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422 42 Splay
42.3.2.2 Mold: Hot Runner Temperatures
Hot runners are really an extension of the machine barrel and as such they can
cause overheating of the material. Just like a machine barrel can overheat the
material so too can the hot runner system.
One key item with hot runners and splay issues is verifying that the hot runner
temperatures are set correctly. If someone accidently enters the wrong value for a
set point the material can degrade causing splay. Also verify that the zone is cor-
rectly reaching the set point and is not running under or over the setting.
Sometimes detailed review of the hot runner manifold is required to determine if
thermocouple zones have been mislocated. In this case either during mold build or
mold maintenance a thermocouple wire is located on the wrong terminal in the
plug. If thermocouple wires are “crossed” the hot runner controller will be trying
to feed amperage to zone “A” but not seeing the feedback from the thermocouple
because the zone “A” thermocouple is actually reading zone “B”. This will lead to
potential cold or hot spots in the hot runner manifold. Newer hot runner control-
lers will detect many of these types of issues, but old controllers will not. One way
of detecting this issue with old hot runner controllers is to manually turn on one
zone at a time and watch if the thermocouple of that zone reacts and shows a heat
rise. If it appears that there is an issue with thermocouples being miswired, the
mold must be corrected. Also bear in mind that a hot runner controller cable can
be miswired from one plug to the other, which will lead to the same problem.
If a hot runner is built with corners that have dead spots or mismatches in the flow
channels material will be trapped in these locations which will lead to potential
splay troubles. Hot runner channels should be as consistent as possible with no
dead spots that will collect material. Unfortunately, this is a problem that must be
addressed at the time the manifold is built, because once you have it in the mold
and in the press there will not be much of a choice but to process around the root
cause. Fixing this issue after the manifold has been built will be a very expensive
proposition, so make sure to verify this up front before the manifold is built.
If a hot runner manifold develops a leak due to damage, the result will be another
spot that will trap and degrade material. Very often hot runner leaks occur from
poor startup procedures. It is important to allow a hot runner manifold to heat up
uniformly prior to starting to inject plastic through the mold. If a hot runner is cold
started it will raise the potential of causing a crack in the manifold. Also remember
that hot runners are built to function in a specific temperature range and the fit of
components is built to accommodate the thermal expansion at this temperature.
If the hot runner is set at a different temperature the thermal expansion of the
components may create gaps leading to splay.