Insulated Runners

The insulated runner system (Figure 41) allows the molten polymer to flow into the runner, and then cool to form an insulating layer of solid plastic along the walls of the runner. The insulating layer reduces the diameter of the runner and helps maintain the temperature of the molten portion of the melt as it awaits the next shot. The insulated runner system should be designed so that, while the runner volume does not exceed the cavity volume, all of the molten polymer in the runners is injected into the mold during each shot. This full consumption is necessary to prevent excess build-up of the insulating skin and to minimize any drop in melt temperature. The many advantages of insulated runner systems, compared with conventional runner systems, include: • Less sensitivity to the requirements for balanced runners. • Reduction in material shear. • More consistent volume of polymer per part. • Faster molding cycles. • Elimination of runner scrap – less regrind. • Improved part finish. • Decreased tool wear. However, the insulated runner system also has disadvantages. The increased level of technology required to manufacture and operate the mold results in: • Generally more complicated mold design. • Generally higher mold costs. • More difficult start-up procedures until running correctly. • Possible thermal degradation of the polymer melt. • More difficult color changes. • Higher maintenance costs. Product and Mold Design d9b604f1c18a4896b020a210866ec775.