Appliance Design - November 2008 - (Page 18)

Metals & Metal Parts lbs. Cool Polymer’s metal alloy can be used on parts weighing a few grams to several pounds. The physical dimensions of a metal injection molded part depend on a number of factors including wall thicknesses. While each application differs, Pelke says that the length to width ratio should be no greater than 10 to 1. Because each application differs, he says the best rule of thumb is to work early in the process with the supplier to best optimize design. Process steps and special equipment can also be a consideration as they vary from method to method. Cool Polymers’ process is the simplest of all. Its metal alloy can be used in any standard injection-molding machine, using the same tooling as that used for plastic molding. For its part, the magnesium injection molding process occurs inside a specially built molding machine that automatically softens the magnesium, flows it into the mold cavity, and forms the part. Metal injection molding, on the other hand, requires several steps and can have higher cycle times and increased material handling. Material handling is typically done through an automated, robotic system, however. The metal injection molding stages PROCESS COMPARISON CHART Process Complexity Simple Part Cost 3 4 Complex Material Tolerance Part Cost Selection Capabilities 5 4 4 4 3 4 High Volume Capabilities 5 5 MIM 5 Screw 2 Machine 3 Castings Machine 4 P/M 2 Die Cast 4 Scale: 5=Excellent, 1=Poor 3 3 3 2 3 1 1 5 5 2 4 2 4 4 5 5 4 1 3 3 Cost/Capabilities for each process vary by application Comparing metal injection molding to other methods to make metal parts. Source: Advance Forming Technology include compounding metal powders, in particles measuring about 20 microns big, with a plastic binder in an approximately 60-percent metal to 40-percent plastic-binder ratio. The material is loaded into an injection molder in which the plastic melts and flows as in a traditional injection molding process. At this stage, the molded component is called a green part because it is in the shape of the finished part, but hasn’t been sintered into its final size and consistency and the plastic binder has not been removed. the metal injection molding process allows for the use of many types of metals as seen in this material list. Source: Phillips Plastics 18 applianceDesIGN November 2008 The part still consists of a 60/40 metal-toplastic ratio. To remove the binding agent, the green part is placed into a debinding machine, which extracts most of the plastic material. The extraction is typically done thermally in debinding furnaces, but can be done with solvents and degreasers. When removing the plastic, voids in the part are created. To close the voids, the part is put in a sintering furnace where it is brought near the melt temperature of the metal. (Stainless steel 17-4, for instance, would be sintered at about 2,100 DegF.) The high heat fuses the powder particles and helps to close most of the voids. Smith likens this process to two beads of water that are near each other and through surface tension becomes one bead of water. During the sintering process, as the voids condense, the part shrinks. Shrinkage rates can vary, but commonly parts shrink about 20 percent. The shrinkage rates are predictable, however, and designers simply include this shrinkage rate into their part size specifications. The process is very similar to traditional powder metallurgy, also known as press and sinter, which also uses powdered metals and sintering. Compared to traditional powder metallurgy, however, metal injection molding can mold geometries that eliminate secondary operations, and offer superior density, corrosion performance, strength, and ductility. The resulting finished part is in the form of a near-net or net-shape part, says Tony Pelke, engineering manager for Phillips Plastics’ metal injection molding facility. Pelke adds that the molding machine’s ability to do multi-cavity parts can lead to very complex parts. It also can help reduce a multiplepiece assembly, by eliminating individual 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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