Using Silicone Rubber for resin casting

Silicone rubber has become the material of choice for making resin molds. Though the history of mold-making goes all the way back in time to ancient Egypt and China, using sand, wax, glue, animal fat, gypsum, alginate, metal, plastic, re-usable vinyl, and even gelatin, today’s resin mold-makers most often choose one of four different flexible rubber materials,  latex, polysulfides, polyurethanes, or silicon rubbers, because of the traits they all exhibit:

1.       they can reproduce exact detail;

2.       flexibility makes demolding (removal from mold) easy;

3.       their durability permits multiple castings;

Silicone rubber has several properties that make it the material of choice for most resin molds. Its inert nature gives it good release properties as well as good chemical resistance, and high temperature resistance. Its major drawback is its cost, particularly that of the platinum-cure which is the only formulation approved as Food Compliant by the FDA.

Resin crafters, especially those who make jewelry typically use silicone rubber for most of their molds.  It is important to prepare for the project as much as possible ahead of time and to read all of the manufacturer’s instructions to make sure that you mix them in correct ratios and thoroughly, leaving enough time to form and if necessary pour your mold before the silicone begins to cure.

Tin-cure silicone (condensation-cure silicones) has two component materials which cure at room temperature (RTV) to flexible, high tear-strength rubbers. Tin-cured silicones can be poured onto models or they can be made brushable by adding a thixotropic additive. They are ideal for molds where easy release or high temperature resistance is required. Tin cured silicones can be used for casting polyurethane, epoxy and polyester resins, waxes, all gypsum products, and low-temperature metals. Though tin cure (condensation) silicone has a shorter shelf life than does its platinum cure (addition) counterpart and it tends to shrink somewhat, it is more cost effective and fine for most resin molds.

Platinum cure silicone rubber, also known as addition cure silicone, is two-component high tear strength, flexible mold compounds. They are recommended as a mold material for casting polyurethane, epoxy, and polyester resins, wax and a wide range of other casting materials. They exhibit extremely low shrinkage and high physical properties. These rubbers are chemically sensitive to latex, sulfur, and certain other materials. Platinum cure silicones can also be used to cast prosthetics for special effects makeup and medical purposes.

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27 Comments

amber

i use a baking silicone mold with easy cast epoxy resin. i love it, but i cant figure out houw to make it shine after i demold it. help!

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Rachel

Hello, I am looking to make some largish silicone molds for casting polyester resin pieces – in the past when i attempted this the resin and silicone reacted with each other, causing veining and stickyness on all of the surfaces….can you suggest a compatible silicone or some sort of barrier/mold release that prevents this? I dont want to fork out more $$$ and have the same problems…. Help! 🙂

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Katherine Swift

Rachel,

I like our Ultra 4 parafilm for a mold release, especially when casting into silicone.

Are you sure you were using silicone rubber and not polyurethane rubber? Polyester resin and polyurethane sometimes don’t play well together.

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Sona

They are beautiful. I rellay love the combination of the materials, it gives Bohr the industrial look and a soft look with the silicon.thanks for the inspiration and have a happy day

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josiah

Please after making my mold with silicone rubber, what type of material is poured into the cavity/ shape created to produce a product like a smartphone cover. I also like to know if I can order it online with shipping to Nigeria. Thanks

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Katherine Swift

You can choose another resin (epoxy, polyurethane, etc.) to pour into your silicone mold. Sadly, we don’t ship to Nigeria.

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Robert DiStefano

witch type of loctite epoxy resin is compatible with platinum silicone putty molds

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Katherine Swift

I have never worked with loctite epoxy resin, so I’m afraid I can’t make a recommendation.

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Nate

I am looking for a silicone rubber hexagon mold (about 3 inches from point to opposite point) to make game board tiles with. I was going to inject epoxy into the mold to make epoxy pieces. I have never done any mold work before. Can you point me to the proper mold and epoxy that you would recommend? Thank you.

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Nate

Thanks for getting back so quick. I simply need hex tiles that are between 2 and 3 inches from point to point (exact size not too important yet. still in design phase, but want a classy looking prototype) and they must be about a quarter inch thick. I want them to be a clear resin that I can color different shades (green, yellow, red, blue, and plain/clear) but still see through them to a game board beneath them. Sides must be perpendicular to faces so both faces are same exact size. Edges can be slightly beveled or sharp… Not important right now. I don’t know what type of molds or epoxy I even should use. Brand new to this. Thanks!

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Katherine Swift

Have you done this before? That will impact my recommendations to you while you are in the prototype phase.

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Nate

Nope. I am brand new. I’m sure I’ll pick it up quick, but something more geared to beginners would probably be ideal. The tiles should come out clear and not too smoky (a little opaqueness is fine as this is just a prototype) and solid feeling.

Katherine Swift

Since you are new to resin, I would suggest starting with the Composimold molding material: https://shop.resinobsession.com/collections/mold-making-materials/products/composimold-molding-material-10-ounce-container It will allow you to gain some skills molding before you move on to the more expensive silicone. It can be remelted and reused in case you make a mistake. As for a resin, most any clear epoxy resin should work for your prototypes. The Resin Obsession super clear resin works well in molds and is great for beginners as well. https://shop.resinobsession.com/collections/resin/products/resin-obsession-super-clear-resin-12-oz-kit Do you have a template to mold from?

Nate

No, I don’t have a template and was just hoping to buy a very simple silicone hexagon mold (preferably one where I could make 6 or more tiles at once) premade. Is this not possible?

Nate

Also, what do you recommend to color the epoxy? Can I use simple food coloring?

Genevieve

Hi! I made several mold out of the Alumilite High Strength 3 rubber. They all turned out to be kind of greasy feeling. I had cast several molds out of the same material a month ago and they all dried very well. I used the whole container, so my mixing ratios should have been fine (I did not measure, but the box said I could mix the entire container of each part together) . Do you know why this would happen? As a result my castings are not turning out as shiny as the previous mold I made. Thanks!

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Katherine Swift

What are you using as a mold release? What is your mold template made from?

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Genevieve

I haven’t been using mold release, I haven’t needed to. I am using plastic safety eyes as my mold template. I put them in little plastic shot glass cups to create the mold. I did the exact same procedure a month or two ago (same eyes, same brand of cups) and the molds turned out great.

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Genevieve

I purchased from Amazon for this round. I bought it, received it and used it all within a 2 week period. I bought at the beginning of December.

Genevieve

I found that thread after I had posted this. 🙂 I did try washing my molds in soapy water. They came out not oily/greasy at all, which was great. I dried them completely and tried them again. I will be demolding tonight. I hope that works. Thanks for all your help Katherine! I’m new to this and it’s great to have this forum and website for supplies and reference.

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