Weight Reduction in Foamed Injection Molded Parts
Until now, methods where the thermoplastic melt is foamed chemically or physically have been used to produce foamed injection molded parts. Particle-foam composite injection molding provides an alternative. With this method, foamed parts are first produced from foam beads containing a blowing agent – for instance, through use of steam. These foam structures are subsequently overmolded in an injection mold. The use of particle foams makes it possible to achieve considerably lower part densities than what is possible with the common foaming methods used for injection molding to date. Moreover, particle foam offers attractive properties in terms of energy absorption, acoustics, thermal insulation and functional integration.
However, in the course of being overmolded with a thermoplastic material, the particle foam is subjected to high temperatures and a high injection pressure. This can cause the foam to collapse if the density of the foam is less than about 0.1 g/cm³. To nevertheless produce parts with a very low foam density, the part must be manufactured in reverse order. This means that a rigid thermoplastic shell is injection molded in a first step. In-line production of the particle foam structure then follows.
Neue Materialien Bayreuth GmbH has conducted a feasibility study of this processing technique. Using a test mold, a composite part consisting of solid PP material and an EPP particle foam was produced in an in-line process. The properties of the composite were then tested and evaluated as a function of mold geometry, material combination and process parameters. Among other things, it was found that it is possible to achieve material bonding with smooth molded parts. Targeted use of rib structures can greatly increase the strength of the composite further as a result of the additional grip.
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