TABLE 1.5(b). Comparison of FRP Composite Manufacturing Processes, Material ROsato and rosato 1990 windi Pultrusion+ Transfer Mauldin 1.5.1 COMPRESSION MOLDING For most high-volume fiber reinforced polymer matrix(FRP) composite parts compression molding is the primary choice. The high-pressure molding process produces of ty of sizes. Matched molds are mounted in a compound or a preform is placed iess. A weighed charge of bulk or sheet molding hydraulic or mechanical molding e open mold, along with a measured charge of esin. The heated mold halves are closed, and pressure is applied. Molding time depending on part size and thickness, ranges from about one to five minutes. Inserts and attachments can be molded in. Compression-molded composites are characterized by good mechanical and chemical properties, superior color and excellent surface finish Trimming and finishing costs are minimal. See Figure 1.6
13 1.5.1 COMPRESSION MOLDING For most high-volume fiber reinforced polymer matrix (FRP) composite parts compression molding is the primary choice. The high-pressure molding process produces high-strength, complex parts of a variety of sizes. Matched molds are mounted in a hydraulic or mechanical molding press. A weighed charge of bulk or sheet molding compound or a preform is placed in the open mold, along with a measured charge of resin. The heated mold halves are closed, and pressure is applied. Molding time, depending on part size and thickness, ranges from about one to five minutes. Inserts and attachments can be molded in. Compression-molded composites are characterized by good mechanical and chemical properties, superior color and excellent surface finish. Trimming and finishing costs are minimal. See Figure 1.6
Preform Mold FIGURE 1.6. Compression Molding 1.5.2 RESIN TRANSFER MOLDING (RTM) Suitable for medium-volume production of rather large FRP components, resin ransfer molding is usually considered an intermediate process between the relatively slow spray-up and the faster compression-molding methods, which require higher tooling costs. RTM parts, like compression-molded parts, have two finished surfaces, but molded parts require trimming. Gel coats may be used. Abrate [2] provides an overall view of resin flow in fiber preforms Reinforcement mat or woven roving is placed in the bottom half of the mold, which is then closed and clamped. Catalyzed, low-viscosity resin is pumped in under pressure, displacing the air and venting it at the edges, until the mold is filled. Molds for this low-pressure system are usually made from reinforced plastics Vacuum-Assisted Resin Transfer Molding (VARTM): after the composite material is entered into the mold, and the part is vacuum bagged, a vacuum of to 14 and cure temperature of less than 350%F is applied. The vacuum compacts the composite nd helps the resin wet out the preformed composite part. See Figure 1.7
14 1.5.2 RESIN TRANSFER MOLDING (RTM) Suitable for medium-volume production of rather large FRP components, resin transfer molding is usually considered an intermediate process between the relatively slow spray-up and the faster compression-molding methods, which require higher tooling costs. RTM parts, like compression-molded parts, have two finished surfaces, but molded parts require trimming. Gel coats may be used. Abrate [2] provides an overall view of resin flow in fiber preforms. Reinforcement mat or woven roving is placed in the bottom half of the mold, which is then closed and clamped. Catalyzed, low-viscosity resin is pumped in under pressure, displacing the air and venting it at the edges, until the mold is filled. Molds for this low-pressure system are usually made from reinforced plastics. Vacuum-Assisted Resin Transfer Molding (VARTM): after the composite material is entered into the mold, and the part is vacuum bagged, a vacuum of to 14 psi and cure temperature of less than 350° F is applied. The vacuum compacts the composite and helps the resin wet out the preformed composite part. See Figure 1.7
Ren Injecton Lid-Up Ml or Wowen Reinforcement FIGURE 1.7, Resin Transfer Molding(RTM) 1.5.3 INJECTION MOLDING Reinforced thermoset molding compounds can be injection molded, Figure 1.8, in equipment similar to that commonly used for thermoplastic resins. The principa difference lies in the temperatures maintained in various areas of the system. with thermoplastics, the injection screw and chamber are maintained at a relatively high mperature,and the die is cooled so the molded part sets up. In contrast, for a thermoset FRP, the screw and chamber are cooled so that the resin does not cross-link and gel, and the die is heated so it does cross-link and cure Injection molding offers high-speed production and low direct labor Combined with the excellent mechanical properties available from long fibered BM result is a capability for high volumes of complex parts with properties comparable to those of compression or transfer molded parts ZOo nger o Screw FIGURE 1.8. Injection Molding 1.5.4 COLD PRESS MOLDING Cold press molding does not use external heat to effect part cure because the Ind cures at room temperature, aided by the self-generated exothermic heat. Cold molding is an economical press-molding method, providing two finished surfaces on parts made, for manufacturing intermediate volumes of products using a low-pressure ure and inexpensive molds of plaster or glass reinforced plastic. These molds do not have sharp edges, so trimming after molding is required
15 1.5.3 INJECTION MOLDING Reinforced thermoset molding compounds can be injection molded, Figure 1.8, in equipment similar to that commonly used for thermoplastic resins. The principal difference lies in the temperatures maintained in various areas of the system. With thermoplastics, the injection screw and chamber are maintained at a relatively high temperature, and the die is cooled so the molded part sets up. In contrast, for a thermoset FRP, the screw and chamber are cooled so that the resin does not cross-link and gel, and the die is heated so it does cross-link and cure. Injection molding offers high-speed production and low direct labor costs. Combined with the excellent mechanical properties available from long fibered BMC, the result is a capability for high volumes of complex parts with properties comparable to those of compression or transfer molded parts. 1.5.4 COLD PRESS MOLDING Cold press molding does not use external heat to effect part cure because the compound cures at room temperature, aided by the self-generated exothermic heat. Cold molding is an economical press-molding method, providing two finished surfaces on parts made, for manufacturing intermediate volumes of products using a low-pressure cure and inexpensive molds of plaster or glass reinforced plastic. These molds do not have sharp edges, so trimming after molding is required
Preform or mat reinforcement is on the lower mold half and a resin/filler mixture is added. The mold is closed noderate pressure or 20 to 50 psi, and the FRP part cures. Cold molding is suited for relatively simple shapes, without ribs 1.5.5 STRUCTURAL REACTION INJECTION MOLDING (SRIM) Structural reaction injection molding shown 19 is suitable for medium- high volume composite parts requiring superior with no loss in toughness or exibility. The SRiM process also produces parts Impact resistance and lower weight, and is excellent for larger part sizes. Like injection molding, resin is injected into a closed mold. However, the SRIM process utilizes a directed-fiber preform or reinforcing mat, which is placed into the mold rior to closure, resulting in even distribution of glass and uniform mechanical properties. SRIM parts offer two finished surfaces, but the polyurethane systems typically sed do not provide a Class A surface finish and are generally considered unsuitable fo applications where cosmetics are important. For structural application, however, the low temperature and pressure characteristics of SRIM lead to lower equipment and lufacturing costs compared to similar processes. FIGURE 1.9. Structural Reaction Injection Molding (SRIM) 1.5.6 HAND LAY-UP The simplest and oldest of the fabrication processes for FRP composites, lay-up, is a labor-intensive method suited especially for low-volume production of components such as boat hulls and associated parts. See Figure 1.10 A pigmented gel coat is first sprayed onto the mold for a high-quality surface finish. When the gel coat has become tacky, glass reinforcing mat and/or woven roving is placed on the mold, and resin is poured, brushed or sprayed on. Manual rolling then removes entrapped air, densifies the composite and thoroughly wets the reinforcement ith the resin. Additional layers of mat or woven roving and resin are added for thickness. Curing is initiated by a catalyst or accelerator in the resin system, which hardens the composite without external heat Hand lay-up offers low-cost tooling, simple processing and a wide range of part ize potential. Design changes are made easily. Parts have one finished surface and require trimming
16 Preform or mat reinforcement is placed on the lower mold half and a resin/filler mixture is added. The mold is closed under moderate pressure or 20 to 50 psi, and the FRP part cures. Cold molding is suited mainly for relatively simple shapes, without ribs or bosses. 1.5.5 STRUCTURAL REACTION INJECTION MOLDING (SRIM) Structural reaction injection molding shown in Figure 1.9, is suitable for mediumto-high volume composite parts requiring superior strength with no loss in toughness or flexibility. The SRIM process also produces parts with high impact resistance and lower weight, and is excellent for larger part sizes. Like injection molding, resin is injected into a closed mold. However, the SRIM process utilizes a directed-fiber preform or reinforcing mat, which is placed into the mold prior to closure, resulting in even distribution of glass and uniform mechanical properties. SRIM parts offer two finished surfaces, but the polyurethane systems typically used do not provide a Class A surface finish and are generally considered unsuitable for applications where cosmetics are important. For structural application, however, the low temperature and pressure characteristics of SRIM lead to lower equipment and manufacturing costs compared to similar processes. 1.5.6 HAND LAY-UP The simplest and oldest of the fabrication processes for FRP composites, hand lay-up, is a labor-intensive method suited especially for low-volume production of large components such as boat hulls and associated parts. See Figure 1.10. A pigmented gel coat is first sprayed onto the mold for a high-quality surface finish. When the gel coat has become tacky, glass reinforcing mat and/or woven roving is placed on the mold, and resin is poured, brushed or sprayed on. Manual rolling then removes entrapped air, densifies the composite and thoroughly wets the reinforcement with the resin. Additional layers of mat or woven roving and resin are added for thickness. Curing is initiated by a catalyst or accelerator in the resin system, which hardens the composite without external heat. Hand lay-up offers low-cost tooling, simple processing and a wide range of part size potential. Design changes are made easily. Parts have one finished surface and require trimming
Fiber/ Resin Composite FIGURE 1. 10. Hand Lay-Up 1.5.7 SPRAY LAY-UP Similar to hand lay-up in simplicity, spray lay-up, shown in Figure 1. ll, offers greater shape complexity and faster production. It too uses a low-cost open mold(one finished part surface), room temperature curing resin and is suited for producing large rP parts such as tub/shower units and vent hoods in low to moderate quantities. Cure is usually at room temperature, but it can be accelerated by cation of moderate heat abImal: pped fiber reinforcement and catalyzed resin are deposited in the mold from a ombination chopper/spray gun. As with lay-up, manual rolling removes entrapped air and wets the fiber reinforcement. Woven roving is often added in specific areas for thickness or greater strength. Pigmented gel coats can be used to produce a smooth, colorful surface Gel Coat ntinuo IGURE I.II. Spray Lay-Up chanical properties and off mold surfaces or contact-molding FRP components can hese techniques speed the cure of p made by lay-up and spray-up, but they also add to processing costs
17 1.5.7 SPRAY LAY-UP Similar to hand lay-up in simplicity, spray lay-up, shown in Figure 1.11, offers greater shape complexity and faster production. It too uses a low-cost open mold (one finished part surface), room temperature curing resin and is suited for producing large FRP parts such as tub/shower units and vent hoods in low to moderate quantities. Cure is usually at room temperature, but it can be accelerated by application of moderate heat. Chopped fiber reinforcement and catalyzed resin are deposited in the mold from a combination chopper/spray gun. As with lay-up, manual rolling removes entrapped air and wets the fiber reinforcement. Woven roving is often added in specific areas for thickness or greater strength. Pigmented gel coats can be used to produce a smooth, colorful surface. Mechanical properties and off mold surfaces or contact-molding FRP components can be improved by a number of techniques. These techniques speed the cure of parts made by lay-up and spray-up, but they also add to processing costs