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  #1  
Old 12-28-2003, 12:50 PM
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rickb rickb is offline
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Inexpensive Silicone Molds w/ RTV?

Hello out there.

If anyone has experience or information using using silicone RTV from tubes (like from Home Depot) to make molds I'd appreciate hearing it. I'm especially interested in drawbacks that may not be obvious -- like shrinkage, aging, strength, etc.

In Philadelphia this summer, I met some sculptors who used 100% Silicone rubber caulking (thinned with xylene if required) as a cheaper alternative to other specialty mold-making silicones (like Rhodia -- which is >$100/gallon).

I just tested it on a few small samples (oil-based clay). Besides taking days to cure, it's pretty convenient (no precision mixing or degassing), seems to pick up detail OK, and required no mold release.

Am I missing something? Or if I can tolerate the long cure time is this a good alternative at about 1/3 the cost?

Thank you very much,

Rick


http://www.richardbecker.com
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  #2  
Old 12-29-2003, 11:31 PM
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Re: Inexpensive Silicone Molds w/ RTV?

Quote:
Originally Posted by rickb
Hello out there.

If anyone has experience or information using using silicone RTV from tubes (like from Home Depot) to make molds I'd appreciate hearing it. I'm especially interested in drawbacks that may not be obvious -- like shrinkage, aging, strength, etc.

....... etc.

Am I missing something? Or if I can tolerate the long cure time is this a good alternative at about 1/3 the cost?

Thank you very much,

Rick


On the suggestion of a local sculptor just beginning his own foundry, who cast a couple of early pieces for me, I used something like that to cast in plaster an oil-clay figure a young friend did, for his mother. This guy said these molds were OK for maybe a couple of casts. I found detail good. My material set up quickly, but was quite thin. Maybe thatís the difference between what I did and what you did.

I made a plaster mother mold and had no trouble, except that it would have been easier to have a thicker rubber.

Last edited by fritchie : 12-29-2003 at 11:35 PM.
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  #3  
Old 12-30-2003, 06:48 AM
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Re: Inexpensive Silicone Molds w/ RTV?

As this type of silicone uses air humidity to cure, you can always stir a few drops of water in to shorten the cure time. Some people use acrylic paint (?) as a catalyst because it gives a visual feedback about the mixture.
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  #4  
Old 01-01-2004, 02:11 PM
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Re: Inexpensive Silicone Molds w/ RTV?

Guys, Thanks for the tips. I will definitely try the water catalyst. Any idea how much water -- you mentioned a few drops -- is that per tube? If you dont know, I'll test it out -- anything to speed it up would be worth trying.
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Old 01-02-2004, 11:36 AM
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Re: Inexpensive Silicone Molds w/ RTV?

No, that's for less than a tube. You probably have to test it out.
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  #6  
Old 08-06-2004, 10:04 PM
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Lightbulb Glycerine..

Been trawling web for similar received wisdom.
www.taxidermy.net/forums/MoldingMenu.html suggests a few drops of glycerine, distilled water, 'bondo' cure accellerator (presumably polyester resin brandname?) and some particularly exotic chemicals which would probably lead to phonetapping from the CIA.
So today I'm off to try clear silicone and glycerine + some acrylic paint indicator. If anyone has noticed a runaway chemical reaction, steaming puddles of ex-oil-clay or other drawbacks with this, write quick.
Heres the next thing for the ingenious collected wisdom out there: easy mold (mould) cases. Plaster bandage is good, but tends to disintegrate. Wooden shuttering works, but requires gallons of moulding compound to fill it for anything notably 3-dimensional. Fibreglass looks good but pricier/stinkier.
How about expanding urethane foam-in-a-can, squirt on, let dry and poof! insta-mold-case. One thing I like about this is its lightness.. I make lost wax shells inside moulds and consequently have to tip the moulds in all directions. On some of the larger hemp-reinforced plaster mould cases this has been a herculanean task.
Has anyone tried this?? or miles of duck tape and cardboard? Has anyone finished a gargantuan moulding project and then thought "Gee, I'd have done this in an afternoon if I'd only used ...". All the professional mouldmakers I have met seem to produce beautiful intricate interlocking moulds using very traditional materials; do they all share some dark secret, that they can achieve the same results with toilet paper and diluted PVA glue?
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Old 08-06-2004, 10:39 PM
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Re: Inexpensive Silicone Molds w/ RTV?; making waxes

Re: Inexpensive Silicone Molds w/ RTV?; making waxes
Toby - This is more to the point of getting your waxes than getting the mold itself, but here are two simplifications I've either used or seen. If the mold isnít too big (Iíve done full figures, but on about 2/3 lifesize scale), you can fill the whole, assembled mold and pour out most of the wax after letting it cool for the right amount of time.

I usually get the wax just above its melting point, when it starts to form a ďskinĒ in the melting pot, and let it sit in the mold for maybe 2 - 3 minutes. You can shift the mold a bit and watch wax buildup at the top, to gauge thickness. Iíve had great success this way, though you may have to paint in extra wax at sharp projections such as armpits and so on. Sometimes this can be done simply by pouring coolish, fluid wax into the complete mold and rotating properly; sometimes by reaching in by hand or with a long, bent wire with a bit of rag attached at the end.

Alternatively, you can paint fluid wax into each mold section, to the proper thickness, assemble the mold, and pour a little warm wax into each joint. This is more problematic, but my founder prefers this method, and I have used it. I seem to get more bubbles on the surface this way, and more surface finish work, but thatís probably a matter of technique.

Also, I have suspended heavier molds flexibly at midpoint over a wax pot when using the pour method, so I can fill the mold, manipulate it a bit, and then empty it at the right time.

Last edited by fritchie : 08-06-2004 at 10:42 PM.
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  #8  
Old 08-09-2004, 07:47 PM
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Re: Inexpensive Silicone Molds w/ RTV?

Quote:
Originally Posted by rickb
Hello out there.

If anyone has experience or information using using silicone RTV from tubes (like from Home Depot) to make molds I'd appreciate hearing it. I'm especially interested in drawbacks that may not be obvious -- like shrinkage, aging, strength, etc.


http://www.richardbecker.com
Hi RickB:
I've used RTV as well as eurethanes and silicones, and the RTV works pretty well for a limited set of parameters.

As you probably discovered, it takes forever to cure, and if it's on too thick, it only cures on the outside and will leave pockets of uncured RTV in the middle for A VERY long time that you discover at very inopportune times. In my experience anyway. Worth it to put on multiple layers over time to build it up--really easy to use plastic wrap as a barrier to smooth in on over large surfaces and work out the bubbles that seem to collect in concave areas. I have also used layers of saran wrap in between layers of RTV to give it more strength, but sometimes it pulls apart oddly when you cut the mold off.

I've used it mostly for pretty small things, and have jeweler friends who use it for their solid wax forms on that scale.

I've wondered about a thinning agent...haven't heard of the xylene before. I also like that it's clear (I tend to use the tub/tile stuff that's a milky clear color because you can see the underlying form a bit) and that it's so flexible.

Downsides, aside from the cure time, is that it rips really easily, and degrades pretty quickly...less on a chemical level than a structural level.

I have also tried casting with it into open face molds to capitalize on that milky pink/white color, which is pretty interesting, and it came out surprisingly well...though that's when I was looking for the thinning agent to deal with it not filling small corners of the mold--too stiff.

I've had it with eurethanes so I do go back to RTV when a project doesn't seem worthy of the expense of silicone. I make a lot of molds of fruit/pods which are often moist, so the moisture cure is great.

Best of luck, and thanks for the xylene tip.

PBAnnie
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  #9  
Old 08-11-2004, 09:45 AM
Toby Toby is offline
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Re: Inexpensive Silicone Molds w/ RTV?

Thanx fritchie for wax tips.. I find the heavier the case is, the less comfortable I am suspending it from one point.
Pinballannie! Did you ever modify the content of the moisture-cure builder silicone? Add accellerators? How thick were you applying it? How long did it take to dry?

Last edited by Toby : 08-11-2004 at 09:52 AM.
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  #10  
Old 08-11-2004, 02:12 PM
pinballannie pinballannie is offline
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Red face Re: Inexpensive Silicone Molds w/ RTV?

Toby!
Ummmm...never used accelerators, as in accelerating set time/consistency like a thixotropic?...curious though so please share if you have tidbits.

I have never had much luck applying layers thicker than 1/8" (sorry, don't know metric conversion offhand) and letting them dry at least a couple of days between coats. I usually gingerly sniff to see if it feels like the RTV has stopped gassing off...that usually seems like a pretty good indicator, though not for huffing! I'm not quite sure how toxic the gases are. Do you know?

The RTV also affects the positive chemically, so if they are vegetables/pods in my case, it essentially rots them so when you cut it off it's pretty disgusting. Not that it's a concern for everyone.

PBAnnie
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  #11  
Old 08-16-2004, 03:04 PM
Toby Toby is offline
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Exclamation Thick quick-drying builder RTV silicone

Followed taxidermist advice. Mixed 4 to 5 drops glycerine + dash of acrylic paint into clear bog-standard builder silicone (1 oz/28g). Paint gives good indication of homogenity. Wear mask, eyewatering vinegar pong (you've probably been there).
Applied to a clay model with a butterknife. Marvellous. Tendency to entrap bubbles; spread thin initial coat myself. 20 mins working time. Applied to 3/4" deep in places (could have been thicker if desired). No need to make more sticky or viscous. Can be applied upside-down.

This leads on to next experiment. (Of course I should reduce the number of variables/experiments per piece but that's why I left engineering)
Expanding-foam-in-a-can mould cases.
Works. Difficult to control depth and even-ness. Impossible to work while wet.
Tends to run off sloped surfaces.. blop.. , don't bother trying to pick up again.
Pretty much lost track of mould seam fence (rows of pins supporting vaselined masking tape) Currently re-excavating. Does'nt stick well to itself once dried
May work best with deep shim fences. Pricier than plaster bandage. Good rigidity + flexibility combination. Bit toxic (isocyanide foaming agent: read cyanide). There is future for this stuff but I think I'll stick to plaster bandage for the next one.
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  #12  
Old 08-16-2004, 03:18 PM
Toby Toby is offline
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Feedback

Two points from re-reading above:
Distinction between Urethanes and Room Temperature Vulcanising Silicones.
Two totally different groups of plastics. Here I differentiate between

A: Mold-makers 2 part RTV silicone and
B: Builders 1 part RTV silicone

Two quite similar plastics, one being pricier but having a different cure mechanism.
As far as I know, the odour (odor in imperial) is that of acetic acid, which is the main component in vinegar. This is the evil stink released by one of the catalysts in builders silicone sealant. I don't know if it's toxic or merely unpleasant (akin to sauerkraut sauna)
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  #13  
Old 01-26-2006, 01:45 AM
Louie Arce Louie Arce is offline
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Re: Inexpensive Silicone Molds w/ RTV?

The idea of using water to accelorate the tube caulking from Home Dept is on the correct path. I will expand on that idea. Take a five gallon bucket , fill it with water and squirt the silicone tube in the bucket. Kneed the silicone with hands and it will ball up into one mass. This proccess is washing out the chemical used in the silicone that retards the drying proccess. In about an hour or so the silicone will set up(cure). Before it sets it can be pushed and molded over the form like an RTV rubber. When you have the desired thickness make a mother mold from paster for the mold support. I have seen this used for bust size sculpture with great detail....good luck. Post results for others to see...thanks Louie
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  #14  
Old 01-27-2006, 05:02 PM
Toby Toby is offline
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Re: Inexpensive Silicone Molds w/ RTV?

I've been using a trick gleaned from a taxidermist website.
Clear builders silicone + 6 drips glycerine + teeny smear of acrylic paint.
Hold nose and mix thoroughly with butterknife.. nearest implement. Slap onto model, build up thick layer and slap a plaster bandage case around it.
Acetic acid (vinegar) fumes released, pongy but I appear to have survived. Problems can be trapped bubbles near surface. I;m using this method in a taught course where administration would throw a wobbly if I billed them for 2-part silicone.
Glycerine: available in chemists, drug-stores.. used for cake icing and nitroglycerine.
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  #15  
Old 01-27-2006, 05:07 PM
Toby Toby is offline
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Re: Inexpensive Silicone Molds w/ RTV?

Read 3 squirts silicone + 6 drips glycerine + smear of arcylic paint (used to indicate thorough mixing). Squirt = caulk applicator gun trigger full squeeze. Yes, english is my first language, mbung
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  #16  
Old 02-03-2006, 08:46 PM
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Landseer Landseer is offline
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Re: Inexpensive Silicone Molds w/ RTV?

I think people pretty answered the issues on this, I know from using silicone caulk that it can skin over and stay liquid inside a thick section for a long time. The stuff also tends to be VERY stiff like tire rubber almost, and it tends to break. I used the 50 year silicone caulk on an RV roof in the seams- one tube per seam.
It lasted well, but when I junked the RV and salvaged the aluminum sheets off the roof, I noticed I could break or tear the silicone as well as scrape it off. It was still pliable etc just that when "bent" over at a good angle it "broke" and left chunks on both sides.

I suspect if you examine the idea of caulk in depth, first ask yourself this question because it answers it for you:

"If this works good and is cheap, then how come EVERYONE isn't using it for molds instead of $100+ a gallon mold rubber you have to order and ship?"
I think the answer becomes obvious right there.
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  #17  
Old 02-07-2006, 04:22 PM
Toby Toby is offline
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Re: Inexpensive Silicone Molds w/ RTV?

It sets because of exposure to atmospheric moisture.
Thats why deep sections never fully set.

This is why glycerine is mixed through it, so that it sets fully in deep sections, the acrylic is to check if it has been thoroughly mixed.
2 part is better for avoiding bubbles (if depressurised) and is less viscous.

You don't believe me? Go try it
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  #18  
Old 02-07-2006, 06:31 PM
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Re: Inexpensive Silicone Molds w/ RTV?

well, dunno if I should weigh in on this
after all, I'm no expert
but ...
I have been using the clear silicone for making molds
(the 30 year kind)
for about three years, now
I get it on special at the local hardware store
3 tubes for 9.99

apply it in thin (up to 1/4" thick) layers
reinforce with drywall mesh where needed
never used any additives
just straight from the tibe

hardest part was getting it to release from the carving
a release agent made by mixing vaseline and camp fuel, or naptha fuel
did the trick
commercial ones didn't seem to work

the silicone is harder to work because it's quite thick
but the clear allows me to see any trapped bubbles
and work them out

a mold lasts for about 30 cast cement copies
maybe not as many copies on intricate figures
the mold will eventually crack with use

it worked for cast paper, too
I'm moving over to casting in resin
a thin application of release agent is needed for that
to avoid sticking to the mold

an example of a small reproduction made this way:

"Hanging off the Traps!"



12" long by 7" high

Clifton
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Last edited by clifton : 02-07-2006 at 06:46 PM.
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Old 02-07-2006, 08:36 PM
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Re: Inexpensive Silicone Molds w/ RTV?

Quote:
Originally Posted by clifton
I have been using the clear silicone for making molds
(the 30 year kind)
for about three years, now
I get it on special at the local hardware store
3 tubes for 9.99

hardest part was getting it to release from the carving

the silicone is harder to work because it's quite thick

a mold lasts for about 30 cast cement copies
maybe not as many copies on intricate figures
the mold will eventually crack with use
Seems to me I remember there was a definite difference between the clear and the caulk with white, silver etc when I use for it's intended purpose of caulking around the house.

That caulk is made to STICK, so I have no doubt release was an issue for you.
The thing is, with the QM 140 Silicone mold rubber, the only "release" I've done is a light coat of lacquer on the model, the stuff doesn't adhere to that at all, it didn't adhere at all to an unfired water clay model with a LOT of deep undercuts and all I did was one light coat of lacquer spray.
That was a drawback with urethane, you had to seal the model and then spray some release crap on over that and it was so slick and greasy that as you brush on the rubber it would start sliding around. The QM140 had no tendency to do that.

I find no matter how carefully I brush on the rubber, I always get some air bubbles trapped, and the thicker the rubber is the worse that factor becomes. So the caulk besides being really think and sticky sticks like strong vinegar, the nice thing about working with the QM140 is the base has NO smell at all, the purple catalist has a slight odor sort of like ink, certainly no fumes!

Ok so you find you get about 30 casts before the rubber cracks or otherwise deteriorates, your caulk is probably fine for what you do, especially if you only make a few casts, or just want one quick mold of a model to get one cast to refine more, and make a permanent mold of that.

I have done that many times when I need to do just that sort of thing,
but usually the materials cost ($9.99 v/s $90.00) is not as important as your TIME spent making and replacing molds.

For me, I would much rather concentrait on casting and making money from my finished casts (which are what bring sin the money not the molds) than spending time I shouldn't have to replacing cracking or deteriorating molds over and over- that's just 100% pure COST and a total loss, and the time is better spent with quality materials to start with so the molds last as long as possible.

By now I must have some 40 molds and most of them are fairly large, many took a good gallon of rubber to make, all but the recent 4 I made are polyurethane and I won't use that again, now I know offhand I have at least 3 molds that have to be replaced because the rubber was from a bad batch and maybe 3-4 that started to tear in thin areas because that rubber is not very strong.
Replacing these molds is each an all afternoon or longer affair, then besides the rubber there's the plaster shell, so two-three hundred pounds of plaster there and then having to dispose of the OLD ones is also a headache.
So you can see why in my case I would want the best material I can find regardless of the cost, because whether a mold costs me $10 or $100 is not an issue when it takes 6-8 hours of my time to set up the model, make the rubber mold, and the plaster shell when I could be casting dozens of pieces to sell in 6-8 hours, or something more productive than backtracking!
I don't know the casting life of the QM140 v/s your 30 casts, but I do have Urethane molds I've taken 100 or more casts out of and they are still fine.
Ive had a black "Tuffy" mold that was professionally made for me before I learned how to make my own more complex molds, and I remember the rubber started to deteriorate around the 20th or so cast if not before, it was getting sticky on the inside surface and leaving bits of rubber on the hydrocal casts. That mold was $400 (1978 or so) and I doubt I got more than 40 casts out of it, so that mold cost me $10 per cast and I was only selling the casts for $38.50 DELIVERED UPS.

Your caulk at $10 for the tubes, 30 casts, say less than 50 cents a cast for the mold cost, but there's your time involved which jumps that 50 cents up a fair amount! So as you can see there's more involved in this than JUST the cost of materials.
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Old 02-07-2006, 10:21 PM
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Re: Inexpensive Silicone Molds w/ RTV?

Anyone have experience with the acrylic and silicone mix kind of calking? Not sure of the exact title but it cost much less then the pure silicone and can be thinned with water. I tried a small amound on a piece of dried water based clay. It picked up detail nicely and did not stick without any release ussed. Not sure about the strength though, I just tested a thin layer so far.
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Old 02-08-2006, 07:03 AM
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Re: Inexpensive Silicone Molds w/ RTV?

True, Landseer
trying a few things at low cost is important to me
I need to know if, and what type of, reproductions might sell

and the quality of the mold does not equal that
made with professional materials
and the stuff does stink!
but it surprised me how well it worked

a water based, silicone-like, caulking would be interesting
thanks for the heads up daaub
I'll look around for it

Clifton
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Old 02-08-2006, 04:53 PM
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Re: Inexpensive Silicone Molds w/ RTV?

More info on the acrylic / silicone calking as it says on the tube.

DAP. Alex Plus. Acrylic Latex Caulk plus Silicone. 35 year. White, Indoor / Outdoor, Paintable, Flexible, Highly Durable, Mildew Resistant, Water Clean-Up.

I just tried this stuff out on a life size sculpture of a standing figure. Mixed it with enough water to give it the consistancy smooth mashed potatoes (best comparison i could think of, hah) and painted it on thinnly with a small brush. Actually, it was just like working with the liquid latex stuff. Took three tubes to cover the whole figure around 1/16" to 1/8" thick in areas. I hope this stuff will stick to itself when I add more layers. Never tested that! Some thinner areas were drying very quickly and I added more on top as I finished the figure and it seemed to bond well.
Never tested it on moist clay yet either, so I hope this works out.

Ohh, didn't post the price, which is what this is all about, a cheap alternative to the expensive mold making products. At Canadian Tire, it comes in a 4 tube package for $6.99 so it is about $1.75 a tube. Not bad if it works.

Last edited by daaub : 02-08-2006 at 07:02 PM.
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Old 02-10-2006, 07:43 AM
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Re: Inexpensive Silicone Molds w/ RTV?

"I just tried this stuff out on a life size sculpture of a standing figure."
Daab

-------

Well, I was thinking of something more the size of a little finger, to try it on ... get a feel for how it works. I guess I'm getting old and cautious.

You have jumped in with both feet. A life size mold is a lot of work. I hope everything comes together well for you. The bonding of the caulk to itself should not be a problem.
good luck and let us know how you make out.

Clifton
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Old 03-09-2006, 08:15 AM
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Re: Inexpensive Silicone Molds w/ RTV?

Hi Daab,

I have been working with the different mixtures.
Subjective results as follows.

I found the mold made from the acrylic silicone mix that you used (I picked some up at Canadian Tire) tore very easily compared to silicone, and the mold is not as flexible. It is wonderfull stuff to use.
Easily thinned, brush it on, no odour, captures detail well with no bubbles. Great that way, but I don't think I could use it except for where I only want to make one or two casts.

I tried washing the silicone out in water and was surprised at how well it worked. The silicone set up a little too quickly but this was probably due to overwashing. One thing that I hadn't counted on was the way it stuck to my hands! I guess it would required gloves that are coated with vaseline or something. The problem I find with this method is that the silicone gets thicker, and it is already plenty thick to work with, just as it comes from the tube. Pressing it to the model and stretching it in place did not appeal to me.
But it certainly set up quickly. I can see that any who used this would find it worked well with practice.

Adding the glycerine proved most interesting. There is some sort of reaction going on there. The silicone is changed to be more resiliant and seems to be more tear resistant. I haven't used it long enough to be certain, but it sure seems that way. The drop of acrylic paint does more than indicate the mix, it is, as I think Toby mentioned above, a catalyst. The setting time is much slower without the drop or two of acrylic paint.

Again my main problem is that I don't care for applying with a butter knife. I would rather just put it on directly from the tube, if it comes to that.

My next attempts involved thinning the silicone with the Xylene mentioned above. I could not find xylene directly but did find a varnish thinner that had the chemical in it. I had thinned silicone with naptha fuel before, but it weakened the finished material and it was too soft to use for molds. This material takes a little mixing but it worked well ( er, shouldn't be mixed in plastic containers ). Thin mixtures can be coated over after a couple of days.

Adding the glycerine to the thinned silicone silicone did speen up setting time, but not like it did with silicone alone. It will require an overnight set before going on to the next step. (Back to the longer set up times.) I don't use mother molds on even quite large pieces. Haven't yet done that large though. I just reinforce with cheese cloth and dry wall mesh.

I have been making molds with the stuff at the local hardware store for about 3 years now, and it is good to finally come up with a decent brush on formula. Thanks to rickb for starting this post, and to others who posted information. Hope it is of some use to others.

Clifton
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  #25  
Old 06-06-2007, 06:59 PM
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Tinkerbell Tinkerbell is offline
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Re: Inexpensive Silicone Molds w/ RTV?

Clifton please can you tell me about yr hardware shop silicone for mold making. Im in Uk thanx
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