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26.3 Dimensions Troubleshooting 247
Differential cooling in a part can result in differential shrink through a part. Differ-
ential cooling is also a major factor influencing warp and warp can impact the
observed dimensions on a molded part.
Make sure that cavity details and coring have adequate cooling to provide uniform
surface temperatures. There are cases where cycle times are dictated due to a
specific location on a mold with poor cooling that in turn impacts the overall
dimensions of the part.
See Chapter 14 on cooling for more in-depth discussion of cooling.
26.3.2.4 Mold: Hot Runner Control
The hot runner controller on a mold must provide reliable and repeatable tempera-
tures for all of the hot runner zones. If a hot runner zone is not running to the
desired set point it can impact the ability to fill and pack the cavity.
Verify that valve gates are actuating at the desired point of the process. If a valve
gate fails to open, the mold will fill and pack differently from the original estab-
lished process. Valve gates need to be serviced over time and the cylinders may
need to be rebuilt to provide proper valve actuation.
26.3.2.5 Mold: Component Deflection
In some cases components of the mold or the mold itself may have deflection that
in turn leads to a dimensional issue on a specific detail of the mold. Deflection of a
lifter could lead to a dimension on a snap detail that is different to that intended in
the design. Thickness of part details can change if the mold or clamp is deflecting.
Case Study: Clamp Deflection
In this case an acetal part was being molded in a 170-ton press. The overall
part was having consistency problems with thickness, which led to stack up
problems. Examination of the part thickness versus the cavity dimensions
showed that the molded part was actually sometimes thicker than the cavity
was cut to. The mold was moved into a 230-ton press and the dimensional
issue was resolved. The mold was actually being blown open and the part
was thicker but did not show flash.
26.3.3 Dimensions Troubleshooting Machine Issues
Potential machine issues include:
Nozzle tip
Velocity to pressure transfer
