Page 110 - Injection Molding Advanced Troubleshooting Guide

96 11 Hot Runners

most cases when there is an issue with burning or inappropriate temperatures the
thermocouple is the root cause and not the heater. A heater is basically like a light
bulb: it is either working or burnt out, but there are times when it appears it is
functioning but is not able to come up to temperature. If a thermocouple is loose or
not held tight against the steel it will be reading the ambient air temperature caus-
ing the controller to send excessive current to the heater and causing overheating.
Overheating can also occur when the thermocouple and heaters are not wired to
the correct zones on the connection plugs. Modern hot runner controllers have the
ability to determine if hot runner heater and thermocouple zones are correctly
matched.

„ „11.3„ Hang Up Areas

With today’s hot runner designs hang up areas in the flow channel are not an issue
in most cases. The one area on the hot runner that causes the most issues leading
to cosmetic defects on the part is the hot drop tip, which depends on the tip style
and insulator caps available. The other area to consider is the orifice on the hot
runner and the orifice on the nozzle tip; they should be one to one in size to pre-
vent hang up of material.

„ „11.4„ Hot Drop Tips

There are three general styles of hot drop tip designs with many variations of each
design. The three types are (1) the low vestige tip, (2) straight through sprue style,
and (3) the valve gate.
1. The low vestige tip is used to direct the gate onto the part with minimal vestige
and also used on cold runners to reduce stringers. These can increase fill
pressures (pressure loss) and contribute to color issues when running multiple
colors. They will also plug more easily with the orifice having less volume/area.
2. The straight through style tip is usually the least problematic with color changes
and contamination because there are fewer areas for the material to hang up.
But even with some of the new designs considering the insulator gap even a
sprue style can create problems. Most systems provide components to fill these
gaps commonly known as color seals.