At
Anoplate, blast finishing can be categorized as strong, mild or
very mild. All of the media is delivered using compressed air, and
the most commonly used media is glass bead, which would be considered
a mild compound. Plastic media is considered very mild and aluminum
oxide is more aggressive.
Blast
cabinets and rotary tumbling units are available in several departments.
One cabinet unit is fitted with an outside rotary table and heavy-duty
track leading into the inside of the blast cabinet, for heavy, safe
material handling. Occasionally, jobs are too large for Anoplate
to handle and the parts, if they can be sand blasted, are
farmed out to approved vendors.
At
Anoplate, blast finishing is primarily used for cleaning, oxide
removal and surface preparation. Occasionally, it is used to put
a satin, frosted finish on parts for cosmetic purposes or to blend
various imperfections. The latter use could be classified as a finishing
operation. Anoplate is not involved with the final application—surface
treatment.
The
principal category under surface treatment is shot peening, for
increased fatigue resistance (aircraft landing gear, for example).
Another use for this peening effect is to remove distortion of a
part. By directing the blast on the reverse side of a distorted
object, an experienced operator can “pull it back” into
shape.
Conversely,
an operator must exercise extreme care on relatively thin parts
that can be cold worked and possibly cause distortion through excess
blasting in one area. A great deal of the success of Anoplate’s
operators is in their ability to blend the variables that are present,
ranging from the type of media, to media size, nozzle angle, and
the distance the nozzle is held from the article to, most importantly,
the pressure.
One
problem Anoplate is constantly confronted with on sand and malleable-type
steel castings is the foundry scale present on incoming machined
parts. Many customers do not realize that most cleaning cycles for
any casting are shorter than cleaning procedures for other metals.
It is possible to get into trouble with prolonged cleaning of castings.
Another
little known fact is that long acid portions of the cycle can damage
the machined areas rapidly. The acid attacks the non-scaled surfaces
(machined) much more rapidly than the oxidized portions (scaled).
Many heavy scales are quite resistant to acid, and prolonged acid
pickling is not a prudent solution.
If
acid is not the solution, what is? The short-term approach is to
call the customer, explain what you have just read and get permission
to glass bead the scale. (Also get permission for the extra charges.)
The long-term solution is to get your foundry to remove the scale
before you start machining. They have the heavy-duty blast equipment
to do the job more cost effectively.
Just
what is this equipment that the foundry has and job plating shops
do not? Wheel blasting equipment that mechanically propels steel
shot by centrifugal action in a wide pattern with lots of force
is much more rapid and much more cost effective. Often, this equipment
is semi-automated.
However,
this new wrinkle on purchasing castings may not be an “easy
sell” with your casting house. They are often backed up in
this area and would most likely try to ignore the plater’s
problem. At least you should check around. At some point it will
save you time and money, not to mention delivery if your plater
scraps the parts.
When
using glass bead to create a fine, satin cosmetic finish, Anoplate’s
main success has been with small parts. In some instances, parts
are racked up on the plating rack and satin finished just before
a shortened cleaning cycle and plating. The advantage of this technique
is that the parts do not have a chance to touch one another after
the blast cycle, which makes the finish prone to chafing and may
cause a blemish or shiny spot.
On
larger parts, a slight variation of pressure, distance, shape of
the media discharge pattern and operator technique makes it very
easy to create a non-uniform finish with the edge patterns. Uniform
satin finishes on large flat areas can be extremely difficult to
obtain.
There
are several reasonable soft, non-aggressive forms of media other
than plastic media. Ground walnut shells and corncob along with
baking soda and agricultural starch can remove foreign substances
with minimal damage to ground surfaces. On occasion burr removal
and deflashing of molded parts is a use for this gentle media.
Serving
the Finishing Industries. Since 1936.
PF Onine and all contents are properties of Gardner Publications,
Inc.
All Rights Reserved.