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Eliminating Zinc Die Casting Defects


Die casting designers can help their supplier avoid defective castings by understanding the best way to design the parts.


JOHN TITLEY, THE BROCK METAL CO. LTD., CANNOCK, U.K. Z


inc die castings can be manufactured as precise, near-net-shape parts and accept a wide variety of finishes. Consistently high quality standards are achievable over large production quantities with the castings. Tis, coupled with the range of physical and


mechanical properties available from zinc die cast- ings, have made the parts the first choice for mass produced components like electronics, locks, security


and hardware for doors and windows, connectors and fittings for hydraulics and pneumatics, and decorative parts for the automotive industry. However, the need for high quality decorative surfaces means finishing


criteria are more critical, which affects casting cost and increases reject rates. Eliminating surface defects is key when manufacturing zinc die castings. The following aspects of the die casting production process can contribute to


casting defects: • Casting design. Section thickness changes, lack of fillet radii, surface textures and profiles all may promote surface defect problems.


• Die design. The calculation of the runner and gate sizes, machine perfor- mance, position of the gate relative to casting geometry, die temperature heating/cooling, use of overflow wells and venting all are critical to defect reduction.


• Ejection. Te use of adequate ejection, size and position of ejector pins, section thickness for ejection, and reduction of mechanical damage/impinge- ments during the ejection cycle are factors.


• Alloy chemistry. Te presence of excessive iron, lead, tin, cadmium, manga- nese and inter-metallics/oxides are possible sources of surface defects.


• Die lubrication. Te diecaster must control the quantity and type of lubrication dispensed and guard against the propensity for die surface deposits/coking/waxing.


40 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2012


• Metal temperature/metal handling. Temperature and temperature variations change casting quality. Introduction of ingot or scrap to the furnace at the wrong time or in the wrong condition also will affect casting quality.


• Secondary operations. Defects can be created through clipping/ break off, grinding and polishing, and mass media finishing. Tese concerns must be addressed at the casting/die design stage.


Casting Design


Well established and documented guidelines can assist diecasters in avoiding finish and finishing defects related to casting geometry. Among these are the following: Use adequate fillet radii and soft


external edges. Internal and external radii improve flow during cavity fill, reduce turbulence, and improve polishing and mass media finishing effectiveness. Tey also increase tool life, reduce die maintenance, increase casting strength by removing notches and stress raisers, and reduce the ef- fect of dramatic section changes. Control section changes. Design- ers should use the three circle rule to control section changes. Reduce the thinnest section to a minimum 0.06 in. (1.5 mm). Adjoining sections should be limited to 0.18 in. (4.5 mm), increasing progressively to the desired or necessary section. Adopt curved surfaces and other design aids to disguise plating and


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