Sep 4 2007
Baking a cake without greasing the tin is usually a disaster: The cake cannot be taken out in one piece. The same principle applies to the manufacture of fiber-reinforced plastic parts such as aircraft wings. Before the engineers can pour the mixture of resin, plastic and textile fibers into the metal mold, they have to apply four to six layers of release agent – mostly by hand.
Similarly to the cake tin, the mold has to be re-greased for each new plastic part. It is done using separating agents in a solvent which evaporates after application. This adds up to a considerable quantity of solvents. Europe’s polyurethane industry alone, which supplies car manufacturers with such parts as tubes, seats and dashboards, emits several thousand tons of hydrocarbons every year.
Together with Acmos Chemie KG, researchers at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research IFAM in Bremen have now developed a new type of coating for metal molds. “Our plasma polymer coating only has to be renewed after thirty to a hundred cycles – in some applications, it even lasts up to a thousand cycles,” says IFAM project manager Gregor Graßl. “This accelerates production and is kind to the environment, as no solvent is released into the air.” It also takes less time to clean the mold after each component, as less material sticks to the surface. Another advantage is that the finished plastic parts no longer need to be stripped before painting. Until now, they were always covered in residues of non-stick solvent, which of course stops paints or adhesives sticking, too. This makes yet another working step redundant, besides that of greasing the mold.
Small molds, such as those used for aircraft window linings, are placed into a vacuum for coating. The researchers then ignite a plasma – an ionized gas similar to that which occurs in thunderbolts and polar lights – and feed it with a gas whose molecules are then split and deposited as a film on the metal surface. “Large component molds like those for 17-meter-long aircraft wing shells do not fit inside a vacuum chamber. The IFAM has therefore developed another new system in which the layering gas is sprayed onto the mold by robot-controlled nozzles. The plasma is ignited inside the nozzles,” says Graßl. The vacuum process is already being used in series production for the first time, while the nozzle variant is still under development. The coating process will be on display at the K2007 plastics trade fair in Düsseldorf (Stand E91, Hall 3) from October 24 to 31.