Rubber for molds

Got a question about techniques, materials or other aspects of physically building a model? This is the place to ask.

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Rubber for molds

Post by TER-OR » Thu Jan 05, 2006 11:19 pm

RTV, baby.
Sticky is not a good thing in a mold.
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Post by TER-OR » Thu Jan 05, 2006 11:20 pm

I've used OOMOO 30 from Smooth-on. For hobby use, I find it works quite well. It's a bit soft, though, so be careful.
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Post by modelnutz » Fri Jan 06, 2006 5:11 pm

Used to use GI-1000
Changed over to Smooth-On Mold Max-30 and 40
Much better pricing. Works pissa ( that's masshole for real good) :D
I'm hooked and will never go back!

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Post by Sparky » Fri Jan 06, 2006 5:34 pm

I've used the Oomoo too. Good starter but you need to degas to pressure cast, or if you have a lot of little surface details. It mixes thick, but mixes by volume (1:1)

I now use Moldmax 30. I used a chef mate scale to measure it out (mixes by weight) in the beginning and never had any problems. You can add a bit more of the watery catalyst just to be sure (you won't have enough for the whole bottle of RTV though).

I always degas with this stuff. But it is much runnier when mixed so one might get it to work on fine detail surfaces with out a vacuum pot. I used it to replace the seal in my pressure/vacuum pot. The seal it replaced was an oomoo replacement seal for the rubber ring the pot came with. The oomoo seal disintegrated over time, and the thousands of micro bubbles can be seen in the tares. The Moldmax seems have made a better seal. I cannot degas the lid since it is the lid for the pot, so I degas the rubber then carefully pour it into the pots lid. This is awkward to the least but the Moldmax lets the bubbles rise out easily making firm rubber seal.
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Post by Umi_Ryuzuki » Sun Jan 08, 2006 11:59 am

Tap Plastic products are an off the shelf thing in my area.
Good for RTV

http://www.tapplastics.com/


They keep a simple guide on line also.
http://www.tapplastics.com/moldmaking_guide/index.php?

(I do not recommend their clearcast product.)
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Post by macfrank » Tue Jan 17, 2006 12:45 pm

I've used the Alumilite Quickset RTV, and I think it works well. Fairly low viscosity, very forgiving component mix ratio (it's 10:1, I do it by volume) good cure time (about 4 hours) and I've never had bubbles trapped in it.

Are all 10:1 mix RTVs pretty much the same (like the Alumilite)? I'd like to find an equivalent that may be eaier to buy/cheaper.

I've tried the OOoommmooo stuff, and while the 1:1 mix is easy, it also tends to trap bubbles.

Frank

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Post by Less Than Super Ostrich » Wed Jan 18, 2006 2:42 pm

That little tub of Alumite cost me $26! I gotta find me a cheaper place to find RTV.
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Post by Sparky » Wed Jan 18, 2006 2:46 pm

I think the Alumite is the stuff Neil uses. Most of the molds he makes I was able to pressure cast without damaging the mold. Some have small air bubbles near complex surfaces of the part, causing little balls of resin to form under pressure. But they come off easily with a sharp #11.

That stuff seems to mix with a low enough viscousity to pour without vacuuming! And molds do fine for the open face stuff we use, good shore hardness too (lets you use less rubber just to keep the mold's form)
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Post by Less Than Super Ostrich » Wed Jan 18, 2006 4:44 pm

Maybe that's why its not lasting long. I probably make the molds too thick!
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Post by Less Than Super Ostrich » Wed Jan 18, 2006 4:52 pm

Here's a good mold making article. Very different than what I'm used to doing:


http://www.jamesrogersstudio.com/molds.html
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Post by Sparky » Wed Jan 18, 2006 6:45 pm

If the mold it self isn't lasting long it's the 15 minute resin they sale with it. It bakes your mold out real quick. One meeting Neil showed us a new mold he had just finished pouring. The next meeting he gave it to me to try pressure casting in it, it was already yellowed as if it was 2 years old. Yikes!

You can also use styrene or acrylic backings so you don't need as thick a mold. This is the idea of a 'mother' for a mold. Some other (cheep) material is added on top to keep the mold from distorting. The rubber serves to reproduce the surface detail, the mother holds the over all shape.

Don't get me wrong, this in-and-of-itself, is a tricky thing to master using.
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Post by Less Than Super Ostrich » Wed Jan 18, 2006 11:31 pm

Molds are lasting fine. I was just talking about how much it cost for the RTV silicone rubber. The stuff gives me three small molds before its gone.

By the way, here is another tutorial on Casting:

http://www.childofmecha.com/Modeling/Re ... y/Clay.htm
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Post by Sparky » Thu Jan 19, 2006 12:02 pm

I didn't know Kleen clay was microwave safe, I've just been putting it in a big zip lock and floating it in hot water to soften it. Microwaving would be much easier and less wet.

Thanks for the link!
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Post by DasPhule » Thu Jan 19, 2006 12:57 pm

It's safe, but if it has a pocket of air in it and you leave it too long, boom, clay coated microwave. I use a heat gun to warm it up.

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Post by Less Than Super Ostrich » Fri Jan 27, 2006 11:06 am

Does anyone have an opinion on ordering RTV Moldmaking rubber from these guys? Looks like its the best deal, I've seen for large amounts of RTV.

http://www.shopmaninc.com/moldmaking.html
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Post by DasPhule » Fri Jan 27, 2006 7:05 pm

That's not silicone, it's polyurethane. You could use it, but the molds would have to have a good layer of releaser on them to use urethane resin in them. If the resin touches the mold without releaser, it'll be stuck to it forever. I use polyurethane rubber for my pot seals, and even with releaser on them resin still gets stuck to them over time.

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Post by Dr. Yo » Tue Feb 28, 2006 1:54 pm

On a slightly related tangent, has naybody used STONER mold release,
and is it compatible with OoMoo rubber?
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Post by MillenniumFalsehood » Wed Mar 01, 2006 3:13 pm

I'm making a 2-part mold(cope and drag) and need to know what to use to keep the RTV from fusing itself to the other half of the mold box. I'm using OOMOO. I'v tried to use oil but it just beads up on the surface and the rubber still sticks to itself. :?
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Post by Sparky » Wed Mar 01, 2006 3:48 pm

This from DasPhule, and I use it all the time:

Paint thinner mixed into vaseline. Just enough so it spreads thin with a brush. Don't wait to long after applying it to make the pour. If you do re-apply it and wait a few minutes for the thinner to evaporate then pour.

I keep a pre mixed jar around for ease of molding (that vial on the work bench is not what you think it is).
Last edited by Sparky on Wed Mar 01, 2006 3:54 pm, edited 2 times in total.
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Post by Dr. Yo » Wed Mar 01, 2006 3:50 pm

That I can answer :) ! Smooth-On makes a variety of mold
releases-the Ease-Release line is what came with my starter set, and it does very nicely keeping the ooMoo from sticking to itself. Here's the
catalog pagefor those products.

http://www.smooth-on.com/releases.asp
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Post by modelnutz » Wed Mar 01, 2006 4:58 pm

Smooth-On's ease release 800 for silicone to silicone works very well.
Follow directions and it works quite well.
I've also used the vasalene cut with mineral spirits.

Modelnutz 8)

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Post by MillenniumFalsehood » Wed Mar 01, 2006 6:22 pm

Since I have the vaseline on hand, I think I'll try that first.
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Post by zaphod » Sat Apr 15, 2006 5:01 am

What mold material do you guys recommend for casting metal? I am considering the lead-free alloys from Micromark.
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Post by DasPhule » Sat Apr 15, 2006 9:54 am

Vaseline thinned with Naptha, not paint thinner. Naptha evaporates much faster. Still, I suggest the Mann's 800, it doesn't dry out the molds lke the vaseline/naptha mix does.

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Post by Sparky » Fri Apr 21, 2006 7:21 am

I just got on the casting group at yahoo. There's a model railroader who's just put together a vacuum chamber. I'll have to follow his updates to see if it breaks down on him, here's his buildup page:

http://www.housatonicrr.com/VacChamber.htm

Looks nice so far I like what he uses for the seal on the lid/bottom. The one I made has started to become a hassle.
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Post by Dr. Snuts, M.D. » Thu Jun 15, 2006 8:06 pm

I'm new to casting. Can anyone give me a run down of the cheapest yet most effective supplies? I don't want to go all out, just experiment on a 9" model I made. If youguys could tell me what the basics i need are, where to get em and a good price, I would sincerely appreicate it.

Thanks in advance,

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Post by Sparky » Thu Jun 15, 2006 10:39 pm

You probably want to avoid getting into the vacuum and pressure setup. If you are going to pressure cast you will need to vacuum degass the ruuber, any pocket of air will collapse under pressure and then re-expand after you release the pressure, repeatly doing this kille dmy omoo 25 molds.

I woudl suggest starting with the moldmax series, the moldmax 30 is has a much lower viscousity (just after mixing) than the omoo, I have heard of folks using a chanking table to viberate out hte bubbles.

You can measure the moldmax 30 by weight with a chef mate scale (from target), when in doubt add a little more of the red catalyst (its the watery liquid). You will run out before using all your RTV but the rubber will cure for sure. Or if your model is big enough you will probably use all of a sample pack to cover it, and you can mix it all up.
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Post by Sparky » Thu Jul 06, 2006 4:50 pm

thought I'd throw up the links to the 'bombs-away' method to degassing from TAP plastics, shinny how to.

http://www.tapplastics.com/uploads/pdf/ ... ctions.pdf
and here's a link to part 2 of the how to video from tap plastics, it shows how even when mixing by hand you end up with air in the RTV, it shows vacuum degassing and the bombs-away method of degassing:
http://www.tapplastics.com/info/video_d ... quicktime&

Here's all the how to videos shinny (thanks to Less Than Super Ostrich for the linky)
http://www.tapplastics.com/info/video.php
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Post by macfrank » Wed Aug 02, 2006 2:36 pm

I made a stupid mistake yesterday that made me think of the "ideal" mold material.

First, I made the lower half of the mold with Oomoo. Came out great! So I remove the Kleen Clay, flip it over, build up a Lego dam, mix up another bath of Oomoo and start pooring.

It isn't until I finished the pour that I realized that I'd forgotten to paint mold release on the cured half! Doh!

I thought I could just cut the cured halves apart, but the masters had different heights and it just didn't come out right.

This got me thinking of the "ideal" mold material:

1 - It would have a fairly low viscosity. Closer to the Alumilite mold material than to Oomoo; this would greatly reduce trapped bubbles.
2 - The cured material would not stick to itself or to uncured material.
3- It would be a 1:1 mix by either volume or weight - not particularly picky (more like Oomoo than the 10:1 Alumilite) Or even a single liquid that magically solidifies.
4 - The cured material could be turned back into a liquid by some method (heat, cold or some safe chemical) and reused.
5 - It would be about the same price as existing hobbyist materials.
6- It would provide excellent reproduction of fine details.

Silly Putty has some of these features, except silly putty never really firms up (un less you cool it) and it really likes to stick to itself.

One can dream...

Frank

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Post by unitruth » Sun Oct 08, 2006 11:19 pm

I use latex .....a few applications...let dry each time...the detail is great.(hobby lobby)then for sturdiness and life...I apply silicon from a tube that is used for glass blocks(home depot)the silicon from the tube sticks to anything and remains flexible forever.I have made many a mold with this double medium.this takes time...but the molds last .
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