Upfront Collaboration Between Casting Supplier and End User Can Improve Post-Casting Processing, from Finishing to Painting
by Joe Miller, Chicago White Metal Casting Inc., Bensenville, Illinois
A plastic bumper joining this two-part case conceals all parting lines, thus avoiding any edge polishing.
In the die casting process, metal molds—or dies—are preheated and coated with a die release agent prior to the injection of molten metal that is forced into the die under extreme pressure (usually from 10,000 to 15,000 psi). But once they are ejected from the die, most parts have a journey ahead of them before arriving at the shipping department.
The large majority of die castings require specific coatings and finishes, polishing and/or painting. These processes are necessary to meet cosmetic/decorative appearance requirements, enhance wear resistance and/or provide a protective barrier against corrosion. Read the entire article here, on the MetalCastingDesign.com site.
The final decision on any post-casting finishing operation should always be made in advance of die design and only after detailed consultation with your casting supplier’s engineering department. The design features of your part have a direct impact on achieving your precise surface finish specifications.
How Die Casting Features Impact Surface Finishing
Superior as-cast cosmetic surface finishes characterize die castings produced by today’s advanced technology. In order to achieve 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 final product assembly. This analysis is as important to final surface finish quality as it is to meeting tolerance specifications.
Die casting section, which are hidden from view and cosmetically non-critical, can be considered for placement of the parting lines and gating. These features can create significant 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 finish is dictated by appearance specifications, the specific surface preparation called for usually depends more on functional design features. Critical edges may require a shave trim, special polishing, a chromate coating and final painting. Specified tight-tolerance holes may call for acid etching or chromating followed by reaming, milling or boring.
The type and quality of the final finish are impacted by the geometry of the design of specific part features. Minor modifications of critical surfaces, edges and mounting features can lead to reduced costs with minimum surface preparation prior to application of a final coating.
Preplanning, well before the final component design is finalized, is the essential step. Design consultation on post-casting machining and surface finishing, prior to tooling design, die construction and die cast production, is the recommended course of action.
Common Areas to Enhance Finishing Results
Design modifications to aid surface finishing quality are not always feasible. But, they can greatly improve results when possible.
Die Cast Part Edges
Die Cast Holes for Machining
Die Cast Mounting Features
Die Cast Bosses
Die Cast Corners
Die Cast Surfaces
Read the entire article here, on the MetalCastingDesign.com site.
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