HUNTORIAL: Plastic casting with brush-on silicone

[Elkman]

Since some people were asking for a huntorial about casting their own bios, I thought I'd share some knowledge about how I've done plastic casting with a brush-on silicone product. The actual project I'm showing here is from an earlier costume project, Zebraman. It's a Takashi Miike film about a guy who's obsessed with a 1970s live-action show that got cancelled, and eventually he becomes a superhero. It's kind of weird, but it's Japanese.

Anyway, here's with the armor. The sculpture I'm casting is the left shoulder. Here's the sculpt, done with Klean Klay:

The mold-making and casting materials are from Smooth-On, using their Brush-On Silicone Starter Kit. The kit includes their Rebound 25 brush-on silicone rubber, Thi-Vex II silicone thickener, Plasti-Paste plastic used to make the support shell, Sonite wax to build a dividing wall in the support shell, and Smooth-Cast 300 casting resin.

The first step is to mix the silicone and to brush it onto the sculpt. Actually, I don't remember if I had to use a sealer or a release agent to prevent the silicone from sticking to the sculpt, since it's oil-based clay. Smooth-On's directions will tell you. The silicone I used, Rebound 25, is mixed in equal parts by volume, so you can measure it out with plastic cups or something, as long as you mix one part from container "A" into one part from container "B".

The first coat shows all the detail, so you want to make sure that you brush a thin coat onto the model and get it into all the nooks and crannies of your sculpt. Actually, my armor didn't have any nooks or crannies -- just sharp corners.

In this picture, you can see the first coat of silicone brushed onto the sculpt, and I've started the second coat. I put in a little bit of blue pigment to indicate that I was doing a second layer, so I could make sure I was making it uniformly thick. Between each coat, you need to wait about an hour, so the silicone rubber is sticky to the touch but not wet.

There are a couple low-hanging areas underneath the bar and the box-like structures, as well as at the top, so I added a little extra silicone thickened with Thi-Vex II. The thickener keeps the silicone from running away quickly, although it's still workable... to a certain extent. The thickened silicone is the tan-tinted stuff in the picture below. You can also use thickened silicone to make sure that there are no undercuts when you create the support shell.

The Smooth-On instructions say that you should apply four layers of brush-on silicone. If your model is surrounded on all sides by silicone, then they also tell you to create a "cut seam" with thickened silicone so you can cut the seam in two pieces. This doesn't apply to my sculpt, since the inside is open to my shoulder.

Here's the completed model with four layers of silicone brushed onto it. I think it was about 1/4 to 3/8 inch thick.

The next part is to create a support shell (also known as a mother mold) that will hold the shape of the silicone. Otherwise, your silicone mold would just flop around all over the place. The support shell will be done in two pieces, or maybe more, so you can take it apart and remove it and then reassemble it. In this example, I did my cut seam right down the middle of the shoulder. I later changed my mind when I did the right shoulder because the support shell locked onto the rolled edges on the sides of the shoulder armor.

To create the support shell, you'll need to build a dividing wall out of clay, so you're troweling the Plasti-Paste support shell onto only one side of the silicone and so there's a dividing wall that you can take apart later. Here's my dividing wall before I started with the Plasti-Paste:

The Plasti-Paste support shell is applied to about 3/8 inch thick. After you apply the first half, you'll remove the clay dividing wall, then seal the cut seam with Sonite Wax. This keeps the first side of the Plasti-Paste from adhering to the second side. Then, you can apply the Plasti-Paste.

I don't have any pictures of applying the Plasti-Paste or anything like that, but here's a rather blurry picture of the final mold with the support shell. There's a layer of aluminum foil in there that also keeps the two sides of the Plasti-Paste from sticking to each other. I also have the Zebraman mask in here. (I told you the movie was weird.)

Next post: I'll show how to cast the plastic.

 

[Elkman]

OK, on to the second step: Casting silicone. Actually, I forgot to show in the first post what the silicone mold and mother mold look like after you take them apart, so here's the picture:

To get everything ready to cast, you'll want to reassemble the silicone mold within its support shell and make sure everything goes together tightly. I actually applied a few screws and some wire to hold it all together:

Here's where my process deviates from the Smooth-On instructions for a bit. Since I didn't have a silicone mold for the inside of the shoulder, I decided to pour the plastic into the mold in a series of roughly flat pours. The idea was to make sure that the outside looked correct, but that the inside (which wouldn't be seen) wouldn't be one solid chunk of plastic that wouldn't fit my shoulder. So, here's the first step.

The plastic, Smooth-Cast 300, is a polyurethane plastic that comes in two parts. Both are liquids, and you mix one part of part "A" to one part of part "B", measured by volume. You can use two plastic cups marked to the same level to mix it. Here's an example of the first pour, where I have the mold at an angle.

The plastic is pretty easy to work with. You mix it one to one, then pour it into your mold. It begins to stiffen up, turn white, and become hard in about five minutes. (That's what she said.) At that point, you can turn the mold and pour another flat section. Keep doing this until you've got everything cast to whatever thickness you want. I think I made my Zebraman armor somewhere between 3/8 and 1/2 inch thick, but you might want to make yours a bit thinner if you're doing a bio or something that you don't want to be big and clunky.

Here's a picture of the inside, after doing several pours in various orientations:

And, here's the finished product.

You may want to do any trimming, sanding, or any other prep prior to painting it. I found (recently) that you need a good primer to get the plastic surface ready for painting, particularly if you've used a release agent. I used Plasti-Kote Sandable Primer, available at a local auto parts store.

Anyway, that's about it for this tutorial. If you've got questions, I've got answers. They may even be better than the kind of answers you get at Radio Shack.

 

[hez1]

Nice job! You can also slush the smooth-on resins around the mold to create a thinner cast. It will take several layers and probably not be as fast, but if weight is an issue at all then it works very well.

 

[PredatrHuntr]

-how did you attach the sculpt to the wooden board? Or does the clay just stick on its own in the upright position?

-how do did you stop the resin from flowing out the bottom open end?

Love seeing stuff like this. Great work here and thank you so much for sharing the pictures and info!

 

[Elkman]

Nice job! You can also slush the smooth-on resins around the mold to create a thinner cast. It will take several layers and probably not be as fast, but if weight is an issue at all then it works very well.

I wasn't aware that you could slush-cast the Smooth-On 300 series. Apparently, they have a product specifically for rotational casting (Smooth-Cast ROTO), but since I'm not doing a fully-enclosed mold, using a roto-casting resin might be overkill. I'll have to try this process, though.

By the way, how thick do you usually cast your pieces in order to keep them lightweight yet strong? I think I've done mine somewhere between 3/8 and 1/2 inch thick, which makes them kind of heavy, as you mentioned.

-how did you attach the sculpt to the wooden board? Or does the clay just stick on its own in the upright position?

-how do did you stop the resin from flowing out the bottom open end?

Love seeing stuff like this. Great work here and thank you so much for sharing the pictures and info!

I can't remember exactly how I attached the sculpt. I think I put about a 1/8 inch thick layer of clay on the board and then affixed the sculpt onto it that way. The bottom of the sculpt was flat, so it worked out just fine.

I also couldn't remember how I kept the resin from flowing out the open end, but looking at the pictures again, it looks like I had a piece of Plexiglas clamped onto the end where the board had been during my mold-making process. The process would probably be different for casting a bio or something like that. You'd probably want to build up a lip on the mold, around the edges of the sculpt, so you can define where the edges of the bio will end.

Thanks for the compliments!

 

[hez1]

I wasn't aware that you could slush-cast the Smooth-On 300 series. Apparently, they have a product specifically for rotational casting (Smooth-Cast ROTO), but since I'm not doing a fully-enclosed mold, using a roto-casting resin might be overkill. I'll have to try this process, though.

By the way, how thick do you usually cast your pieces in order to keep them lightweight yet strong? I think I've done mine somewhere between 3/8 and 1/2 inch thick, which makes them kind of heavy, as you mentioned.

The 300 series is a pain to slush, but it can be done. The Roto I found was not rigid enough unless it was backed with fiberglass. It will deform under it's own weight unless cast very thick, which sort of defeats the point IMO. The 320 series slushes beautifully.

I find anywhere around 1/4" thick is fine. That's what I try to get to as a minimum, with the edges being slightly thicker.

 

[xdmray]

good tut!!!


but if i may, next time for such a shape i would lay it flat so you can get a wall all the way around the sculpt. that way you dont have to worry about the resin pouring out. might have a little bit less wasted silicone when molding as well.

cheers mate!