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Thermal cycling of molding tools increasingly used for automotive applications

Thermal cycling of molding tools increasingly used for automotive applications

Plastic products produced by using the current injection molding process usually have many defects such as short shot, jetting, sink mark, flow mark, weld mark and floating fibers. These defects have to be eliminated by using post-processing processes such as spraying and coating, which cause environment pollution and waste in time, materials, energy and labor. These problems can be solved effectively by using a new injection method- Variotherm injection molding. Injection molders in the developed countries of Europe and now in USA are increasingly finding benefits of providing thermal cycling to surface temperatures of tool during the molding cycle instead of keeping a constant. It is called heat/cool process, and often referred to as Variotherm molding process. In this new process, mold is raised beyond the glass transition or crystalline melting temperature during the filling stage, followed by rapid cooling. As per, it provides benefits such as:
•  More uniform holding pressure even in the areas of gate
•  Longer holding pressure time bringing better uniformity
•  Longer flow lengths
•  Reduction of internal part stress
•  Reduction of weld lines , silver streaks or sink marks
•  Better reproduction of minute mold surface detail
•  Improved surface finish of molded part
•  Eliminate post-mold downstream operations like sanding, annealing, priming, and painting to hide surface defects
Molders can raise the temperature of their mold surface using the three processes of convection, conduction and radiation :
•  Convection heating can be done with water or oil through single or multiple heat/cool circuits in the tool. Hot dry air or steam is also used-turned on and off during injection/cooling. Steam is being used mainly by developers in Japan
•  Conduction heating with electric or ceramic heating cartridges in the mold
•  Radiation from infrared lamps

It is finding an increasing diversity of applications from automotive bumpers to television flat-screen frames, cell phone cases, laptop covers, automotive lenses, and niche areas such as lab-on-chip devices with molded micro-capillary structures. It increases process ability and product performance in applications such as automotive, packaging, optics. Heat/cool molding is being increasingly considered for thin-wall applications due to the reduced filling pressure and injection pressure requirements. Automotive part manufacturers were the first users of this Variotherm process. The process was first developed and used commercially in Germany and France in Europe and also in Japan
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