Rapid Heat Dissipation from Critical Areas of Injection Molds
Until now, it was not possible to apply the process of selective laser melting (SLM) to copper alloys. Now, researchers at the Fraunhofer Institute for Laser Technology ILT, Aachen, Germany, have overcome the technical problems by modifying the method. As part of the InnoSurface project, funded by the Federal Ministry of Economics and Technology, the researchers managed to produce intricately shaped parts that were hitherto extremely laborious to accomplish, if at all, with conventional technology. They have thus opened up new possibilities, for example, in the manufacture of molds for processing plastics.
Such projects had been doomed to failure till now due to the high thermal conductivity of copper and copper alloys. True, copper has a lower melting point than steel, but it does not absorb the laser light as well, and it dissipates the heat better. The result is that the melt track tears and tiny spheres of melt start to form. These are much higher than the layer thickness and interfere with the workflow. They also create cavities that lower the density of the final component. "To compensate for the high level of heat dissipation and low absorption of copper during melting, we use a 1.000-watt laser for SLM instead of the usual 200-watt laser," says project leader David Becker. For satisfactory results, he opted for a laser that has a very uniform beam profile. Becker and his team simultaneously modified the entire installation so that the high energy input would not create any interference; e.g. they changed the protective gas supply and the system mechanics. "Experiments on the copper alloy K220 Hovadur have already yielded excellent results," says Becker, before adding: "the density of the workpieces is nearly 100 percent." The process is therefore ready for industrial use.
It is precisely the very high thermal conductivity of copper and its alloys that makes it ideal for so many applications. Thus, inserts made from such materials accelerate heat dissipation at critical points in steel molds used for injection molding plastic parts. With SLM, it is possible to additionally provide these copper inserts with conformal cooling channels for a coolant, such as water. Cycle times and warping are minimized because the entire mold cools evenly and rapidly.
The Aachen-based researchers now intend to go a step further and process not only copper alloys, but pure copper into dense parts. The thermal conductivity of pure copper is almost twice that of Hovadur K220.
Fraunhofer-Institut für Lasertechnik ILT
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