Powder Molding of Thermoplastics

In-mold decoration can be done with either injection molding of thermoplastics or compression molding of thermosets. Thermosetting plastics are decorated with a two-stage process. For melamine products, for example, the mold is loaded with the molding powder in the usual manner and closed. It is opened after a partial cure, and the decorative "foil or overlay is placed in position. 

Polyethylene is readily fabricated by all methods of thermoplastic processing. The principal methods used are film and sheet extrusion, extrusion coating, injection molding, blow molding, pipe extrusion, wire and cable extrusion coating, rotomolding, and hot melt and powder coatings. 

Although it would be desirable to recycle laminate scrap, this has been difficult because of its thermoset nature. However, a 1993 patent (18) suggested a means whereby scrap consisting of cellulose, thermoset resins, and partially reacted resins can be ground to a powder which is used as a filler in a thermoplastic resin. The filled thermoplastic resin is then used for molding of various articles. 

They can be processed by techniques commonly used for thermoplastics, such as extrusion, injection molding, rotational molding, dip, slush molding, and powder and fluidized bed coating,16 and can be expanded into foams.17 Compression and transfer molding of FEP resins can be done, but with some difficulty. Extrusion of PEP is used for primary insulation or cable jackets and for tubing and films. 

A measured amount of powder or liquid thermoplastic (TP) is placed in the cavity that is mounted on a turret arm capable of rotating the mold. The mold in the oven spins biaxially with rotational speeds being infinitely variable, usually ranging up to 50 rpm on the minor axes and 12 rpm on the major axes. A 4 1 rotation ratio generally is used for symmetrically shaped parts. A wide variety of ratios are necessary for molding unusual and complex shapes. 

RTP produces series of engineering thermoplastics lubricated with PTFE, molybdenum disulfide, graphite powder, silicone and glass, carbon and aramid fibers. Grades AR, TEE and SI include respectively aramid fibers, PTFE (10-20%) and silicone fluid (0.5%). Matrix materials used include PA-6, PA-66, PA-6,10, PA-12, PC, POM, PET, PBT, PSF, PPS, PEI, PEEK and polyphthalamide. The blends have been developed for injection molding of various parts for engineering applications. 

For molding urea and melamine materials, pressures of approximately one and one-half times that needed for phenolic material are necessary. In compression molding of thermosets the mold remains hot throughout the entire cycle. After the molded part is ejected, a new charge of molding powder is introduced. Unlike thermosets, thermoplastics must be cooled to harden. So before a molded part is ejected, the entire mold must be cooled, and as a result, the process of compression molding is slow with thermoplastics. 

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