Thread: HDPE joinery

Originally Posted by bradleyheathhay

What's the best way to secure 1/2" walls of HDPE together for a wood/resin mold? Drill and tap holes in the edges and use course thread screws?

Yes - you can even use wood screws if you don't want to tap a heap of threads.
To tap the threads I used a cordless screw driver with the clutch set on about 5 - drill the hole slightly smaller (0.1-0.1mm) than recommended
Do a practice run of course.
At work we often just used stainless wood screws - the kind that look like self tappers with thread all the way along the screw,
We used a lot of countersunk screws but if you don't want to risk the holes blowing out eventually use this kind of screw
Screen Shot 2021-03-28 at 10.20.31 am.png

If the mould is to be repeatedly used, the HDPE will creep over time (that's why CS screw holes also blow out over time) so some form of joinery dads etc will keep it better in shape.

A sketch of what you have in mind might enable further advice to be provided.

Thanks BobL. At first I was considering using metal inserts screwed into tapped holes but after thinking more I think the inserts might splay the form out some so I've settled on going with straight screws into tapped holes.

I use a bead of silicon and cheap wood screws

Thanks dai sensai, yeah I'm always over thinking things. I was wondering about sealants with HDPE. So you say silicone sticks to it fairly well? Wouldn't imagine it would be too hard to pry off.

Originally Posted by bradleyheathhay

Thanks dai sensai, yeah I'm always over thinking things. I was wondering about sealants with HDPE. So you say silicone sticks to it fairly well? Wouldn't imagine it would be too hard to pry off.

Yes it comes off easy but stays to seal the joint. I cast under up to 100psi and the joints leak if I don't use silicon.

Very little will stick to HDPE but we also used (MANy) tubes of silicone to seal HDPE boxes/joints at work. A physically stronger seal can be obtained by welding the HDPE. but this requires some skill as its easy to overheating burn the plastic.

Originally Posted by BobL

Very little will stick to HDPE but we also used (MANy) tubes of silicone to seal HDPE boxes/joints at work. A physically stronger seal can be obtained by welding the HDPE. but this requires some skill as its easy to overheating burn the plastic.

You don't want a permanent seal, the cast will get stuck, so need to be able to get the mold apart.

A small continuous bead of silicon is all that is needed. It does stick to the material to some extent, enough to form a gasket that seals the joint. I do it every day

Good to know the HDPE is 'sealable' now. Do you use anything else to help hold your HDPE molds together, screws or clamps maybe?

Originally Posted by bradleyheathhay

Good to know the HDPE is 'sealable' now. Do you use anything else to help hold your HDPE molds together, screws or clamps maybe?

definitely, I use wood screws.