I was asked this question by a molder who was having trouble balancing a hot runner system…
We have this new mold which is running 16 cavities in PBT. Usually we can balance the mold using the drop temperatures, but that does not seem to work on this mold. Any thoughts?
Essentially, semi-crystalline polymers with a high degree of crystallinity such as PBT and nylon are not highly affected by temperature. In such a case, you will have to adjust the gates or the hot runner system itself to balance the shear of the mold.
Cold runner systems are the best for these materials. I know many molders who use a press-side low-RPM grinder to effectively grind up the material and re-introduce PBT back into the process.