1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17  
A program to improve and control the quality of a metal or plastic 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-507 can provide the designer with helpful information.

Significant Surfaces
A most 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/chromium
or nickel/chromium finishes are generally subjected to abrasive polishing
with belts or wheels 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 belts.
  • Avoid sharp edges and protrusions which cause excessive consumption of wheels or belts.
In small parts which are to be barrel processed the above rules apply plus
a requirement that the parts must be sturdy enough to withstand
the multiple impacts of barrel rotation. Small flat parts which tend to nest
together should be provided with ridges or dimples to prevent this.

Design for Racking, Draining and Air Entrapment
Most metal or plastic parts weighing more than a few ounces 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 in the following paragraphs.

Products which would occupy a large volume in processing tanks, large
in proportion to surface area, should be designed to be plated in sections
for assembly after coating.

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. In design, avoid rolled
edges, blind holes and spot-welded joints. Drain holes are especially
important in irregular shapes and tubular parts.

Avoid 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.
Back to IndexNext Page