why are my plastic bearings melting

if you cleaned the bearings with mineral spirits and then used lithium grease to pack them the mineral spirits weren't dried out engouh and it eitehr ate the grease up or you didn't pack them right. In my professional career I NEVER used plastic wheel bearings because people who did had the same problems. Get steel ones. Also properly torqued means to torque to 50' lbs and back off to first hole on nut on a chevrolet. You may have tightened it to much.

Thanks for the tips. I will definitely never use the plastic bearings again and i was a little hesitant when i got it at the parts store, but it matched the one i took out so i figured it would be fine. I didn't use any cleaner and I am pretty sure i packed them really well and filled the inside of the Rotor hub with plenty of grease as well. I did find a couple guides online that said to torque it to 180' lb to 240'lb and not 50. That did seem a little tight, but when i got it there, the hole for the cotter pin was lined up perfectly so i went with it. once i get it off, i will be getting a new metal bearing.

ok my mistake ont eh torque but you have to back it off 1 notch for the cotter pin.

180 to 240 foot pounds is for the axle nut on the one-piece bearing and hub assembly on a 4wd truck or front-wheel-drive car. That torque is critical in preventing the bearing from becoming noisy.

I never used a torque wrench on tapered bearings but 50 foot pounds would seem to be about the same as "common sense". Failing to back the nut off to the nearest cotter pin hole can leave the rollers too tight so they squeeze the grease out rather than let it run in between the axle and rollers. I've run into a few people who think tight is good and tighter is better.

Hi guys. I've been following this to learn what is meant by "plastic" bearing. If you mean the outer cage is plastic, that is common. It just holds the rollers in position. Are you saying that causes problems or am I missing something else?

Yes, plastic cage that holds the rollers.
Looking again, I think you may be right about the torque. It's not a 4X4, but just a rear wheel drive and therefore it shouldn't need the full torque. And thinking back to the first time I took the nut off, I did it with my hand(no socket).

like i said 50 ft lbs torque then back off to first notch. you don't need a lot of grease in the rotor either. Just make sure you pack the bearings well. if they are new, spray with brake clean and let dry for a few minutes before packing them. Make sure to put a light coating of greason the outer side as well so they don't start dry.

UPDATE - got the rotor off with a 7 ton 3 jaw puller(o'reily has these to 'borrow') and then re installed everything including a full metal outer bearing and a lightly torqued axle nut. Everything seems to be working out great.
Thanks for the input guys.