Polyurethanes have approx. 10–21 times greater thermal expansion coefficient than steel. This must be taken into account during design, so that space is provided for the polyurethane material to expand when heated.
Mold shrinkage
The majority of mold shrinkage is due to the thermal expansion coefficient and only to a small extent to chemical contraction. Linear mold shrinkage for cast urethane systems lies between approx. 1.4–2.2% depending on, among other things, composition and reaction temperature.
However, in certain critical cases (complex moulds), the slight chemical contraction may cause internal cracks before the material has reached sufficient strength.
By lowering the mixing temperature and the mould temperature by approximately 6–9°C and increasing the oven temperature by 6–9°C, the urethane rubber can instead be made to expand slightly, thereby preventing the formation of internal cracks.
Mould shrinkage also depends on wall thickness and hardness. For injection-moulded thermoplastic polyurethane, mould shrinkage is also highly dependent on the injection pressure. In addition, post-conditioning has an effect (applies to all polyurethanes).
Mould shrinkage must be given particular consideration when polyurethane is combined with metals or ceramics.