Structural foam injection

Production Considerations Structural Foam Injection Molding Sheet Molding Compound... 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

Compression Molding of Low-Density Foam Casting of Soft Plastisol Foam Extrusion of Structural Foam Injection Molding of Structural Foam Injection Molding Elimination of Sink-Marks... 

FIGURE 2.13 Structural foam injection molding, (a) During injection under high pressure there is very little foaming, (b) After injection, pressure drops and foaming occurs at hot core. 

A variety of foams can be produced from various types of polyethylenes and cross-linked systems having a very wide range of physical properties, and foams can be tailor-made to a specific application. Polypropylene has a higher thermostability than polyethylene. The production volume of polyolefin foams is not as high as that of polystyrene, polyurethane, or PVC foams. This is due to the higher cost of production and some technical difficulties in the production of polyolefin foams. The structural foam injection molding process, described previously for polystyrene, is also used for polyethylene and polypropylene structural foams (see Figure 2.61). 

Figure 4.5 Layered structures with high bending stiffness, (a) Glass-fibre-reinforced skins on an aluminium honeycomb core (b) structural foam injection moulding with maximum density at the skins.

Production considerations Structural foam Injection molding Sheet molding compound... 

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