Using Old Smooth-On OOMOO 30 Silicone Mold Making Rubber

Have some older pink Smooth-On OOMOO 30 mold making rubber that’s gone a bit, well, rubbery, but it still a bit liquid (like warm sticky taffy or silicone caulk)? Don’t throw it out!

Old OOMOO 30 Silicone Mold Making Rubber used to make an ugly mold in a lego box
Looks ugly, but it works! ๐Ÿ™‚

Well, okay, if you want to use it the way you’d use pourable fresh OOMOO, then definitely throw it out. If you are open to being a little more creative in your mold making and hate throwing things away (like me!)… read onwards. ๐Ÿ˜€

NOTE: This method has worked for me multiple times in the past, but is not guaranteed. You may end up having to clean unset OOMOO off your item.

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Mixing the Taffy– err, Old Silicone!

When you first get OOMOO 30 the pink and blue sides both mix and pour easily. After a while the pink side will start to harden and eventually you will end up with a chunk of pink rubber.

But before that, the pink will be sticky, thick, and a good workout to stir…. and you can still use it!

Old OOMOO 30 Pink thick like taffy or caulk still works
Pink OOMOO 30 taffy-style

First scrape out as much of the old pink (that’s still gooey) as you can. In my case an inch thick layer of pink had hardened on top, but underneath there was still usable material.

Now come the part where things get a little creative!

Since the pink has thickened I didn’t try weighing or measuring it like you would do with normal mixing. Instead I just added blue until it was approximately the same color as my existing molds.

Keep adding blue OOMOO 30 until it's the right color
Blue + Pink = Ready to mold!

I was tempted to ask the Smooth-On folks for help, but somehow I think making tutorials for using expired product isn’t something they’d encourage! ๐Ÿ˜‰

Mixing the blue and pink can be quite a bit of work. I broke one popsicle stick and I ended up using three layers of cups to make it sturdy enough to withstand the stirring. Just make sure you mix it thoroughly, the pink really resists combining at first, but the more blue that is added the easier it gets.

Making the Blobby Mold

Now that you have a cup of purple goo, it’s time to make a mold!

Only this is not going to pour. At all. In fact you are going to have to fight with it to get it out of the cup.

OOMOO 30 Moldmaking Lego Molds
Getting ready for the madness!

So we don’t pour, we paint! …Well, we use a brush to glob on the goo, which is sort of the same thing.

Grab a brush that you don’t plan on using for anything non-fluffy again. The OOMOO will get down in the base of the bristles and make it fan out, no matter how well you clean. I used the brush that I do my wood glue and plaster painting with.

Use a brush to dab on the thick OOMOO 30
Use a brush to dab on the thick OOMOO 30

My method of mold making (which I need to make a post on) has me using rubber bases to the molds that I superglue items to. You can use any base you like as long as the item is attached securely.

Painting on the goo will feel a lot like painting with thick peanut butter. You want to use the same pushing or tapping motion that you would with a stencil brush, so that you are forcing out any bubbles. Keep doing this until the object is covered with a nice layer of goo and you can’t see the sculpture material clearly. (Like the tail of the pony in the photo.)

Let that sit for just a minute or two. The goo will be slightly liquid and will settle out a bit. Letting it rest will help those air bubbles escape the thin-ish layer before you add the rest.

Gloop on the old OOMOO 30 mixture second layer
These are not pretty molds, but they work!

Then gloop on the rest of the mold material!

As noted before, the material will not want to gloop. Do your best to make sure the object is covered and that you place the material on top of the item. The goo will still settle out a bit (how much will depend on the thickness), but you need to make sure you’ve built up as much around the object as you can.

The cure time for the old material seems to be a little faster than the fresh stuff, but I still let it sit overnight (or all day) before I try and demold.

Plaster Mother Molds, GOGOGO!

So now you have a nice mold… that is an awful shape. Time to make a plaster mother mold to hold it level! ๐Ÿ™‚

Trim the mold so there aren’t any bits that would get trapped in a plaster cast. For the most part the gloop mold should be smooth and generally free of undercuts thanks to the oozing-by-gravity that it underwent.

Make a plaster mother mold to support the OOMOO 30 rough mold
Mother molds leveling ALL the things!

Then pick a container that is slightly larger than your item and plaster cast away! I use the Pink Square Divider Trays By Recollections that I bought for the new silicone caulk experiments because they are wider at the top than the bottom so the plaster comes out easily. (Post on that later!)

I used a tiny bit of rubber cement to hold the OOMOO mold in place, but you can use any temporary glue. I waited until the plaster was quite thick before I started pouring. When it’s more of a thick pudding than a liquid it minimizes seepage under the edges of the mold.

Make sure you make the cast deep enough so there is at least a half inch of plaster past the deepest part of the mold. Plaster can be fragile and you want to make sure it’s thick enough to hold up to use. You can also use the wood glue mixture to harden things, but I didn’t bother.

Making resin casts in your OOMOO 30 gloopy mold
And now it’s time for ponies…

Once the plaster is dry (or dry enough) pop it out of the container. Flip it over and then carve around the edges of the silicone mold until you can lift it free. You may have to sand the bottom of the cast to level the plaster, but otherwise you are good to go!

Have fun experimenting! And let me know if you discover and more tricks or tips on using old OOMOO 30 to make molds! ๐Ÿ˜€

Using Old OOMOO 30

Martha Bechtel

My name is Martha Bechtel and I write fantasy and science fiction stories, paint small model horses silly colors, cast resin and plaster magnets, code random code (and Wordpress plugins)... Come on in and join in the fun!

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