Modern Plastics Handbook 2011 Part 10

Tham khảo tài liệu 'modern plastics handbook 2011 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter Eight cost differential can cover quite a volume before injection molding becomes more competitive. However as product designers create structural foam designs with detail equivalent to injection-molded parts or require gas counterpressure molds that advantage has diminished. When the cost of a structural foam mold is nearly that of an injection mold the process will be selected for its unique attributes. Such features as weight reduction filling of hollows and the ability to make larger parts due to the low pressure become the principal selection criteria. For some parts there might be one more stop on the pecking order before going to injection molding that is gas-assisted injection molding. This process provides a rigid wall with a hollow channel within its thick sections and a solid wall elsewhere. Molding pressures can be as low as 15 of injection molding pressures with the resultant increase in the size of the part which can be molded in a given molding machine. Gas-assisted injection molding is of particular value in the elimination of assembly operations used to fabricate box structures. There have been successful applications in the elimination of parts in automobile door frames where the gas has been used to create the box structures within the frame. For most parts the ultimate in production volume economies can be reached with injection molding. At a size of about 5 ft2 of surface area however injection molding drops out of the competition because molding machines which can handle that size are very scarce and are usually reserved for proprietary applications. In that case structural foam molding may be the most efficient process. Unfortunately structural foam molding can handle a much more limited palette of materials than injection molding can. The materials that are readily molded with structural foam are ABS ionomers polycarbonate polyethylene and polypropylene. Most of the remainder can only be handled with difficulty and some like .

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