Trying by Hand At: Mold Making and Casting with Silicone RTV

This is a great technique for making multiple copies of a piece out of resin. It's used in sculpting, prop making, special effects makeup, and many more applications. This Instructable is the documentation of the class in which I found a piece to copy from the part bins available at the TechShop, cast a mold out of silicone, and filled the mold with resin.

We discuss the process of casting beginning with choosing a piece to cast, making the mold, and casting a piece from the mold.

Step 1: Before You Start: the Setup

If you have a silicone mold already available to you, the casting process does not take very long. If you're starting by making the mold, however, be prepared to wait at least two hours for the mold to cure. Be sure you have the following before beginning:

Time: at least 2 hours for a small project
Tools:
-Nitrile gloves (NOT silicone gloves)
-utility knife
-4 paper cups for measuring
-2 paper cups for mixing
-paper or plastic cup to hold the silicone
-hot glue gun
-wooden mixers (popsicle sticks)
Materials:
-something to mold
-silicone RTV
-urethane liquid plastic resin
-Ease Release agent

You should coat the surface that you're working on in a disposable material to catch any spills. It would also be a good idea to have paper towels on hand to clean up messes. 

Step 2: Molding: Choose and Prep Your Piece

When choosing what piece to mold, make sure to consider the following guidelines:

Make sure that your piece doesn't have any nooks and crannies for the silicone to get stuck in; complex parts that cannot be lifted from the silicone will tear the mold.

If any of the components of your piece are loose, you may want to hot glue them together to prevent the silicone from shifting into unwanted places during the mold-making process. 

Your piece should be rigid--a good rule of thumb for this kind of casting is that a soft mold like silicone should be used to cast hard objects while a hard mold should be used to cast soft objects.

Be aware that while your piece will probably survive the molding process, it might not. Do not try to cast anything unless you are okay with it potentially being damaged.

Step 3: Molding: Creating a Molding Container (Part 1)

Begin by placing your piece on an index card or piece of cardboard. 

Step 4: Molding: Creating a Molding Container (Part 2)

Using a knife or similar tool, cut the bottom out of a paper cup. 

Step 5: Molding: Creating a Molding Container (Part 3)

Place the cup over the top of your piece. Using a hot glue gun, glue it onto the card. Make sure that the seal is tight--you don't want the liquid silicone to leak. 

Step 6: Molding: Mixing Your Silicone (Part 1)

This particular kind of silicone is made by mixing two liquids that are sold together as a kit. You don't have to wear gloves for this part, but it wouldn't hut. 

Step 7: Molding: Mixing Your Silicone (Part 2)

After opening the containers, be careful to not let the liquids come into contact with one another except during the mixing process. Use separate stirring sticks and mixing cups. If contaminated, the silicone will start to cure inside the bottles.

Step 8: Molding: Mixing Your Silicone (Part 3)

Stir the bottles with separate stirring sticks until you have a nice, even consistency.

In order to determine how much silicone you need, you can fill your empty mold container with water. You should completely cover the object that you're molding in at least half an inch of material. Be aware that you should wait for the mold to dry completely before pouring in the liquid silicone.

Your silicone will be composed of a 1:1 ratio of the two liquids. Therefore, pour half of the amount into a separate cup from each bottle.  Different ratios produce different consistencies, curing time, and lifespan of the mold. Experiment or research this further if you'd like to know more about different ratios.

Step 9: Molding: Mixing Your Silicone (Part 4)

Once you have measured your liquids, pour them together into one cup and stir vigorously until the liquid is a single consistent color. The photo is about halfway stirred--there should be no marbling.

Step 10: Molding: Pour Your Silicone

The curing process begins as soon as you mix the two liquids so you should pour the liquid into your prepared container within a few minutes of making it. Tilt your mold slightly, pinch the cup of liquid to form a narrow spout, and pour a thin, steady stream of liquid onto the index card. Pour from a higher height to achieve a thinner stream. The tilting is to prevent air pockets from being trapped in your mold--make sure to fill the container slowly and steadily to ensure the entire piece is covered.

To remove air bubbles, pick up your container and gently tap it against the table surface for about a minute. The bubbles should rise to the surface.

Step 11: Molding: Removing Your Mold (Part 1)

The mold will take at least 75 minutes to cure. Check the package that the silicone came in for an approximate wait time. When the mold is done curing, it should not be tacky to the touch. If you prod it with a popsicle stick, it should be a firm consistency. The popsicle stick should not leave a mark.

When the mold is done curing, carefully cut or peal away the cup. It may help to cut it away in pieces. Be careful not to damage your mold.

Step 12: Molding: Removing Your Mold (Part 2)

Remove the container.

Step 13: Molding: Removing Your Mold (Part 3)

Flip your mold over and gently pry out your piece. The silicone should retain its shape when pulled or squeezed gently, but it can tear if you aren't careful.

Step 14: Casting: Mixing Your Resin (Part 1)

Now that you have a mold, you can begin to mix the liquid resin that you will use to cast from it. 

This material is extremely irritating to the skin; be sure to wear gloves for this part. If you spill any on yourself, rinse it immediately.

Follow the same procedure as in the silicone mixing. Begin by opening the jars and stirring the liquid with separate popsicle sticks. Pour an equal amount of each into separate cups. Be careful not to cross contaminate the liquids. 

Step 15: Casting: Mixing Your Resin (Part 2)

After you have determined the amounts you want to use, pour them into the same cup.

Step 16: Casting: Mixing Your Resin (Part 3)

Stir vigorously to get an even texture. The curing process will begin immediately and is much quicker than the silicone; be ready to begin casting.

Step 17: Casting: Pouring Your Resin

If your piece is likely to be difficult to remove from the mold, first prepare the mold by spraying some release agent into your mold. 

Pinch the cup and pour a thin, steady stream of the liquid into your mold. 

Step 18: Casting: Wait for It to Cure

The curing process should take about ten minutes. The chemical reaction should cause the liquid to heat up. 

Step 19: Casting: Removing Your Finished Piece

The process should be complete if you can prod the material without it shifting or smudging. It should be completely rigid before you remove it from the mold.

Gently pull the silicone away from your piece and pop it out of the mold.

Step 20: Congratulations!

Your piece is finished! You can use the mold a number of times before it wears out. 

Step 21: From Start to Finish

This is the final image of the three stages of the process: original object, mold, and final cast.