Shrinkage in Rigid Polyurethane Foam
Shrinkage — visible contraction, concave faces, or panel warping — is one of the most damaging defects in rigid foam production. Unlike most issues, it often appears 12 to 72 hours after demould, when the part is already packed or installed.
The most common root causes are insufficient closed-cell content, blowing-agent ratio issues, premature demould, polyol functionality or hydroxyl number drift, cell-gas diffusion, and incomplete cure. Rigid foam relies on trapped blowing-agent gas to maintain pressure inside cells; any condition that lowers that pressure produces shrinkage.
First-line checks: measure closed-cell content (target 90%+ for most rigid systems), compare free-rise and in-mould density, run a 70°C / -20°C dimensional stability test, review demould times against reactivity profile, pull polyol blend OHV against spec, and inspect mould temperature logs.
Shrinkage is expensive because it's discovered late. The root cause is almost always a combination — borderline blowing-agent ratio, marginal cure profile, polyol blend slightly off spec. Solving it requires looking at all three together.