Cryogenic high speed shot blast
deflashing is a process used by the rubber industry to remove
flash from molded rubber parts. The process uses liquid nitrogen,
high speed rotation and media (shot blast) in varying combinations
to remove the flash in a highly precise, economical and expedient
Parts that have thin flash
can be quickly and thoroughly cryogenically deflashed. This
process is exceptionally good at removing the inner dimensional
and complex flash that cannot be removed by any other method.
There are two basic styles
of cryogenic deflashing machine - basket and belt. The basket
style was designed to process small parts and offers 100% parts
containment. The belt style was designed for larger/heavier
parts which require more room to tumble and a stronger rotation
FOR BEST RESULTS
The old saying "garbage
in, garbage out" is very appropriate. To consistently get
a quality finished part, you have to consistently put in a quality
Ideal Flash Configuration
Everyone who molds rubber parts
feels that the ideal flash configuration is not flash at all.
Flashless molding is being developed and will have a future,
but given the vast quantities of existing molds, cryogenic deflashing
will be around for quite a while, So what is the ideal flash
configuration? Make the flash as thin as possible, with as good
a flash base as possible, or, in the case where sealing surfaces
are involved, try to move the flash away from the critical areas.
The location of overflow in
reference to the part has an impact on the cycle time and deflashing
temperature, as well as general deflashability of the part.
Overflows should, if they are necessary for the molding process,
be moved as far away from the part as possible (x> plastic
*(x= distance between outer
edge of part and inner edge of overflow. Plastic media must
be smaller than x in order to get inside this area and deflash
The closer the overflow gets
to the part, the more difficult it is to remove; the plastic
shot cannot penetrate between part and overflows to remove the
flash,. If enough room is left between the part and the overflow,
the following advantages are usually achieved:
Tear Trim Design
Tear trim design was developed
to eliminate the cryogenic deflashing operation. The overflow
is placed extremely close to the part (x = o)** so that, when
this overflow is removed by hand, no flash remains.
**(actually, x approaches
zero - there is virtually no thin area between part and overflow).
This design usually works well
until the mold starts to wear, and this usually does not take
very long because of the knife edge required between the part
cavity and the overflow cavity. Trying to cryogenically deflash
these parts is very difficult. When the part is cooled down
and becomes hard, the overflow essentially becomes part of the
part, and the shot media cannot penetrate the minuscule area
between part and overflow. A solution to permit cryogenic deflashing
of this part design is to fill in the overflow cavity, thereby
leaving only a skin (thin flash) to remove. This improves the
deflashability and quality of the part and also results in a
mold that has far superior wear properties.
Everyone knows that the parting
line and flash base configuration determine the overall deflashing
quality. If there is no difference between the thickness of
the flash and the part, the deflashing unit will remove both.
No cryogenic deflashing unit will eliminate molding problems.
In these cases, if part quality
needs to be improved, mold rework is necessary.
When qualifying your part:
Best results can be
achieved when the parting line does not exceed 0.005",
or 0.127 mm thickness
Parts need a clear,
consistent demarcation of flash
Liquid nitrogen (N2) is injected
into a highly insulated chamber in which molded rubber parts
are tumbled and blasted. The flash, which should be significantly
thinner than the parts themselves, is embrittled by the low
At the same time, a precision
throwing wheel, turning at high speeds (up to 8,000 rpm), throws
plastic shot at the tumbling parts, and the plastic shot breaks
off the brittle flash on impact. The deflashed parts remain
in the chamber, and the machine separates reusable media from
debris (flash and dust).
There must be room within the
blasting chamber for the parts to tumble. The tumbling action
exposed the parts to both the N2 and the media stream. The actual
size of the chamber should be at least twice the size of your
load. Average load sizes are from 3/4 cu. ft. of parts to 4
cu. ft. of parts.
Average cycle time in most
cases is three to five minutes.
Ultimately, the best way to
qualify your parts as candidates for cryogenic deflashing is
to have a sampling of your parts processed (tested) in a cryogenic
The new generation of cryogenic
deflashing machines uses the latest in technology offering options
such as programmable controllers with numerous deflashing "recipes"
for automatic operation, networking capabilities, bar coding
abilities, message centers, SPC reporting, telephone modem hook-ups
and RS 232 ports which allow the use of printers and report