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  A program to improve and control the quality of a metal product should
start at the desk of the designer. The metal finisher is restricted in what
he can do by certain basic principles of mechanical finishing and of
electroplating. The engineer should understand the limitations imposed
by shape and size of components to facilitate quality finishing at an
acceptable cost. The designer can exert as much influence on the
quality attainable in finishing a part as can the electroplater himself.
ASTM Standard B-5O7 can provide the designer with helpful information.

Significant Surfaces
An important term used in specifying metal finishes is "significant
surfaces". In most products the same standard of quality is not required
over every square inch of surface. Instead, the quality specifications apply
and compliance is expected only for the so-called "significant surfaces"
defined by mutual agreement between the producer and purchaser
as follows:

Significant surfaces are defined as those normally visible (directly
or by reflection) which are essential to the appearance or serviceability
of the article when assembled in normal position, or which can be
the source of corrosion products that deface visible surfaces on the assembled article. When necessary, the significant surfaces shall be
the subject of agreement between purchaser and manufacturer and shall be
indicated on the drawings of the parts, or by the provision of suitably
marked samples.

Design for Mechanical Finishing
Metal products which are to be coated with copper/nickel/decorative
precious metal or substrates utilizing non-nickel plating processes followed
by the decorative precious metal finish are generally subjected to abrasive
polishing with wheels or mass finishing techniques in preparation for the
plating operations. This is done to aid in securing an attractive, uniform,
mirror-like or satin appearance on the finished part. Mechanical finishing
is an expensive operation. To reduce costs and assist the metal finisher in
improving the appearance and quality of the product, the designer should
consider certain rules applicable for parts requiring mechanical finishing.
  • Avoid blind holes, recesses and joint crevices which can retain polishing compounds and metal debris.

  • Avoid intricate surface patterns which will be blurred in polishing.

  • Significant surfaces should be exterior, reachable by ordinary polishing wheels or mass finishing media.

  • Avoid sharp edges and protrusions which cause excessive consumption of wheels.

In small parts, which are to be barrel processed, the above rules apply.
This includes the requirement that the parts must be sturdy enough
to withstand the multiple impacts of barrel rotation and will not entangle
causing damage or incomplete finishing. Small flat parts, which tend
to nest together, should be provided with ridges or dimples to prevent
such nesting.

Design for Racking, Draining and Air Entrapment
Most metal parts weighing more than a few ounces or that require a high
degree of surface finish or a jewelry finish, are not plated in bulk in barrels
but are mounted on racks for processing in cleaning and electroplating
tanks. Design considerations relating to racked parts are described below.
  • Consult the plating department to make certain that parts can be held securely on a plating rack with good electrical contact without masking a significant surface. Many difficult racking problems can be solved by design modification.

  • Provide for good drainage of processing solutions from racked parts. Certain shapes tend to trap solution which then causes contamination by carry over, possible corrosion of the part and waste of materials. Carry over aggravates the problem of waste disposal and adds excessive cost due to chemical losses. In design, avoid rolled edges, blind holes, and spot- welded joints. Drain holes are especially important in irregular shapes and tubular parts.

  • void shapes which can trap air on entry into processing tanks if this air could block access of solution to areas requiring treatment. Wherever air can be trapped, hydrogen or oxygen gas may also accumulate during a cleaning or plating step.
 
 
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