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Bayer MaterialScience develops new in-mold coating process
Updated: 2006-03-30 00:00 Source: share:

The application of a coating to an injection molded plastic part is still a time-consuming and cost-intensive procedure. A process currently being developed by Bayer MaterialScience AG promises to improve the situation. It is a special variant of the in-mold coating process (IMC) which combines injection molding with reaction injection molding (RIM). Trials with prototypes and a number of development projects with cooperation partners have already produced highly promising results. In a two-step process, the part is first injection molded and then, with the aid of a turntable mold, swivel platen or sliding table, is transferred to a second cavity. There a reactive two-component polyurethane system is injected using the RIM process. This cures within the cooling time in the closed mold. In this way, injection molded components can be provided with functional coatings, decorative finishes or skins.

“The injection molding process and the curing of the two-component polyurethane system take place simultaneously. This ensures short cycle times of the sort typically found in the normal injection molding of thermoplastics. And that means high productivity,” explains Rainer Protte, a specialist with Bayer MaterialScience's Polycarbonates Business Unit. Combining the injection molding process with the RIM process opens up enormous savings potential. Because the two-component poly-urethane system cures in the thermoplastic mold, a second mold is unnecessary. Another advantage of this process is that textured surfaces like fine grains can be reproduced in outstanding quality. Bayer MaterialScience sees enormous potential for this application in the surface coating of parts for the automotive and electrical/electronic industries.

Significantly lower coating costs
Compared with a separate coating line, coating a thermoplastic substrate by the new process means lower investment
costs, easier logistics and reduced space requirements. There is no need to transport the injection molded parts to the coating line – and perhaps store them there – which considerably reduces the risk of the parts becoming soiled or damaged. The reject rate is considerably lower. Another advantage is that there is no longer any overspray, which can be quite considerable when coating offline.

The new “closed” in-mold coating process also has a cost advantage over the “open” IMC process already established in production line coating. With the old method, the cavity surface of the injection molding tool is first sprayed with a coating and the cavity then filled with the melt. Because the coating and the injection molding are performed consecutively, cycle times are substantially longer.

“For car interior applications, for example, our IMC process could be used to apply a decorative finish to parts such as the glove compartment flap, add-on parts for the instrument panel and post-finishers. When combined with the Film Insert Molding (FIM) process, decorative surfaces with a deep gloss finish can be obtained. Potential applications in the car body segment are coated panels and mirror housings,” says Torsten Pohl, a coatings expert in the Coatings, Adhesives & Sealants Business Unit. Low-viscosity systems are used as two-component polyurethane coatings, based, for example, on Desmodur® and Desmophen® from Bayer MaterialScience. They cure quickly, can easily be demolded and adhere well to Desmopan® thermoplastic polyurethanes, Makrolon®  polycarbonate and various PC blends like Bayblend®and Makroblend®. Apart from that, they do not contain any solvents, so emissions are minimal.

Colored, lightfast decorative skins with pleasant tactile properties The new IMC process can also be used to apply a decorative polyurethane skin at least a millimeter thick to an injection molded substrate. This results in a surface with good adhesion, pleasant tactile properties and an attractive appearance. For this, Bayer MaterialScience uses the low-emission polyurethane raw materials, Crelan® and Bayflex®.Aliphatic isocyanates are frequently chosen as a raw material component, as they produce lightfast polyurethane systems that do not yellow. “Because of their light stability, the RIM skins can be produced in light colors, which means we can satisfy the trend towards brighter colors for car interiors,” explains Andreas Klein, a member of the development team in the RRIM group of the Polyurethanes Business Unit. The team is currently focusing its attention on decorative trim, but Klein also sees potential for applications such as decorative skins for larger components like armrests and door liners. Trays coated with a non-slip finish for the center console are other possibilities. Such coatings could also help dampen troublesome grating noises produced when plastic parts rub against each other.


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