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have a die lubricant surface residue, which will increase dross levels and cause fumes. Adding sprues to the melt also will increase oxides because of their surface area-to-alloy volume ratio. Some of the oxides released can become entrained in the melt, caus- ing casting defects and increasing the risk of failure or fracture. Entrained oxides will appear on the surface of castings as oxide folds, which cannot be removed by polishing and will show through a plated surface. T e interface between these folds and the cast alloy structure may absorb plating solu- tions during the plating process and be released after plating, causing the surface to blister and lift off . Peeling off the blisters often will show a clean zinc casting surface, with the bottom of the blister showing a copper surface.

Casting Design

Well established and documented guidelines can assist diecasters in avoiding fi nish and fi nishing defects related to casting geometry. Diecasters should: • Use adequate f illet radii and soft external edges. Internal and external radii improve flow during cavity fill, reduce turbulence, and improve polishing and mass media finish- ing effectiveness. They also increase tool life, reduce die maintenance, increase casting strength by remov- ing notches and stress raisers, and reduce the effect of dramatic sec- tion changes.

• Control section changes. Designers 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 polishing blemishes. Large flat sur- faces exaggerate the edge build up associated with plating and powder coating. Minute curvature is easier to polish and improves material flow during cavity fill, reducing turbulence and flow defects.

• Avoid deep blind pockets or holes. Plating will not move into these

Zinc castings must be free of surface defects because they often are designed with high quality coatings in mind.

areas. They also tend to become blocked with polishing debris and plating solutions and need cleaning after finishing. As a casting feature, holes and slots are useful for plating fixture points. However, this can leave scars, which are unsightly and can cause corrosion defects.

• Avoid gate scars and parting line defects. Clipping and break off, machining and polishing often reveal porosity and fracture defects at the gate scar or along the part- ing line. These defects are process generated or caused by temperature imbalances during the casting stage. Because these defects will occur even with good die design, the part designer should specify the faces that should not be used for gating or positioning of overflow wells.

• Avoid vertical parting line changes. The casting designer should always

specify the maximum draft allow- able to prevent the casting from sticking in the die. Generous draft promotes easy ejection, low die face wear and efficient die cycling. Maximum draft also should be implemented across the parting line, where zero draft has severe consequences. Normally, zinc die castings need a minimum draft angle of 10 degrees per side, but this is insufficient if used on die face parting lines. Angles in excess of 50 degrees on die parting line features are advisable.

• Follow proper paint curing tempera- tures. Specify finishes with low cur- ing temperatures to diminish the risk of blisters, porosity and exag- geration of casting imperfections. High gloss paints also will amplify the evidence of surface defects, and the use of matte or semi-matte finishes will assist the diecaster. In some instances, aesthetic consid-

erations and/or the mechanical func- tion parameters of the part override good design practice.

Die Design and Ejectors Quality die castings are manufac-

tured from quality dies. T e number of impressions, die cycle times and minimal scrap will be infl uenced by the decisions made at the die design stage. It is important to establish the

Zinc castings such as the heat sink cover shown must have adequate fi llet radii and soft external edges to avoid cosmetic defects.

September 2012 MODERN CASTING | 27

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