The program to improve and control the quality of a metal product
start at the desk of the designer. Basic principles of mechanical
and electroplating impose important restrictions on the size and shape
of components. The designer should know enough about these principles
so that he can design to minimize costs of quality finishing while
products which will have a long service life. Metal finishing processes
are at least as complicated as metal stamping, casting or forging
which the designer is usually more familiar. Proper selection of finishes
and processes offers many opportunities to improve quality, reduce
and increase production. ASTM Designation B-507 can provide
the designer with helpful information.
A most important term used when specifying metal finishes is "significant
surface" because on many products the same standard of quality
required at all points on the surface. The significant surfaces can
defined as those normally visible directly or by reflection which
essential to the appearance or serviceability of the article or which,
if they are a source of corrosion products, can alter the appearance
or performance of visible surfaces. Significant surfaces preferably
be agreed upon between purchaser and manufacturer and should be indicated
on drawings. Furthermore points at which thickness
measurements are to be made should be identified.
Design for Barrel Processing (Electroplate or Mechanical Plate)
Metal parts which are to be zinc or cadmium plated do not ordinarily
require polishing with belts or wheels before plating. Those to be
or smoothened are often treated by barrel finishing or tumbling or
vibrating processes. In designing to improve quality, consideration
be given to certain rules applicable to such processing, whether this
be surface preparation or barrel plating:
Design for Racking, Draining and Air Entrapment
- Avoid blind holes, recesses and joint crevices which can retain
tumbling compounds and metal debris.
- Avoid intricate surface patterns which will be blurred by barrel
- 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 good entry and drainage of solutions during rotation
by using simple shapes.
- Significant surfaces must be exterior for barrel work in order
to undergo proper mechanical preparation and cleaning or to receive
their share of metal deposit. They should be convex, if possible,
rather than recessed.
There are design considerations other than the above for parts which
are to be mounted on racks for processing in cleaning and electroplating
tanks. Among them are the following:
- Products which would occupy a 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 any significant surface. Many difficult racking
problems can be solved by design modification.
- Provide for good drainage of cleaning and other processing solutions
from racked parts. Certain shapes tend to trap solution which
then cause contamination by carryover, possible corrosion of the
part and wastage of materials. Carryover also aggravates the problem
of waste disposal. In design avoid rolled edges, blind holes and
spot-welded joints. Provide drain holes in recessed areas.
- Avoid shapes which can trap air on entry into a processing tank
if this air can prevent access of solutions to areas requiring
treatment. Wherever air can be trapped, hydrogen or oxygen may
also accumulate during cleaning and plating.