The following exerpts are from The designer should understand the the Quality Metal Finishing Guide for limitations imposed by shape and size Decorative Copper/Nickel/Chromium: of components to facilitate quality he focus of this month’s finishing at an acceptable cost. The P&SF is “design for A program to improve and control the designer can exert as much influ- finishing.” Designers have as much quality of a metal or plastic product ence on the quality attainable in influence as the surface finisher on should start at the desk of the de- finishing a part as can the the quality attainable in the finish of a signer. The metal finisher is restricted electroplater himself. ASTM part. Often the finisher has to be by certain basic principles of me- Standard B-507 can provide the creative and jump through hoops to chanical finishing and . designer with helpful information. achieve a good finish on a difficult-to-plate part. Good design—with the Influence of Design on Electroplatability finish in mind at the Feature The distribution of electroplate is Better Design beginning—doesn’t indicated in an exaggerated fashion. always happen. Where can finishers and Ideal shape. Easy to plate uni- designers go for help? formly, especially where edges are ConvexConvex surfaces surfaces One source is the rounded. Metal Finishing Not as desirable as crowned sur- Suppliers’ Association faces. Use a 0.4 mm/25.4 mm (1/ (MFSA), which has 64 in./in.) crown to hide undula- published eight Quality FlatFlat surfaces surfaces tions caused by uneven buffing. Metal Finishing Guides* that contain a Undesirable. Reduced thickness on wealth of practical center areas. Also requires in- information on the creased plating time for deposit- many different aspects Sharply angled edges ing a minimum thickness of du- rable electroplate. All edges should of . All be rounded. (Edges that will con- of MFSA’s Guides that tact painted surfaces should have a pertain to surface minimum radius of 0.8 mm [1/32 finishing present similar in.]). information on design, tailored to each specific Large flanges with sharp inside finish. Subjects of the angles should be avoided to mini- eight Guides include: mize plating costs. Use a generous radius on inside angles and taper FlangesFlanges the abutment. • Decorative Copper/ Nickel/Chromium Narrow, closely spaced slots and • Zinc and Cadmium holes reduce electroplatability and Coatings Slots cannot be properly plated with cor- • Mass Finishing rosion-protective nickel and chro- • Tin and Tin Alloy mium unless corners are rounded. Coatings • Hard Chrome Must usually be exempted from Blind Holes minimum-thickness requirements. • Electroless Nickel Where necessary, limit depth to Plating 50-percent of width. Avoid diam- • Decorative Precious eters less than 6 mm (7/32 in.) Metal Plating • Chemical Surface Increase plating time and costs for Preparation for a specified minimum thickness and Electroplated and Sharply angled indentations reduce the durability of the plated Metallic Coatings part.

12 PLATING & SURFACE FINISHING ignificant Surfaces position; or, which can be the source mirror-like or satin appearance on the A very important term used in of products that deface finished part. Mechanical finishing is specifying metal finishing is “signifi- visible surfaces on the assembled an expensive operation. To reduce cant surfaces.” In most products, the article. When necessary, the signifi- costs and assist the metal finisher in same standard of quality is not cant surfaces shall be the subject of improving the appearance and quality required over every inch of the agreement between purchaser and of the product, the designer should surface. Instead, the quality specifica- manufacturer, and shall be indicated consider certain rules applicable for tions apply and compliance is on the drawings of the parts, or by the parts requiring mechanical finishing: expected only for the so-called provision of suitably marked samples. “significant surfaces” defined by • Avoid blind holes, recesses and joint mutual agreement between the esign for Mechanical Finishing crevices that can retain polishing producer and purchaser as follows: Metal products that are to be coated compounds and metal debris. with copper/nickel/chromium or • Avoid intricate surface patterns that Significant surfaces are defined as nickel/chromium finishes are typically will be blurred by polishing. those normally visible (directly or by subjected to polishing with • Significant surfaces should be reflection), which are essential to the belts or wheels in preparation for the exterior, reachable by ordinary appearance or serviceability of the plating operations. This is done to aid polishing wheels or belts. article when assembled in normal in securing an attractive, uniform, • Avoid sharp edges and protrusions that cause excessive consumption of Influence of Design on Electroplatability wheels or belts. Feature The distribution of electroplate is Better Design indicated in an exaggerated fashion. For small parts that are to be processed in Inside and outside angles should a barrel, these rules be rounded generously to apply, in addition to a minimize costs. Flat-bottomFlat-bottom grooves grooves requirement that the Deep, V-shaped grooves cannot be parts be sturdy V-shapedV-shaped grooves grooves satisfactorily plated with corrosion- enough to withstand protective nickel and chromium the multiple impacts and should be avoided. Shallow, of barrel rotation. rounded grooves are better. Small, flat parts should be designed Increase plating time and costs with ridges or Fins for a specified minimum dimples to prevent thickness and reduce the durabil- ity of the plated part. them from “nesting” together as they are Narrow ribs with sharp angles processed. RibsRibs usually reduce electroplatability; wide ribs with rounded edges esign for Rack- impose no problem. Taper each ing, Draining & rib from its center to both sides Entrapment and round off edges. Increase Most metal or plastic Concave recesses spacing if possible. parts weighing more Electroplatability is dependent than a few ounces are upon dimensions. not plated in bulk in barrels, but are Increase plating time and costs mounted on racks for for a specified minimum processing in clean- thickness. ing and electroplating Deep scoops tanks. Products that would Buildup on jut will rob corners occupy a large volume from their share of electroplate. in processing tanks— Crown the base and round off all large in proportion to corners. surface area—should Spearlike juts be designed to be plated in sections for assembly after Electroplatability is dependent coating. upon dimensions. Round off Rings corners and crown from center Consult with the line, sloping toward both sides. plater to make certain that parts can be held

February 1998 13 securely on a plating rack with good as possible. At the same time, uneco- crowned areas receive more electrical contact, without masking a nomical waste of metal by excessive uniform coatings. Use a 0.4 mm significant surface. Many difficult build-up on both non-significant and per 25.4 mm (0.015 in. per in.) racking problems can be solved by significant areas must be avoided. crown minimum. design modification. It is possible to estimate metal • Edges should be rounded to a radius Provide for good drainage of distribution ratios from models or of at least 0.4 mm (1/64 in.); processing solutions from racked mock-ups, but there are also empirical preferably 0.8 mm (1/32 in.). parts. Certain shapes tend to trap rules. These can guide the designer to • Re-entrant angles or corners should solution. This causes contamination improved uniformity of thickness, be filleted with a generous radius, by carry-over, possible corrosion of resulting in improved quality with making the radius as large as the part and waste of materials. Carry- greater economy. These general possible. over increases waste disposal prob- principles illustrate what has been • Avoid concave recesses, grooves or lems. When designing a part, avoid learned from practical experience. slots with width less than one-half rolled edges, blind holes and spot- the depth. welded joints. Drain holes are • Avoid concave or perfectly flat • Minimize the number of blind holes, especially important in irregular significant surfaces. Convex or because these typically must be shapes and tubular parts. Avoid shapes that can trap air on entry into processing tanks, if the air could block access of solution to areas requir- ing treatment. Whenever air can be trapped, hydrogen or oxygen gas may also accumulate during a cleaning or plating step.

esign for Good Distribution of Electrodeposit Experience and cost accounting show that simple shapes are always finished more uniformly and more economically than complex shapes. This is rule number one for the designer. One of the most important factors determining the quality of a coating is its thickness on significant surfaces. Fundamental laws of electrochemistry operate to prevent a perfectly uniform deposi- tion of an electrodeposited coating on a cathode of any practical shape and size. Portions of the work nearest the anodes tend to receive a heavier deposit. Sharp edges or protru- sions at all current densities tend to steal a disproportionate share of the current. The goal of the designer and the plater is to make thick- ness variations as small

14 PLATING & SURFACE FINISHING exempted from minimum thickness • If fins or ribs are required, reduce • Adopt recessed in preference to requirements. Where necessary, their height and specify a generous raised letters and insignia, but round limit their depth to 50 percent of radius—1.6 mm (1/16 in.)—at each off edges and provide gentle their width. Avoid diameters less base. Round off tips with radii of at contours. than 6 mm (7/32 in.). least 1.6 mm (1/16 in.). Multiple • Integrated studs for fasteners • Countersink threaded holes to parallel fins should have spacing should be shortened as much as minimize electroplate thickness at between centers equal to four times possible and inside angles at each the peripheries and to facilitate the width of the fin. Broad, hollow ribs base should be rounded gener- insertion of fasteners after plating. are preferred over slender, solid ones. ously. Tips should be similarly rounded. • Studs or bosses with hollow centers should be shortened as much as possible and angled 90 degrees from the major plane of the part. All bosses should face the same direction. • Assist the plater by clearly marking significant surfaces in part drawings. • Avoid use of a variety of basis metals in any one part to be plated. The contact of dissimilar metals may interfere by galvanic action with covering power or with adhesion of the deposit. Cleaning may also be complicated because of unequal reactions with the different metals.

esign Features That Influence Electroplatability The effect of the basic design of a product or component upon the effectiveness or durability of the plating used has been the subject of much study and research. Many failures for which the plater has been blamed can be attributed to the original design. A major contribution to the plating industry was made by the Zinc Institute, Inc., when it spon- sored a design study by Battelle Memorial Institute, which has resulted in the establishment of basic design principles to be applied to zinc castings. These prin- ciples can be applied to other substrates. The various shapes illustrated in this article are examples of design configurations and their relationship to electroplating quality. (Courtesy of the Zinc Institute, Inc.) P&SF

*All Quality Metal Finishing Guides are available through MFSA for $15 each ($100 for full set). The member price is $12 each ($75 full set). Order through the National Association of Metal Finishers (NAMF), 209 Elden St., Suite 202, Herndon, VA 20170-4815 (Phone 703/709-8299; FAX: 703/709-1036).

Free Details: Circle 108 on postpaid reader service card.

February 1998 15