With the proper temperatures set, the mold must be filled as fast as consistently possible. This means that the mold should be able to be filled to at least 90 percent of the injection rate of the production molding machine without adverse effects.

There are exceptions to this rule in areas such as lens molds, but you always want to have a mold that can be filled rapidly without adverse effects, even if you want to fill slowly later. This means that you shouldn’t have jetting, burning or flash just because the mold is filled fast. Filling fast doesn’t mean ramming the plastic into the end of the cavity. You must decouple the fast fill from the sudden stop. This molding technique should be used during all new mold tryouts.

Separating the fast fill from the sudden stop uses up the inertia of the fast flowing plastic before the cavity is full. The opportunity to look at fill balance is available by making a fill only shot where the mold is filled to a point of 90 to 95 percent.

This is the balance during flow, and is different than static or pack balance. Looking at short shots is really a key to doing a rigorous mold tryout.