Two Part Pourable Silicone Pumpkin Mold

December 1, 2016

For this project, we will use pourable silicone to create a two part mold of a pumpkin. Two part silicone mold requires two separate pours of silicone, so these types of molds usually take two or three days to make.

Materials needed:

• Small pumpkin
• AeroMarine 128 Pourable Silicone with Catalyst
• Non-sulfur, non-drying modeling clay (Plasticine, Plastalina)
• Cardboard (or foam board)
• Hot glue gun
• X-Acto knife
• mixing cups and mixing utensils
• Scale that weighs grams
• Small plastic straw, swizzle stick or plastic toothpick (to vent the stem of the pumpkin)
• Silicone-to-Silicone mold release
• Gloves
• Sharpie pen

Your work area:

• Clean, level work surface, covered in paper or plastic for easier clean-up.
• All materials comfortably within reach.

Project steps:

• First, we need to construct a mold box to fit the pumpkin from cardboard and hot glue. I measure and cut the cardboard for the sides and bottom of the box first, but don’t glue them together yet.

• Next, we need to embed half the pumpkin in modeling clay. The clay acts as a place holder for the second half of the mold. I like to clay the piece in before I glue the sides on the mold box, I find it easier than trying to work the clay into the already constructed box.

• Then, use the hot glue gun to glue the sides of the box into place. Next, I apply a seam of hot glue to the outside of the box where the cardboard pieces meet. This ensures that the box is tightly sealed, with no place for silicone to leak out. I then use a small amount of clay to seal any gaps between the mold box and the clay.
• Because of the curve of the pumpkin’s stem, I’m worried that an air bubble will form at the tip of the stem when it is time to cast the piece. So I’m going to press a plastic toothpick into the clay from the tip of the stem to the edge of the mold box.
• We also need to create registration marks, or keys, in the clay.  These indentations and matching protrusions will help the two sides of the mold lock together correctly, so when I cast the piece there isn’t a lot of flash, which is casting material leaking between the two sides of the mold. I use the end of a sharpie pen to make indentations in the clay, as you can see in the picture. You can also see the toothpick embedded in the clay as well.

• Now that the mold box is ready, we need to weigh out and mix the silicone. The 128 Silicone is a tin catalyzed, condensation cure silicone that is mixed 10:1 with the pourable purple catalyst. For the first pour of this mold, I put the empty mixing container on the scale and zeroed the scale out. This is so I’m only weighing the silicone, not the plastic mixing container. I weighed out approximately 2600 grams of silicone and 260 grams of catalyst. As you can see in the following picture, the catalyst is quiet a bit thinner than the silicone. Just mix carefully and slowly, taking care to scrape the sides and bottom of the mixing container, until it is one uniform color.

• When I pour the silicone into the mold box, I pick a corner away from the piece and pour the silicone “high and thin”, that is, I hold the container a foot and half above the mold box (high) and pour slowly so the silicone comes out in a thin ribbon (thin). This helps reduce air bubbles in the silicone and lets it flow slowly around the piece, which further prevents any air bubbles.

• Once the silicone is all poured, I lift the mold box about an inch or two of my work table and drop it several times. I do this for a few minutes. This helps encourage any bubbles to rise to the surface of the silicone and pop.
• This silicone takes 16-24 hours for a full cure, so I’ll come back the following day to finish the second half.

• After the first pour is cured, I rip apart my carefully constructed mold box, taking care to keep the pumpkin embedded in the silicone.

• Next, I pull all the clay off the pumpkin. Make sure you don’t pull the plastic toothpick out because it needs to stay in the silicone like the pumpkin.
• I use another toothpick to clean off any little bits left behind after I remove the bulk of the clay.

• Then I cut more cardboard to make another mold box to fit around our first half of the mold. Before I glue to mold box in place, I generously spray the mold release on all sides of the first half of the mold. That way if any silicone leaks down between the box and the first pour, it won’t bond and make my parting seam hard to find.
• Once the box is constructed, I spray more mold release on the first half of the mold. I focus on making sure the silicone, not the pumpkin, is well coated with the release. Some will get on the pumpkin and that’s fine, but the important thing is to make sure the silicone has a good, thick coat of mold release. I’m using Petrolease, which we carry, but you can also use plain, old Vaseline.

• Now I weigh and mix more silicone, this time approximately 1800 grams of silicone and 180 grams of catalyst because this side is slightly smaller than the other. I pour high and thin into a corner, pick up and drop the mold box for a few minutes and leave it to cure until the following day.
• The second pour is cured.  Again, I tear apart the carefully constructed mold box. You can see in the next photo where some of the second pour leaked down between the first half and the box, but because of the mold release, it pulls way easily and can be trimmed away.

• After trimming the excess silicone, I can see the parting line and easily pull the two halves apart. Next, I can remove the pumpkin and the tooth pick to see the finished mold. Sometimes little flecks of clay can be left behind on the mold. The best way to clean a silicone mold is with warm water and dish soap. Do not use and abrasive soap or an abrasive sponge because you don’t want to scratch the mold.

That’s it!  Here is a lovely two part mold of a pumpkin. From here, you could use a urethane casting resin to cast a pumpkin and paint or decorate it anyway that you wish.

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