PCBs warp when heated. It is in their nature to do that because the fiberglass material is a poor thermal conductor. When the copper on one side heats up, that side expands more than the other side. It has to warp. It has no choice.
If you really want to know how to do it right, see: [
www.instructables.com]
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]