design, die construction and die cast production, is the recom- mended course of action.
Design Guidelines for Enhancing Finishing Results
Design modifi cations to aid sur-
Fig. 1. A plastic bumper joining this two-part case conceals all parting lines, thus avoiding any edge polishing.
optimal results at the lowest cost per part, early discussions are essential to clarify precisely how the part will mate with other components in the fi nal product assembly. T is analysis is as important to fi nal surface fi nish quality as it is to meeting tolerance specifi cations. Die casting section, which are
hidden from view and cosmetically non-critical, can be considered for placement of the parting lines and gating. T ese features can create signifi cant cost penalties if they are placed on a viewable, cosmetic surface of the parts. Likewise, a potential sink mark on a non- cosmetic surface can be largely ignored, or steps can be taken to overcome its possible appearance by wall redesign—for example, internal support features which will be invisible to the user. While the exterior fi nish is dic-
tated by appearance specifi cations, the specifi c surface preparation called for usually depends more on functional design features. Criti- cal edges may require a shave trim, special polishing, a chromate coating and fi nal painting. Specifi ed tight- tolerance holes may call for acid etching or chromating followed by reaming, milling or boring. T e type and quality of the fi nal
fi nish are impacted by the geom- etry of the design of specifi c part features. Minor modifi cations of critical surfaces, edges and mounting features can lead to reduced costs
Fig. 3. Flush bosses for mounting mated parts (left) may cause adjacent paint damage, and cast-in boss shoulders protect adjacent painted areas during assembly.
Nov/Dec 2015 | METAL CASTING DESIGN & PURCHASING | 35
with minimum surface preparation prior to application of a fi nal coating. Preplanning, well before the fi nal
component design is fi nalized, is the essential step. Design consulta- tion on post-casting machining and surface fi nishing, prior to tooling
face fi nishing quality are not always feasible but, when possible, they can greatly improve results. Die Cast Part Edges: Part designs that hide trimmed edges within the fi nal product assembly eliminate the need for post-casting edge polishing. Early consultation on cosmetic features assures proper placement of necessary parting lines to conceal trimmed visible edges (Fig. 1). Die Cast Holes for Machining: Countersinks (chamfers) or counter bores placed on holes assure the integrity of the surface edge of tapped holes. Leading threads will
Fig. 2. A cast-in countersink (left) protects lead threads, while a counter bore also serves as a thread protector.
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