Technology has introduced a new core pin made from a special grade of tungsten
carbide that directly addresses the problems of thermal conductivity and
deflection in the plastic injection molding process. The tungsten carbide core
pin has a very high thermal conductivity with extreme rigidity. In such
applications as medical parts and consumer components, the use of tungsten
carbide core pins has resulted in cycle-time savings of as much as 20 to 40
percent without sacrificing the quality of the molded part.
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In high-volume production of plastic injection molded components, cycle
time is critical to profitability, and one of the limiting factors is the
removal of heat from the mold. Some plastic injection molded parts have deep
internal features that require the use of long core pins. During solidification
and cooling, the plastic contracts on the core pin; thus, the rate of cooling
is controlled by the heat transfer through the core pin. Whether the core pin
has a bubbler, heat transfer is dependent upon the thermal conductivity of the
core pin material.
Before the development of tungsten carbide core pins, hardened copper
alloys typically were selected as the material of choice for long core pins.
However, the copper alloys are not very rigid, and for high-aspect ratio
configurations, they will deflect during the injection phase.† The deflection results in unacceptable
dimensional stability. In situations where deflection occurs, hardened tool steel
is used.† But because steel does not have
high thermal conductivity, cycle times suffer.