Injection molding is a complex scientific manufacturing process. By following a methodical set-up process, molders aim to minimize or eliminate common molding defects that affect part quality. Generally, molding defects are either part design, mold design, or process related. This training tip focuses on short shots.
There are various potential design-related causes for short shots. One of the most common relates to part wall thickness. Significant wall thickness variations in a part can create underfill conditions where the polymer material flow cannot fill the thin wall sections.
It is important to allow for part and mold redesigns to avoid design related short shots and other defects. Overall nominal wall thickness distribution in combination with part features should be optimized. The number of gates and gate location in combination with polymer material flow behavior needs to be evaluated in order to avoid thin sections near or at the end of the flow path. Once the part and mold design are optimized with the polymer material, design related short shot conditions can be avoided.
In this animated demo you can see a short shot developing during filling and the resulting molding defect on the part.
There are also numerous possible factors in the process set-up that could result in a short shot. The following is a checklist to help avoid shot short conditions:
- The appropriate amount of raw material should be loaded into the machine hopper
- The material feed to the cylinder assembly must be set up properly
- All supply lines must be open when using an automatic material loading system
- The feed throat needs to be open to allow material to feed into the cylinder
- The feed throat temperature must be set to prevent over-heating the material and creating a blockage
Running various process sensitivity tests can help determine whether a part should be redesigned prior to mold build to enhance a molding window and ensure part quality.
If you are interested in learning more about process-related short shots, check out the full demo lesson.
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