Appliance Design - November 2008 - (Page 20)

Metals & Metal Parts the metal injection molding production line at Advanced Forming Technology. their products from plastic to metal. By converting from plastics to magnesium, the finished part will be lighter, thinner, and stronger. Magnesium is a great choice, he says, for any manufacturer that is pushing the limits on plastics, in terms of higher temperatures, heat dissipation, heat conduction, heat resistance, EMI shielding, and other factors. Because it is a magnesium part, it is lightweight, and strong. It can also create very thin walls, as thin as 0.5 mm. In one example, a computer housing that had a 1.5 mm thick plastic housing was replaced with a 1 mm thick magnesium housing and got greater strength during impact tests. One drawback of the process is that it produces parts that can creep at elevated temperatures. According to LeBeau and others, creep can cause a magnesium component to deform under load at elevated temperatures (250 DegF) over extended periods of time, even if the stress applied is below the yield stress of the alloy. The company is also working to expand its material selection. While currently Thixomolded parts are made solely from magnesium feed stock, the company is researching the potential for using zinc and this drive gear for a copy machine was developed using Thixomat’s magnesium molding process. aluminum metal materials. As it happens, zinc and aluminum are the components that make up Cool Polymer’s metal alloy blend. What sets apart Cool Polymers’ metal alloy from the other molding processes is its ease of use. While similar to metal injection and magnesium molding, the Cool Polymers alloy requires no special equipment beyond a traditional injection molding machine and the tooling. Designs that currently use plastic or even die-cast metal are target applications. According to Jim Miller, Cool Polymers’ product manager, what separates this method from the others is the fact that it uses a standard injection molder and the infrastructure and knowledge base of plastic injection molding is already in place. Because of this, Miller is confident that the alloy’s use will continue to grow. While there is a kinship with plastic injection molding, they are two different processes. For instance, design engineers should be aware of the thermal conductivity differences between plastic and metal, which can affect such things as cool down rates in the cavity and heat transfer in the molding machine’s barrel. These differences can cause flow and freeze-off problems, especially if the part to be molded is long and thin. Both the mold maker and the designer can play a part in helping to deal with this challenge. Designers can make for short sections or, in the case of a larger part length, design in supporting structures. The molder either has to get into the tool cavity quicker or the Delta T that exists between the melt temperature and the tooling temperature has to be increased. When it comes to tolerances, Miller says that it is part dependent, but is on the order of thousandths of an inch, per inch. Relative to plastics, the M950 is more controllable because the shrinkage rate is predictable and low, about 0.5 percent, which is at the low end of what is seen from plastics. The material is isotropic, and will not undergo the residual stresses that can cause warpage in certain plastics. In the long run, Miller says this will create flatter parts. Part complexity is similar to injection molding. It allows for thin walls and deep draws. The mechanical properties of the new Cool Polymers’ material include a tensile strength of 325 MPa; yield strength of 250 MPa; elongation at break of 2 percent; modulus of elasticity of 90 Gpa; specific heat capacity of 0.1 20 applianceDesIGN November 2008 www.applianceDesIGN.com http://www.appliancedesign.com

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Appliance Design - November 2008 Contents Editorial Shipments/Forecasts News Watch Metals & Metal Parts Gas Technology Displays Quality & Standards New Products Design Marts Association Report: AHAM Advertiser's Index

Appliance Design - November 2008

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