Unless you're using a twin injector the temp that the belly plastic was injected at means zip to what temp the top plastic is. One thing that creates issues is if the bellies are made and kept for a week or two then finished. They will skin over with the oil the plastic is known to exude after the baits are made. That oil has to be removed. #20 degree plastic is more than enough to weld the halves on small frys especially if one keeps some pressure on when the rod stops.
