Thermoplastic foam molding

Thermoplastic foam molding combines mechanical strength with lightness. The parts have a cellular, lightweight internal structure and a smooth, closed-pore outer skin. 

FIGURE 3.19 Process chain, thermoplastic foam molding [48]... 

With the exception of NORYL phenylene oxide-based resin most engineering foams are reinforced. Loadings range from 4 to 40% (4). Thermoplastic resins molded in the form of structural foams include die following (5) ... 

Because of the low pressure (0.3 MPa) in the mold as compared to those encountered in thermoplastic injection molding (150 MPa) this process is suitable for the production of thin part with large surface areas. A significant application for this is in panel formation in the automotive industry, including fascias, door panels, spoilers, grills, and bumpers. While a rigid foam part with a flexural strength of 700 MPa would require a thickness of 7 mm, a RRIM part, because of the reinforcement, would allow a much smaller thickness (< 3 mm) by virtue of the increase in flexural modulus. 

Modified Poiyphenyiene Ether Thermoplastic polyphenylene ether alloys with impact polystyrene. Has good impact strength, resistance to heat and fire, but poor resistance to solvents. Processed by injection and structural foam molding and extrusion. Used in auto parts, appliances, and telecommunication devices. Also called MPE, MPO, and Modified Polyphenylene Oxide. 

Styrenic Resins Styrenic resins are thermoplastics prepared by firee-radical polymerization of styrene alone or with other unsaturated monomers. The properties of styrenic resins vary widely with molecular structure, attaining the high performance level of engineering plastics. Processed by blow and injection molding, extrusion, thermoforming, film techniques, and structural foam molding. Used heavily for the manufacture of automotive parts. 

Injection compression molding (ICM) is, similarly to the gas injection technique and thermoplastic foam injection (TFI), particularly well-suited to making thick-walled moldings with little warping and few sink marks. However, thin-walled parts with long flow paths can also be produced efficiently by means of ICM. 

When the beads are heated in steam (or expanded ), the pentane volatilizes to expand the beads into a thermoplastic foam. These foam beads are subsequently packed into molds and fused by heat into the shape desired. The EPS can also be extruded into sheets and thermoformed into products such as meat trays and plates. EPS has good thermal insulation and shock-absorbing properties. 

IWo specialized injection-molding processes will be considered in some detail. These processes are structural foam and reaction injection molding. In the case of structural foam, a gas is distributed throughout the molded plastic to form a foam. When cooled, the product is a structural thermoplastic foam. The reaction injection-molding process involves mixing two reactive components and injecting them into a mold, where the product is formed. 

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