Molding and Casting: Part 2

When we last left off in Molding and Casting Part 1, we had a one-part rubber mold that was ready for pouring casts. Here in Part 2, it’s time to pour one!

Here are the supplies I will be using:

  • Smooth-Cast 320 by Smooth-On
  • Mann Ease Release
  • Black silicone pigment
  • 2 small plastic cups
  • 1 stir stick
  • Protective gloves and goggles

The first step is to give your mold a quick spray of Mann Ease Release on the inside. This is important, because otherwise the resin could stick to the walls of your mold, and rip pieces off when you remove the final cast.


Smooth-Cast 320 is a very reliable resin that cures in just 10 minutes. I buy it in the big 1 gallon jugs, but there are smaller sizes available as well. Similar to the mold making material, you mix equal parts of A and B by volume. Also, these need to be well shaken before using.


I have poured this same mold dozens of times, so I know the exact amount of resin that I need. But for your own mold this might take some trial and error. I fill each up with the same amount of resin, one Part A and one Part B.


Next, I put one drop of the black silicone pigment into Part B. You could put it into Part A, but the instructions recommend extra stirring for Part B, so this is a good excuse to stir it. I am wearing protective gloves and goggles, because these chemicals are basically liquid plastic, and you don’t want that on your body or in your eyes!


When I stir in the pigment, it will turn the resin jet black. However this resin cures white naturally, so one drop of black pigment actually results in the final product being gray.


The next step is to pour Part B into Part A. It is ok to leave a few drops of Part B at the bottom of the cup, as long as 99% of it gets into the Part A cup.  Once you start this process you only have about 90 seconds to work with the resin before it starts to cure.


Now that the two parts have been combined, it is time to stir the mixture like crazy. Make sure to scrape the sides and bottom of the cup. Stir it for about 30 seconds.


Set your stir stick aside and pour the mixture into the mold slowly. The resin should be a very thin stream.


Let the mixture come to just below the top of the mold. From this point it will take 10 minutes to cure. I recommend removing it from the mold within an hour.


After it has finished curing, begin by gently pulling the top of the mold away from your part. Next, start sliding your finger in between the mold and the part, all the way around. These molds are very stretchy, so you won’t damage it by doing a small amount of pulling. Finally, push up from the bottom of the mold, and you will be able to pull your part out.


Store the mold out of direct sunlight. You should be able to get about 50 casts out of one mold before it starts to break down.


I don’t have any new parts to mold that require a 2-part mold, but as soon as I do, I will write a new article on that.

Happy molding and casting!

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