Polymer supplier develops cost-efficient production tech for lightweight vehicle interior parts

Germany’s Covestro has developed an integrated polycarbonate-based material and process concept suitable for a cost-effective production of lightweight decorative components for the automotive interior.

The concept is based on polycarbonate and enables customised design and styling elements. An example is a glovebox cover made from the polycarbonate ABS blend Bayblend, whose surface shows high-gloss and structured areas.

Covestro says it supports the trend in the automotive industry towards lightweight components with customizable Class A surfaces that can be fabricated economically in a single step.

At the K 2016 plastics trade fair (Stand A 75, Hall 6), the company is showing prototypes of a glove compartment cover to demonstrate the technology. They were manufactured in an injection mould with dynamic temperature control by means of physical foam molding, from Bayblend grades (PC/ABS) surface-optimised for use in automotive interiors.

Visitors can also watch the part being manufactured via the ProFoam method at the stand of injection moulding machine specialist Arburg (Stand A13, Hall 13). In this case, the company selected Makrolon SF800Z, a glass fibre-reinforced polycarbonate from Covestro specifically developed for physical foam moulding. It already is in use today for printer housings and other non-automotive applications. It has a UL94 listing for flame retardance.

Even lighter components

“In addition to the lightweight strategy of physical foam moulding, which reduces density and thus component weight, we also deliberately took advantage of the opportunity to make process-related design modifications,” said Rainer Protte, head of Special Injection Molding Processes at Covestro’s Polycarbonates business unit. “Increasing nitrogen concentration in the material reduces the viscosity of the melt and improves flow.” This enables mold wall thickness to be reduced without exceeding the permissible injection pressure. The combination of the two effects – reducing density and wall thickness – enabled the weight of this prototype to be reduced by about 30 percent. Polycarbonate and its blends display good mechanical properties, meaning that this material class is also suitable for components subject to high mechanical stress.

Thanks to the process conditions and material, premium surfaces with high-gloss and textured areas can be achieved even with physical foam molding. An additional coating step is unnecessary.

Mirror-smooth surface

This is made possible by dynamic temperature control, where superheated water is used to heat the surface of the mold cavity to the glass transition temperature or higher prior to injection. Once the melt has been injected, it is allowed to cool to demoulding temperature. The high mold temperature makes the surface of the molded part virtually mirror-smooth. Structures can also be rendered very effectively using this method. 

The ProFoam process in operation at the trade fair is a physical foam moulding method developed by Arburg, which is based in Lossburg, Germany. The plastic granulate is already enriched with liquid propellant in a granulate lock upstream of the injection unit. During the plasticising process, the blowing agent dissolves in the melt and is not released again until the pressure increases during injection, when it is then emitted in the form of microcellular ‘bubbles’ that ‘freeze’ during solidification of the melt. Fiber-reinforced plastics can likewise be processed by this method without any additional shearing.

In addition to Arburg, Covestro says it also worked closely on this project with gwk Gesellschaft Wärme Kältetechnik in Meinerzhagen (temperature control processes), Hiddenhausen-based mould manufacturer Krallmann, and J. & F. Krüth in Solingen, which specialises in mould texturing, the focus in this case being on 3D laser texturing. 

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