Rough of defective surface are smoothed by mechanical polishing on rotating wheels or continuous, abrasive coated belts, spin finishing, or vibratory finishing. Fissures, skin blisters, skin blisters, and other defects with a depth of 25 to 50 µm can usually be erased with these metal removal methods, when conditions are adopted to reach the bottom of the defects. Deeper defects are infrequent.
Rough or defective surface are often polished on wheels or belts, while parting lines are polished by the same operator. If polishing is mechanized to advance die castings attached to a conveyor through successive belts or wheels to polish different areas, a manual operation might be required later to complete the smoothing of parting lines if they are too curvy. The finish range from 0.2 to 0.4 µm, depending on the abrasive and the pressure.
Smoothing by spinning in abrasives is accomplished by attaching die castings to spindles or drums rotated with a peripheral speed of about 600 m/min in a slurry of abrasive material such as ground corn cobs or nut shells mixed from 5 to 10 min and the finish from 0.1 to 0.2 µm, depending on the abrasive.
Spin polishing, like wheel polishing, usually induces scratches with directional characteristics, which must subsequently by smoothed by buffing. Any subsurface porosity only 25 to 50 µm below the surface, which in encountered with some casting, usually is exposed by wheel polishing or spin polishing. Depending on the size of these pores, relatively thick leveling copper is required to avoid defective electroplates when subsurface pores are exposed.
Vibrating tubs loaded with plastic media, such as polyurethane impregnated with an abrasive such as aluminum oxide, smooth the surfaces of die castings in 4 hr or less when frequencies are in the range of 1600 to 3000 cycles/min and amplitudes are adjusted to 0.8 to 6.4 mm. Vibratory machines produced a finish of 0.15 to 0.20 µm. Media and zinc parts are usually loaded in a ratio of 5:1 or 6:1. Surface gauges might occurs with a smaller ratio.
Die casting can be buffed to produce a mirror-like finish suitable for plating with conventional solutions. Buffing is omitted, however, for die castings which have good cast surfaces or which have been uniformly polished to a finish of a 0.15 to 0.2 µm. If solution with good leveling power are available for copper and nickel plating.