The purpose of a short shot during 1st Stage Injection is to ensure that all cavities remain short at transfer during normal process variation.
Question:
I was told to short the mold around 98% based on final part weight, is that enough?
My Response:
Basing any short shot on the final part weight is an inaccurate and misleading process. This is faulty because there are so many factors which contribute to the percentage of material that is added to the mold between when the first cavity fills and the part is completely packed out. Aspects such as wall thickness, material shrinkage, filling imbalance, degree of semi-crystallinity, pressure loss, material consistency, packing pressure, etc. all make a general rule based on final part weight impossible. because of these factors, a short shot of 95-98% based on a fully packed part will likely be full in most if not all cavities at the time of transfer as well as show screw bounce.
The best approach is to use a short shot percentage based on 100% being equal to the weight of the shot when 1) only one cavity fills and 2) no packing is present. We typically recommend 90-95% short under these circumstances. If the packing is established correctly, the process will have enough pressure to keep the screw moving forward to fill and pack out the parts without flash. This will compensate for 1-2 percent check ring variation as well as 2-3 percent material viscosity variation.
Additional Thoughts:
If you are processing very unstable material such as post-consumer regrind having as high as 10-20% viscosity variation from shot-to-shot, then a short shot as low as 75% may be necessary to maintain a consistent final part weight.