Technical benefits from using hot runner for preform mould manufaturing

Posted by TINA at 28.02.2020

• Simplification of the design of certain types of plastic mould. Use of cold runner molds with an extra parting line (moulds known as three-plate moulds or with a so-called intermediate plate) have major restrictions. Hot runner system is widely used in preform mould making also. The functioning of such preform moulds is difficult to automate because of the sprue removal aspect, and because of the tendency for the sprue to get stuck between the mould plates. The heavy moving plate of the PET preform mould can be a cause of rapid wear to the plate guide system. The same could be said of its drives and mechanical travel limiters. Besides this, the opening and closing time for a preform mould of this type is always longer than for moulds with a single parting line. The proportion of the sprue in the overall injection mass in three-plate moulds is also greater.

hot runner nozzle

• Eliminating the fall in melt temperature which occurs in a cold runner, has enabled the use of a longer flow path in the cavity. This is of significance in the case of partiallycrystalline plastics, which feature a narrow temperature window that prevents compensation of the temperature drop in the CR by raising the injection temperature.

• The plastic flowing into the cavity is at a controlled temperature (assuming precision temperature regulation in the hot runner system).

• The lower pressure loss in the hot runner system means a higher cavity fill pressure is available.

• In moulds for large products, there is greater freedom to select the optimum location for the injection points, which leads to a more uniform filling, with a smaller loss of temperature and pressure in the mould cavity. Engineering mouldings show less differential shrinkage and lower internal stresses.

• A reduction in the injection pressure required to fill the cavity, which enables the required injection moulding machine locking force to be reduced.

• The potential to regulate the holding time by controlling the time for which the gate is live or by mechanically closing it. The reproducibility of self-cooling conditions for a plastic under pressure, and thus also of moulding shrinkage, are thus improved. Holding pressure loss is also less than for cold runner, since the gating runner has a larger operating crosssection which does not diminish through the formation of an insulating layer.

• Hot runner make it possible to design preform moulds, for example, the modular type for thinwalled packings, bottle preforms, and so forth, and also moulds for sequential moulding, moulds for in-mould decoration and lamination, and so on.

• The potential for further development of stack moulds and for design of moulds with gating via a long core.

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