Even a very tiny black speck can ruin a micro-molded part.
That was just one of the challenges facing Knowles Electronics when designing a
miniature audio component used in a hearing aid. The part is smaller than a
pellet of resin.
Incredibly, the materials used are glass-filled nylons,
including one loaded 50 percent with glass. Wall thicknesses are 0.001 inch or less, and the part must match more
than 60 colors, including 20 different flesh tones.
"Our customers include almost all of the major hearing-aid
manufacturers," says Bob Walter, global commodities manager for Knowles, which
is headquartered in Itasca,
IL. "They are very specific about
the color of the parts we sell them. There are very exact color specifications
and we have to match that color standard time after time, which isn't always
The part required a specially compounded resin from Clariant
Masterbatches and a specialty molding house.
"With components this
small, a hundred pounds will yield more than 50,000 parts," says Tom Murphy,
vice-president of production at Accumold,
"There are not a lot of companies who are willing to work with us on these
small quantities, but Clariant will and we can really rely on them to repeat
the colors accurately from lot-to-lot."
Clariant compounds nylon 6/6 with three different levels of
glass fiber: 50 percent, 35 percent and
30 percent. Even in very tiny parts, glass reinforcement is required to provide
stiffness and dimensional stability. Clariant can deliver these materials in
customer-specified colors. The nylon supplied for Knowles' parts is specially
formulated for improved flow so that it more easily fills the tiny mold
"The color dispersion needs to be so precise," says Murphy.
"In the parts we mold, there is a definite transparency factor. If you evaluate
a 2- or 3-inch sample chip, the color might look just fine, but in these thin
wall sections you can immediately see any inconsistency. The other thing we
worry about is black specks."
An occasional by-product of the compounding process, the
black specks Murphy refers to, are tiny bits of polymer that can appear in the
finished compound. Some are microscopic. "If you can see it in the finished
part," he says, "the customer will reject it. It was a big problem with other
vendors but Clariant seems to have gotten it under control."
Bob Wick, product manager at the Specialty Compounding group
at Clariant Masterbatches of Holden,
MA says, "we use a testing
procedure developed by TAPPI, the pulp and paper industry association, to
measure exactly how many and what size specks are in a given sample. It's one
thing to say you have no black specs and quite another to certify it on a
lot-to-lot basis, but that's what we have done for the materials we produce for
molding is growing as many parts, particularly in the electronics industry,
shrink in size. Micro-molded parts at Accumold range from about 1/2 inch sq in
size to parts measured in microns. These components often have complex geometry
and very tight tolerances. In one of its jobs, molded rotors have 18 tiny teeth
around a rotor's 0.092-inch diameter.
Accumold builds specialized molding machines to meet unique
requirements. The machines handle volumes from prototypes to 5 million or more
parts per year. Typical lead times for micro molds are between two and three