This is the second in a series about making silicone molds for 3D prints. If you missed the first one, you can read it here.

While the last article focused on objects made of simple extrusions and flat backs or bottoms, most 3D prints aren’t going to fit within those criteria. Something like 3D Phil has complex curves, is too tall to use his in an open face mold, and has a hole between his legs. The hole alone is enough to make a mold out of the question, but all three would prove too difficult to attempt, unless you use a different technique: a two-part mold.

What is a two-part mold?


Exactly as the name suggests, a two-part mold has just that, two parts. Rather than one solid block of silicone with a depression in it (like a one-part mold), a two-part mold has a depression in two pieces of silicone, that when pressed together form a cavity to pour into. This does mean some considerations need to be taken when deciding on the orientation the 3D print is going to be molded in, because you need to be sure you can actually remove a casting when you’re finished and it’s not locked in the mold.

A finished mold and finished casting that both look awesome.
A finished mold and finished casting that both look awesome.

Deciding on A 3D Printed Model

Unlike one-part molds, most objects can be made in two-part molds, but rarely you may find your object needs, three or more parts. The methodology is the same, just repeat the steps for each new part.

Materials and Tools:

  • 3D Print
  • Mold release
  • Oil based, non-sulphuric clay
  • Sculpting tools, printed or otherwise
  • Hot glue gun
  • Hot glue
  • Foamcore
  • Wood
  • Tape
  • Ruler
  • Knife

Step 1: Figure Out The Parting Line On Your 3D Printed Part

Once you know what to look for, parting a 3D print is pretty easy. You need to look at your model and consider:

  • Will the seam be in a high-visibility spot? If you’re molding a bust, while it may seem ideal to put the seam along the point of symmetry (from front to back), that will put a noticeable line straight through the face of it. It would be better to put the seam separating the bust into a front and back to keep the parting line hidden from first glance.
  • Are there any holes that need to be molded? This means simple holes, like the hole in a cup handle and not internal channels. You want the parting line to be along a side of the object where each half of the mold can essentially pull straight back from the opposite side, without much if any flexing to remove the part.

Depending on the features of the object, it’s possible that only two parts to the mold will lock in the object, and more separate parts are necessary. The steps are basically the same, just figuring out the various parting lines.

With most models, it's easy to tell where the parting line should be.
With most models, it's easy to tell where the parting line should be.

Step 2: Build The Mold Box

Just like in an open-face mold, you’re going to need to build a box to contain the silicone, but there are some key differences.

  • Make sure that the height of the box is at least an inch taller than your object, and that it’s wider and longer than the object by an inch as well. This will give you ½ inch of space on all sides of the 3D print. You can ere on the side of caution and make the box an inch and a half taller to give more room if you overestimate your silicone.
  • The difference this time, is you don’t need a base for the mold, or rather you don’t need to make permanently adhere a base to it, because in the end you’ll need to flip the intact box over and work on the 3D print from the underside while it’s in silicone.
A mold box is key in a successful two-part mold.
A mold box is key in a successful two-part mold.

Step 3: Clay Up the 3D Print

To make the first half of the mold, you need clay to create the negative of what the first side will look like.

  • Place your 3D print on top of a ½ inch thick piece of wood or cut a piece of clay to a half inch thickness, to properly elevate your 3D print.
  • Begin filling in the space around your 3D print within the mold box with clay, making sure to keep the clay completely parallel to the table where it touches the print. Don’t let the clay creep up the side or down the side of the model, otherwise it’ll make casting a little difficult.
  • Once you have the first side completely flat and shaped properly, use a small tool to press divots into the surface of the clay, or other tools to carve channels. These are registration keys to register the silicone in the same position every time you make a casting, and with a two part mold (made like this) you can never have too many registration keys. Just make sure that you don’t go too small or too deep with your divots, otherwise they’ll give you problems later, creating a suction between the two halves or tearing away.
  • Also something more complex models will need to be considered are vents, sprues and gates, which allow air to escape and resins to flow into all of the casting instead of trapping bubbles. It’s an art and a science figuring it out, and an entire professional skillset on its own.
  • You will need to figure out which end of your model you’re pouring into though, and making sure that side has a spout for it.

Note: If you've never worked with a combination of clay and silicone (like plastalina oil based clay and platinum cure silicone or water based clay and tin cure silicone), make sure to mix a small test batch and apply it to the surface of some clay. This is so you can test and make sure that the silicone will actually cure, because sulfur in clay will inhibit its curing abilities and never harden, leaving you with a ruined and gooey mold

Don't worry too much about keeping your model clean, just focus on keeping the edges perpendicular to the model.
Don't worry too much about keeping your model clean, just focus on keeping the edges perpendicular to the model.

Step 4: Pour The First Half

Just like the open face mold, you need to spray mold release and properly mix up your silicone. You can also degass your silicone before you pour it to create an even higher quality mold.

  • Same steps from open face
Pour the first half of your silicone ensuring your model is covered in at least half an inch of silicone.
Pour the first half of your silicone ensuring your model is covered in at least half an inch of silicone.

Step 5: Pour The Other Half

The second half is essentially the same, just some differences in prep work.

  • Gently flip over the mold when the silicone has cured, and begin removing the clay. You want to be careful not to disturb the 3D print and to make sure it stays seated in the silicone. If it comes loose, your finished mold may be oddly shaped if you try to reseat it and it isn’t properly seated.
  • Once the clay is removed, use some mineral spirits to clean out any clay residue and any remaining clay. Make sure even the smallest speck of clay is removed, because that speck will be apparent in every casting if it’s allowed to stay.
  • IMPORTANT - Spray down the second half with mold release. This will keep the second pour of silicone from fusing to the first, turning your delicately crafted mold into a solid silicone prison around your 3D print.
Make sure to spray in some mold release before pouring in the second half, otherwise encase you model in silicone.
Make sure to spray in some mold release before pouring in the second half, otherwise encase you model in silicone.

Step 6: Liberation

When the second half has had adequate time to cure, you can begin demolding.

  • Remove the mold box and separate the two halves of silicone. It may take some persuasion to split it, where some spray release didn’t get full coverage, but once you get it started, it should start to separate like a zipper. Silicone is pretty resilient, but don’t tear at it if it isn’t budging. You may need to take a knife to the edge to get a start to the seam.
Sometimes you'll need to do a little digging to get your model out, depending on spills, leaks, and how much mold release you used.
Sometimes you'll need to do a little digging to get your model out, depending on spills, leaks, and how much mold release you used.

Step 7: Casting Preparations and Casting

Your mold is complete, but it needs some rigidity; as it is, it’s a little floppy.

  • Cut a piece of wood to act as the mold jacket, two pieces the same size. You’re making a silicone sandwich, with a piece of wood on each size.
  • I’ve seen a lot of different methods of keeping this together, from rubber band to bolts and clamps. I like to use cheap masking tape; it’s something cheap and disposable but it won’t apply a clamping pressure, squeezing together the mold and distorting the casting.
  • When I’m using casting resins, I’ll keep each half separate, and give the inside a light dusting using baby powder, to help break the surface tension when pouring in the resin, then I’ll close the mold up and blow out most of the powder.
  • Casting is the same from here, and you can use a variety of materials, as silicone is pretty tough to damage, but the quicker your resins cure, the hotter they get, which does shorten the life of the mold. Just something to consider.
You can cast in a variety of different materials now that you have your mold. In this case I used a quick curing urethane resin.
You can cast in a variety of different materials now that you have your mold. In this case I used a quick curing urethane resin.

And that's it! We hope you learned some great skills. If you have any questions regarding molds, feel free to leave a comment below!