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Urethane mould substitutes for Silicon moulds

 

The Situation:

A customer switched from a conventional silicone compound to our more economical polyurethane to make moulds for casting parts. After switching they noticed bubbling while casting urethane into these new moulds.

 

They believed that the bubbling was due to water on the surface of the mould reacting with the Isocyanates in the urethane mixture.


The Basic Requirements

All requirements had been met by the urethane they were using. The only issue was bubbling during casting, at the mould/casting interface. It was suspected that this was adsorbed water on the surface of the mould interfering in the casting process. Assuming this was true the customer was requesting a formulation change to address the bubbling issue. Moisture scavengers were being considered as a possible option.



The Formulating Problem

The Importance of Trouble Shooting Before Formulating

The customer noted that they were getting bubbling especially during the higher humidity months. On the worst days the recorded humidity was over 50%. All things being equal between their old set up and new set up, this would indicate that the increase in humidity was the causes of the problem. However, it was assumed in this instance that the humidity was not the cause as experience has shown that cured polyurethane moulds would not react with moisture, and no prior problems existed with the casting urethane on either high or low humidity days. This prompted us to investigate the processing conditions. In order to test the processing conditions we needed to replicate as close as possible a makeshift humidity chamber. This chamber was constructed using a gallon can containing water. It was placed in an oven at a temperature which reflected the operating temperature at the customer’s site. Samples of all processing components (urethane moulds, casting urethane, and mould release) were introduced into the test.


Castings were poured in the makeshift humidity chamber using the customer’s materials and simulating the customers processing conditions. The results showed no bubbling. There were no other reactions occurring in this closed test that could have caused bubbling other than water and Isocyanates. With chemical reactions excluded, a physical phenomenon was the next likely choice. A volatile organic compound flashing off could cause bubbling similar to what was observed in the cast urethane. Analysis of the MSDS’ of all the substances showed that the mould release was the only substance that would flash off within the operating temperature. Trials were run under dry and humid conditions. For each of these conditions either mould release was sprayed on and allowed to flash off before pouring the urethane or the urethane was poured before the mould release had time to flash off. The only instances of bubbling were noted in the trials were when the mould release was not allowed to fully flash off. The conclusion was that they were not allowing enough time between spraying the mould and pouring their urethane. This makes even more sense when we consider the fact that with their old silicone moulds they would not have used mould release at all. The processing suggestions were passed along to the customer who has had no further issues with the product.


In this case if formulations were initiated as opposed to careful troubleshooting, research and development time would have been wasted chasing a solution for a problem that did not exist. Furthermore the client would not have ended up with a product that solved their problems. This is why it is crucial to get a full grasp of the problem before moving forward to the formulation stage.

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