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Mold Temperature
Controlling the mold surface temperature is critical
to profitable molding, yet sufficient attention is rarely
given to this aspect of multi-component injection
molding. Most simulation programs consider only the
water line temperature, rather than the temperature
of the mold itself, and assume that adjustments to
the water line temperature will uniformly affect the
mold. In multi-shot
molding, the heat from the first component is critical
because when it's too high, the colors bleed together.
The ability to consider the mold surface temperature
in a multi-component simulation is crucial to
the successful production of the final product.
Voids and Sinkmarks
When there is excessive heat in certain areas of
the part and the mold cannot absorb it, these areas
will solidify more slowly, are isolated from packing
and tend to shrink more. For example, transparent
areas may be frozen while colored areas are still
liquid. These isolated areas will also tend to sink
and may have voids, which, in turn, create gaps
which the second shot of material may flash over.
SIGMASOFT® allows
the engineer to identify
these problem spots,
investigate the root cause
underlying the problem,
and adjust the design or
the process to correct
the issue before the part
goes into production.
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Inclusion of Over-molded Inserts
In the manufacture of, for example, a toothbrush,
the bristles are placed into the mold and the first
component is injected to encapsulate them. A
positive bond between these two components is
critical to ensure the bristles don't fall out: after all,
what good is a toothbrush without bristles? In order
to ensure this bond, the simulation needs to calculate
the amount of heat exchanged between the mold
and the bristles, and between the bristles and the
first shot. If the surface temperature is too low when
the polymer arrives at the bristles, bonding will not be
sufficient. The virtual production model automatically
calculates the exchange of heat between all of
the components throughout the complete cycle.
Air Entrapment and Venting
The multi-component simulation model must also
consider the matter of air entrapment within the
cavity. Fine detail areas, which may not fill completely
during the first molding cycle due to the presence
of entrapped air, can leave an opening for the second shot of material. Large vents can be placed throughout the mold to reduce air trapped within the cavity, but the possibility for flashing during filling increases dramatically. Using a comprehensive simulation tool allows for the optimal placement of appropriately sized vents in order to accommodate higher filling pressure without flashing the part.
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