Machinery's Handbook, 31st Edition
610 POLISHING PLASTICS To minimize surface heating during machine polishing, it is recommended to avoid lingering at any one location, and to keep speeds and forces low. Otherwise, surfaces can quickly heat to temperatures of 400°F (205°C), and many plastics will soften and melt at these temperatures. Heating also can cause plastics to give off toxic gases. So machine polishing should be performed in vented enclosures, and masks worn during manual polishing to filter gases and dust. As with metals, polishing operations proceed from coarser to finer surfaces. If mold - ing, additive manufacturing, or machining processes left a rough surface, sanding may be needed prior to polishing. Larger defects, including parting lines, imperfections, scratches, saw lines, and scars resulting from fabrication, also can be treated with abra- sives prior to buffing. Wet or dry abrasives, such as silicon carbide or aluminum oxide, generally are used in grain sizes from 60 to as fine as 1000. Surfaces are ready for fine polishing when a satin finish is achieved. If the plastic parts are sufficiently robust, tumble polishing can be used with abrasive media in a vibratory bowl. Tumbling processes can operate as a batch process, with high loadings of parts. Cycle times are typically a few hours, with the usual media to parts ratio approximately 2 or 3 to 1 by volume. A wide variety of abrasive media are available, varying in shape, density, hardness, and grit. For burnishing and polishing of plastics parts, angle-cut cylinders, with finer grit and lower density and hardness, are recommended to avoid excessive material removal and loss of feature detail. Tumbling of plastics parts in ceramic media usually works best when used as a wet process, with intermittent or continuous filtering of the generated particulates. Economical polishing of edges—for example, of acrylic sheets—may be accomplished by flame polishing. If the surface is rough, edges should first be sanded with 320 grit paper. Quickly passing an oxy-acetylene torch over the edge surface melts it with the local flow and solidification results in a polished appearance. To minimize rounding of melted edges, a small strip of plastic or other material can be clamped on each side. Note that polycarbonate and many other plastics may not be easily flame polished, given their propensity to oxidative degradation and flammability. Polishing with buffer wheels is common, with speeds for polishing comparable to those for turning (see Table 12), typically 800 to 1,500 feet (360 to 580 meters) per minute, with rotation speeds varying with the buffer wheel size. For example, a wheel measuring 6 in. (150 mm) operating at 750 rpm will provide a linear velocity of 1,180 feet/minute (360 meters/minute). For most plastics, soft cotton buffs are best, using a wet or greasy tripoli or silica compound. If a cleaning solvent is used, it should be checked to see that it does not dissolve the plastic and used only in a well-ventilated area. Each round of polishing should be performed with a separate, clean polishing bonnet. Finishing and Decorating Plastics.— There are a number of other methods for finish - ing and decorating the surfaces of plastic parts, such as spray painting, vacuum met - allizing, hot stamping, silk screening, and even plating. Conductive coatings may be applied to inside surfaces, usually by flame- or arc-spraying, to dissipate static electric - ity and provide electromagnetic shielding. Thorough cleaning is essential. Materials such as polyethylene, polypropylene, and acetal have waxlike surfaces that may not be painted easily or may need pretreatment or special primers. Many amorphous plastics are easy to paint. Suitable coatings include polyurethane-, epoxy-, acrylic-, alkyd-, and vinyl-based paints. Oven curing must be carried out at temperatures well below the de - flection temperatures of the plastics in order to avoid distortion of parts. See Deflection temperature under load (DTUL) on page 574. Vacuum Metallizing and Sputter Coating require application of a special base lac- quer and protective clear top coat before and after treatment. Resistance heating or an electron beam can be used to melt the coating metals such as aluminum, silver, copper, and gold, which usually are pure elements. Wires of the coating metal are hung from tungsten heating elements in an array that gives them direct views of the surfaces to
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