HDPE joinery

[brad hays]

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?

[Jim Matthews]

This might get more knowledgeable replies in the boat building sub.

I believe HDPE may be bonded with some epoxies.

If the mold is large, the HDPE should have a plywood "scaffolding". Any exposed fasteners on the side that will hold the resin mix must be coated with a release agent.

https://www.mcmaster.com/glue/for-use-on~hdpe-plastic/

[Dan Hunkele]

There are plenty of mold making videos on You Tube. Check some of those out. Appears they just use sheet metal screws.

[Erik Loza]

Brad, I know basically know nothing about epoxy pours and that sort of thing but did in work in the plastics industry for a bit. You may already know these things but a couple of thoughts:

1.) The only true way to join HDPE is to weld it, which is exactly what it sounds like. Maybe mechanically fastening it will solve your needs but if the joint needs to be liquid-tight, would have to be weleded, which requires some very specialized equipment.

2.) HDPE is extremely soft. I’ve never it seen it tapped. In fact, I have some adirondack chairs that are HDPE and you can easily pull the screws out if you try. “Through-hole drilled to mechanically fasten to something else?”. Yes. “Actually tapped to receive and hold a screw?” No

3. Not sure the size mold you plan to use but HDPE is pretty wimpy as far as structural strength. It often needs to be backed up by plywood or something. Again, you may already know this.

4.) I’ve heard 3M makes some epoxies that will bond HDPE. Might be OK for patching a small hole or something but I can tell you that for anything industrial or structural, gotta be a welded joint on HDPE.

Hope this helps and good luck in your search.

Erik

[Ole Anderson]

Not an epoxy person, but have watched plenty of videos. Isn't the standard epoxy mold melamine with a mold release applied it it? https://www.youtube.com/watch?v=ShG4tNcJzFg&t=8s

[Tom M King]

Sounds like a hard way to do it. I make wooden forms, like the finished product you want, and use the flexible, pourable mold making gunk for parts that otherwise can't be pulled out of fixed molds. Once you have the mold, coat it with mold release, and make as many parts as you want. All the ingredients, except the wood are available off Amazon.

[Jim Tobias]

What Tom King said is the best long term plan. If you just want molds for an occasional pour of a certain size, the HDPE work fine. You can either just use sheet metal screws or for longer life on your molds, drill out and use threaded inserts and 8/32 small bolts. If you do this I strongly advise getting the right sized https://smile.amazon.com/gp/product/B015CAPAPM/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Also for sealing it, use https://smile.amazon.com/gp/product/B01N6L7FK0/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
This works great and prevents leaks. You should use mold release spray, Stoner, https://smile.amazon.com/Stoner-236-Urethan-Mold-Release/dp/B00Y3NWYRM/ref=sr_1_2?crid=IO9Q564AEKDD&dchild=1&keywords=stoner+mold+release+spray+for+resin&qid=1616947602&sprefix=stoner+mold+%2Coffice-products%2C157&sr=8-2
The idea of using the screws and inserts is that you can unscrew one end if necessary to help release the resin form without damaging the mold or (eventually) stripping out the points where you used sheet metal screws.
Keep us posted on your progress.
Jim