A pre-stretch bubble that changes height or shape from cycle to cycle produces a different material distribution before the mold even begins to form the part. The finished dimensions may remain acceptable while minimum wall thickness drifts.
Variation can come from sheet temperature, initial sag, air pressure, valve response, leaks, clamp slip, incoming gauge, or material orientation. A position sensor can also create false variation if its lens is dirty or its trigger point does not match the actual bubble geometry.
Diagnostic sequence
Initial sheet position, measured temperature, bubble pressure curve, and maximum height should be recorded together. If pressure repeats but height does not, sheet condition or clamping is involved. If height follows pressure changes, the air circuit and valve require attention.
A closed-loop height trigger can improve repeatability, but it should not hide an unstable thermal process. The bubble must reach the same geometry for the same physical reason, not merely stop at the same sensor point.