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EspañolBumper molds have no runners or hot runners
When the plastic melt is thermally stable, the need to regrind the runners using runners' molds can be eliminated. One common type is the hot runner mold. In this bumper mold, plastic is injected into an insulating runner plate, which is held slightly above the melt temperature by a set of individual torch heaters, and then enters each cavity through runner channels and insulating nozzles body. This is a runner's operation because there is no runner regrinding. The air gap thermally insulates the hot runner plate.
Hot runner bumper molds offer many advantages due to the removal of runners and runners. There is no material loss or contamination from regrinding, and there is no additional cost for automotive companies to mix regrind with virgin resin. Polymer properties, especially optical and electrical properties, are not altered by regrinding. Neither melting nor cooling of the recycled material wastes any energy. Effectively increase the injection volume and reduce the mold opening stroke. In the case of cold runners, this can happen during ejection and is eliminated by hot runners. The bumper mold fills faster and makes it easier to achieve balanced flow to the cavity.
Hydraulically operated gate valves or valve gate systems have been developed to control drooling and prevent freezing. Valve gates are compact hydraulic cylinder/valve pin assemblies used to actively close individual gates. Hydraulic cylinders retract at each gate at preset times, allowing the melt to flow. Auxiliary timers set the forced closing of each valve to occur after the optimal preset fill time has expired. This eliminates overpacking or underfilling at each gate.
System manifolds and nozzles are temperatures controlled to prevent thermal degradation of the melt. This sophisticated system helps control cavity filling and end bumper product characteristics, especially for large multi-gated parts and home molds. Home molds make different parts for a common assembly and inherently have an unbalanced flow.
A simple alternative to heating hot runner bumper molds is to provide sufficient thermal insulation in the runner plate to prevent the plastic from freezing. In insulated runner molds, a skin of low thermal conductivity plastic formed at the end of injection near the surface of the runner wall insulates most of the plastic flowing through the channel.
