Sheet molding compounds

An SMC is a reinforced plastic compound in sheet form. Most SMCs combine glass fiber with a polyester (TS) resin. Any combination of reinforcement and resin can be produced. The reinforcements can have continuous long fibers or any size of chopped fibers laid out in a different orientation from that of the resin. The different orientation makes it feasible to use SMCs on flat to complex-shaped molds. These SMCs will contain various additives and fillers to provide a variety of processing and performance properties (Table 6-1). 

SMCs are made to meet the shelf life required. These B-staged compounds are usually used in a few weeks or months. Some have a shelf life of six months, for example. Suppliers recommendations should be followed in keeping these compounds at a low temperature, or a curing action will occur (Chapter 6). 

TPs are also used in sheet form with different reinforcements and resins (Table 7-2). They are called stampable sheets rather than SMCs. These compounds provide unique properties with a quick and easy processing capability. 

See the Appendix for English to metric conversions. ASTM test methods used. 

The material leaves the mixer looking (but not smelling) like pancake batter, and is spread continuously onto layers of polyethylene film (Fig. 20.3). Chopped 

Before use, the sheets are cut to size and the polyethylene stripped oft They are then placed in a heated compression-type mold (several layers may be stacked for greater thicknesses) where true cure (crosslinking) of the polyester takes place. 

Sheet molding compounds provide a high strength-to-weight ratio, with low cost and easy, rapid processing. They are used for automotive body parts such as front ends and fender extensions and for other structural parts such as business machine and air-conditioner housings. They (along with RIM polyurethanes and polyureas and injection-molded thermoplastics) are in the thick of the competition for increased use in auto body panels (doors, hoods, fenders, etc.). They have recently been adopted for the body panels in the GM minivans. 

Bulk molding compounds (BMCs) are similar in composition to SMCs, with somewhat shorter fibers ( in.), and perhaps less glass (15%) and more filler. They are produced in chunk form and compression molded like other thermosetting compounds. 

Collyer (eds.). Mechanical Properties of Reinforced Thermoplastics, Elsevier, New York, 1986. 

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HDPE, high density polyethylene PP, polypropylene EVA, ethylene—vinyl alcohol SMC, sheet-molding compound ERP, fiber-reinforced plastic LDPE, low density polyethylene PE, polyethylene BMC, bulk mol ding compound TPE, thermoplastic elastomer. 

Composites. The history of phenoHc resin composites goes back to the early development of phenoHc materials, when wood flour, minerals, and colorants were combined with phenoHc resins to produce mol ding compounds. In later appHcations, resin varnishes were developed for kraft paper and textile fabrics to make decorative and industrial laminates. Although phenoHcs have been well characterized in glass-reinforced composites, new developments continue in this area, such as new systems for Hquid-injection molding (LIM) and sheet-molding compounds (SMC). More compHcated composite systems are based on aramid and graphite fibers. 

PhenoHc sheet-molding compound is seeing increased use in interior and exterior appHcations in mass transit. The trains in service on the English Channel tunnel have several panels made from phenoHc SMC which had to meet the strict European safety standards for fire and smoke generation. 

SMC = sheet molding compound BMC = bulk molding compound. 

Two-component urethane adhesives are used to bond sheet molding compound (SMC) panels for automotive OEM and aftermarket applications. Two-part urethanes are used as laminating adhesives in the RV industry. 

Sheet molding compound (SMC) consists of randomly oriented chopped fibers in a matrix of resin and filler. SMC is produced in the continuous manner shown in Figure 1-16. Note that the polyethylene film protects the roller system from getting gummed up with the resin-filler paste. The rug-like rolls of SMC are then used in compression molding machines to create large parts such as the sides of cars and trucks. 

Design Parameter Resin-Transfer Molding Spray-up Hand Lay-up Mat/Preform Sheet Molding Compound... 

Shear modulus, polyamide, 138 Sheet molding compounds (SMCs), 30 Shoe sole products, 205 Shore hardness gauge, 243 Side-chain liquid crystalline polymers, 49 Side reactions, in transition metal coupling, 477... 

Sheet-molding compound, 19 557, 20 117 injection molding of, 20 118 phenolic, 18 793 Sheet(s)... 

Sheet molding compounds (SMCs) and bulk molding compounds (BMCs) are the dominant materials used in automotive applications. These composites of unsaturated polyester resin, fillers and fiberglass have advantages of high stiffness, heat resistance and low coefficient of expansion. Coupled with low creep resistance, which is a distinct advantage over thermoplastic competition, and low-profile additives, which can yield Class A surfaces, these materials are well suited for applications from exterior body panels to under the hood components. 

Genamid 250 and Versamid 115 were obtained from Henkel Company. The steel substrates used were cold rolled steel and Bonderite 40 phosphated steel from Parker. The composite was sheet molding compound from Rockwell. 

A.2.4 Discontinuous-Fiber-Reinforced Polymer-Matrix Composites Sheet Molding Compound. Of the parameters influencing the mechanical properties in short-fiber-reinforced polymer-matrix composites, fiber composition, matrix composition, fiber geometry, and manufacturing method will be elaborated upon here. 

Figure 5.101 Various forms of sheet molding compounds (a) random, SMC-R (b) continuous/ random, SMC-CR and (c) criss-crossed continuous/random XMC L and T refer to longitudinal and transverse directions, respectively. Reprinted, by permission, from Composite Materials Technology, P. K. MaUick and S. Newman, eds., p. 33. Copyright 1990 by Carl Hanser Verlag.

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