I've been making molds for a while now, but I often found myself frustrated by the high costs of materials. I love the quality that silicone molds like Smooth-On give, but I wanted to find a cheaper alternative for small casts. Hot glue was the obvious choice for me because of its price, its flexibility, and its incredibly quick cure time. The one major problem with hot glue is that it sticks to almost everything. I needed to find a mold release that would fill any small pores in the positive, allow the object to de-mold easily, and be as inexpensive and readily available as hot glue to avoid defeating the purpose. I tried everything. Oil, petroleum jelly, even some Smooth-On mold release as a test but none of them worked well enough to not damage the molds when I removed the object. I didn't figure it out until we had a power outage and I was digging through my collection of scented candles. The liquidy, melted wax was perfect for coating objects. Hot glue doesn't stick to it but it goes on thin enough to not obscure the details of the mold. Here's how I finally got the perfect hot-glue mold:
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Step 1: Building a Well
The first thing you need to do is plug in your glue gun. It will heat up while you make a container for your mold. Using your clay on a bit of wax paper, build up some quick walls. You want the well to be just a little larger than the object you're casting. Don't worry if you don't have any sculpture skills. It doesn't need to be fancy, just functional. Try to make sure the clay is stuck firmly to the wax paper to avoid any leaks.
Step 2: Prepare Your Object.
Light your candle and let it sit for a few seconds until it melts into a clear liquid. Using your old paintbrush, coat your object in a very thin layer of the wax. You need to work quickly as the wax will begin to solidify in a few seconds. If you need to spread it any thinner you can carefully hold your object a few inches over the flames and the wax will liquefy again very quickly. When you're dipping your brush into the wax be very careful not to let the bristles touch the flames. It may help to tilt your candle and let the hot wax collect on the inside of the jar. If your object has a large hole in it like my seashell, fill it up with clay before you coat it in wax.
Step 3: Making the Mold
Fill your well most of the way with hot glue. To avoid any unwanted air bubbles you should keep the tip of the hot glue gun submerged in hot glue the entire time you're filling the well. Make sure you have a few extra glue sticks handy because you may need more than one. When it's filled, quickly push your waxed object into the glue, being careful not to burn yourself. You can wait for the glue to cool naturally but I opted to run it under cold water and it cured in about 5 seconds, allowing me to demold immediately.
Step 4: Casting
You can use any cold cure casting material with this mold, but I opted for UV-Cure resin because I had it on hand. Since cold hot glue doesn't stick to anything you won't need to use any other form of mold release. Simply fill the mold with resin and cure it. I cured it in a UV tank that's meant to cure SLA prints but sticking it in sunlight for a few hours, or under a UV light for 30 minutes works just as well. To demold the print pinch the mold between your fingers and push your thumb against the bottom. The cast will pop out easily.
Step 5: Marvel at the Incredible Detail
The seashell that I found was a test to see how detailed I could get these cheap molds. The result was stunning. My camera can't quite capture the delicate ridges in both the original shell and in the cast version but I'm so excited to take a crack at making 2-part hot glue molds and using this method in future projects. I hope you find this as useful as I do and that this Instructable helps bring down the costs for aspiring mold-makers all over Instructables. If you use this tutorial, please post a picture here. I'd love to see how you use this!
First Prize in the
Hot Glue Speed Challenge