Making a one-part rubber mold and casting an object

Picture of Making a one-part rubber mold and casting an object

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.

Picture of Create a mold sized to your object.

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.

Picture of Tape and seal off the cardboard mold.

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.

Picture of Glue the original object down.

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.”

星蜗8 months ago

I think it can't too cute!

T_om8 months 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.

5454SteveF T_om8 months ago

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:

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.

Kinnishian T_om8 months ago

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_om8 months ago

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

blatantimage8 months ago

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.

Trevor548 months 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.

mayan cthulhu priest.jpg
chudman8 months 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)  chudman8 months ago

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

danny61148 months 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.

Triclaw8 months ago

mmmmmmmmm doughnuts

danzo321 Triclaw8 months ago

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.

danzo3218 months 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.

danzo3218 months ago

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

danzo3218 months 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!

danzo3218 months 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.

danzo3218 months 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.

danzo3218 months 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.

danzo3218 months 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.


Verga8 months 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.

jrs6238 months 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!
smledgerwood8 months ago
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!
doodlecraft8 months ago

Donuts! Love this! :)

Awesome. I would make tons of donuts! haha