Introduction: Making a Silicone and Ultracal Matrix Mold

About: I like to make stuff. I teach stagecraft to high school kids, and have won some awards for it. In my dream world, I will get to make wacky stuff all day and teach other people how to make wacky stuff. Teach…

A hard shelled mold with a squishy interior that captures and works well with undercuts.  Capture the finest details from any object you wish to replicate with this sturdy mold process.  Silicone is USUALLY self-releasing from almost any casting material, from stone to resin to low-temp metal (350 degrees max.)

Step 1: Materials

You'll need lots of stuff.  Fortunately, it's pretty easy to find.

From an SFX house (I like Silpac inc and Motion Picture FX Company):
RTV silicone.  I like Rhodosil 1065.

From your shed:
Old crappy slotted screwdriver
Chip brush (to cut down)
Power Drill
1/4" twist bit
1" spade bit
duct tape

From your trash:
Old plastic container
small water bottle
disposable foil pan or takeout container (clean)
Popsicle stick

From your artbox:
craft knife
sulfur-free plastecine
sculpting tools
hot glue gun
scissors (to cut down your chip brush)

small scale
whatever material you are going to use for your final cast (I used low-temp metal, but resin, sfx gelatin, plaster, etc will all work fine with this type of mold.)

Step 2: Make Your Base Blocks

Base blocks cut from clay will form the silicone interior for very small objects. They should fit together as close to perfect as possible.  If you're casting a larger object, you would cover it with saran wrap, wet paper towels, or damp fabric and entomb it with clay.

I like sulfur-free plasticine as opposed to water based clay, because you can work with it over an extended period of time with no shrinking or cracking.  Plus, you can reuse it seemingly forever.

Make sure the clay is labeled as sulfur-free; even saying the word "sulfur" may cause silicone to fail to cure.  Ok, not really, but I wouldn't take any chances.

Step 3: Add Matrix Channels

With your base block on a disposable pan, cut logs of clay and add them to the top and sides of one of the base blocks.  Eventually, this will all be replaced with silicone.

If you're casting a larger object, also build a clay dam around the object at its widest point.

Step 4: Prepare to Pour Your Stone

Use cardboard, lauan, or vinyl base cove to create a pour base.  I like to duct tape the corners and hot glue it to the disposable pan to prevent leaks.

Step 5: Pour Stone

Add your ultracal to water slowly until it sits on the surface of the water.  When mixed, it should look like thick paint.  Pour to cover the clay by about half an inch. 

Do not breathe ultracal while in dust form.  Do not allow ANY of it down your drain.

If casting a larger object, mix ultracal to the thickness of a milkshake and brush it onto half of the clay entombed object on one side of the clay dam. Build it up until it's half an inch thick at the thinnest point.

Step 6: Bust It Out

Wait until the ultracal has started to cool off from the chemical reaction.  Remove the wall (carefully- you'll need it later.)  Flip your mold.

While the ultracal is hard but before it's completely cold, carve out a large, conical pour channel and bleeder channels.  Use a large drill bit and drill shallow mold keys.  Some people like to make these in clay prior to the first pour, but I think carving them afterward is easier.

Spray liberally with several coats of crystal clear spray paint.

Step 7: Build the Top Half of the Inner Mold

Lay the other half of the base block on top and build matrix channels.  This should look like a mirror image of what's under the ultracal.  Fill your pour channel and bleeder channels with clay.

Step 8: Apply Mold Release

I like a 50-50 mix of vaseline and baby oil, but that's just me.  I use a disposable chip brush.

Step 9: Pour Part 2

Rebuild your wall.   Apply more duct tape and hot glue.  Mix more ultracal.  Pour until it's half an inch higher that the thinnest point.

If you spring a leak at the base, wad up some paper towels and stick them in the leak.  You may lose water, but you won't lose any more stone.

Step 10: Pop It Open

Remove wall.  You won't need it again. 

CAREFULLY pry apart the two halves with a screwdriver you don't care about.  If you used enough clear spray paint and mold release, it will pop apart fairly easily.  Otherwise, let the swearing commence.  If you didn't get enough mold release in your key holes, they will break off funny.  This is ok; the uneven break will serve much the same function. 

Drill or carve bleeder holes in the sides of one half of the mold.

ON ONE HALF ONLY (preferably the one with the roughest surface,) carefully pick out all of the clay.

Step 11: Clean the Mold

Using a cut-down chip brush and rubbing alcohol (99% works best,) clean the clay residue out the half of the mold you just pulled the clay out of. 

Step 12: Insert Object Here

Add your objects (in my case, clay sculpts) to the top of the clay in the half of the mold that still has clay in it.  Add flashing so the resin/plaster/metal etc can travel from the pour channel, through the objects, and out the bleeder holes on the sides.  This is what allows the air to escape.  Add small, pyramid shaped keys and dig out round keys in the flat part of the clay.

Step 13: Prepare Empty Half to Pour Silicone

Drill a large pour hole in the thinnest part of the mold using a spade bit.  I find that a 1" hole is a perfect fit for a cut-off water bottle.  Cut off water bottles make excellent funnels for silicone.  Add small bleeder holes at the top and edges so air can escape, and so you know when the mold is full.  Do this at the thin points of your mold.

Step 14: Clamp and Attach Funnel

CAREFULLY match up your mold halves.  Clamp tightly with rubber tipped clamps so your mold won't crack.  Screw your cut off water bottle into the big hole.

Step 15: Mix Silicone

I like Rhodosil 1065.  Follow the manufacturer's direction and weigh components carefully.  Mix slowly and don't introduce any extra air or bubbles.  The silicone should be a uniform color.

Step 16: POUR!

Pour from 18" or more in a thin stream or ribbon to break up any remaining bubbles.  When every bleeder hole has silicone coming out, you're done.  Mixed too much silicone?  Find something else to cast.  QUICKLY. 

Step 17: Separate Your Halves

Once the silicone is fully cured, unclamp and separate the two halves.  Remove clay except for the objects and flashing (which should be stuck in the silicone in the other half,) pour channel and the bleeder holes.  Clean the empty half as before. 

Drill funnel hole and bleeder holes for silicone in empty half.

Brush everything on the "full" side with mold release.

Step 18: Fill Other Half With Silicone

Match the two halves, clamp, mix silicone and pour.  Just like last time. 

Step 19: Crack It and Clean It

Carefully open the two halves and remove all traces of clay. 

Step 20: Cast Some Stuff

Your mold is complete!  Cast to your heart's content.  These decorative gears were cast in low melt metal, and are ready to have the flashing cut off and be added to jewelry pieces.