The invention provides a free flowing copolymer between disparate plastic layers to thermally balance between the two differently expanding/contracting materials. The “primer” layer or “thermal balance layer” fully encapsulates the PET layer and can absorb the difference between the internal PET layer with any layer bonded to the encapsulated PET without cracking, bending or pitting and can “reset” into position when the heating/cooling has ended to make planar plastic core sheets for products such as credit cards, gift cards and the like.