You are probably right about off-gassing. It is unlikely that the polyethylene itself is breaking down at 150 C. However, there may be processing aids in the PE which are volatile at high temperature. For example, most injection molded parts use a mold release agent to prevent the part from adhering to the mold. Zinc stearate is a common mold release agent. Compounds like this could be off-gassing at 150 C. You can scrape a small amount of the residue from the door and have GC/MS analysis run on it. This analysis will identify the compound. Your PE may also contain a filler which could be colored and volatile. You should be able to find out from the manufacturer what substances are in these bars besides UHMWPE.
LDPE and HDPE are known to have "crystalinity" or a repeating structure. Maybe UHMWPE also has a crystal structure. This structure may rearrange under heat and cause an appearance change..
Wayne, Please look at the results of the GCMS testing on the residue from our ovens. I know it points to the vacuum pump oil but I don't see this residue when I run the oven without the crosslinked UHMWPE bars. Could the "Mineral based hydrocarbons" possibly be from the bars? I need to rule out the vacuum pump before we can process material. I am going to try a vapor trap and run some scrap bars to see if the residue goes away. Any advice you can give would be appreciated. You can email me at firstname.lastname@example.org if you would like to.
I look forward to your reply