How can the foaming index (ISO index) of PU foam be controlled to optimize sandwich panel dimensional stability?

ISO index = (actual isocyanate used / theoretical amount for complete reaction) × 100. The ideal range is 110–120, where isocyanate is slightly in excess, forming biuret and allophanate crosslinks, increasing foam rigidity and reducing thermal expansion coefficient. If index <100, the foam is soft, cures slowly, and exhibits high shrinkage (up to 2%), leading to surface depressions; if index >130, the foam becomes brittle and hard, reducing impact strength of bonding with metal facings, and residual isocyanate absorbs moisture causing blistering. Professional manufacturers use online near-infrared (NIR) to monitor the index in real time, controlling deviation within ±2, ensuring sandwich panel length tolerance <1mm/m and flatness ±0.5mm.

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How can the foaming index (ISO index) of PU foam be controlled to optimize sandwich panel dimensional stability?

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