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How To - Casting Resin Part I Making the mold

02-12-2004, 02:11 PM
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 airhttp://images5.fotki.com/v58/photos/1/11686/415169/IMGP1496-vi.jpg

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

02-12-2004, 03:24 PM
excellent explanation!

02-12-2004, 03:47 PM
Well made tutorial

maybe one day I will make my own resin part.

02-12-2004, 03:55 PM
Great stuff. Looks like I know what I'll be doing this long President's Day weekend.

02-12-2004, 04:05 PM
Thanks alot!! When do we get the Second part?!?!?!

02-12-2004, 04:12 PM
Thanks alot!! When do we get the Second part?!?!?!


02-12-2004, 04:15 PM
awesome dag!

02-12-2004, 04:19 PM
Hey Dag, would you mind giving us a list of prices for the materials? And what company supplied it (and where it was/can purchased)?

From what I hear RTV can get quite expensive.


02-12-2004, 05:02 PM
Hey guys I revised the first postand includied a link and picture to what I used.The RTV yielded all the molds you see, I am now out. I think it was a great starter kit for $70.
Another good place is alumilite.com and I am sure there are others. THese products have a limited shelf life so you don't want to buy more than you can use before it goes bad. I have had mine for 1 year and the Silicone was on the verge of being too old. And there are different types of silicone with different cure times and durometer ( hardness ) ratings. Some mix 1 to 1 others a re 10 to 1. There is no way I could cover it all in my little how to so I encourage you to do a little looking around and learn what you can
As far as the second part goes I am anxious to get home and start using the new ones I made last night so I hope to have some more for you guys tommorow.

Thanks for the feedback guys

02-13-2004, 12:26 AM
Very informative Dag. The pictures are great, really demystify the process. Makes me think I could give this a try one day. Looking forward to part two.

02-13-2004, 12:48 AM

I want to see more!!!!!!!!!!! :D

02-14-2004, 06:12 AM
cool idea i love it

02-14-2004, 01:46 PM
Amazing tutorial. Thanks man. :bigthumb:

02-14-2004, 05:42 PM
Hi guys,

I've been pondering casting parts for a while now. My main gripe has been the cost. I found that resin and the mold material to be outrageously expensive!

Also, has anyone tried "reusable" mold material? I've seen a big block at my LHS go for $23, and supposedly also has built in release agents. All you do is heat it in the oven to melt it, then mold to your part. I am just wondering how good it (ie. how well it will conform to details, etc..).


02-14-2004, 07:43 PM
it's great! thx man!!
the steps is so detail and now i understand how to do the mold. thx!!
waiting for part 2 :smile:

02-15-2004, 04:29 AM
it's great! thx man!!
the steps is so detail and now i understand how to do the mold. thx!!
waiting for part 2 :smile:

Wait no longer, Dag has already posted part II of his impressive lesson:

02-16-2004, 02:01 AM
Hey dag65 (or anyone), what's a good chemical for cleanup on RTV? I made a bit of a mess making a mold, and I also want to clean out those measuring cups for use later, but the RTV is pain to clean up.

06-28-2004, 09:10 PM
just a quick question. for a two part mould, do you wait for the first half of the mould to dry, apply the releasing agent to the dry surface of that, and then pour in the second half of the mould?

06-28-2004, 09:24 PM
That's exactly how you do it.

06-28-2004, 09:28 PM
cheers man

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