are not necessarily very useful when selecting resins for micromolding. That's
one of the conclusions from an interesting study conducted by Accumold, a leading
micromolder based in Ankeny, IA. And don't expect to get meaningful results
from flow analysis. Even the best programs may show parts to be unmoldable, or
may give incorrect data.
necessarily believe it when a resin supplier tells you it's impossible to
achieve the seemingly impossible when designing very tiny parts.
shows how, in terms of micromolding, picking resins based on data sheets can be
misleading or confusing," says Aaron Johnson, marketing manager for Accumold. "It's
important to consult with your molder when designing parts that may be
considered outside the norm."
sheet, may describe a thermoplastic as "easy flow", for example. There's a big
sheets are often calculated using much larger sample bars and their stated
parameters may not apply," says Johnson. For example, most recommended gate sizes
are larger than many micromolded parts.
injection molding test was conducted with 11 commercially available
thermoplastics, ranging from high-density polyethylene (HDPE) to polyetherimide
(PEI) and polyethertherketone (PEEK). The critical dimension in the test part
is a wall thickness of 0.003-inch over a length of 0.1276-inch.
was to see how far we could push the different resins and take note how they
compared," says Johnson. "The mold was not modified for each resin, including
the gate or runner system. We used the standard processing windows as specified
for each resin as the only variable to give each process a chance to fill the
part on its own."
critical dimension was to keep the 0.003-inch thickness. Results show how resin
selection affects the desired output when designing a part. The mold was built
with a thick-to-thin transition to create an optimal opportunity for the parts
to fill. That, of course, is not always possible in the real world.
concern in the test was to maintain the 0.003-inch height. Length of fill was
not the key issue. "Simply increasing
the pressures and blowing open the mold would not have given us the right data,"
says Johnson. The mold was not adjusted either for performance or shrink.
of the study was to show how various thermoplastics stack up in a given
Click here for a table with detailed
findings and click here
for a copy of the Accumold report.
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