You need release paper or better yet, a release board between the heat platen and the piece being mounted.
Have you tried to Google Seal 210M?
Bienfang and Seal, same thing.
There's tons of info available..
Most distributors that have matboard will have release boards. I keep a sheet of flawboard cut to size on top of the pad when using 1/8 foam board for mounting and remove it for 3/16 board. This keep the thickness of the package consistent so that you don't compress the pad. They are expensive to replace.
Take extra pains to keep anything from getting on the pad and clean off the platen right away if something adheres to it.
You might want to get Chris Paschke's book of mounting, The Mounting and Laminating Handbook. https://www.designsinkart.com/book.htm. You might find it used on Amazon. One of the older editions should be fine for your purposes. The newer editions have additional information about more recent mounting methods. Maybe more than you wanted to know...