Introduction: Advanced Silicone Mold Making Techniques for Epoxy Casting

By Narwhal Labs
About: Narwhal Labs is a community and makerspace that encourages and supports creative building, learning, and experimentation.

In our last instructable with Phillip, we went over the basics for making simple silicone molds in a home workshop with as few specialty tools and supplies as possible. We're stepping up our game and introducing some more advanced techniques for making complete acrylic mold masters and working with different silicone formulations that require some extra tools and know-how. Check out the full video below to see his process as you follow along in this guide.

Supplies

Step 1: Mold Preparation on the Laser

Much like in our last video and instructable with Phillip, we start by engraving and cutting the parts for our master in the laser. For this particular mold, we're cutting a base and lid from 1/8" acrylic, two wall pieces from 3/8" acrylic, and our master from 3/8" cast acrylic. Cast works better for engraving and cutting on the laser than extruded acrylic. We've included handy registration holes in the base, lid, and walls that fit standard shelf pins so our lid can be held on or removed easily. Phillip does all his design work in Inkscape, which is a free and open source alternative to other expensive vector based design software. You could build your designs in Illustrator, Autodesk Fusion 360 - or whatever fits your workflow best.

Thank you to Thunder Laser for supporting Narwhal Labs and enabling us to do cool laser work like this - but remember you can make your masters with a scroll saw, band saw, and do engraving with handheld rotary tools - you don't need a fancy laser to make cool stuff. Phillip uses a modified inexpensive K40 laser at home for mold-making.

Be sure to clean the acrylic with isopropyl alcohol. Avoid touching the edges of the acrylic with alcohol to avoid crazing.

Step 2: Assembling the Mold and Using Acrylic Solvent

With all the components to our mold engraved and cut, we're ready to assemble. We're using Sci-Grip Weld-On 3 to glue our acrylic together. This substance is an industrial solvent and typically only available from industrial suppliers like McMaster Carr. It is methylene chloride based, is not recommended for skin contact, evaporates quickly, and dangerous to breathe in excess. Please refer to the manufacturer SDS for detailed safety information. We'll also cover some safety and handling recommendations.

Weld-On 3 is an acrylic solvent that doesn't get cloudy, and sets up quickly. Initial bond in 3 minutes, 800PSI bond in 2 hours, 2000psi bond in 24 hours.

Please wear an appropriate respirator and gloves when working with this solvent. Because of its fast evaporating nature, the first precaution we're going to take is to not remove the tin safety cap underneath the screw cap. Puncture a small hole with a punch, awl, or small screwdriver to limit evaporation from the can.

Using a syringe with a long wider needle tip, extract some solvent. Fill the squeeze bottle with it. The squeeze bottle should not be more than half way filled. That way you can squeeze out some air and maintain a vacuum in the bottle. This prevents excess solvent from leaking/dripping on our mold. While It doesn't cloud, Weld-On 3 can cause some minor imperfections in the acrylic's surface finish.

In this case, we'll start by gluing our walls together. Remove any protective sheets or tape on the acrylic and stack them like you intend when they are assembled. Using the needle tip on the dropper bottle, carefully squeeze some sovlent along the seam between the components, while dragging the needle tip along. The solvent will wick into the seam and light pressure for only about 2-3 minutes will form a strong enough bond to move onto the next step.

Follow the same steps to glue the rest of the mold together. Place the walls on the base, and run a bead of solvent along the seam. Place the master in the center and do the same. We recommend waiting at least a few hours before pouring your silicone.

Step 3: Silicone Preparation - Degassing Silicone in a Vacuum Chamber

For this project we're using Smooth-On Mold Star 30. This is a platinum cure silicone with an approximate shore hardness of 30A when fully cured. Its higher viscosity requires degassing to be used effectively, but it results in a more durable, long lasting mold.

Platinum cure silicones quite literally use platinum as their curing agent, while tin cure silicones use tin as the name suggests - this means with the high cost of platinum, platinum cure silicones are much more expensive, but have a near unlimited library life compared to more short-lived tin cured silicones. For a platinum cure silicone that does not require a vacuum chamber, consider Mold-Star 15 as used in our previous instructable and video on this topic.

Start by mixing the two parts of the silicone individually. Some solids and precipitate will have seperated and will be sitting on the bottom of each pot. We want to make sure they are fully incorporated before moving forward. DO NOT share drill mixers between pots without extremely thorough cleaning and drying. You don't want to risk triggering the whole pot to cure, or inhibiting the cure.

We measured it out by weight, which is more precise than measuring by volume. While this silicone is 1:1 by volume, the ratio by weight is 100:96.

We mixed the parts together in a paint pot with a drill mixer - it's getting degassed so air introduced in this step is not a big deal. Transfer the mixed silicone to a second bucket - one at least double the volume of the silicone, and try to resist the urge to scrape out the remaining mix in the first bucket. This will ensure we have no unmixed silicone from the walls or bottom of our first pot.

Place the paint pot in a vacuum chamber and begin to pull a vacuum. You'll see it bubble up. If it gets close to the top, release the valve and let some air in slowly so it goes down. Close the valve and remove more air. Repeat until little or no bubble are escaping the mixed silicone. You'll often see the bubbles rise to the top during this process and collapse back down on their own- this is a good sign you have sufficiently degassed.

Slowly reintroduce air to the vacuum pot. Quickly reintroducing air can add bubbles back in and cause splashing.

Step 4: Pour the Silicone, Wait for Curing, and Use Your Mold!

Double check you have no dust or contaminates in your mold master. Pour the silicone into your assembled acrylic master. Use the high pour, thin stream technique, pouring into the lowest point of the master to minimize any chance of bubbles.

Mold-Star 30 has a 6 hour cure time, so be sure to wait at least that long before demolding.

Demolding is easy - carefully pull up the edges of the cured silicone working around the edge little by little. It should release easily.

Use a cuticle cutter to quickly trim off any flashing - it's super satisfying too!

And check out the finished mold, and one example of the result when casting epoxy in it! Phillip's wife Elizabeth did an awesome job on that pour. A similar effect was achieved in her video below she recorded with TotalBoat. Thanks so much for checking out this Instructable! What kind of molds have you made with these techniques - we'd love for you to show us!