Monday, November 7, 2011

Two part mold making tutorial - Resin Casting

Making the 2 part mold.

Lets make these.

Smooth-On Oomoo 25.
This product is mixed by volume at a 1:1 ratio.  It does not have an foul odor, in my opinion.  It has a 15 minute pot life and sets up in 75 minutes.

This product does not need to be degassed or weighed, it is mixed by volume.

Making it much more user friendly.


Bad Photos, blame the plastic wrapper that I was trying to photograph.  On the second frame the label says no sulfur, your gonna have to trust me on that.  I would recommend if possible by a natural colored clay instead of blue or whatever.  The color is a pain in the ass to clean out of skin, tools, and clothing.

You have to use sulfur free clay so that it does not mess up your mixed silicon.

Mold boxes.
This step will take longer than it most would people think it would, take your time and do it right.  Using a sulfur free clay, build up a layer of clay to around half way up the original.  Make sure to leave no gaps and not get the clay on the top half of the original.  Build your mold on a flat portable surface, I used a piece of 1/8 inch thick plastic card.  I also Use Legos for the walls of the mold box.  Where you stop with the clay will be the mold line, you want to be aware of this so that you can minimize mold lines in you detailed areas.  Leave plenty of room on all sides of the original, so that the mold halves maintains strength.

Adding vent channels.
At the bottom of the photo you can see the end of a syringe that I use as a pour gate for resin, the white plastic rods were added to make channels for air to escape when pouring the resin.

The holes in the clay will form registry keys that will help to keep the halves lined up properly.

Math Hell.
If you are still with me this far along, then I am going to assume that you can read and have a basic understanding of the MATHs.  At this point you need to figure out how much silicon to make.  So some basic calculations of volume are going to be needed, along with at least one graduated container.  So my two boxes had interior measurements of 6.5 cm 9.5cm x 1.5cm  &  6.5 cm 11.25cm x 1.5cm, giving me two volumes 92.625cc & 110cc.  Total volume needed approximately 202.3cc minus the filler(scroll down) and volume of the original to be cast.
This is America here but metric is so much better for things like this (everything really).

Two Compounds One Cup.
I use clear throw away drink cups, using the graduated container measure out 100cc of water and pour into the cup.  Mark a line with a sharpie  at the water line.  Now pour another 100cc of water in and mark the next line.  Make sure the cup is completely dry  I put tape over my lines so that the mark would not rub off.

Mix it up.
Follow the manufacturers instructions on mixing the A&B sides of the silicon.  I keep two labeled large paint stir sticks like from Lowe's and two measuring cups one for each bucket with my gear. After stirring I add the thinner of the two to the cup up to the first mark, then add the other side up to the second mark.

Stir with a third stick until the color is even, you need to stir this and pour the silicon in around 5 or so minutes you have a work time of 15 minutes total.

Left over silicon.
Left over silicon can be cut into small bits and added to a new mold pour. Keep it to small amounts added after the first part of the pour.  When the resin has coated the clay floor and original.

Keep and failed molds and left overs for this reason.

Poured Silicon.
Start pouring in the most empty corner of the mold box and allow the silicon to flow by itself throughout the mold box.  That will help to ensure complete detail capture.

In this photo you can see the air bubbles caused by mixing and pouring rising up and popping leave them alone and let the silicon do its thing.  Gently shaking, tapping, or even vibrating the mold will make this go faster.

(You can buy a professional vibrator for this, please post pics of it delivered to your door step if you do.)
The manufacturer says 75 minutes to cure, I recommend pouring the first half and between 2-4 hours.  Let it really set up and cure well before the next steps.

Remove the walls.
Remove the walls of the mold.

Remove the clay
Remove the clay, notice how the blue stains the plastic card.  Buy a natural unstained shade if possible.

Make sure to get all the little flecks and pieces.  Try not to remove the master from the mold silicon.

Prep for 2nd pour
You must apply a rubber to rubber mold release agent to all of the silicon, be extra careful not to get any on the master being copied.  Use a old paint brush for this.

Ready to pour the other side.

Rebuild the mold box, mix & pour your silicon.

2-4 hours later.
Gently pry the 2 halves apart.  You will have to clean out the pour channels and venting sprue gates.  Use a very sharp knife and possibly a good quality sprue cutter.

Give the mold at least 8 more hours to fully cure before you start pouring resin in it, yeah the manufacturer says its ready but do your self a favor and wait.


  1. Very nice post, thanks for sharing the information. Keep up the good work.


  2. wonder if a sonic cleaner would work well for vibrating.

    also any way to see pics of a resin made model? like the final finished project? also how did you make your guide holes and rods that fit them?

  3. also what type of resin did you use?

  4. Thank you for sharing your mold making supplies materials and also this tutorial post, I really appreciate it.

  5. This is the information i have been looking for.. thank you so much admin

  6. This comment has been removed by a blog administrator.

  7. Simply wish to say your article is as astonishing. The clarity in your post is simply great, and I could assume you are an expert on this subject. Same as your blog i found another one Sohman Epoxy .Actually I was looking for the same information on internet for
    Black 2RL
    and came across your blog. I am impressed by the information that you have on this blog. Thanks a million and please keep up the gratifying work.