Sprue plug

Although runners should deliver melt to each cavity at the same time, they may be naturally balanced or artificially balanced. In naturally balanced runners the distance from sprue to all gates is the same, whereas artificially balanced runners have the same pressure drop from sprue to all gates. Cold runner molds also incorporate cold slug wells at each turn in the melt flow. These collect the cold melt from the sprue plug (melt frozen at the nozzle), thereby preventing this melt from entering the cavities. 

Before drilling the sprue, remove the plug insert from the top mold half. Then put the mold halves back together and mark the... 

For amorphous plastics, where this window is very wide and also includes, for example, thermoforming, the sprue in an HR nozzle gate solidifies slowly, and at temperatures lower than Tp, it does not form a plug hard enough to block the gate. The value of AT should therefore be taken as no more than an approximate window of openness of an HR gate for amorphous plastics. 

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