Hard Plastic For Molding Masters

[alru19]

I am looking to make multi-cavity 2-piece pop mold, so I'm going to need multiple masters. I have seen that most people just make the multi-cavity molds by using plastisol baits as the masters, but I would like for a more rigid material that I can sand the seams off of. Is there any hard plastic that I can pour into a pop mold that will give me a rigid model that I can then sand and use to make more molds? Thanks.

[MEckl8321]

I am looking to make multi-cavity 2-piece pop mold, so I'm going to need multiple masters. I have seen that most people just make the multi-cavity molds by using plastisol baits as the masters, but I would like for a more rigid material that I can sand the seams off of. Is there any hard plastic that I can pour into a pop mold that will give me a rigid model that I can then sand and use to make more molds? Thanks.

You might try Alumilite two part resin. I've had fair success making masters from RTV Molds. Have yet to try it with POP, though.

Alumilite white resin

[Vodkaman]

If you try to make a hard casting from PoP, you risk destroying your mold, by the cast getting locked in.

Either cast soft plastic and spend some time with a hot knife, to dress the seams up, or use RTV to make a mold to cast your hard masters.

Dave

[MEckl8321]

If you try to make a hard casting from PoP, you risk destroying your mold, by the cast getting locked in.

Either cast soft plastic and spend some time with a hot knife, to dress the seams up, or use RTV to make a mold to cast your hard masters.

Dave

Dave,

I agree it may be dangerous to try to make a resin master of the POP mold, but would a release agent sprayed/brushed onto the POP help at all?

[Vodkaman]

Dave,

I agree it may be dangerous to try to make a resin master of the POP mold, but would a release agent sprayed/brushed onto the POP help at all?

It is possible that you could get away with it, depending on the master shape, depth etc. Also, if the resin shrinks at all, this would be in your favor. But having destroyed a mold a few years ago, that I had put a lot of work into, I am very wary about trying that again. If I could easily replace the mold, then I would try it, for fun. But if the mold has value in time invested or cannot easily be replaced, then I don't think it is worth the risk.

I often make PoP molds from hard masters and they are hard enough to retrieve. This is just my opinion and thought. If someone can offer you more hope of success, I hope they jump in here.

Dave

[wchilton]

I've got some ideas for you but have never tried them myself with POP (only with silicone).

First thing I'd consider is paraffin wax. It should not stick to your POP mold and if anything goes wrong you can warm the whole mold up to get the paraffin out. The wax is more flexible when warm so that would also help in de-molding. You couldn't really sand the masters but would have to cut or scrape with a pen knife to remove material. There are "casting" waxes...I've tried them and they are harder when cool but more difficult to work (doesn't get as flexible as paraffin) because of the more definite melting point.

The material I've used for silicone mold masters and like is hot melt adhesive. You'd have to MAKE SURE it doesn't stick to your POP mold with a little testing and I would use a release agent. There are hot melt "sticks" that are low-tack so they don't stick so strongly. Again, if you can't get the master out of the mold you can resort to heating it to soften the hot melt.

One material I haven't seen talked about too much here is modeling clay. Modeling clay is basically just clay, paraffin, and oil. It can be melted and poured just like wax but it doesn't get quite as hard when cool. My problem with modeling clay is just that it is so soft I find it takes too much patience (for me) to work with the stuff.

There is a low-melting-point metal called cerro-safe that gunsmiths use. It melts below 200 deg F and it has a unique property of shrinking just a little bit as it cools and then expanding back to original size when fully cooled. It would be more expensive for you but would certainly give you a "hard" material to use as a master and again you could heat the mold to remove it if there were any problem. If your original is smooth and has no under-cuts this might work. I got some from Brownells several years ago, looks like prices have gone up quite a bit on it since then.

Last thing is I've been using a casting resin from AeroMarine that I really like working with. I think it's a urethane resin. It's used for making cast objects and sets up to a sort of creme/white color. Anyway, what I've noticed is that when it first sets up it is still somewhat soft and can be bent/shaped just a bit. That might be just flexible enough to allow you to de-mold without problems. Downside is if it doesn't come out easily you'd have a real problem on your hands. If I were gonna try this I'd probably make a soft-bait "master" and then a test mold that I could try things out with without risking my original mold.

Good Luck.

Warren

[alru19]

Thanks gents.

My original master is wood, and a very simple design. It's lathe-turned, so my 2-sided pop molds don't have any undercuts and removal shouldn't be a problem. But I have already made several pop molds of this wooden master, and don't care about ruining the plaster molds if infact they get lodged in there. I have decided to give the urethane resin route a try. I assume I will need to seal the plaster and use mold release so this resin doesn't stick to the mold? I plan on sealing the molds with elmers like I do for the plastisol, but don't know about mold release. Thanks again.

[Vodkaman]

Great post Warren. All good suggestions.

alru19 - Please report back on this one. A lot of people will be interested in your results, including me. Pictures would be a bonus, but understand if you do not want to.

Dave

[wchilton]

I would definitely use a mold release of some kind. Don't know what to recommend but probably PAM or one of the other standby's mentioned on the forum would be fine. One thing you'll have to think about is that the casting resins are VERY THIN until they set. The one I've used is almost water-thin. You may have a problem with resin running out the edges of a 2-part mold and you also will need to make sure and put release on the mold faces (not just the cavity) to prevent resin from gluing the faces together. Maybe just a little bit of modeling clay around the edges of the cavity would help control that "leakage". You could also try a grease or petroleum jelly as a sort of sealant. You could also try to pour a plastisol (melted soft plastic) "gasket" around the cavity and maybe have less of a mess...just a few thoughts.

If you can get the urethane casting resin to work I think you'll like how it behaves once it has hardened. It's easy to shape by sanding, very much like wood, in fact, but with no grain to have to deal with. The stuff I use is a bit oily when I first demold so I've wondered if it has some sort of release agent "built in". When you first remove from the mold and it is a bit flexible it can be cut with a sharp pen knife so that's also an opportune time to do some trimming, might save you some work.

Good luck and let us know how it works out.

Warren

Edited April 19, 2011 by wchilton

[alru19]

Alright, I gave it a shot, but never was successful. Here is my list of how the polyurethane resin reacted to the plaster. Unsealad plaster:Poly adhered. Plaster sealed with Elmers: Poly adhered. Wet unsealed plaster: Poly removed easily but it had a bubbly surface. Unsealed plaster with vaseline: Bubbly poly surface. Sealed plaster with vaseline: Bubbly poly surface. Sealed plaster with pam: Bubbly poly surface. Plaster sealed with paint: Closest to success, but the paint came off on the poly. I ended up just doing what vodkaman said first and made a silicone mold. I am sure there is a way to do this successfully in plaster, but it definitely involves sealing the plaster so no bubbles get into the poly. Thanks all for the help.

[Vodkaman]

Alright, I gave it a shot, but never was successful. Here is my list of how the polyurethane resin reacted to the plaster. Unsealad plaster:Poly adhered. Plaster sealed with Elmers: Poly adhered. Wet unsealed plaster: Poly removed easily but it had a bubbly surface. Unsealed plaster with vaseline: Bubbly poly surface. Sealed plaster with vaseline: Bubbly poly surface. Sealed plaster with pam: Bubbly poly surface. Plaster sealed with paint: Closest to success, but the paint came off on the poly. I ended up just doing what vodkaman said first and made a silicone mold. I am sure there is a way to do this successfully in plaster, but it definitely involves sealing the plaster so no bubbles get into the poly. Thanks all for the help.

Good effort and thanks for reporting back.

What ever you try, the plaster always has to be sealed. I notice that you didn't try thinned D2T epoxy, quite a few PoP users seal with this. All the bubbling that you mentioned, is air in the plaster expanding, due to the exothermic reaction of the resin. Correct sealing will prevent this.

You did not mention having any trouble retrieving your casts from the mold, if this true? It makes sense, as resin does have some shrinkage. If so, I may give PoP a go myself, as I cannot find RTV locally and I have a need to do some casting in the near future.

Dave

[Marks Lures]

Sealing a POP mold

What about the old fashioned way of hand wax that is used for furniture?

I have dried out POP molds and used thinned downed shillac (or clear laquer) to soak in and hand waxed.

POP is cheap was not going to use expensive epoxy to seal up the mold.

Also used thinned down laquer in the boat seats I made for my boat that I had 15 years, the coated wood was still in good shape when I sold it 5 years ago.

Mark