Introduction: Mold Making: Two Part Silicone Mold

About: I'm M@. If you know Prototype This, TechShop, The Best of Instructables, Show Me How, or AVPII: Requiem, you've seen some of my work and the cool stuff I've been involved in. I build and design and make and th…

*UPDATE* I'm going to include some FAQ's to answer some of the questions I've received. {FAQ}

This tutorial will demonstrate how to reliably reproduce cast objects using a two part silicone mold. This technique, when skillfully applied, can have incredible resolution, effectively cast very complex shapes, and can yield parts that need little cleanup. This type of mold is most useful for parts that are fairly small, and therefore will not consume a massive amount of silicone to cast. To save material on large castings, techniques such as glove molding and matrix molding are often applied.

Step 1: Select, Clean, and Add Sprue

I chose a figurine from a comic shop as my demo to mold. This will be what is called the master. It has a fair amount of surface detail, and some small parts that, although tricky, are faithfully reproduced in the final casting.

Begin by inspecting, cleaning, and securing your model. I glued down the small parts that my figurine came with which originally snapped together. I took a cotton swab and got out all the grit and dust from every tiny crevice. Remember that everything you see on your part will be cast into the mold, even down to fingerprints. You will not be able to correct these mistakes once the part is cast, so take the time to do it right.

You will need to create a sprue or sprues on your part. These will become funnel shaped holes in the mold. You need them so you can get your casting material into the mold so don't forget to put them in. I usually use the tops to bottles or machine my own, but you can make most any tube or funnel shaped object work. If you'd like, you can even make your sprue out of clay. I usually smooth the seam in between the sprue and the master as to not catch bubbles on the very bottom of the part. Try to think ahead when you're molding a part, if something looks like it will trap bubbles, try to orient things to avoid it. Your goal is to create a mold that will produce the best quality parts with the least amount of work and fiddling.

Step 2: Clay Up

Once you've prepped your part for the show, it's time to lay up the first half of the mold. Draw a line that defines the very outermost edge of its curved surfaces. You want one continuous seam tangent to a majority of the curves on your part. The goal is to evenly split the part in the mold so it comes out easily without damaging anything. Also, molding this way will make the seam line easier to clean in your final part.

Find or construct a border for the part. You want it to be some sealed volume that at least extends an inch above the part, and an inch to the sides in all directions. I glued the top of a paper mixing bucket to a masonite panel with hot glue. Lay your part in this container securely with some clay. The clay is going to have a tendency to stick to your part, and will need to be cleaned when you begin the second half of the mold, so do yourself a favor and coat the part in a spray release, or wrap it in clingfilm and cut the film away at your mold line. Cleaning clay off your part will most likely result in it pulling up out of the mold before you begin the second half, which will mean that you'll get some silicone dribbling in between the part and the mold and screwing up your texture. Begin making a rough border around the part with clay, coming up to the line you drew earlier. Work the clay layer until it is smooth and even around the part. Leaving a texture to the clay will only make the mold harder to pull apart.

Step 3: Inscribe a Key

For most organically shaped parts like action figures or prosthetics, your mold will fit together in a very specific way. The seam line that these organic shapes have just naturally locks together in one position. For more geometric or mechanical parts like legos or usb drives, your mold seam may be nearly flat, and therefore difficult to align or register. To make sure the mold goes together the same way each time, you add a seam line or a key to the mold. I prefer to make a quarter inch trench along the border of the part, not only to fit it together well each time but to also prevent material from leaking out when I'm casting the final parts.

Step 4: Mix and Pour Your Silicone

Silicone is a great molding material because of its natural tendency to not stick to things. This makes it ideal for casting a wide variety of things from plastics to plaster. However, it does love to stick to itself. If you don't make sure the mold is properly sealed in between castings, you'll find yourself with a big block of silicone with an action figure prize inside. I'm serious.

I recommend you use a digital scale to measure out your materials. I try to gauge the volume of my mold by some simple geometry. Always mix a little more than you think the mold will actually take to fill. It's very difficult to mix up a new batch of material on the fly when this happens. Also, before casting, inspect your entire mold to make sure that there are no possible holes or leaks. Even a tiny pinhole leak in your mold could mean you find a pool of cured silicone on your rug and a lightly silicone glazed part in your mold when you come back to check it the next morning.

Mix as per the manufacturer's instructions, and pour in a high, thin, even stream to eliminate air bubbles.

Step 5: Walk Away

I have a tendency to fiddle with the mold, hoping to improve things as it cures. This doesn't often work to my advantage. The more fiddling you do, the more chance you have at screwing things up. What you should do is vibrate the table the mold is on to dislodge bubbles for a few minutes, and then go somewhere and read a nice book. You should let the mold do it's thing. When the silicone is cured (this will vary depending on the manufacturer) come back and take another look.

If all goes well, you should be able to simply flip the part over, take off the clay, and pour the second half. For me, that meant cutting the bottom off of the mold container (the paper mixing bucket) and turning the whole thing over. Remember to apply a generous coat of sealant. I use Smooth-on SuperSeal spray. It requires two thorough coats spaced twenty minutes apart. Check for bubbles in your sealant, smooth them out with a soft brush if there are any.

Mix and pour again.

Step 6: Molding: Complete

After the second silicone piece has cured, you're done with the mold. You can take your master out and inspect the mold. Now you're ready to clap it back together and cast your final parts. I usually hold molds together with rubber bands wrapped tightly enough around the mold to keep it together without distorting it. I'll use a bucket or something similar to hold it in the right position for casting.

This kind of mold is perfect for a wide range of liquid casting agents, from hot wax, to plastics, to resins, to plaster. Remember a few key factors:

  • Each molding material has a different cure time, viscosity, and surface tension. Some of these materials are prone to capturing bubbles in the mold. To prevent this, make sure to read up on how to prep the mold, whether it's adding talc, or a release agent, to get the best quality casting you can.
  • Learn what the mold tends to do, and make procedures to correct the errors. I found that this mold tended to catch bubbles in the little spheres that stick out from the base. I found that by shaking the mold, and poking at the bubbles with a small rod, I could get a flawless casting.
  • You can make multiples of this mold by repeating step five over and over again. Professionals tend to make a plastic or silicone copy of their part just for this purpose, so they can duplicate as many molds as necessary without damaging the master.
  • Recycle your scrap silicone. Since silicone is so eager to stick to silicone, it makes sense to extend your materials by mixing in chunks of cured scrap silicone with large castings. It saves a lot of material. You can also cast excess silicone into sheets in a loaf pan or muffin tins to use as mixing boards. They make great palettes as paint and epoxy won't stick to them.

For video tutorials on mold making, visit Freeman Mold Making and Casting Supply. There is a great tutorial on a more complex molding technique on ConceptArt. You can also check for information on various molding materials and suppliers.

Step 7: FAQ

*UPDATE* I'm going to include some FAQ's to answer some of the questions I've received.

  • Q: What resin did you use? What sealant did you use to prevent it from sticking to the mold?
    • A: I had decent luck using crystal clear spray varnish as a sealant. I would recommend you use mold soap or SuperSeal to coat the mold. Spraying a coat of PVA in the mold before casting will also provide a solid seal. For casting parts out of your mold, silicone usually doesn't need any release or sealant, but the particular resin I was using met some cure inhibition against the RTV. You can find a non polyester clear casting resin and it will probably cast fine without needing any release or sealant.
    • Q: Is the mold reusable?
    • A: The mold is completely reusable. Cured silicone has an average shelf life of five years and will hold up to thousands of castings. It will break down depending on what material you use, but that's still in hundreds of castings with the most abusive materials. You'll know when the mold's at the end of its life when it's not flexible and begins to yellow.
    • Q: Where can you get silicone and other molding materials? How much will they cost?
    • A: Good silicones can go for $100/gallon and more. You're going to pay more per oz by buying smaller batches, but it's easier to live with the expense. The material I used comes in a 2 pint batch at $22 { manufacturer's site }. The polyester resin I used was CastinCraft clear resin, and cost $15/pint. You can pick this and the colorants I used up at most craft stores. I also used SmoothCast for the white model you see below and C-1515 for the translucent model. Each of these are about $15/pint as well. For more resources, you can check Smooth On, Freeman Casting Supply, and MicroMark.
    • Q: Silicone is too expensive. I heard you can use molding rubber. How is it used?
    • A: Actually, if you're using a molding rubber, your process is a little different. When you take out your original part, and lay it up in clay. Before you pour your rubber over you'll need to coat it in a molding sealant and then a release. I recommend you use something like PVA, liquid floor polish wax (wax is both a seal and a release), SuperSeal, or Mold Soap. SuperSeal is pretty easy to find, and usually comes in a spray bottle. Personally, I love the mold wax. You'll want to clean up everything with a soft brush if you see any streaks. Then you apply a release like EZrelease, or Universal Mold Release. After doing all that, you can pour up your rubber. Wait for it to cure, flip the mold, clean out the clay, repeat the seal/release process and pour again. Once your mold cures, you remove the piece, apply a coat of seal to the mold, and then a coat of release (this is why I prefer silicone, because you don't have to fiddle with releases.) After everything is sealed and released, you should be ok to cast. If everything's done right, you should be able to produce some nice shiny pieces. Allow the plastic to cure overnight and wake up to some brand new toys.