Before we begin this lesson, make sure that you always read the Safety Data Sheets that come with your materials for important safety precautions. Not all silicones or polyurethanes are the same. Be sure to read the Technical Data Sheet for mixing ratio, cure time, and cure temperature before beginning your project.
Here’s what you’ll need:
- An object to mold
- Foam core board
- 2 Mixing sticks
- 2 Mixing cups
- Hot glue
- Hot glue gun
- Sharp knife (X-acto or box cutter)
- Kitchen scale (one that measures grams)
- Pourable 2-Part Silicone Elastomer
- Disposable brush
- Mold release
- Pourable 2-Part Polyurethane or Resin (Smooth On makes good products for both the silicone and the resin that you’ll need for this project)
Check out our Amazon shopping list here to get all your supplies in one place!
Making a one-part mold is a fun and relatively easy way to make multiples of a simple object.
We suggest you watch the video in its entirety first, then follow along with Kent as you make your own one-part silicone mold. Remember, you can pause the video any time you need.
Step One: Choose Your Object
The one-part mold works best on objects with few undercuts. What is an undercut, you ask? It’s difficult to explain, but it’s basically any areas of an object that might get stuck in the mold. The more complex the object, usually the greater number of parts your mold must be made in, in order to accommodate the undercuts.
Here’s an example:
Let’s say the green is our object and the grey is our mold. The object is simple and has no undercuts, so getting it out of a one-piece mold will be easy.
This object is more complex and has two undercuts (one on each side) where the mold will get stuck. Imagine trying to push the green object straight up out of the mold. The cut-outs in the sides of the object would get stuck, which would make de-molding it very difficult! Objects like this with multiple undercuts require more complicated molds in two or more parts (yes, you can make molds in multiple parts!).
Silicone is a pretty flexible material (unlike plaster), making it easier to mold semi-complex objects as you’ll see in the video tutorial.
Choose your object wisely!
Step Two: Prep Your Materials
Lay out your foam core board on your surface (it’s best to cover your surface first in newspaper for easy clean-up in case of spills). Make sure you have all your materials handy. Plug in your glue gun so it has time to heat up.
Step Three: Build Your Walls
Put some hot glue on the base of the object you’ll be molding and stick it down on your foam core board. Measure the height of your object and make sure you have enough room around it to make tall enough walls. Kent just eyeballs this measurement in the video and ends up with just enough room for his object (he is a professional, after all!). For beginners, we suggest measuring to avoid any frustration.
Let’s say the green box is your object and the dotted line around it is the base of your mold. There’s a little bit of room on each side of it for the silicone to go around it. The arrows are the same size as the green box, which is the minimum height you’ll need for the walls.
Once your object is glued down, grab your X-acto knife or box cutter and cut a long rectangular strip of foam core board at least the height of your object (see above illustration!). Put the straight edge of your foam core “wall” near your object, giving about 1-2” around your object for the silicone. Once it’s in place, put a strip of hot glue at the base of the wall to hold it there.
When the glue has dried, score (make a light cut–not all the way through the foam core) on the outside of your wall and bend it around the other side of your object. Continue doing this until you have all four walls built around your object. Then use your hot glue gun to secure the walls in place. Make sure you seal up the seam between your first wall and fourth wall by adding some hot glue along the vertical seam. You don’t want your silicone to leak out!
Step Four: Apply Mold Release
This step isn’t shown in the video, but it’s a very important step! You’ll need to apply mold release to your object, otherwise, it will not come out of the mold. The silicone will stick to your object and you won’t be able to get it out.
Always apply aerosols OUTDOORS (never indoors!).
Step Five: Mix & Pour Your Silicone
The silicone you purchase will come in two parts–the silicone and the catalyst–the latter being what helps the silicone to cure, or harden. We are using a silicone that uses a one-to-one ratio, but check the technical data sheet that came with your silicone to see what the mixing ratio is. We are going to continue under the assumption that the silicone has a 1:1 mix ratio for the sake of clarity and ease. Make sure you follow the mixing ratio of your particular silicone!
Get out your kitchen scale and turn it on, making sure the measurement is set to grams. Place your measuring cup on the scale EMPTY and weigh it. Now click the “tare” button–this will reset the scale so that it goes back to zero and does not weigh the measuring cup. This is helpful because you can weigh just the material itself and not the cup. Now you will weigh out Part A of your silicone. How much you need really depends on the size of the object you’re casting as well as the size of the mold itself (or the amount of space between your foam core walls and the object). Kent is starting with 400 grams of mixed material, so 200 grams of each part of the silicone. Be sure to pour your silicone slowly so that your measurement is accurate! Once you have 200 grams, hit the “tare” button on your scale again so it resets to 0. Now you are ready to pour in 200 grams of part B to the same measuring cup. You’ll notice in the video that Kent actually needed around 600 grams to fill the space of the mold, but we’ll discuss that a little bit more later!
Using a disposable wooden stir stick, mix your silicone ingredients together very well. You want to make sure that they’re well combined so that the final result sets up properly. Don’t take too long on mixing, though, because the silicone may start to set in your measuring cup (if it’s a quick-set formula like Kent is using). Once it’s been mixed, use a disposable brush to start brushing the silicone onto your object. This helps to make certain that you get all the tiny details and a minimum of air bubbles. The silicone won’t stick to your object all the way and that’s fine–you just want to make sure you dab on at least a thin layer of the silicone to all the detail areas. After that, you can gently pour the rest of the silicone into your mold until it covers the top and goes over it by about ½”. It’s at this point in the video (12:10) that Kent realizes he needs more silicone. The new batch of silicone must be added while the first batch is still wet, otherwise, the two layers won’t bond correctly. Because Kent is using a quick-set silicone, he must work quickly to get the rest of the silicone mixed and poured!
Now that you’ve poured your silicone, it’s time to take a quick break and wait for your silicone to cure. Again, check your technical data sheets to see how long your silicone needs to cure completely.
Step Six: De-Mold Your Object
Once your silicone has cured completely, you can de-mold your piece. Use your X-acto blade or box cutter to gently cut the corners and seams of your foam core mold and pull it off of the foam core base. You can pry the hot glue off the bottom of your object and then remove the outer walls of the mold until you just have a cube of silicone. Gently squeeze the edges of the mold to loosen the silicone around your object, then push on the bottom of the mold to get the object out of the opening left by the hot glue.
In the video, the object molded has a slight undercut, making it difficult to remove. You’ll notice that Kent and the cameraman used a tripod to help push the object out of the silicone.
Step Seven: Mix Resin
If you need to take a break, this is a good time to do so. You can wait to cast your resin piece at a later date if needed. Store your mold in a cool, dry place until you’re ready to use it again.
In this video, we are using a polyurethane to cast our piece in the mold. The polyurethane comes in two parts and needs to be mixed in the same way we mixed the silicone before. Read the technical data sheets of your polyurethane, resin, epoxy, or acrylic to find the mix ratio and cure time specific to your material, just as you did with your silicone. You’ll need a new mixing container and mixing stick as well as your kitchen scale for this part. Just like before, place your empty mixing container on the kitchen scale and reset the weight by hitting the “tare” button. Once the scale reads 0 with the container on it, you can start mixing your casting material. The amount of casting material you’ll need depends on the size of your mold. Mix the correct ratio of parts A & B together in your mixing container and pour it into your mold. In the video, Kent uses the back end of a brush to move the casting material around inside the mold to make sure there are no air bubbles.
In about 4 minutes, our cast is set! Make sure you check your technical data sheets for the cure time of your material. Once it is completely set, you can de-mold your piece the same way you did the original. Work the edges of the mold to loosen it up a bit then push the piece up and out. Voila! You have now completed a single part silicone mold and resin casting.
The original piece is on the right and the newly cast piece is on the left.
To clean up, MAKE SURE you put the correct lids back on their containers. For two-part materials like the silicone and resin, it’s important not to mix up the caps–it’s possible for the materials on the lid and container to combine and set the lid in place.
You can pull leftover set material out of the mixing buckets and recycle or reuse them if they’re not too dirty. Sometimes, though, they just have to go in the trash.
You can continue to make castings in the same silicone mold as long as it holds up! At some point, the mold will degrade and likely tear apart, but you should be able to get at least a few castings out of it.
When you’ve finished your project, send us a photo! Student submissions are eligible for our monthly Amazon gift card drawing, so don’t delay–send us a photo now!