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When is it Practical to have a Cold Sprue?
With the advancement 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, we have to consider the purpose of the cold sprue itself.
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'). With the advancement of feed system technology, there are better ways of transferring polymer from the machine to the cavity.
Designing a Cold Sprue into a Part
Considerations when designing a part with a cold sprue include:
1) The nominal wall thickness of the part. This will dictate how big the sprue should be on the part (or at least it should).
2) The longest flow path of the polymer through the cavity. This must be considered and will depend on the material being used.
3) Accounting for possible sink marks in the part design on the opposite side of the cold sprue.
4) Determining if the cold sprue will control cycle times.
5) Removing a cold sprue after ejection (post-molding process = $$$). The sprue will then need to be designed so that sharp edges are relieved with sufficient radii in order to prevent the polymer from shearing as it exits the sprue which could cause surface defects on the part.
6) The location of a single sprue on a part. The sprue location is typically dictated by the part as it lies in tool position.
Part Design
With the demand for thinner, more complicated parts, it may be necessary to have multiple gate locations. If one gate location is tolerable, it is very rare that it drops into the center of the part in tool position.
Without the use of analytical tools, it's difficult to prove that a part will not be optimally designed if a single, cold sprue is used. However, it's often proved that it wasn't the best design after the part was made. It's old technology and only has a place in very specific conditions.
Hot Runner Technology
With the advances made in hot feed systems, it's typically more cost effective:
- To employ a hot runner in a tool than it is to have a post process (removing cold gates or sprues), or
- To eliminate scrap due to using cold runners which may or may not be degated during ejection. Many times, scrap cannot be reused and it is not cost effective.
- Hot drops allow many drop locations to be considered on a part based on the requirements of the part design and material being considered.
Sometimes it is necessary to utilize a cold sprue in conjunction with a hot runner system because a hot drop may fall onto a part surface with a steep angle, therefore a cold sprue must be designed into the drop tip. Even so, technology now permits hot drops to be designed at an angle so that a cold sprue is no longer required.
So, to answer the question, yes, sometimes it is practical to design a cold sprue onto a part but there are many considerations when doing so and it is typically best not to have a cold sprue at all.
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