Originally Posted by Poly
Virtually no glue sticks to polyethylene. The screws issue requires forward planning at the design stage. They can mould bolt inserts into the hull at manufacture or design so it can through bolted. If you need to screw aftermarket stuff in place then self tappers should be seen as temporary... and regularly checked. You can use cavity wall type fittings that expand behind the material but keeping that watertight becomes more complex.
we use a lot of UHMWPE/LDPE (different grades of polythene) at work & it's virtually impossible to glue as it exudes oil over time. All our components are mechanically fixed. We don't even use self tappers or other "screw in" type fixings, everything is nut & bolted with form G washers (big thick ones
) There are one or two specialist companies that can weld polythene but I wouldn't trust it for anything structural.