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Making a one-part rubber mold and casting an object

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Picture of Making a one-part rubber mold and casting an object
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This Instructable is connected to Arlene Shechet’s exhibition Meissen Recast currently on view at the RISD Museum.

Meissen Recast plays with the idea of molds, the casings that shape an object’s final form. Shechet reinterprets plaster molds by casting them in porcelain and calling attention to their usually unseen details. The process of mold-making and casting invites creative experimentation, but follows rules of precise timing and size.

Taking us through the steps involved in casting objects, Marisa Marofske (RISD BFA Painting, 2014) uses the form of a dog toy to create an array of iconic, colorful donuts.

Read the full article on our website and explore other objects made with molds on Pinterest.

 
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Step 1: Create a mold sized to your object.

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This could be an elaborate form made out of acrylic, but for some materials it works just as well to create a mold out of cardboard.

“I started by tracing out the shape of the object. You want the walls of the mold to be [higher than the original object]. You would want to score, and then obviously you’d want to cut.

And then what happens is this becomes a really simple box.”

Step 2: Tape and seal off the cardboard mold.

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It’s essential that none of the material escapes the box.

“You can just use any kind of masking tape, anything. You’re not going to have any leaks, which is enemy number one in casting. You could [also] use packing tape on the inside.”

Step 3: Glue the original object down.

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This way, the object won’t rise when the casting material is poured around it.

“Since the bottom is flat like this, the way I’m going to do it is glue it down flat on the surface of the mold itself and fill it above. And then [the flat bottom] will become the top of the mold.

You just want to make sure you have a decent amount of rubber between the object part of the cast and the top of the mold.”

Step 4: Carefully prepare your casting material.

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Marofske chose Dragon Skin, Medium Set, a kind of silicone rubber. Both parts are individually mixed in a ratio of 1:1, then carefully mixed together. For this part, it’s important to wear gloves and mix in a well-ventilated area. The tools you use also matter—wooden paint stirrers work better than makeshift stir sticks. The rule of thumb: mix well, but carefully.

“So you have Part A and Part B, it’s one to one. The thing that’s great about Dragon Skin is it’s super stretchy. When you’re casting, you want to do soft mold for hard object, hard mold for soft object.

The stuff we got, the medium set, the pot life is just around 8 minutes, so that means you only have 8 minutes before it starts setting up. So this takes about 5 hours, between 5 and 7 hours to fully set up, but you, it’s I think worth it for the extra time you have in mixing it, because you want to get all the bubbles out, so it’s nice to not be racing the clock.

Having the right mixing tools makes your life a lot easier. And then when you’re mixing them, you want to use separate stirrers, just cause if you mix one and then you’re stir the other one, it’s going to start to set.

So again I’m just going to stir this and make sure it’s mixed. So you can see some bubbles have risen to the top. It’s impossible not to have some.”

Step 5: Pour the mixture into the mold.

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Start pouring in the corner, and as the material fills up the mold, make sure it covers the entire object. Because the dog toy is rubber, Marofske didn’t spray it with a mold-release agent—she felt sure the mold material would separate from the rubber without sticking. For other materials, it’s best to cover the object with mold-release spray.

Step 6: Tap the mold to help bubbles rise to the surface.

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“You can just kind of tap the sides, to get some of the bubbles. You can even pick it up and tap it. But you can see this is totally fine, nothing’s going to seep out. And then, since it’s a waste mold, we’ll just rip it off and it’ll go right into the trash.”

Step 7: After the mold has fully set, remove the cardboard waste mold and discard.

Picture of After the mold has fully set, remove the cardboard waste mold and discard.

“It looks like it did stick a little bit, but it should be fine. If I were going to do it again I might just put tape on the inside of the box.”

Q: “Could you use Vaseline?”

“Yeah, you could, Vaseline is a pretty common casting material. It’s just horrible. I hate Vaseline so much. But if you’re casting plaster into plaster you have to use Vaseline. It acts as a resist.”

Step 8: Remove the dog toy from the Dragon Skin material.

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Some of this had crept over the top, so Marofske used a knife to cut a clean line around the open bottom of the mold. “And now I’m just going to trim this, just so when you pour the casting it doesn’t dip into the material.”

Step 9: Spray the inside with mold release, and mix the casting material.

Picture of Spray the inside with mold release, and mix the casting material.

“And since I’ve done so many of these, I actually know the exact amount to mix, but one way you can do it if you wanted was you could fill it with rice or water and then pour it into a thing with volume, so you could find out how much you needed. I know that it’s just around five ounces, which is the volume of . . . the object itself.

And this works just like the rubber. So you’re going to want to stir them individually before combining them. And again, try to not add any air into the liquid.”

Step 10: Mix the casting material and pour into the mold.

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We are using Smooth-Cast 300. The mixture needs to poured soon after its parts are combined, before it begins to harden.

“So I’m going to stir them together and almost immediately pour them into the mold. And then you’ll see that they’re both pretty transparent but it’ll cloud up and become opaque white. And it also gets pretty hot when it happens because it’s a crazy chemical reaction.

And then you just tap the sides to get some of the bubbles from pouring it to come to the surface.”

“It takes about five minutes. This one is the fast-setting, so there’s medium set for this kind of stuff too. But since it’s such a small cast you don’t really need the time. Sometimes you’ll even see the vapor coming off it once it starts reacting, so you definitely want to do this is a well-ventilated area. It almost looks like it’s smoking or something, but it’s subtle. Again, you don’t need a respirator, but you don’t want your face in it.”

Step 11: Pop the plaster object out of the mold.

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Step 12: Things to keep in mind.

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Finally, we asked Marofske about things to keep in mind when casting and making molds. Below are her top five considerations for the process.

1. Mix well, but carefully.

2. Having the right mixing tools makes things a lot easier. For example, it makes a difference to buy paint stirrers. You can just use anything, obviously, but this makes it super simple.

3. Try not to get it on you because it’s horrible and gross.

4. A good way to think about casting is to choose your casting method based on the object, not the other way around. So if you wanted to cast a rubber duckie, for example, you’d probably do that in plaster. But soft molds are always easier. The thing is if, this doesn’t require another mold, because it’s not big enough and there’s also just enough space around it for it to hold its shape.

5. Also I think the mantra of casting is: expect to fail. Because it happens. It’ll leak out the side and then that’s it.

星蜗1 year ago

I think it can't too cute!

T_om1 year ago

The stuff from Smooth-On is wonderful... but extremely expensive.

A good hone-made casting silicone is a mix of regular silicone sealant (the "Regular", not "Odorless"... get the one that smells like vinegar) Get a box full of the tubes for what Smooth-On would charge for just a "trial kit".

Mix one part silicone sealant to one part corn starch. Mix well and press/pour around whatever you want to mold.

You can also thin silicone sealant with tetrahydrofuran to make it brushable. This allows you to build up layers of rubber similarly to how latex molds are made. It has to be pure silicone to work.

What function does the cornstarch perform?

the non odorless silicone he is referring to is moisture activated cornstarch pulls moisture out of the air and cures the silicone faster. think of the starch as a catalyst.

However, you can't use cornstarch right out of the box, you need to let it absorb the moisture first. Here's the Instructable on Oogoo:

http://www.instructables.com/id/How-To-Make-Your-O...

maybe if you live in a very dry environment I live in Washington and i have used it straight out of the box to make Oogoo with perfect results every time.

Hmm... Thinking back, I think the cornstarch I had came in a moisture-tight package. Maybe that was the difference. I live in Northern California where it's somewhere middle of the road in terms of humidity. I just put some in a ziplock and left it open for a few days.

I buy store generic cornstarch it usually comes in some kind of paper pouch in the box. Some times it is wax paper but even that has never caused issue but i would assume if it was packaged in a plastic bag that it would need a day or so to pull moisture out of the air. you could also put it in a box with a cup of water that you brought to the boiling point.

At the ratios we're talking about it also has a material function, so it's part catalyst part filler. Different ratios will result in a different end product.

Basically "Oogoo Sugru Substitute" so consider to look into those instructables for more info on mix ratios and user experience.

A word to the wise Oogoo isn't a perfect sugru substitute. Do Not use it on anything that will have long contact with the skin. Even after 4 weeks to cure the ear buds i tested caused irritation with just short term use.

risdmuseum (author)  T_om1 year ago

Fascinating! We would love to see you demo the process.

Sorry, but not impressed by this one. It's like a documentary on someone who has never cast before, announcing quoted play-by-play of some artist we are supposed to know. The art show has little relevance, and spending 3 minutes browsing the PRF will yield better instructions on real-world mold/casting techniques.

Trevor541 year ago

I make little plaster statuettes. I find that I can get good results by using liquid latex with a thickening agent. Ideally paint on a layer of unthickened latex first to capture fine detail, then cover with the thickened latex. You can leave the first layer to cure slightly, and then paint another couple. You can leave this to cure for the next couple of days [I'm very patient], or cure it in an oven on a low heat. Watch out for pockets of uncured latex, so give it time. I then normally make a two-part casing from plaster bandage. Remove the mould from the master, and fill with plaster. This method is quite slow, and unsuitable for resin casts, but cheap and effective. The downside is that latex tends to shrink over time, and go a little stiff, but I've got moulds which are about 20 years old and still serviceable.

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chudman1 year ago

Plaster object? the above title says plaster when in the last step you used a two part mix of something other than plaster. Can you use plaster?

risdmuseum (author)  chudman1 year ago

For the demo we used a Smooth-On product, but plaster works fine! The donuts in the first image are made of plaster.

danny61141 year ago

So, is the finished product safe for dogs? My malamute destroys chew toys and the possibility of him ingesting some of this is concerning.

Triclaw1 year ago

mmmmmmmmm doughnuts

I once made a mold of a real donut! We were making funny trophies for the motorcycle squad which escorted the crew when they made "The Firm" here.

danzo3211 year ago

"Expect to fail" ??? Hey! Expect to very faithfully carry out every step !!! and you'll never fail !!

If you see liquid rubber leaking, you grab some plasticine clay which you always keep handy and cram it to seal the leak.

I am sorry but this is not well written, and clearly by someone who has barely learned the process.

danzo3211 year ago

OK - how did SmoothCast 300 urethane become plaster?? Sure it's white and hard.. Plaster costs almost nothing; resin is far from free.

danzo3211 year ago

So she's going for a hard, impact-resistant urethane plastic cast. To make more dog toys you would choose a tough urethane rubber. Want a chocolate donut? Melt chocolate.. To evaluate the danger of liquid components and vapors, READ the Material Safety data sheets!

danzo3211 year ago

Vaseline, especially thinned with petroleum distillates, is the recommended release when making a silicone mold.

When casting plaster in plaster, oil soap solution is the preferred release agent.

danzo3211 year ago

"Waste mold" has a different meaning altogether. You are making a silicone mold which will last a long time - NOT a waste mold. Your cardboard form is what you're going to throw away. You only do that if your mold is stiff enough to keep its shape even when filled with casting material. If you made a reusable mother mold (surrounds the silicone) you could use much less silicone.

Tapping frees the air bubbles to rise to the surface. I would raise it a few inches and let it drop.

danzo3211 year ago

Mold-release sprays may contain silicone, or wax, or grease. You don't spray with silicone if mold will be poured of silicone, as it makes the rubber lock into the pattern. have seen no mention of what the casting material will be but that should be talked about early.

danzo3211 year ago

<you want to do soft mold for hard object, hard mold for soft object.> You choose molding rubber by a few rules, and you choose a soft rubber if the pattern [original object] has difficult undercuts. You also think about the casting medium, and how it might deform your mold.

RISD 73

Verga1 year ago

Thank you for this Ible. My wife had a statue from her brother that got broken when we moved. This just might be the way to repair the damage by making a whole new statue. Good photos and clear instructions made it very easy to follow and giving the link to the website makes it even better.

jrs6231 year ago
Isn't this one done in resin? Smooth Cast is a plastic, not plaster.

smeldgerwood, Clear resin molds however can take expensive materials including sometimes a vacuum/pressure chamber. Sometimes it depends on the material's viscosity.

Over all though, good tutorial!
Thanks for the tutorial. Have you done any resin castings? A transparent blue donut would be cool. Or you could marbleize it by pouring two differently colored transparent resins!

Donuts! Love this! :)

Awesome. I would make tons of donuts! haha