With the advancements in feed system technology, when is it truly practical to design a cold sprue into a part or feed system? Before we answer this question, the purpose of the cold sprue must be considered

The cold sprue is simply a means to get polymer from the injection molding machine to the cavity (or at least that’s how it was in ‘the beginning’)  However, cold sprues are typically large, which leads to waste, and are typically thick, which may contribute to long cycle times

With the advancement of feed system technology, there are additional and potentially more efficient ways of transferring polymer from the machine to the cavity However, costs will need to be considered

Designing a Cold Sprue into a Part

Considerations when designing a part with a cold sprue include:

1. Wall thickness: If the cold sprue is designed to drop directly onto the part, the nominal wall thickness of the part must be considered The nominal part thickness will dictate the size the diameter sprue should be on the part (or at least it should) based on sink marks and freeze times
2. Properly sized: If the cold sprue drops onto a cold runner which then feeds the part, the cold sprue must be sized such that both the cold runner and cold sprue are not so large that they control cycle time On the other side of the spectrum, they must not be too small as to create a severe pressure loss and also allow for sufficient pack times to allow the gate to freeze AFTER the part freezes
3. Flow Path: The longest flow path (distance) of the polymer as it flows through the cavity  The flow path must be as short as possible and balanced within the part It will also depend on the material being used Some materials can flow much further than others
4. Sink Marks: Account for possible sink marks in the part design on the opposite side of the cold sprue Sometimes a small mound may be required to be designed onto the part opposite the cold sprue to prevent excessive shear which may cause visual defects such as splay However, a thick region on the part may lead to a sink mark If there is potential for a sink mark on the part opposite the sprue, it may even be necessary to model in a small dimple opposite the sprue
5. Removal of Sprue: Removing a cold sprue after ejection is an additional process