Page 460 - Injection Molding Advanced Troubleshooting Guide

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temperatures should be measured on the mold and documented for future refer-
ence and troubleshooting. When checking mold temperatures make sure that the
mold has been running on cycle for long enough for the steel to reach its true run-
ning temperature. Studies have shown that this can easily take an hour or longer of
run time, depending on the size of the mold and the cooling layout. See Figure 46.2
for an example of how a mold heats up during the cycle. Both temperature curves
are recorded with mold thermocouples. The flat horizontal part of the curves on
the far left is the baseline mold temperature and the steel temperature increases as
the molten plastic touches it.

Figure 46.2 Process monitoring curves for mold thermocouples

One of the key impacts of getting the melted plastic to a temperature below its HDT
is the cooling efficiency of the mold. To ensure maximum cooling, turbulent water
flow through the mold needs to be achieved. To achieve appropriate water flow, the
thermolator must have a sufficient amount of pumping capacity to adequately
reach the desired flow rate through the cooling channels. Water flow is another
parameter that must be documented during process development, so that during
troubleshooting the original flow can be compared to the current flow.
Another important check on mold cooling is the inlet water temperature versus the
output water temperature. Depending on how critical the molded parts are, the
maximum temperature rise for the water should be less than 4 °F or 2 °F on criti-
cal jobs [1]. This temperature can be checked with a handheld pyrometer with a
surface probe: place the probe right on the water fitting to check temperature. If