Prototypes solve automotive moulding puzzle

Edited by the Engineeringtalk editorial team Oct 9, 2006

Space Puzzle Molding is used regularly in the prototyping and production of early-series plastic components for the automotive and other industries.

The Space Puzzle Molding (SPM) technique, patented and operated by Bavarian based company Protoform, is used regularly in the prototyping and production of early-series plastic components for the automotive and other industries.

All grades of injection moulding materials can be processed.

Inserts, outserts, co-injection and gas-assisted methods can also be accommodated appropriate to the designer's specification.

In this automotive application the unusual technical achievement made was the result of an idea introduced by Protoform's technical staff when co-operating with designers from Mitsubishi.

Working to a brief from automotive component specialist Dr Schneider, the task was to find an economical solution to a composite application for a directional ventilator moulding destined for one of the Smart range of vehicles.

The design dictated that the main body of the housing should be in PP/PE TV20, a hard durable plastic suitable for both the ventilator vanes and the swivel mechanism, but that the interface between the moving part and the rear housing should be an inner band of a softer grade of plastic to act as a bearing surface and partial seal.

The material chosen for this was TPE Santoprene which has a lower melt temperature than the PP/PE.

Tradition, and also advice from the materials experts, suggested that the only feasible route would be to inject the harder material first and to do this would mean creating a cavity into which the softer material to be injected later.

This method would necessitate creating a collapsible core for the centre of the tool, a simple and inexpensive exercise for the SPM Space Puzzle tooling, but considerable more expensive for the production tool.

With economy in mind for both prototyping and later production it was agreed to try an unconventional approach.

Space Puzzle Molding tools are renowned for their flexibility and the creation of two sets of "horseshoe" forms to create the outer surfaces of the component enabled the softer, lower melting point Santoprene to be injected first.

It was not necessary to create a whole new separate tool to complete the product as the Space Puzzle technique of assembling the tooling into a common support bolster allows for such switch operations to be readily accomplished.

The first matching "horseshoes" were then replaced by the larger ones illustrated in the CAD tooling image, leaving the first moulding in situ, and the higher temperature material injected.

It was anticipated that there would be a small degree of melt at the interface of the two materials but, as predicted, the working surface of the Santoprene remained perfectly formed and entirely functional.

Sean Halstead of Protoform explains: "It was of critical importance that the inner functional face of the Santoprene, which has an injection temperature of 220C, did not reach deformation temperature".

"As a quick test we applied a strip of TPE Santoprene, of the same thickness as that in the design, to a heat source corresponding the injection temperature of 250C for the PP/PE.

We than measured the temperature on what would be the inner face".

"We established that with the 1 second of injection time needed, the deformation temperature was not reached".

"We also optimised the design of the TPE part to reduce the possibility of heating due to friction".

"The idea worked beautifully in practice and we were able to run series production of identical mouldings for prototypes in production intent materials".

"It was bending the rules a little but it achieved the desired result".

"The economy of this approach was appreciated by the client and the technique was reflected onwards into production".