Injection Molding Process Optimization
Optimizing the molding process requires technical skill, knowledge and expertise of the equipment as well as a methodical scientific approach. You can run endless trials and experiments and collect data but that will not change the end result. The process reliability is limited and only as good as the first four inputs will allow it to be.
Optimizing the process can be achieved using a practical and methodical scientific approach.
- Optimize mold fill time to minimize the effects of changes in relative viscosity of the material.
- Optimize the injection hold time by determining the gate freeze off time so you can most effectively use the holding pressure to optimize packing of the part and its ability to consistently hold dimensions.
- Based on material and maximum wall thickness, determine the optimal cooling time that will allow parts to completely solidify and prevent excessive part shrinkage and/or other defects caused by premature cooling.
- If needed, a process window can be determined by testing the plastic melt profile at the low and high end of the manufacturer’s recommended range and regulating the pressure from low to high at each temperature extreme while monitoring a process critical dimension to determine the safe operating window that will keep the parts within the acceptance range. The bigger the window, a more robust and reliable process can be expected and the less likely you are to find any unacceptable product in the lot of molded parts. The smaller the window, the more difficult it will be to maintain dimensionally consistent parts, thus more frequent inspection would be warranted. For semi-crystalline materials, mold temperatures and cooling time also have to be optimized to make sure the parts have developed the required crystallinity before they are ejected. If not, post molding crystallinity development will affect the long-term dimensional stability of the part.
The five inputs of the molding process as well as the acceptance criteria of the parts play a vital role in predicting how likely you are to achieve a robust and reliable process for the parts. A process with optimal material flow, packing, and cooling, will yield the most consistent dimensionally stable parts. This does not ensure that the parts will meet all the dimensional and visual acceptance criteria nor will it ensure that the variation from part to part or lot to lot will be in the acceptable tolerance range. It will depend on how big of a process window can be achieved and how wide the acceptance criteria are. On the other hand, it is possible for a process that is not optimized to consistently make good parts as long as the process window and acceptance criteria are large enough.