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Eliminating Zinc

Diecasting Defects Z

Diecasters must understand what defects can occur when working with zinc to identify the cause and possible remedies for the issues. JOHN TITLEY, THE BROCK METAL CO. LTD., CANNOCK, U.K.

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 in large production quantities. Tis, coupled with the range of

physical and mechanical properties available from zinc die castings, 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 decora- tive parts for the automotive industry. Te need for high quality decorative

surfaces means finishing criteria are more critical, which affects casting cost and increases reject rates. Eliminat- ing surface defects is necessary when manufacturing zinc die castings. Te diecaster should consider the following as a possible cause of casting defects: • Alloy chemistry. The presence of excessive iron, lead, tin, cadmium, manganese and inter-metallics/ oxides are all possible sources of surface defects.

• 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.

• Casting design. Section thickness

changes, lack of fillet radii, surface textures and profiles all may pro- mote surface defect problems.

• Die design. The calculation of the runner and gate sizes, machine performance, position of the gate relative to casting geometry, die heating/cooling, use of over flow wells and venting all are critical to defect reduction.

• Ejection. The use of adequate ejec- tion, size and position of ejector pins, section thickness for ejec- tion, and reduction of mechanical damage/impingements during the ejection cycle are factors.

• Die lubrication. The diecaster must control the quantity and type of lubrication dispensed and guard against the propensity for die sur- face deposits/coking/waxing.

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

Die Casting Alloys Metallurgical changes are the

least likely to generate serious casting problems or surface defects in zinc die castings. However, changes in the aluminum and magnesium content of zinc alloys can affect other factors that determine surface finish—die tem- perature, metal temperature at the end

of the cavity fill, fill time, fill pattern and gate speed. Aluminum content below the

specified minimum will reduce the fluidity of the alloy, changing cavity conditions and increasing fill time. In the event the mechanical proper- ties of the alloy deteriorate when the aluminum content is 4.5%-5.5%, it can be improved at 8% and yield greater creep resistance in hot cham- ber pressure diecasting machines. Magnesium is added to zinc to

improve hardness and control the corrosion effects of certain impurities. Excessive magnesium has detrimental effects on surface finish, reducing fluid- ity and impeding cavity fill conditions. Low magnesium zinc alloys have been introduced to overcome the problem. Te alloys offer the designer a mate- rial that can be cast in thinner sections (down to 0.5-mm) while retaining the high surface definition associated with more traditional zinc alloys. Iron is the most frequent con- taminant in the zinc diecasting process. Most hot chamber diecasting machines use cast iron- or ferrous- based melting/holding furnaces. Te casting alloy comes in direct contact with the furnace and will attack the iron, over time causing contamination. Te presence of aluminum in the alloy prevents excessive iron pick up, and close control of the melt temperature will prevent excessive erosion of the

September 2012 MODERN CASTING | 25

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