Page 109 - Injection Molding Advanced Troubleshooting Guide
P. 109
11 Hot Runners
11.1 Hot Runner Advantages
Hot runners are used for a variety of reasons and have many more uses today with
the advances in technology than they had years ago. They provide the following
advantages:
Are used to eliminate the cold sprue/runner, which can save you money with
scrap when regrind is not an option. Of course there is an added cost to the tool
up front so justification is always needed with savings to offset the cost.
Allow direct gating to the part with a low vestige tip or valve gate and eliminate
the need for three-plate runner systems.
Allow gating on the outside edge of the part when a part is centered in the mold.
On larger parts where knit lines or flow lines are a concern sequential valve
gates allow control of the flow and to move the knit lines.
Can use multiple hot drops to reduce fill pressures.
Can be used to eliminate long cold runners to reduce the pressure loss and im-
prove the process window.
11.2 Heaters and Thermocouples
Think of a hot runner as an extension of the molding machine itself. Its purpose is
to keep the plastic molten at a proper temperature on its path to the cavity. The
steel that contains the flow channels needs properly placed heaters and thermo-
couples to ensure the flow channel is at and maintaining the proper temperature.
The thermocouple itself is the most critical component of this process. The thermo-
couple is what measures the temperature of the component and tells the controller
how much current (amps) to send the heater to maintain the set temperature. So in
