1. Purity
First of all, there is no doubt that the foam scrap must be clean with no other impurities or rubbish. That’s the basic requirement.
2. Foam density
There is no absolute good or bad about density actually. Good or bad depends on what kind of rebonded do you want. Regarding the foam scrap, the density you choose should be lower than the rebonded density you foam. Low density foam scrap can be foamed low density rebonded. If you want to foam hard feeling rebonded, you can also use low density foam scrap. But low density foam scrap will cost more rebonded glue. High density foam scrap can be only foamed high density rebonded, and it can cost less glue.
3. Foam skin
Foam skin is much cheaper than pure foam. It can make the rebonded feel firm and not limp. So you can add more foam skin to foam hard rebonded.
