Sheet Molding Compounds, Thermoplastics

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. 

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. 

The matrix can be either a thermoset or thermoplastic. The oldest and still widest used material for compression molded products is phenolic. The thermoset materials used to manufacture fiber reinforced compression molded articles is unsaturated polyester sheet or bulk, reinforced with glass fibers, known as sheet molding compound (SMC) or bulk molding compound (BMC). In SMC, the 25 mm long reinforcing fibers are randomly oriented in the plane of the sheet and make up for 20-30% of the molding compound s volume fraction. 

The resin matrix can be either thermosetting or thermoplastic. Thermosetting resins such as epoxy, polyimide, polyester, and phenolic are used in applications where physical properties are important. Polyester and epoxy composites make up the bulk of the thermoset composite market. Of these two, polyesters dominate by far. Reinforced with glass fiber, these are known as fiberglass-reinforced plastics (FRPs). FRPs are molded by layup and spray-up methods or by compression molding either a preform or sheet molding compound (SMC). 

In use are stamping compression molding presses. Plastic used can be TS sheet molding compounds (SMCs) and stampable reinforced thermoplastic sheet (STX) material (STX is a registered tradename Azdel Inc., Shelby, NC). It is usually composed of a glass fiber-thermoplastic RTP (Chapter 15). 

Alkyd resins have been the workhorse for the coatings industry over the last half century. The term alkyd was coined to define the reaction product of polyhydric alcohols and polybasic acids, in other words, polyesters. However, its definition has been narrowed to include only those polyesters containing monobasic acids, usually long-chain fatty acids. Thus thermoplastic polyesters typified by polyethylene terephthalate (PET) used in synthetic fibers, films, and plastics and unsaturated polyesters typified by the condensation product of glycols and unsaturated dibasic acids (which are widely used in conjunction with vinylic monomers in making sheet molding compounds or other thermosetting molded plastics) are not considered as part of the alkyd family and are beyond the scope of the present discussion. 

UD, unidirectional GMT, glass mat thermoplastic TMC, thick molding compound SMC, sheet molding compound. (From Refs. - . )... 

Sheet molding compound (SMC), glass mat thermoplastics (GMT), chopped strand mat (CSM)... 

Special types of glass fiber for specific processes (Chapters 4 and 5), such as sheet molding compound (SMC), pultrusion, and reinforcement of thermoplastics, is a marked trend in current development. Examples follow ... 

Silicone plastics Styrene acrylonitrile copolymer St ene-butadiene rubber Styrene butadiene styrene block copolymer Sheet molding compound St50 ene acrylonitrile copolymer Styrene butadiene block copolymer Styrene butadiene styrene block copolymer -a thermoplastic elastomer... 

Bulk molding compounds (BMC) and sheet molding compounds (SMC) have gained increased commercial importance in recent years, particularly in the automotive industry. Every yeeir new applications are introduced. BMC and SMC products consist of a thermosetting resin, such as an unsaturated polyester, containing a filler and reinforced with glass fibers. Often the recipe includes a thermoplastic additive known as a low profile additive (LPA) to eliminate cure shrinkage and improve surface quality. Other additives may also be present to control viscosity, add color, and promote mold release. 

Apart from thermoplastics, two other materials labeled as SMC (sheet molding compound) and reaction-injection-molded (RIM) were utilized as body panels (Figure 11). This type of body panel has higher material costs... 

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