Because zinc castings often are designed with high quality coatings in mind, they must be free of surface defects. Under ideal conditions, evaporation leaves the die face
covered in a microscopic coating of lubricating oil. Control of the volume of spray and use of the correct type of lubricant are critical to maintaining a good casting surface finish. Surface defects can arise from oil deposits burning onto
the die cavity face and forming fine coking, which shows on the casting surface. Tis usually is the result of exceeding the die temperature guidelines, an oil rich die spray, improper selection of lubricant or excessive application of lubricant. Te application of die lubricant to a die that is too cold re- sults in cold flow defects, excessive porosity from fumes and steam in the casting and interrupted cycling.
Post Casting Defects For a zinc alloy die casting to accept high quality finishes,
several stages must be considered after the part is complete. Most of these stages involve post-casting operations that identify, create or reject defective parts after finishing. Secondary operations include clipping, degating, grind- ing and polishing, vibro- or mass media finishing, machin- ing and plating/painting, among others. Rejects can arise from each of these operations, but from a finishing point of view, some forms of reject can be ignored (e.g. machining scrap due to dimensional inaccuracy). Te movement of parts after casting generally is re-
sponsible for the same number of rejects or surface defec- tive parts as the casting process. Multiple handling points make impingement damage common unless care is taken when parts are transferred. Often, the level of polishing or mass media finishing is insufficient to remove the edge cuts, nicks and impingements introduced by handling after the casting leaves the die. Brock Metal is an independent supplier of zinc alloy ingot; for more information visit
www.brockmetal.com.
44 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2012
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