Novel mold lets soft foam be thick, thin—or both

In what is being claimed as an injection molding first, a slide-table mold developed by Swiss moldmaker Georg Kaufmann Formenbau AG (Busslingen) enables processors to vary the thickness of a layer of soft-touch foam exactly where it is required on a part, via the mold’s geometry. ‘Varysoft’ is the name the moldmaker has affixed to its novel system.

The company is well known in moldmaking, especially for molds used to process door panels and other automotive interior components. For its newest feat, the company built a mold with two cavities, one for the conventional injection molding of a supporting structure (typically polypropylene) and one for preforming of a decorative material—film, fabric, or imitation leather. For the foam layer, the machine needs either an additional injection unit for expandable thermoplastic elastomer (TPE) or a polyurethane (PUR) processing unit with PUR mixing head.

Let the processing begin
How are parts formed? First the pre-cut decorative material is placed in the second cavity of the open mold and fixed in place. The mold closes and the decorative material is preformed, while in the first cavity the polypropylene (PP) is molded and then cooled. When the mold opens, the PP supporting structure remains in the moving mold half and the pre-formed decorative material remains in the stationary mold half.

The two halves of the open mold are then repositioned by means of a sliding table so that the PP supporting structure and the preformed decorative material face each other. The mold then closes but a gap remains between the halves. This gap represents the thickness of the foam layer and is determined by the geometry of the second cavity; it is deeper than the geometry of the first cavity by a distance equal to the desired thickness of the foam.

This gap is then filled with foam, with either expandable TPE injected into the gap via a gating system and diverter mounted on top of the mold, or via the PUR mixing head, docked against the underside of the mold. Once foaming is complete, parts can be de-molded and any surplus decorative material can be trimmed.

In another decorative development, Kaufmann also says it can make injection molds suitable for back injection molding of metal films. Such films could be thick enough to give the feel and look of a genuine metal surface, more so than with electroplating, but without requiring a secondary operation.

Here again, the process is deceptively simple. The metal film is placed and held in a mold in essentially the same way as any other decorative surface would be for back injection molding.

With this development, any surface structures machined into the walls of the cavity (logos, for example, or other decorative elements) can be transferred to the molded part (see photo, previous page). While the mold is closed after injection, an integrated punching tool separates surplus film from the molded part, but leaves the edges of the film wrapped around the part to produce a smooth edge.

Kaufmann says it already has formed parts using aluminum and stainless steel films and is testing other materials. The moldmaker worked with the university in Rappperswil, Switzerland on development of the coupling agent necessary for a reliable metal-plastic bond. —Matt Defosse