Upfront collaboration between casting supplier and end user can improve post-casting processing, from finishing to painting.

Jon Miller, Chicago White Metal Casting Inc., Bensenville, Illinois

(Click here to see the story as it appears in the November/December issue of Metal Casting Design & Purchasing.)

In the diecasting 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.

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.

Design Guidelines for Enhancing Finishing Results

Design modifications to aid surface finishing 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 final 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 be protected from deburring or polishing. (Fig. 2).

Die Cast Mounting Features: Wherever possible, create raised “shoulders” on bosses that will receive painting masks; scuffing thus can be avoided on surrounding painted surface areas during fastener torqueing and mounting of mated parts (see Figs. 3).

Die Cast Bosses: Include correctly designed gussets to improve die fill and avoid resulting sink marks on Class “A” surfaces. Short and stocky bosses are preferable to tall, thin designs to optimize metal flow and insure integrity of the feature (Figs. 4).

Die Cast Corners: Use the maximum allowable radius for all internal 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 die cavity and maintaining the integrity of the corners of the part (see Fig. 5).

Die Cast Surfaces: Subtle textured surfaces can be produced, as-cast, on selected areas of a component by special preparation of the die. These cast-in textures are created by photoengraving techniques during die construction and are sometimes recommended for use on the underside of complex parts to aid in the smooth filling of the die (Fig. 6).

Finishing Steps and Alternatives

Depending on the specifications on durability, protection and cosmetic appearance, most aluminum, magnesium and zinc die castings will receive one to three post-casting finishing steps: deburring, conversion and/or combined conversion functional coating, and a final surface finish coating.

Post-Trim Deburring: Vibratory processes including automated operations using a range of media types, are used to round sharp edges, remove burrs, loosen flash and debris, and smooth and brighten surfaces. Liquid, selective media, calibrated vibration and special compounds can also be used to cushion against damage. Most die castings go through a mechanical deburring operation prior to post-trim finishing, though additional operations may be required for 100% burr-free specifications.

Surface Conversion Coating: Deburring is usually followed by a conversion coating to remove any remaining oil, diecast part release agents and other contaminants. Where final painting is specified, this coating serves as preparation and primer. In many non-cosmetic applications, this conversion coating serves as the component’s final finish.

Environmentally friendly Trivalent clear chromium is now a proven, economical alternative to widely used hexavalent chromium coatings, offering high corrosion resistance for aluminum, magnesium and zinc die castings. With a bright finish, it meets stricter EPA regulations and RoHS European Union mandates, avoiding concerns with prohibited toxic hexavalent chromates.

Combined Conversion Coatings/Functional Finishes: Where a diecast component has a specific functional requirement, such as added corrosion protection, added durability and/or semi-decorative appearance, one of the combined conversion-functional finishes is often recommended. A combined coating replaces the use of a surface conversion coating, which serves as either a paint base or the part’s final finish.

Final Cosmetic Surface Finish: While final painting or plating of diecast housings and components is most often specified for cosmetic purposes, and to sustain their decorative appearance over the life of the part, other functional purposes of a final applied finish may be of equal, or of even greater, importance. These include: maximizing corrosion resistance, heat dissipation, and surface performance under abuse, and adding greater insulation properties.

A wide range of painting applications is available for die castings in matching finished, mating parts produced in other production processes. These final finishes include powder coatings in finishes from fine to coarsely textured, liquid paint polyurethane and water-based finishes applied at various thickness/texture levels.

Component masking is an essential part of virtually all cosmetic surface finish applications and required to assure part areas that must not receive finish coatings are protected during processing. Required protective masks must be applied during finishing production and later removed. Costs are proportional to part complexity and the resulting masking required. Unique to diecast parts, special fixtures mated to the die castings can be constructed, in the case of longer finishing runs and those parts suited to such fixturing. If feasible, such fixture masks can reduce finishing production costs.

Painting (Powder or Liquid): Non-solvent-based powder coatings are environmentally friendly, enabling non-toxic waste disposal. Powder coatings for die castings produce a durable, uniform surface finish, from matte to semi-gloss, available in a range of surface textures. Custom colors can be formulated at extra cost.

Powder coating is the only recommended painting process for die castings operating in the field at temperatures above 300F (149C). Custom formulations are available to enable maintenance of coating integrity at even higher operating temperatures.

Polyurethane and other wet paint chemistries are the most common final color finishes for die cast components, and have now been joined by water-based wet paints. Finishes from matte to high-gloss are available.

Total production costs and lead times for most liquid paints are lower than for powder coating, especially for short production runs, particularly those projects requiring a custom formulation.

Electro-deposition of a metallic coating on a die casting can provide the most attractive, durable and wear- and corrosion-resistant finish of any surface treatment, at proportionately added cost. While die castings offer excellent built-in EMI-RFI shielding as cast, internal board design sometimes indicates additional plating to achieve maximum possible shielding protection.

Considering all that a diecast component must go through after the molten metal solidifies, purchasers and designers should work with casting suppliers early in the process to ensure parts move efficiently from the early stages of design through casting and post-casting processing. The final results will be higher quality, lower cost castings—something every casting purchaser strives for. 

This article is based on “The OEM Quick Guide to Surface Finishing" from Chicago White Metal Casting Inc., available at www.cwmdiecast.com.