Cheap molding material/pink foam?

[Risu]

I'm working on a prop that I'm going to build by stacking layers of wood or foam and carving/sculpting the final shape, then I want to make the piece hollow by making a cast copy. I don't have any silicone handy and I don't want to buy any, so what's the best low cost option? The piece is perfectly flat-backed, the mold would be one sided, and the piece doesn't have any kind of ridges that would require the mold to be flexible. The piece should theoretically pop right out if the mold is flipped over.

 

[robn1]

Re: Cheap molding material?

Plaster is a common molding material. Make sure the master has plenty of mold release. After molding, seal the plaster with car wax, and use more mold release when casting your part.

 

[Risu]

Re: Cheap molding material?

Will the plaster sold in cartons at Home Depot/etc work fine or do I have to have a specific type? Anything I should know ahead of time to prevent bubbles? With the car wax, do I just get a good coat of it and rub it into the plaster, or is there something in particular I have to do to seal it? And finally, what would make for a good mold release, I've only ever used this spray on stuff in my boss's shop that I can't remember the name of, but I'm pretty sure it came from Smooth On. I read somewhere that Vaseline does the trick, are there any drawbacks I should know about?

 

[robn1]

Re: Cheap molding material?

Will the plaster sold in cartons at Home Depot/etc work fine or do I have to have a specific type?

I don't know about that, I've only used the kind from the hobby store. Durham's Rock Hard Water Putty is also good, I'll be using that for a project soon myself. Home Depot should have it.

Anything I should know ahead of time to prevent bubbles?

Best advice is to mix it slowly to prevent bubbles from forming. Pour some on your master and work it into the details with a throw away brush, this will also help prevent bubbles. Then pour the rest. Sometimes if I get some bubbles, I'll mix a thick batch of plaster to fill them in.

With the car wax, do I just get a good coat of it and rub it into the plaster, or is there something in particular I have to do to seal it?

Any brand of wax will do, just make sure it's carnauba wax. Just apply it as you would on a car, wipe it on, allow to dry a few minutes then buff. This will give the plaster a good smooth finish and will help prevent your casting from sticking.

And finally, what would make for a good mold release...

The Smooth-On release would be fine. Vaseline works, thin it with water so it can be spread smoothly with a brush. If your master is very smooth, the wax may be enough, but I'd recommend using a release just to be sure.

 

[Risu]

Re: Cheap molding material?

I don't have access to the smooth on release anymore, so the vaseline will have to do. The good thing is that there's nothing on the piece that can catch. I know that mold release can change the surface texture of a pulled cast, am I correct in assuming that the vaseline/water mix will be more likely to do so than the pro stuff?

 

[EricHart]

Re: Cheap molding material?

Anything I should know ahead of time to prevent bubbles?

Start off with a small batch of plaster the consistency of heavy cream. Do a beauty coat by flicking, pouring or brushing on a thin layer over the entire surface of your model.
Immediately after, mix a thicker batch of plaster and pour it over top.
(If using your hands to manipulate the plaster, make sure you are wearing gloves. As plaster sets, it can heat up enough to burn skin).

Mixing your batch slowly and carefully will help keep air bubbles out. Mix from the bottom as much as possible to keep from stirring more air bubbles in. Before you pour your batch of plaster, bounce the container against the ground a little and tap the sides to help drive air bubbles to the surface.

 

[JBomm]

Re: Cheap molding material?

Just make sure you don't let any plaster go down your drains. You don't want a build up of that down there. Clean off in a bucket and throw it outside somewhere or drain it and put it in the trash. Temperature of the water and irritation changes your set time. The warmer the water, the faster your plaster will set up.

 

[Star Wars Chick]

Re: Cheap molding material?

I recently made a mold out of Bondo- pretty cheap and worked quite well. Used Vaseline as the release agent.

 

[tictoc]

Re: Cheap molding material?

If you choose the plaster from Home Depot, do not pick plaster of Paris. Choose the big bag of plaster called Dowman’s Fixitall. That plaster is stronger and about the same price.
When you mix up the Dowmans, use warm water because it mixes easier.
Follow the advice from The Prop Builders Molding and Casting book by Thurston James. Fill your mixing container with water, then slowly sift in the plaster. Let the plaster soak and fall to the bottom of the container. Sift in the plaster until it fills up the container and starts to collect at the top like an island, then you will know you have reached maximum capacity.
Wait about 3 minutes prior to mixing. Then mix slowly to avoid bubbles. If you follow James advice, the plaster works pretty well and is cheap. Remember, it will take a few days to weeks for your plaster to “dry” out before you can use it, especially if it is "curing" in a cold garage.
Vaseline works well as a mold release. I have used it. Someone on here made the suggestion to coat the mold and then use a hair dryer to warm the Vaseline. The Vaseline will lose the brush lines and make a more uniform/ smooth coat.
Good luck. Show us how things worked out.

 

[EricHart]

Re: Cheap molding material?

Ah yes, the dry time. While you can certainly remove a plaster piece from its mold after 20 minutes to 2 hours (varies widely due to differences in plaster-type, ration of water, humidity, etc.), it will still take 2 or more days to fully dry, even in a warm, dry room. You can speed that up with fans, heat lamps, or even regular lamps that give off a bit of heat. Just don't heat it too much or too fast; wait until after it's "cured" (a few hours after it's been poured). You're trying to simulate a hot day in the desert, not bake a cake.

 

[metrosonus]

Re: Cheap molding material?

so what's the difference between resin and plaster?

 

[kimncris]

Re: Cheap molding material?

plaster is basically stone - powdery little bits of ground up stone, mixed with water it hardens to a material like concrete. Resin is a two part polymer that when mixed hardens to a material like plastic.

 

[metrosonus]

Re: Cheap molding material?

So there's no real difference then and it comes down to preference?

Ah see....

 

[IronCostumer]

Re: Cheap molding material?

I figured this was sort of on the same topic and didn't want to post a new whole thread for this question. Is there cheaper plastic alternative to smooth cast roto or 300? I'm making something that is rounded and fairly thin, so I checked out those two products but they are way to expensive for me. Anyone know of any cheaper alternatives?

 

[exoray]

Re: Cheap molding material?

I figured this was sort of on the same topic and didn't want to post a new whole thread for this question. Is there cheaper plastic alternative to smooth cast roto or 300? I'm making something that is rounded and fairly thin, so I checked out those two products but they are way to expensive for me. Anyone know of any cheaper alternatives?

Any comparable quality resin will be comparably priced for the most part, you will only be penny pinching at the end of the day... There are 'cheap' resins but you almost always get what you pay for, bottom of the basement cheap resins are bottom of the basement quality... The only real way to save money on resin is purchase in bulk, 10 gallon kits or the 110 gallon kits...

With silicones it's another story, there are some great 'lower' cost silicones out there to be found that work very well... Again bulk purchasing will leverage much better prices...

If you have a local Hobby Lobby, take their almost every week 40% off coupon ( Weekly In-Store Specials : Hobby Lobby - Hobby Lobby ) it's only 25% this week... But anyway in the craft isle by the model parts you can get small packages of Alumilite resin, most stores carry the white and natural colored resin, as well as a starter kit with silicone... If you head over to the 'clay' isle you can find even smaller packages of 'Amazing Resin' it's Alumilites bargain line...

 

[Risu]

I've hit a hitch so I've got a new question. I've been to both Home Depot and Lowe's and neither of them carries the pink insulation foam I was planning to use for my build any more. Where can I still get the stuff?

 

[exoray]

I've hit a hitch so I've got a new question. I've been to both Home Depot and Lowe's and neither of them carries the pink insulation foam I was planning to use for my build any more. Where can I still get the stuff?

Do they carry the light blue variety? Same stuff different manufacture... They almost certainly have to carry it locally, as it's very popular now (code in many places) as an insulator for foundation walls prior to back filling...

 

[EricHart]

Try local lumber stores. If they don't have the pink foam, they should have the blue foam; as far as carving is concerned, the only difference between the two may be the price.

 

[Risu]

Nope, the blue either, all they have is this white stuff that REALLY doesn't carve well.

EDIT: Well this is just frustrating, Home Depot has it on their website, but not in store.

 

[sabergirl]

I've found that many stores in California don't carry insulation foam at all, because there isn't much call for it in construction due to the weather being so warm.