Visual inspection is recommended for identifying castings of poor surface quality, which should be rejected or marked for salvage before they are plated. Remelting and recasting is less costly than plating. A study of lighting conditions for inspection resulted in the conclusion that reliability could be improved by increasing lighting intensity to 200 or 250 ft-c. Inferior castings with defects that are not always detected with a normal light level of 50 to 70 ft-c were detected with 200 ft-c illumination, which avoided costly rejects after plating. Rejects after plating were reduced by two-thirds in another plant that installed high-level illumination for inspection.
Salvaging procedures for defective casting include mechanical smoothing procedures that erase surface defects usually limited in depth to less than 50 μm. Information on inspection and salvage procedures was developed during research programs administered by the International Lead Zinc Research Organization, Inc.
Parting lines are smoothed by:
1. Mechanical polishing with abrasive-coated wheels or belts.
2. Tumbling with abrasive media, or.
3. Vibration with wheel or continuous cloth belts that run over flexible backup wheels are usually used for mechanical polishing.
Wheel diameters range from 5 to 40 cm, depending on the complexity of the shape. Wheels are rotated with a maximum peripheral speed of 2500 m/min. A peripheral speed of 2100 m/min should not be exceeded with belts. Lower speeds of the order of 1100 to 1400 m/min are fairly common for small die castings. Abrasive belts should be lubricated with a small amount of grease. Die casting usually are polished individually in 30 sec or less. If small, larger castings sometimes required 5 to 6 min.
Tumbling in horizontal barrels loaded with abrasive stones such as preformed and fused aluminum oxide, granite or flint, and a lubricated such as soap solution removes parting line burrs from die castings in 4 to 12 hr. The barrels may be rotated at 5 rpm, but higher speeds reduce the time cycle and costs. The weight ratio of abrasive stones to zinc die casting is approximately 2.25:1.0.
Vibration in a bed of abrasive ceramic media usually removes parting line burrs in 3 hr or less. Frequencies range from 700 to 3000 cycles/min and amplitudes from 0.8 to 6.4 mm. A vibrating tub with capacity of 0.5 m3 can be loaded with about 900 kg of abrasive media and 180 kg of zinc die casting.