The CWM Die Cast Component OEM Quick Guide to Surface Finishing A Condensed Resource on Die Cast Part Surface Finishing for OEM Design Engineers & Specifiers A preliminary guide for evaluation of die cast surface finishing alternatives for product appearance, durability and protection to aid in cost-effective specification of parts produced in Aluminum, Magnesium & Zinc—and ZA-8—die casting alloys. A companion piece to the CWM Comprehensive Surface Finishing Guide and Sample Portfolio, which contains detailed finishing recommendations and die cast plates in the most widely used alloys, with surface finishes applied. Chicago Browse CWM’s DC2 ® White Metal (Die Cast Design Center) Casting, Inc. Over 70 die casting design guides & bulletins are available to OEM product designers and engineers for download 24/7 at our website: http://dc2.cwmdiecast.com CONTENTS Introduction to Surface Finishing here. Advanced die casting processing has the for Die Cast Parts unique ability to deliver extremely close tolerances as-cast, with machining often entirely eliminated; Introduction to The three main reasons for the application of spe- where such as-cast processing is not feasible, Surface Finishing for cific coatings and finishes to die cast components pre-die-design consultation with CWM is essential Die Cast Parts are these: to assure successful post-casting machining 2 ■ Meet cosmetic/decorative appearance require- results. ments Surface finishing test results, executed under How Die Cast Part controlled laboratory conditions, are available to ■ Enhance resistance to wear Features Impact CWM customers for many of the finishing opera- Surface Finishing: ■ Provide a protective barrier against environ- tions outlined in this Quick Guide. Contact the Design Guidelines mental or galvanic corrosion CWM Sales Department regarding your specific for Enhancing In certain cases, die castings are also called upon finishing questions. Finishing Results to meet very special shielding specifications or 2-3 special pressure tightness requirements: How Die Cast Part Features ■ Special EMI/RFI shielding requirements Surface Impact Surface Finishing Die cast housings and components, in all Finishing alloys, provide assured built-in EMI/RFI shielding. Superior as-cast cosmetic surface finishes char- Steps and Coating When shielding requirements are particularly criti- acterize die castings produced by today’s Alternatives cal, with special designs or products subject to advanced technology. To consistently achieve 4 extreme abuse, surface plating can be used to such results at the lowest cost per part, it is achieve such optimum specifications. The result- essential that early discussions clarify precisely Die Cast Finishing ing plated surface finish will also provide an how the part will mate with other components in Cost-Performance enhanced appearance. When specialized EMI the final product assembly. This analysis is as Comparison Table gasketing must be used, plating may also be important to final surface finish quality as it is to 5 required. meeting tolerance specifications. Those sections of the die casting which will be ■ 100% pressure tightness requirements “hidden” from view, and cosmetically non-critical, Surface Finishing Impregnation is a post-casting sealing process can be considered for placement of the part’s Requirements for which, under vacuum, brings organic material into gating for metal entry and for its required parting 9 Typical the surface and subsurface pores of a die cast- lines. The presence of either of these features of Die Cast Parts: ing, providing highest pressure tightness. It is rou- die construction can create a severe cost penalty and suggested tinely recommended for critical air or water if they fall on a viewable, cosmetic surface of the coatings for each sealing applications, and, in addition, assures part. Likewise, a potential sink mark on a non- 6-7 high output for any final surface finishing required. cosmetic surface can be largely ignored, or steps can be taken to overcome its possible appear- The CWM This CWM Condensed Quick Guide, or its com- ance by wall redesign—for example, internal sup- Surface Finishing panion—the CWM Comprehensive Surface port features which will be invisible to the user. Portfolio: Finishing Portfolio with sample finished plates— While the exterior finish is dictated by the Comprehensive should not be viewed as self-selection tools for required appearance specifications, the specific descriptions and specifying precise finishes for die cast parts. They surface preparation called for is usually more sample library are published as aids to understanding the range, dependent on functional design features. Critical of coated die cast characteristics and relative costs of the post-cast- edges may require a shave trim, followed by spe- plates ing finishing operations available to OEMs from cial polishing, a chromate coating and then final 8 CWM to meet final product requirements. painting. Specified tight-tolerance holes may call Over 70 years of CWM experience, and that of for acid etching or chromating followed by ream- its surface finishing production partners, are CWM Design ing, milling or boring. reflected in their content, to aid in assuring that Resources for OEMs The type and quality of the final finish produced CWM Al, Mg and Zn die cast housings and com- Summary listing is impacted by the geometry of the design of spe- ponents meet stringent protection, durability and of CWM sales and cific part features. Minor modifications of critical decorative specifications. surfaces, edges, and mounting features can engineering The final decision on any post-casting finishing change a costly finishing scenario to highly cost- resources for operation should always be made in advance of efficient processing with minimum surface prepa- designers and die casting die design and only after detailed con- ration prior to application of a final coating. specifiers sultation with CWM engineering. The specific Preplanning, well before the final component 8 design features of your die cast part in almost design is locked in, is the essential step. Design every case have a direct impact on achieving your consultation on post-casting machining, as well precise surface finish specifications. © 2012 Chicago White as final surface finishing, prior to tooling design, Metal Casting, Inc. This is also true in the case of any post-casting die construction and die cast production, is the All Rights Reserved. machining requirements, a subject not treated recommended guideline. 2 CWM Surface Finishing Quick Guide Design Guidelines for Enhancing ■ Die Cast Bosses Finishing Results Include correctly designed gussets to improve Design modifications to aid surface finishing qual- die fill and avoid resulting sink marks on Class “A” ity are not always feasible. Where they can be surfaces. Short and stocky bosses are preferable made, significantly improved results will follow. to tall, thin designs to optimize metal flow and insure integrity of the feature (Figs. 4a & 4b). ■ Die Cast Part Edges Part designs which hide any trimmed edges within the final product assembly will eliminate any need for post-casting edge polishing. Early CWM consultation on cosmetic features will assure that necessary parting lines will be placed to conceal trimmed visible edges (Fig. 1). Fig. 4a Tall bosses can lack Fig. 4b Shorter bosses with integrity for support. gussets optimize designs. ■ Die Cast Corners Use the maximum allowable radius for all inter- nal and external corners to permit vibratory deburring media to reach all part surfaces. This design guideline for die cast corners of housings is also vital to assuring the complete filling of the Fig. 1 A plastic bumper joining this 2-part case will con- ceal all parting lines, thus avoiding any edge polishing. die cavity and maintaining the integrity of the cor- ners of the part (see Fig. 5). ■ Die Cast Holes for Machining Design in chamfers (countersinks) or counter bores on holes to assure integrity of the surface edge of tapped holes. Leading threads will be pro- tected from deburring or polishing. (Figs. 2a & 2b). Fig. 5 While advanced technology can execute most corner designs, gentle radii, as above, will reduce finishing steps. ■ Die Cast Surfaces Fig. 2a A cast-in counter- Fig. 2b A counter bore also Subtle textured surfaces can be produced, as- sink protects lead threads. serves as a thread protector. cast, on selected areas of a component by spe- cial preparation of the die casting die. These ■ Die Cast Mounting Features cast-in textures are created by photoengraving Wherever possible, create raised “shoulders” techniques during die construction and are some- on bosses which will receive painting masks; times recommended for use on the underside of scuffing can thus be avoided on surrounding complex parts to aid in smooth die fill (Fig. 6). painted surface areas during fastener torquing and mounting of mated parts (see Figs. 3a & 3b). Fig. 3a Flush bosses for Fig. 3b Cast-in boss shoul- mounting mated parts may ders protect adjacent paint- Fig. 6 The addition of cast-in surface textures, as in this cause adjacent paint damage. ed areas during assembly. internal frame, can aid smooth filling of the die cavity. CWM Surface Finishing Quick Guide 3 Finishing Steps and Alternatives parts produced in other production processes. Depending on requirements to meet durability, pro- These final die cast finishes include powder coat- tection and cosmetic appearance specs, most Al,