https://www.google.com/patents/US3095400
“These are the patents you are looking for”
Looks like the final answer is mixing small particles of a thermoset with a thermoplastic during the molding.
https://www.google.com/patents/US3095400
“These are the patents you are looking for”
Looks like the final answer is mixing small particles of a thermoset with a thermoplastic during the molding.