Acceptable Part Weight Variability

Yesterday, I received a question which focuses on the common issue of variability…

Currently we are running a mold (8 cavity) with TPU Estane material with part weight 2.68gms.(Holding time – 5.5sec)

Based on the Part 2D drawings the theoretical part weight calculated from 3D Solid works is 2.71 gms. From the gate freeze study the exact holding time for this part 7.5sec. With this time the part weight getting 2.70 gms nearer to the theoretical.

What is the permissible allowance (variation) while processing?. Is there any standardization for this? How much minimum variation can be allowable for any part? 
My Response
First, I want to say that your calculations were very accurate. Assuming you are running a process which has a short shot during first stage… the typical variation is often less than 5%. Many processes can maintain even closer tolerances within a specific material lot.
The best way to determine this is to mold the process for a period of time, and measure samples of parts over time. After a couple lots of material have been processed, then you can determine the actual statistics and standard deviations to be expected.
Additional Thoughts
It is critical that you monitor the part weight closely since it is very common for molders to make process changes that increase the part weight during troubleshooting. For more on this, please review my previous blog: Calculating Material Usage Requirements

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