If this is to bond the foam sheet uniformly and strrongly onto the ply, then I suggest a spreadable adhesive is the right type. PVA will do the job on many foams like expanded polystyrene, but those that are waxy will need a solvent based 'impact' type. Further alternatives, Copydex, which is an easily spreadable latex solution, or a spray adhesive like my old friend 3M spraymount, or similar; these make a weaker bond, but are very quick and easy to apply very evenly.
Ask the foam supplier for any incompatabilities and recommendations, followed by testing at least 24 hours in advance of the intended use, to ensure no immediate degradation of the foam.
(It's a tricky field, I could cite many examples where manufacturers have 'done their research' yet some years later have faced customer claims because the foam has become hard and incompressible, or a soggy sludge, or a brittle substance crumbling into powder: sometimes all these results from nominally the same foam and adhesive combination. I was fortunate to know a chemist who served ICI Plastics Division as their chief analyst in the previouis century and his comment on such events was 'problematic, we are working in it'. There has been much progress since ...)
Ask the foam supplier for any incompatabilities and recommendations, followed by testing at least 24 hours in advance of the intended use, to ensure no immediate degradation of the foam.
(It's a tricky field, I could cite many examples where manufacturers have 'done their research' yet some years later have faced customer claims because the foam has become hard and incompressible, or a soggy sludge, or a brittle substance crumbling into powder: sometimes all these results from nominally the same foam and adhesive combination. I was fortunate to know a chemist who served ICI Plastics Division as their chief analyst in the previouis century and his comment on such events was 'problematic, we are working in it'. There has been much progress since ...)
