Instructables

Trying my hand at: Mold Making and Casting with Silicone RTV [Pittsburgh Tech Shop]

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Picture of Trying my hand at: Mold Making and Casting with Silicone RTV [Pittsburgh Tech Shop]
Today at the Pittsburgh Tech Shop, I took a class in mold making and casting. This is a great technique for making multiple copies of a piece out of resin. It's used in sculpting, prop making, special effects makeup, and many more applications. This Instructable is the documentation of the class in which I found a piece to copy from the part bins available at the TechShop, cast a mold out of silicone, and filled the mold with resin.

I will discuss the process of casting beginning with choosing a piece to cast, making the mold, and casting a piece from the mold.

More information about the Pittsburgh TechShop can be found at http://techshop.ws/

I made it at TechShop!
 
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Step 1: Before you start: The Setup

Picture of Before you start: The Setup
If you have a silicone mold already available to you, the casting process does not take very long. If you're starting by making the mold, however, be prepared to wait at least two hours for the mold to cure. Be sure you have the following before beginning:

Time: at least 2 hours for a small project
Tools:
-Nitrile gloves (NOT silicone gloves)
-utility knife
-4 paper cups for measuring
-2 paper cups for mixing
-paper or plastic cup to hold the silicone
-hot glue gun
-wooden mixers (popsicle sticks)
Materials:
-something to mold
-silicone RTV
-urethane liquid plastic resin
-Ease Release agent

You should coat the surface that you're working on in a disposable material to catch any spills. It would also be a good idea to have paper towels on hand to clean up messes. 

Necessary materials can be purchased from websites such as http://www.smooth-on.com/
mscott2711 months ago
The colors, and ratio are Smooth-on specific. The color thing is something that Smooth-on does to make it easier to see when it's mixed properly, and I believe they also formulate their products to give you the easy mix ratios. It's a good tutorial, if you keep that in-mind. Anyone not using smooth-on products to follow your guide is very likely to not have those colors, mixing colors to indicate consistency, or ease of use. The basic approach is the same, but you might want to emphasize that point, rather than say "materials can be bought somewhere like Smooth-on," in-case anyone reading this has another supplier in-mind, and thinks it'll be the same thing. Or, tell them to "mix the components in the ratios indicated by the manufacturer, and wait until it cures, which may appear different, and have different requirements than shown here."
Personally, I think it reads better, and more accurate if you just point out that the tutorial is for a specific Smooth-On product, and in the materials list, be specific as to what they should order, rather than general terms that invite purchasing mistakes. It's a great tutorial though!
spylock1 year ago
Release agent?Like a cooking spray,or something just made for that purpose?
Silicone usually releases from non-porous surfaces without anything but if you do, anything greasy (except silicone oil) will work fine.
Thank you Sir for getting back to me so fast with the info.
lamerc spylock1 year ago
Researching making molds like this for casting jewelry pieces I've seen a lot of people recommend olive oil highly as a mold release (especially in lieu of the expensive sprays usually marketed for this). FWIW.
Olive oil, canola oil, nose oil.. extremely thin coating.
danzo3211 year ago
If the bottom half of the object is nested in plasticine or something, you can pour or paint the top mold, make a mother mold if necessary, then turn it over, remove the clay and make a mold of the bottom half. Plenty of other tricks, like molding in a funnel for pouring, and register-bumps so the halves will line up perfectly.
Pignanelli1 year ago
"This material is irritating to the skin" is somewhat of an understatement. The Methyl ethyl ketone peroxide (MEKP) used as a hardener for the resin is an extreme organic oxidizing agent, and is very dangerous to your body and long-term health. You should work with it (if at all) in open air with the wind blowing away, or in a vacuum hood. You should take every precaution that it does not touch your skin. (Imagine bleach which does not respect the epidermal barrier.)
thisisanexparrot (author)  Pignanelli1 year ago
I made the language in that step a bit stronger to reflect this; it's definitely important to stay safe when casting.
I use safety glasses when using MEKP.
legoyzueta1 year ago
I have used silicone while molding face and body casts with an artist.
We often used an excelerator to speed up the curing process. You might ask this company about this. We bought by the gallons, but the blue silicone is very fun.
Two hours is pretty fast! Lifecasting grade is way faster, like 15 minutes.
thisisanexparrot (author)  legoyzueta1 year ago
That's a good idea; I should look into that. Thanks for the tip!
jbh1231 year ago
Nice I'ble, especially for someone like me with no experience with doing something like this. One question: I'm a little confused by the second photo in step 18 - the one with the horseshoe-shaped pieces. It looks a little out of place. Is that the one you meant to post?
Those look like dental impressions to me, but I was wondering why they were pictured, too. I'm all curious to see clear resin teeth as well, if that's what they are. :D
thisisanexparrot (author)  Lunarius1 year ago
The teacher for this class had sculpted and molded a lot of cool teeth to use on masks that he makes; I can't find any pictures of the teeth we made but I'll make a note to take a few.
Your dentist makes silicone molds of teeth all the time, and casts them in hard plaster.
Ah, cool! If you do get the photos I'm eager to see them.
thisisanexparrot (author)  jbh1231 year ago
Good point; while we were waiting for the silicone to be done, we cast some teeth molds that had been used for another project. They don't really fit here, though, so I took them out.
Edgar1 year ago
"Norwegian Blue, beautiful Plumage... It's just stunned!"
Gonna do this one day with Oogoo, methinks...
Gone to my Blog, along with a stunning Maplin new venture, 3D Printer on a kit:
http://faz-voce-mesmo.blogspot.pt/2013/06/medusa-da-vos-asas-e-impressora-3d-que.html

thisisanexparrot (author)  Edgar1 year ago
Hah, glad someone got the joke. Also, thanks for the blog post!
I saw the original Monty Pithon, I'm that ooold, cough, cough... :)
TruFord1 year ago
First off you did a great job and I enjoy the tutorial. Ive been playing with molds for awhile and Ive found that legos are great for making the containers. just place your object on wax paper and then build a container with legos. Then when youre done you just put the legos away for next time. Just figured Id share what Ive found helpful/useful in mold making. Once again great job and great mold.
Spokehedz1 year ago
I made a vacuum chamber out of a large glass sun tea jar (had a hole in the glass already) and a brake bleeder kit from HarborFreight. Both are under $30 total. I use it to re-bubble all my molds and my resin so I get perfectly clear casts. The glass is handy so you can see how many little bubbles you have, so you can slow down the vacuum pressure if it begins to bubble out too much. I have pulled as much as 25 inches of mercury (25hg) and had no ill effects on the glass. Obviously, you can test yours first.
boxcarmj1 year ago
Thank-you, I can use this.
Mike.
thisisanexparrot (author)  boxcarmj1 year ago
You're welcome! Make sure to do some more research on this subject. There are plenty of molding and casting techniques out there and there might be one better suited to your needs.
Slabysz1 year ago
Do you glue the object to the note card so the silicone doesn't flow under it, especially when you lift and tap the cup to free the bubbles?
thisisanexparrot (author)  Slabysz1 year ago
I don't believe I did for this one; that's a good point though. You'd just have to be careful about the glue being completely under the piece so that it doesn't show up in the final mold.
dangre1 year ago
Not an ible but a good explanation of two sided molding.
http://www.alumilite.com/HowTos/2PieceMold.cfm
thisisanexparrot (author)  dangre1 year ago
Excellent link; thanks!
trophygeek1 year ago
Great article.

You might want to change latex gloves to "Nitrile gloves" and add "NOT silicone gloves". The powder that sometimes found in latex (as well as many hand lotions) can cause the silicone to not set correctly.

http://www.silicone.pro/2011/06/what-are-bad-materials-to-have-around-lsr/
thisisanexparrot (author)  trophygeek1 year ago
Updated accordingly. Thanks for citing your source!
Brilliant Tips and Tricks for beginners !

ive already been playing with clear casting resin but used all sorts of things to make the mold, finally admitted defeat and bought some RTV Silicone to try out, GREAT Tip on using the paper cups !

Thanks so much for taking the time to put this instructable together !!
No problem! I learned it from good teachers.
Aloz1 year ago
Good job on the piece and the instructable .
will you do a ible on two sided molds? I could use it to duplicate moshlings for my nephew.
thisisanexparrot (author)  Aloz1 year ago
Two sided molds are definitely possible. I don't have a project documented for that, but you should be able to find another Instructable for it.

The technique is essentially to submerge your object, leave a small hole in the top for pouring in the resin when you're ready, and very carefully cutting an irregular edge around your piece so that when you take the two halves apart you can easily tell how they fit back together (kind of like a puzzle piece).

http://www.instructables.com/id/Two-Part-Silicone-Casting/ Here's a good example.