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The job of a molder is to make specification
parts reproducibly at the agreed price. So, the minimum essential
test for raw material is to ensure that the material can be made
into useful parts in the molder's machines. This evaluation has
three levels, depending on what is found in the earlier steps:
minimum essential material evaluation; system check; and
everything else. The molder must go as far into these procedures
as necessary to ensure successful parts and a satisfied customer.
In the laboratory, melt flow rate (melt index, melt viscosity, and
so forth) is the one single simple test that gives the best answer
to the basic question of quality. If a molder does nothing else,
this is the one laboratory test that must be performed on all
thermoplastic materials: This basic test can be extended to more
than one shear rate, or holdup time can be extended. These simple
modifications will ensure material processihility.
The next test is a standard system check used by many modern
molding shops to test entire systems. Set the machine to molding
conditions found on the setup sheet with the tool in place; then
make some air shots and mold a few parts. The temperature of the
air shots should be measured by a melt probe (thermocouple or
pyrometer) that is accurate to ±2 deg F or 1 deg C. A minimum of
five parts should be made and weighed to ±.01 percent to ±05
percent (or less, depending on part tolerance).
Part number and weight should be reported along with average
weight and range of values. (Some old-fashioned people like myself
like to plot this on probability paper, but this is another
topic.) If melt temperature and part weight, color, and shape are
right, then the material can be assumed to be acceptable. The
final size can be measured after the part cools-24 to 48 hours
later in a controlled temperature and humiditv environment. Other
tests (color, warpage, mechanical properties, environmental
resistance, flammability, and so forth) can be performed at this
time.
The above is basic QC for everything and should he run every time
the mold is hung or the material or process is changed. The setup
sheet must contain these data. The above will catch problems, but
only after the tool is hung.
If you have problems with any of the parts from this test or if
you wish to test the material before committing to hanging the
tool, then you need a series of laboratory tests to answer the
three basic material questions:
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Is it the right chemical composition (LLDPE
and not LDPE, for example)?
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Can the part he fabricated from it
(considering flow, shrinkage, processing temperature, and so
forth)?
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Does it meet appearance and performance
standards (measured on the fabricated part, not the raw
material)? If it is less expensive to mold test bars and
relate these to the final part, then this option can be
exercised so long as it can he related to the true part in the
real world.
Details of these tests are covered nicely in the series "The
Materials Analyst" by Michael Sepe in the August 2000 issue
of Injection Molding Magazine
-E. Coleman, CP Technology Inc., Stamford, CT, (203) 329-3693, eccpt@aol.com
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