In order to more accurately select the shrinkage rate of large-scale cast polyurethane elastomer products, the impact of process conditions, environmental factors and product structure on the shrinkage rate was systematically studied, and The change curve of shrinkage rate with time was measured. The reasons are analyzed. The results show that the product structure has the greatest impact on the shrinkage rate, and the process conditions also have a greater impact on the changes in the shrinkage rate, while fluctuations in environmental conditions are the cause of the later fluctuations in the shrinkage rate.
The selection of shrinkage rate is one of the key factors that determine the success of mold design. Especially for large-scale (part size above 1 000 m/n) polyurethane elastomer products, due to their large shrinkage rate (generally around 1.5%), if the shrinkage rate is improperly selected, the modulus repair amount will be increased and the mold life will be extended. The manufacturing time increases and the cost of the mold is increased; in severe cases, the mold will be scrapped due to excessive size.
Therefore, analyzing the reasons for the shrinkage of polyurethane elastomer and studying the qualitative relationship between shrinkage rate and process conditions are of great significance to the design of cast polyurethane elastomer molds. Through a large number of process experiments, this paper comprehensively studies the impact of process conditions, structural factors and environmental conditions on shrinkage.
The shrinkage rate of polyurethane elastomer changes as follows:
1) Increases with the increase of mold temperature;
2) Zigzag changes with the prolongation of aging time;
3) Decreases with the increase of ambient temperature;
4) Fluctuations occur as the ambient humidity increases;
5) Different mold structures have different resistance to shrinkage, with the smallest in the longitudinal direction and the largest in the transverse direction;
6) Before a certain length, the shrinkage rate increases with the increase of the flow path length. After exceeding a certain length, the shrinkage rate decreases with the increase of the flow length.
It can be seen that the process conditions have a certain impact on the shrinkage rate, and the impact of temperature is the most significant; the structure of the product itself has a greater impact on the shrinkage rate. The elastic structure of polyurethane is mainly caused by the different flow paths; environmental factors are late-stage influencing factors, and their changes are the main factors that cause fluctuations in shrinkage.
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