FYI... Possible workaround.
I left a stack of badly curled 67 lb cover stock that I printed this morning under a s huge stack of textbooks. When I got back home about 10 hours later, I anxiously lifted the stack of books to see if it flattened out.
The result of 10 hours under 20-30 pounds of books? Nothing. The print job was just as curled as it was in the morning.
So, I tried something new. I created a print job with almost nothing on it, except a small dot, then ran it through the printer, upside down.
The result, the second curl effectifely un-curled the first curl. Although it increases my print time, the virtually blank back page prints quickly and doesn't use much toner.