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Primer: Moldmaking

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Mythbusters' Adam Savage teaches you how the pros replicate objects.

In principle, moldmaking is a simple process, but with every object you want to replicate comes a new series of pitfalls, innovations, and solutions. This article explains how to make a two-part, underpoured block mold, which is a versatile and beginner-friendly type that's great for small, detailed objects such as jewelry, game pieces, masks, picture frames, and figurines.

Savage learned this technique by apprenticing under some of the great moldmaking masters in the special effects industry, and this article reveals their unpublished tricks.

We'll make our mold out of silicone rubber, an excellent casting material, but it costs about $100 per gallon. This process uses as little of it as necessary, and it's important to follow all of these instructions, because a mistake can be costly. Then we'll cast our duplicates in opaque urethane resin (clear resin requires a more difficult process).

This primer first appeared in Make: Technology on Your Time Volume 08, which is sold out and no longer available.

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Why a Two-part, Underpoured Mold?

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MOLDMAKING

www.makezine.com/08/primer

Why a Two-part, Underpoured Mold?

1. Set up your object on a piece of foamcore and

Two-part molds can handle more shapes than one-piece molds, which work only for simple, completely convex objects. And underpoured molds minimize problems with bubbles in the resin.

Underpouring means that you pour the resin into a main intake vent (or sprue) that curves around to fill the mold up from below, rather than simply pouring into the top (Figure A, opposite). Meanwhile, smaller vents on top allow the displaced air to escape. As you pour, resin splashing down forms bubbles which can stay in the main cavity and ruin the surface of the casting. These bubbles also tend to collect in fine-detail areas, where they are the most difficult to deal with.

The advantage of under-pouring is that it generates fewer bubbles, and lets them rise up into the vents where they won’t cause trouble. Top-pour molds are sometimes acceptable, but pouring from underneath is generally worth the extra silicone required.

 

Design the Mold

PDF

MOLDMAKING

www.makezine.com/08/primer

Why a Two-part, Underpoured Mold?

1. Set up your object on a piece of foamcore and

Two-part molds can handle more shapes than one-piece molds, which work only for simple, completely convex objects. And underpoured molds minimize problems with bubbles in the resin.

Underpouring means that you pour the resin into a main intake vent (or sprue) that curves around to fill the mold up from below, rather than simply pouring into the top (Figure A, opposite). Meanwhile, smaller vents on top allow the displaced air to escape. As you pour, resin splashing down forms bubbles which can stay in the main cavity and ruin the surface of the casting. These bubbles also tend to collect in fine-detail areas, where they are the most difficult to deal with.

The advantage of under-pouring is that it generates fewer bubbles, and lets them rise up into the vents where they won’t cause trouble. Top-pour molds are sometimes acceptable, but pouring from underneath is generally worth the extra silicone required.

 

Pour the Silicone

PDF

MOLDMAKING

www.makezine.com/08/primer

4. Use a hot glue gun to secure the box to the base, and to glue the (single) seam of the box (Figure F).

Carefully examine your entire glue seam to make sure it will hold the liquid silicone, which will leak out of the smallest of holes.

You can also add an accelerator, in the amount of

2-10% of the total weight, but the resulting silicone won’t be as durable and will tear more easily. Curing times will be 24 hours with no accelerator, 4 hours with 2% accelerator, and 1 hour with 10% accelerator.

Pour the Silicone

1. Figure out how much silicone you need by calcu-

3. Mix the silicone, activator, and accelerator, if used

(Figure G, opposite). The activator is blue to give you a visual cue when you’re done mixing. If you see any streaks, you’re not nearly done. Make sure you’re using a mixing stick that’s strong enough, and scrape the sides and bottom, where it’s hardest to get a good mix. I usually mix by pushing the mixing stick down the sides of the bucket, and dragging it across the bottom, turning the bucket as I go, for as many as 6-10 full revolutions. Silicone is thick, so it helps to have someone else hold the bucket.

 

Here's What You'll Need

PDF

MOLDMAKING

www.makezine.com/08/primer

4. Use a hot glue gun to secure the box to the base, and to glue the (single) seam of the box (Figure F).

Carefully examine your entire glue seam to make sure it will hold the liquid silicone, which will leak out of the smallest of holes.

You can also add an accelerator, in the amount of

2-10% of the total weight, but the resulting silicone won’t be as durable and will tear more easily. Curing times will be 24 hours with no accelerator, 4 hours with 2% accelerator, and 1 hour with 10% accelerator.

Pour the Silicone

1. Figure out how much silicone you need by calcu-

3. Mix the silicone, activator, and accelerator, if used

(Figure G, opposite). The activator is blue to give you a visual cue when you’re done mixing. If you see any streaks, you’re not nearly done. Make sure you’re using a mixing stick that’s strong enough, and scrape the sides and bottom, where it’s hardest to get a good mix. I usually mix by pushing the mixing stick down the sides of the bucket, and dragging it across the bottom, turning the bucket as I go, for as many as 6-10 full revolutions. Silicone is thick, so it helps to have someone else hold the bucket.

 

Cut the Mold

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4. Now the silicone is ready to pour into the box.

Don’t just pour it willy-nilly; this is a surefire way to trap bubbles in parts of your mold. Instead, pour in one location, away from your part, and let the silicone slowly fill up the mold around it (Figure I).

Pour as thin a stream as you can, about as thin as a pencil lead. This starts to remove some of the tiny, invisible bubbles formed during mixing. Use the stir stick to scrape every last bit of silicone out of the bucket.

Should you discover silicone leaking from a small hole in the box, stop it with a wet paper towel or, better, a small piece of water-based clay. The water helps the silicone coagulate. Trying to patch the hole with hot glue might just make it wider.

it with an orbital palm sander. Just hold the sander against the table, right next to the mold, for about 5 minutes. A layer of bubbles should rise to the surface.

6. Wait until the mold “kicks,” 24 hours for most RTV silicones without an accelerator.

Cut the Mold

1. Remove the box and silicone from the mold base by inserting your X-Acto knife blade at an angle from just above the hot-glue seam around the bottom

 

Cast It!

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Q

R

Fig. Q: Pouring the resin.

Fig. R: The main casting unpacked from the mold.

Fig. S: The arms removed from their mold. Fig. T: The finished casting, cleaned up, glued together, and painted with primer.

S

Cast It!

1. Remove the original part, sprue, vents, and pouring gate, and dust the inside of each mold half lightly with cornstarch or talcum powder. I put my powder into a small cloth sack, secured with a rubber band, to spread it evenly without clumping

(Figure P). Silicone rubber gets a slippery tack to it, as the silicone gradually sweats out. This means that silicone molds generally need no mold release, because the silicone itself prevents sticking. But a light dusting still helps the two halves align together perfectly, and also acts as a sponge to the resin, drawing it into the fine details of your mold and inhibiting small bubbles which can gather at the high points.

2. Put the mold halves back together, box them in their original box, and secure it all with rubber bands, pouring side up.

 

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