When your mold is 98% full, packing is already beginning to take place. This makes it nearly impossible to completely separate filling from packing.
Question: I configured my mold with a 98% fill and finish with packing. Why do I still get sinks and short shots?
My Response: We always recommend 90-95% because 98% fill causes the following problems:
1) Since only a small portion of the mold is filling at 98%, packing has already begun to take place in most of the mold cavity. This results in a blending of packing and filling. Whenever the fill varies, the degree of packing will also vary again.
2) Plastics tend to have 5-10% normal variation in viscosity. A 98% fill can only compensate for small variations in material viscosity. In such a process, a moderate drop in viscosity is likely to cause flash, overpacking, overweight parts, warpage, large dimensions, cycling problems, or part sticking. Likewise, a moderate rise in viscosity can cause shorts, sinks, voids, warpage small dimensions, and cycling problems.
3) Common variation in a 'healthy' check ring is typically between 1-3%. This means a process established with a 98% fill can vary between 96.5% and 99.5% full with a 'healthy' check ring. The parts produced from a 99.5% fill are going to be much different than those produced from a 96.5% fill.
4) When the fill reaches 98% full, the mold cavity is beginning to pack. This results in a higher requirement for clamp tonnage since it is required to overcome injection pressure rather than packing pressure.
For more about why 90-95% fill, please feel free to read the following: