Just because the mold is flashing, does not mean the part weight has increased…
Question: Hello, we have a part which is flashing, but the part weight is lower than before. Does this make sense?
Answer: When this is the case, it is usually caused by too much material entering the mold during injection. As the mold becomes full, the pressure builds in the mold cavity. When this process transfers from high pressure injection to low pressure packing, the screw bounces back causing material to exit the mold cavity. The high high pressure may cause flash, but the screw bounceback loses material which can not always be compensated for with packing. The result is often a lower part weight. Beware, that it is common for sinks or voids to also be present in such a part. This is a great example of why you always want to document the final part weight as well as the short-shot weight (without packing) for all your processes.
Additional Thoughts: The best solution for such a problem is to have the process transfer before any of the mold cavities are full. This allows you to use packing pressure and time to more accurately control the final part weight.