Hotrunner Molds

FIGURE 1.11 Hotrunner mold dart , 32 cavities, with electro magnetically operated shut-off needles... 

With real hotrunner molds, the continuous heating compensates for heat losses caused by conduction, convection and radiation Therefore insulation plays a crucial role. The heat capacity is usually dimensioned so that the heat-up time is acceptable which means between 15 to 25 minutes. When analyzing the permanent heat losses, an energy consumption of less than 60% of the installed power can mostly be seen [1 to 7,11,19, 20, and 23]. 

In general there are two different ways of heating when using hotrunner molds internal or external heating. 

If the effort for hotrunner systems are not justifiable, then a tunnel gate (Figure 1.36) which is the most common gate type, is being used. Advantageous is that the automatic separation of the runner from the molded part occurs inside the mold without any additional mechanism. 

FIGURE 1.36 Tunnel gate at the end of a sub-distributor, fed by a hotrunner nozzle a, b Runner ejector c restraining cone d runner e cutting edge f molded part g hotrunner nozzle [15]... 

Three-plate molds are used, when the product must be gated centrally (Figure 1.39) and an injection via hotrunner is not possible. 

The additionally needed opening width used for runner demolding can be disadvantageous. The combination with a heated sprue channel decreases in the bulkiness of the runner is therefore not rare. Three- plate mold- gates are used for applications with very small gate distances not achievable by hotrunner nozzles because of their space requirements. A typical example is day light of 7 mm for the individual gating of narrow chambers which are positioned next to each other in molded parts for medical use. 

Hotrunners are basically an extension of the machine nozzle into the mold [23]. This definition already explains that they are equipped with an active heating system and with a well-functioning insulation to maintain the flowing capacity of the compound. 

The hotrunner system is located in the middle plate of the stack mold. It is centrally fed by a heated runner pipe which is also called snorkel . 

FIGURE 1.75 Hotrunner system for a two component injection mold [41]... 

Multi-component molds are mostly, especially with high cavity numbers, equipped with a hotrunner system which must provide a perfect heat separation (Figure 1.75). 

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