The size shrinkage of cast polyurethane products is mainly based on the following two reasons.

  When the pouring material changes from liquid to solid, the molecular cohesion increases, which makes the product size smaller.

  The thermal expansion coefficient of the casting material is larger than that of the mold material. The resulting size shrinkage increases with the increase of the molding temperature, that is, the higher the molding vulcanization temperature, the greater the size shrinkage. The size shrinkage of products cured at room temperature is minimal.

  In addition, the shrinkage rate is also related to the shape and size of the product, and the hardness of the product. The shrinkage rate of products in different directions is also different, and the shrinkage of hard products is generally slightly larger than that of soft products. Products with a metal skeleton shrink less and shrink toward the metal. The shrinkage rate is related to many factors, among which the molding vulcanization temperature is most easily overlooked. It is often found that the size of the products formed by the same mold is quite different, most of which are caused by the fluctuation of the molding vulcanization temperature. In addition, according to the author's many years of practice, the size of vulcanized products varies greatly, mostly caused by fluctuations in the molding vulcanization temperature. In addition, according to the author's many years of practice, the length of vulcanization time, hot and cold demolding, atmospheric vulcanization, pressure vulcanization, etc. will also affect the shrinkage of the product. Therefore, for products with strict size requirements, in addition to strictly controlling the molding and vulcanization temperature, other operations must also be standardized.

  The shrinkage rate of CPU elastomer moldings is generally 1% to 2%. It can be designed according to 1.5% to 1.7% first, and there is a certain margin for modification in the key dimensions of the mold. In addition, the vulcanization molding temperature (such as 100-120°C) can be appropriately changed to adjust the shrinkage rate of the product within a certain range.