Casting polyurethane question

[challenger]

I'm looking for advice on how to cast polyurethane. Anyone do much of this?
Thanks

[plunger]

Go to the SMOOTH ON youtube videos. All you need to know is there.

[Dan Dubeau]

There is a TON of info on youtube about casting, moldmaking etc. Smooth on is a great resource of info as plunger said. Another great channel I just stumbled on is Robert Tolone https://www.youtube.com/channel/UCPA...ebL6RBW5-bnOrg . He primarily does toys and sculpture, but the techniques are the same whether you're casting toys, a tool, or in my case fishing lures.

Any specific questions? I'm still a complete beginner, but am nearing the end of my first bottle of resin so am pretty much an internet expert .

[gellfex]

I've done a fair bit. Fun stuff! My secret trick was making molds from reusable tool dip, the stuff that's sometimes on new saw or router blades. For simple shapes it works great, and is much cheaper than silicone. Plus, if you screw up, or are done with a mold, just drop it back in the pot!

If you describe what you want to cast, more specific advice could be given.

[Dan Dubeau]

I've done a fair bit. Fun stuff! My secret trick was making molds from reusable tool dip, the stuff that's sometimes on new saw or router blades. For simple shapes it works great, and is much cheaper than silicone. Plus, if you screw up, or are done with a mold, just drop it back in the pot!

If you describe what you want to cast, more specific advice could be given.

AH HA! Great tip, thanks I'm going to try it. I've played around far too much with silicone house caulk and various additives for small stuff. Alumilite has a reusable molding material i've been wanting to try, but I've got lots of tool dip laying around here and it sounds like a much better (as in cheaper) way for small stuff I only want to cast once or twice. Playing around with some on my desk right now I can see how it would be perfect.

Silicone is relatively expensive, but I've started cutting up and recycling old molds as filler to lower mold cost a bit. Works great. The silicone house caulk bits work good as filler too, but they mess with the durometer a bit too much. I also gave up on matchplate molds too. Takes too long to resin print a master, and I find cut molds make better castings now.

[gellfex]

I have my doubts whether your printed resin master will hold up to the 300° heat of the tool dip. But I have made masters out of acrylic that worked well, so it's worth a shot. I use a small cheap electric fryer to get good temperature control.

Making match plates is not hard at all with the tool dip. Where for silicone you would use water clay to sculpt your parting line, for tool dip just use Sculpey modeling clay that simply hardens under heat. Some pieces that were halves of something screwed together I would just lay flat on a surface, build a box around it and pour over it. Then I could flip it, trim it, cut keys in it, and pour the second half.

PVA works great for parting the dip, even if it melts the opposite side a bit the PVA film keeps it parted. I have also used the dip to make fiberglass molds, sometimes just using a thin layer and then doing a plaster and burlap mother mold around it. I made a rather large fiberglass skull from a client's sculpture that way.

[Dan Dubeau]

So far my 3d resin (elegoo black) has held up to a couple plastisol pours at ~300-330 without really breaking down. I wouldn't count on doing it for long runs, but it's worked for under 10 pours/injections. I didn't think it would either, but was pretty surprised. Same with pla molds for injecting. Technically shouldn't work, but it does.

We must be thinking different about different tool dip plastics, as the stuff I'm thinking about melts in a crock pot (translucent green). At least I think it does anyway, unless I'm thinking about something else (which is entirely possible). I've scrounged some from the shop and am going to play around this weekend if I get some time.

[oxford]

If you describe what you want to cast, more specific advice could be given.

Based on a different thread of the OP I think he is looking to cast cylinders roughly 2” diameter by 14” long over a center shaft. Roughly something in the density close to nylon.

[gellfex]

Dan Dubeau I think this is the product I have, it's been a while. McMaster 7762t72

oxford I think he could totally use the dip rubber to cast that around a shaft. Do I recall correctly that he wants to put flutes in it?

[challenger]

Based on a different thread of the OP I think he is looking to cast cylinders roughly 2” diameter by 14” long over a center shaft. Roughly something in the density close to nylon.

This is what I was thinking of duplicating with polyurethane. I made one fluted roller of the dimensions you listed and proceeded to F up the second one. I milled it and didn't realize that it moved on the table and totally screwed up the "timing" of the flutes so the two rollers can't mesh even the slight amount I need them to.
So now that I have a master, to make a mold from, what formula/material do I look for that will give me a hard final product? Are these compounds listed as the cured durometer?
Many thanks.

[dian]

pu resin is soft. i believe they top out at 80 shore a. pretty expensive too. is that what you want? there are plenty of other resins that are harder and can be filled, which reduces the cost.

[Noitoen]

pu resin is soft. i believe they top out at 80 shore a. pretty expensive too. is that what you want? there are plenty of other resins that are harder and can be filled, which reduces the cost.

Not all PU resins are soft.

[loose nut]

I have cast items in RTV moulds using cooking spray as a release agent with no trouble. Bite the bullet and use RTV, it will be simpler in the end.

[challenger]

I will be researching which product to use. I do need it to be somewhat hard but not necessarily as hard as the nylon I currently have. I also don't wish to spend more than I need to.

[gellfex]

I will be researching which product to use. I do need it to be somewhat hard but not necessarily as hard as the nylon I currently have. I also don't wish to spend more than I need to.

You should calculate your volume, PU resin ain't cheap. Polyester is much cheaper. Neither is very strong without filler or reinforcement. PEs impact resistance is low. But something like chopped glass reinforcement inhibits pouring. You could put a 'gel coat' of thixotropic (thickened) chopped glass in each side of the mold, let it cure, then put the halves together to pour the piece.

Crazy idea: gypsum cement like Hydrocal or Ultracal. Stuff is like stone. Might actually work for your honeycomb rollers.