polishing blemishes. Large flat sur- faces exaggerate the edge buildup associated with plating and powder coating. Minute curvature is easier to polish and improves material flow during cavity fill, reducing turbu- lence and flow defects. Avoid deep blind pockets or holes.
Plating will not move into these ar- eas. T ey also tend to become blocked with polishing debris and plating solutions and require cleaning after fi nishing. As a casting feature, holes and slots are useful for plating fi xture points. However, this can leave scars, which are can cause corrosion defects and are unsightly. Avoid gate scars and parting line
defects. Clipping and break off , ma- chining, and polishing often reveal porosity and fracture defects at the gate scar or along the parting line. T ese 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 on parts which should be avoided in the gating or positioning of overfl ow wells. Avoid vertical parting line changes.
T e casting designer should always specify the maximum draft allowable to prevent the casting from sticking in the die. Generous draft promotes easy ejection, low die face wear and effi cient die cycling. Maximum draft should be implemented across the parting line, where zero draft has severe conse- quences. Normally, zinc die castings need a minimum draft angle of 10 de- grees per side, but this is insuffi cient if used on die face parting lines. Angles in excess of 50 degrees on die parting line features are advisable. Follow proper paint curing tem-
peratures. Specify finishes with low curing temperatures to diminish the risk of blisters, porosity and exag- geration of casting imperfections. High- gloss paints 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 quantity of parts required per year or batch, as this will aff ect machine size and manufacturing costs and be used to establish the die design. Casting quality criteria, fi nish, ac-
curacy and other factors infl uence die design. During the quotation stage, the die designer can calculate the feed
system and gate size required to achieve the fi nish, cycle time and machine size required and predict the fl ow path of the molten material in the cavity. T e size of ejector pins and position
of fi xed and sliding cores are critical to the performance of the die. T ese factors are infl uenced by the casting profi le, section thicknesses, aesthetic features and limitations. Positioning ejector pins on areas of the casting thick enough to support the ejection force when the casting alloy is hot and ductile will reduce surface bumps. Placing ejectors on bosses within the
Fig. 1. The surface of this black powder coated automotive part shows a blister (circled) due to entrapped gas under the surface of the casting.
Fig. 2. The picture shows a cavity under the surface of the casting due to a blister. The thickness of the zinc layer directly beneath the paint is approximately 0.45 mm. The areas circled show the separation cracks between the inner surface of the laminated blister and the basic casting substrate.
Fig. 3. Numerous large pores (approxi- mately 1 mm in diameter) are evident in this example of gas porosity.
Fig. 4. Shown is an enlarged view of micro- porosity directly under a plated surface.
Fig. 5. This photograph shows an enlarged view of an impingement defect on the surface of a casting.
Dae ra sum que perum cullabo. Exped quidi nate elisimus sum est, quia nos ipsanturio. Derepudi dolupta tendae quid modis vollectis int
Fig. 6. Sub-surface porosity has a signifi cant effect on fused powder paint fi nishes. This common defect, known as cissing, indicates the presence of minor gas, air or steam pockets below the surface of the casting.
Sept/Oct 2012 | METAL CASTING DESIGN & PURCHASING | 41
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