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Old 02-12-2004, 02:11 PM   #1
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How To - Casting Resin Part I Making the mold

Well I have been promising this for some time now and I finally have had some time to make some progress.
First I have a couple of disclaimers. I am not claiming any information contained here is original. I have compiled my method from many sources , the Interenet, Scale Auto, and personal acquaintances. This article is for informational purposes only.
I also have to mention copyrights and so on. I do not condone making copies of kit parts and selling them for profit. I am sharing this info so that you might make copies of your favorite parts or original works.
Now lets get to the fun.
Basic materials needed for simple one and two piece molds.
RTV Silicone ( Room Temperature Vulcanizing Silicone for Mold Making )-There are many types and companies out there. I personally use a 4 hour cure. Do some research and see what will work best for your needs.
Rubber to Rubber Mold release
Spray Mold Release
Greasless Modelling Clay
An old paint brush
Graduated Mixing Cups
Craft Sticks for Stirring
Masking and Electrical Tape
Elmers ( White ) Glue
2 Part Thin Resin
Plenty of Cleaning Rags

At this point I should mention that the part you are casting needs to be as clean and free from flaws as possible. Spend the extra time to make it perfect now otherwise you will only be duplicating flaws that will need to be repaired on every part. Your work area and mixing material need to be free from dust and dirt. In some cases as in this hollow cast tire you will need to fill the inside with modelling clay before making a mold, note the blue clay in the center

We will use the Legos to make a box to pour the mold in. It needs to be a little bigger and deeper than the total area of your pieces.
Here is one that I made for doing a two piece mold for a tire.
You will notice I marked the center of each side and drilled a hole for a sharpened nail to fit into, these hold the tire in place while you pour the first half of the mold.

A variety of other molds, notice they are wrapped in tape to make them hold liquid well

I like to work on an old piece of glass or plexiglass. For one piece molds make sure the back of each part is flat then use a little Elmers and glue it to the glass. This keeps it from floating up into the silicone once it is poured. In the next picture I am applying a small bead of modelling clay to the underside of my mold box. This acts to seal the box to the glass.

Next apply the Rubber to Rubber Mold release to the inside of the mold and to the part. If you don't you will never get the part out. Here I am applying it to a tire before pouring the second part of the mold. There may be other cheap substitues for the mold release but make dure you test them for comptibility with the silicone forst. And the must be thin, if they go on thick and leave brush marks the brush marks will show on every piece you cast

Do this for each mold you plan to make, I usually prep more than I will pour so I use every drop of Silicone. What I don't do this time I do the next.

Now its time to mix up a batch of silicone, heres where the graduated cups come in. Its very important to be accurate, pour out equal amounts of part I and II into sperate containers

Then carefully transfer both parts to another contianer , be careful not to introduce any air into the mixture

Now carefully mix with a craft stick , again take your time and don't make any air bubbles. Air is your enemy. It should be a uniform color once its all mixed. At this point if I had a vacum chamber I would put the silicone in there and suck out all the air. But I don't yet so on we go.

Carefully pour the silicone in a very thin stream into the corner of the mold allowing it to ooze over the part until full. By pouring a very thin stream you will help to break up any small air bubbles in the mix.

I found that if I use a small plastic cup and stick it in the resin and plunge it up and down it will pull most any air bubbles to the surface.

Let everything set up fully and then gently remove the box from the glass and push out the silicone. Then gently remove the master piece. I usually trim off any ragged edges around the edges of the mold with an exacto knife.
Heres what you should have now

Here is a detail of a tire
I used the area where the two nails were for a vent to pour in the resin and one to vent out air

Here you can see a few air bubbles in the mold, these will be okay as once the are cast they will only be a small nib to remove

And here is what happens if you don't use the rubber to rubber mold release. I guess I forgot the center of the tire. Luckily this one will still work as the two pieces tore apart nicely.

A set of Brembos

Next up Part II pouring the resin

Last edited by dag65; 02-12-2004 at 04:50 PM.
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