Recently a technicians asked me this question…
During a gate seal determination (part weight vs. second stage time), we saw the part weight almost stabilize and then it shot up until it finally leveled out. What might have caused this?
When doing a gate seal determination, you might see this two-plateau result. Essentially, this is causes by material shrinkage in the gate area allowing more material to flow through the gate before the gate finally freezes.
The material can either flow through the center of the gate as the material shrinks around itself, or the material can flow around the gate as it shrinks away from the mold surface.This is most common when you are molding parts with thick gates or materials with especially high melt flow rates.
Since the first plateau is caused by stagnation of flow before a second flow of material into the mold cavity, it is always good to verify that such a process does not result in delamination, brittleness, or low mechanical strength.