creep resistance of polyurethane

huangyhg

creep resistance of polyurethane
hello,
in a low pressure (20 psig)valve, we were using a stamped,
1/8" diameter, .032 thick polyurethane disk of 90 durometer shore hardness. the disk got bonded to the plunger. now we switched to a plunger where the seal is molded to the plunger. we still are specifying 90 durometer polyurethane. to our great surprise, during a time period of a month, the molded polyurethane flows into the 1/16" diameter bore it is supposed to seal off. the spring force pushing the seal to the seat is .65 lb. using the same materials, why does the molded seal flow when the seal, stamped from sheet stayed put in hundreds of valves for over 5 years? is there a spec. that we overlooked?
we picked polyurethane for wear reasons. is there something that wears as well, has the same resiliency and does not flow? medium to seal is air (moist).
many thanks in advance,
friedhelm