) As represented in Fig. C.4, fibres and mats are pulled from creels into the precatalyzed resin bath, and then pulled through series of preformers (in order to distribute fibre evenly, squeeze out excess resin, and carefully form the cross-section). Then they are pulled through a long heating (gradual tapering) die where lateral pressure improves resin 11

12 infiltration and wet-out), and curing takes place. The most important factor controlling the mechanical properties is the fibre wet-out. Unsaturated polyester, vinyl ester and less commonly epoxy based composites are processed by pultrusion. The resin viscosity in commercial pultrusion lines may range from 400 to 5000 cp. The highest known line speeds are about 5 m/min, however the quality may deteriorate at such high speeds. Control of pulling force and design of fibre guidance is important as they influence fibre alignment and wet-out. Defects found in pultrudates (fibre bunching, shifting, wrinkles, folding of mats) are related to these factors. Among the common pultruded products are structural beams, hollow tubes, flat sheets, mobile phone antennas, sporting equipment and other. Process and component characteristics are summarized in Tab. C.2 Table C.2 Pultrusion process and component characteristics (From Åström, ref. [2], pp ) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns Long Long with open bath, low with injection Component Geometry Straight or curved, prismatic or non-prismatic, solid or hollow Size Any length, cross-section limited by machine size Holes and inserts Longitudinal holes and inserts possible Bosses and ribs Longitudinal ribs possible Undercuts Longitudinal undercuts possible Surfaces Good Fibre arrangement Random or oriented, continuous or discontinuous Typical fibre volume fraction Void fraction Mechanical properties to high Quality consistency 12

13 Resin Transfer Moulding (RTM) Liquid Composite Moulding (LCM) is a generic name for a number of processes involving injection of resin into a pre-form of dry fibres. Two of the most important processes are Resin Transfer Moulding (RTM)(less known as Resinject, VARI or TERTM) and Reaction Injection Moulding (RIM). The latter one will be discussed separately, ahead. The RTM is a two-stage process. First, a pre-form consisting of fibres and possibly a foam core, even metal inserts, is prepared (pressed) in the general shape of the finished part. A binder is used to preserve integrity of the preform. Second, the pre-form is placed in a closed mould and the liquid resin is injected under pressure. The resin and curing agent are premixed before injection into mould, and a low viscosity mix then injected into the mould at relatively low pressure (0.1 to 1 MPa), at room or elevated temperatures. Due to low pressures and temperatures moulds can be inexpensive. As examples, boat hulls, tennis rackets, marine propellers, crash helmets and relatively large body parts for the automotive industry are made in this way. Process and component characteristics are summarized in Tab. C.3 Table C.3 RTM process and component characteristics (From Åström, ref. [2], p. 228) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns to intermediate Short to intermediate Component Geometry Any Size Any Holes and inserts Possible Bosses and ribs Difficult Undercuts Difficult Surfaces Good Fibre arrangement Random or oriented Typical fibre volume fraction Up to 0.6 Void fraction 13

14 Mechanical properties Quality consistency Vacuum Infusion Moulding (VIM) (or Process, VIP) (or Vacuum Injection Moulding) VIM is a composite manufacturing process suitable for producing high-quality largescale (say, up to 1000 m 2, up to 15 cm thick) components. A stack of dry fibres is placed between a rigid mould half and a flexible and airtight bag. The bag is sealed to the mould, except at certain positions being open for resin inlets and outlets. Liquid resin is then forced into the stack by reduction of the pressure at the outlets, while keeping the pressure atmospheric at the resin inlets. The flow-enhancing layer facilitates the infusion by allowing the resin to spread quickly over the lateral extent of the part. A schematic of the process is shown in Fig. C.5 Figure C.5 Schematic of vacuum infusion process. (From Andersson, [ref. 3].) Advantages of this process are: possibility to produce large surface area components increased fibre to resin ratio stronger laminate low void content reduced volatile organic compounds (VOC) emissions 14

15 Common applications include: boats, yacht hulls, offshore structures, vehicle body panels, wind turbine rotor blades and others. The process and component characteristics are summarized in Tab. C.4 Table C.4 Vacuum Infusion Process and component characteristics (From Åström, ref. [2], p. 230) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns Component Geometry Size Holes and inserts Bosses and ribs Undercuts Surfaces Fibre arrangement Typical fibre volume fraction Void fraction Mechanical properties Quality consistency low Short Long Any Any Possible Difficult Difficult One good surface Random or oriented Up to 0.4 (fabrics) Vacuum bag, pressure bag, autoclave moulding There are various ways used to realize the idea of consolidating a composite using a flexible airtight bag. The airtight bag moulding process in an autoclave is used predominantly in aerospace industry where high rate of production is not an important consideration. The starting material are β-stage epoxy prepregs which should be tacky and of good drape. They contain ~ 50 vol.% fibres. To prevent moisture pickup (condense on removal from the deep freezer) the prepregs should be allowed to dry 15

16 before use. Schematic of the bag moulding process in an autoclave is shown in Fig. C.6 Figure C.6 Schematic of autoclave moulding. (From Åström, ref. [2].) The entire assembly is placed inside a preheated autoclave where a sequence of vacuum and pressure + temperature is applied to consolidate the laminate. Typically: apply vacuum and slowly increase temp. to 130 C dwell at 130 C (with vacuum on); when viscosity is at minimum switch on pressure (~7 bar) for about 60 min; during this time excess resin flows out Excess* resin flowing out of the prepreg during dwelling is desirable because it removes the entrapped air and residual solvents, which reduces the void content towards high quality composites *) There is an interest though to reduce this excess without lowering the quality of the composite. Traditionally up to 10% resin flowed out, today this amount can be around 1%, and methods to achieve high quality without resin flowing out are investigated. further increase temperature to 175 C and cure for 2 h slowly cool and switch of vacuum and pressure. 16

17 Temperatures up to 380 C are also in use, depending on the epoxy grade. Applications include: high-performance racing vehicles and yachts, aircraft control surfaces and skins, sport equipment, etc. The autoclave moulding process and component characteristics are summarized in Tab. C.5 Table C.5 Autoclave moulding process and component characteristics (From Åström, ref. [2], pp ) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns Short to intermediate Long Component Geometry Any Size Limited by autoclave size Holes and inserts Possible Bosses and ribs Possible Undercuts Difficult Surfaces One good surface Fibre arrangement Oriented and continuous Typical fibre volume fraction Up to 0.7 Void fraction Mechanical properties Quality consistency Vacuum bagging usually refers to a simple workshop-like kit containing a flexible airtight bag allowing to consolidate composites by means of atmospheric pressure. Compression moulding Compression moulding is frequently used to process moulding compounds, mainly SMC and BMC. The principal advantage of compression moulding is its ability to produce in one-go, at a low cost, large series of complex geometry parts. 17

18 One-piece car grilles, wheels, bumpers, leaf springs, and others are typical examples of processing by compression moulding. The schematic of compression moulding is shown in Fig. C.7 Figure C.7 Schematic of compression moulding press. (From Mallick, ref. [1], Fig ) Typically the processing conditions are: pressure 1 to 35 MPa, temp C, time 1-6 min (preheating of the charge can shorten the time). The charge (e.g. a stack of SMC layers) is smaller than the mould surface (60-70%). Flow in the mould is of great importance, it must take place towards high quality, and is complex and determines fibre orientation, distribution, and defects Compression moulded SMC parts, when poorly controlled, may contain a wide variety of 18

19 surface and internal defects. The surface defects create a poor appearance, and the internal defects will affect the performance. The origin of defects are: entrapment of air resulting in porosity, blisters due to excessive gas pressure from un-reacted monomer or blisters again due to entrapped air. Weld lines (also called knit lines) are linear domains of poor fibre orientation and are formed at the joining two divided flow fronts. Since fibres tend to align themselves along the weld line, the strength of the part in a direction perpendicular to the weld is reduced. A similar effect can appear if the mould leaks. Warpage is critical in thin-section mouldings and is caused by variations in cooling rate between sections of different thicknesses or different fibre orientation. In-mould coating is used to mask surface defects. The process and component characteristics are summarized in Tab. C.6 Table C.6 Compression moulding process and component characteristics (From Åström, ref. [2, pp ) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns to intermediate Long Short Component Geometry Any Size Limited by press size Holes and inserts Possible Bosses and ribs Possible Undercuts Difficult Surfaces Poor Fibre arrangement Random continuous or discontinuous Typical fibre volume fraction 0.1 to 0.3 Void fraction Mechanical properties Poor Quality consistency Good Reaction Injection Moulding (RIM) 19

20 In the RIM process two resins are mixed and injected into a mould. The major differences between RIM and RTM is that with RTM the resin and curing agent are premixed before injection into mould, low pressure is used, whereas with RIM the liquid resin and curing agent are mixed by impingement as they are injected into the mould, thus high pressure is needed. RIM tends to be faster but gives worse mechanical properties. Two types of RIM process are used: Reinforced Reaction Injection Moulding (RRIM) using discontinuous and short fibres, and Structural Reaction Injection Moulding (SRIM) using long fibre pre-form. In the RRIM process milled fibres or glass flakes are added before injecting. In SRIM the reinforcement is first placed in the mould (just as in RTM) and following mould closure the highly reactive impingement-mixed resin is injected into the mould to impregnate the reinforcement. The schematic of RIM process is shown in Fig. C.8 Figure C.8 Schematic of RIM process: distinct stages of RIM process are shown from left to right. (From Åström, ref. [2], Fig. 4-41) Process and component characteristics are summarized in Tab. C.7 Table C.7 RIM process and component characteristics (From Åström, ref. [2], p. 233) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Long 20

21 Cycle time Health concerns Short Component Geometry Any Size Holes and inserts Possible Bosses and ribs Difficult Undercuts Difficult Surfaces Good Fibre arrangement Random and oriented Typical fibre volume fraction Up to 0.4 Void fraction Mechanical properties Quality consistency Good Thermoplastic composites processing Injection moulding Both, thermoplastic and thermosetting polymers can be processed by injection moulding. In thermoset injection moulding the injection chamber is cooled and the mould heated, whereas in thermoplastic injection moulding it is the opposite. Furthermore, the processing screws used for thermoplastics are generally longer than those used for thermosets, and the compression ratio tends to be higher. However, the main use is for thermoplastic matrices. For composites processing, the polymer is mixed with short cut fibres to form pellets (or granulates) few mm in length, which are fed into the hopper of the injection moulding machine, and injected into a mould in the same manner as conventional, neat thermoplastics. This technique is probably the fastest manufacturing route for composites. It is used for decorative and semi-structural parts. Intensive fibre breakage takes place and, as a result, a strong fibre length distribution has to be expected. Due to varying conditions of flow in the mould there is a strong fibre orientation distribution, and hence different anisotropy in different places of the part. It should be also noted that the wear on the machine is high due to the abrasive action of some fibres. Compression moulding (GMT) This process is similar to cold sheet metal stamping and has a short cycle time. GMTs can use nearly any thermoplastic for the matrix, however, in practice choices have been limited to polypropylene, polyvinyl chloride, polyamide, polyesters, polycarbonate and polyphenylene sulphide, with polypropylene accounting for about 95% of commercial use. 21

22 GMTs have been adopted by the automotive industry. Applications include seat frames, battery trays, bumper beams, load floors, front ends, valve covers, rocker panels and under engine covers. Typically for polypropylene GMT, a sheet is heated in an infrared oven up to 150 C and swiftly placed in a stamping press, to produce a composite component. Process and component characteristics are summarized in Tab. C.8. Table C.8 GMT compression moulding process and component characteristics (From Åström, ref. [2], ref ) Process Equipment cost Mould cost Labour cost Raw material cost Feasible series length Cycle time Health concerns to intermediate Long Short Component Geometry Any Size Limited by press size Holes and inserts Possible Bosses and ribs Possible Undercuts Difficult Surfaces Poor Fibre arrangement Random and continuous or discontinuous Typical fibre volume fraction Up to Void fraction Mechanical properties Poor Quality consistency Good Hot press moulding of thermoplastic prepregs The thermoplastic prepregs are stacked and hot pressed to form the advanced thermoplastic composite (ATC) final product. Prepregs are available in many materials including polypropylene, polysulphone, and PEEK. Originally, ATCs used amorphous resins such as polyethersulphone and polyetherimide for the matrix. However where increased solvent resistance was required semi-crystalline polymers such as polyether ether ketone and polyphenylene sulphide may be employed. A limited number of pseudo- 22

23 thermoplastics (pseudo-thermoplastics exhibit some characteristics of both, thermosets and amorphous thermoplastics; they are formed during the processing, where they polymerize and crosslink but the degree is very low; thus they can be reprocessed and reused) including polyamide-imide and polyimides have also been used. The continuous reinforcement phase may be in strand, woven, knitted or braided fabric forms and be made from carbon, aramid and/or S-, R- and E-glass. Carbon is the most popular material for higher temperature application, while E-glass dominates lower temperature applications. REFERENCES 1. P.K. Mallick, Fiber-Reinforced Composites, Materials, and Design, Marcel Dekker, B.T. Åström, of Polymer Composites, Chapman & Hall, H.M. Andersson, Visualisation of Composites, PhD Thesis, Luleå University of Technology,