Bulk Molding Compounds, Thermoplastics

Most types of TPs can readily be compounded with reinforcing materials that include different forms of glass fibers and high 

For heavy-duty purposes such as bearings, tough strong fabrics like cotton duck are substituted for the paper. Fabric-based laminates may be further modified with graphite, fluorocarbons, or other low friction materials to provide low-friction RP bearings requiring no lubricant. Many different RP laminates continue to be produced and used in different markets (principal circuit boards, etc.). 

Nonfibrous reinforcements are also employed as reinforcements and fillers. They result in increased tensile strength and deflection temperature, but usually with TPs decrease impact resistance. Nonfibrous reinforcements are included when fabricating with exceptional flatness. The nonfibrous include mica, glass beads, and minerals such as wollastonite (talc, calcium carbonate, and kaolin are considered fillers). Like fibrous reinforcements, the nonfibrous reinforcements can be processed by many different technologies. 

There are also flexible RP compounds. These RTP elastomeric materials provide special engineered products such as bushings, conveyor belts, mechanical belts, high temperature or chemical resistant suits, wire and cable insulation, and architectural designed shapes. 

The workhorse of the RTS industry is TS polyester (also called polyester-TS) with glass fiber. The fiber reinforcement may be in the form of chopped fibers, porous nonwoven mats, woven fabrics, or continuous fibers. The combination of plastics and reinforcements results in versatile materials with unusual characteristics. The reinforcement adds strength and toughness to inherent weather resistance, moldability, and colorability. Thus RTSs are used because of their increased tensile, flexural, torsional, and impact strengths increased modulus of elasticity increased creep resistance reduced coefficient of thermal expansion increased thermal conductivity and, in many cases, lower costs. 

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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. 

Figure 4.8 Example for producing thermoplastic bulk molding compounds via an extruder (courtesy of FTP Co.)...

Injection Molding. Injection molding was first developed for metals and then for thermoplastics. With further progress in both materials and equipment, it has been applied successfully to thermosetting plastics. It is used mostly for phenolics and is being used more and more for polyester bulk molding compounds. 

Material to be molded is placed in a matched-metal-die mold cavity and the mold closed. The part is formed by compressing the material under heat and pressure until the resin is cured or sufficiently cool to allow removal. Suitable for she (SMC) or bulk molding compounds (BMC), jH-eforms, w resin applied to reinfracemoit in the mold (wet system molding) or reinforced thermoplastic sheet (hot stamping). 

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. 

Complex reflectors within new vehicle headlamp assemblies typically must withstand temperatures of up to 400°F. Appropriate thermoplastic materials must be selected not only for the lamp backcan and lens, but also for the electronic/electrical connectors. Variations on filled polyester, poly(imides), bulk molding compound (a sheet molding compound [SMC] analogue), and nylon/poly(phenylene oxide) alloys are typically utilized in these applications. Poly(carbonate) (PC) or impact-modified PC is typically used in the lens applications where high heat is required. Acrylic plastics can and are most typically utilized in lens apphcations that do not require high heat, such as rear stop mount designs. 

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. 

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). 

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