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. I learned this technique by apprenticing under some of the great moldmaking masters in the special effects industry, and this article reveals their unpublished tricks. I hope they don’t get mad.
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).
Why a Two-part, Underpoured Mold?
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. 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.
Many mold materials are time-sensitive, so after each step, you lay out all the things you’ll need next.
Molds are too often destroyed by impatience; not spending an extra 2 minutes mixing the silicone (for example) can throw away several days’ work.
Some materials, such as the thickening fillers, are also toxic, so wear eye protection, gloves, and a smock or apron, and work in a well-ventilated area.
Finally, no matter how careful you are, be prepared to mess up until you have more experience.
Here’s what you’ll need
This is a lot of stuff, but it’s all essential; the price of a missed step is often catastrophe. Also, you can use these tools and supplies to make many more molds.
Silicone RTV (room temperature vulcanizing) rubber and activator—Silicone is soft (low durometer), durable, and accurate for detail. Many varieties and colors are available, which will all work for our purposes, and I’ve found moldmaking suppliers to be helpful about which products are best suited to different projects.
Urethane resin or other casting material
Talcum powder or cornstarch
Wax and shellac (optional)—It is necessary to seal the pores and keep the silicone from sticking if your object is unglazed terra cotta, wood, or other highly porous material.
Styrene strip or equivalent—Most hobby stores carry Evergreen or Plastruct brands.
Hobby knife with a curved blade and regular #11 blades
Matte knife Or snap-blade knife
12″ or 18″ straightedge Armature wire 3/16″ or 1/4″—At any art store
Plastic hemisphere(s), at least 3/4″ diameter—From plastics supplier or art store
Cyanoacrylate (CA) glue—Such as Zap a Gap or Krazy Glue
CA glue accelerator (or baking soda)—This makes the CA glue “kick” almost immediately; regular baking soda works well for this, and with no smell!
Paint mixing sticks
Tongue depressors or popsicle sticks
1-gallon paper paint bucket
Hot glue gun and
3/16″ or 1/4″ foamcore
Small cloth sack
Permanent marker—Such as a Sharpie
Scale—Digital is best, but a triple beam balance will do
Dremel tool with fine cutting wheel
Orbital palm sander—Or large massage vibrator
Rubber or nitrile gloves—From drugstore or medical supply
Safety glasses or goggles Respirator—Silicone is relatively benign, but a respirator is always a good idea