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Usually, the problem is caused by polyol quality issue.
Possible reasons:
High water content as Gaosu mentioned.
High content of K+, Na+ or other metal ion in polyol
Too lower acid value of polyol and/or TDI.
Sorry, I have problem to type Chinese.
一般都是聚醚问题,k Na离子含量高,自生热就非常大,在合成预聚体时加一点苯甲酰氯或磷酸