Making better RTV molds.

[bizzybody]

One item that's very useful for making RTV molds is a Dremel type rotary tool with a mandrel to hold fiber reinforced cutoff disks.

How I make molds is I use a sink cutout from a kitchen countertop as my board. They're available free or very cheap from most any place that makes custom countertops. The plastic laminate makes for a smooth parting face on the mold and it's heat resistant enough that hot glue doesn't bother it.

I made sets of mold forming boards by gluing and nailing together pieces of 3/4" thick pine into L shapes. Four of these fit together to make any size mold I need. I've found that except for really large molds, spring clamps are fine for holding them together. I coat the inner faces and edges with high temperature mold release wax a couple of times, mainly so the hot glue I use to stick the assembled frame to the mold board will peel off. Since the wax soaks into the wood, a single initial waxing has been enough.

For the sprue hole in the first half I use a piece of half-round wood molding, then insert a full round piece (or two half rounds glued together) in the first half when pouring the second half. Use molding sized so a small and cheap plastic funnel can be wedged into the hole when the mold is assembled. The funnel helps with filling larger molds, mix up the amount of resin needed then dump it all into the funnel at once. Bubbles in the mix will be forced towards the center and up instead of going into the mold.

A tiny drop of super glue secures the half sprue former to the mold board. Don't bother putting anything on the board to make a connection to the cavity. It's easier to use the Dremel to cut it after the second half of the mold is cast.

Other than the sprue former and using modeling clay to seal the original to the molding board and form up to the parting line, I don't do anything to form vent channels or keys in the first half.

Here's where the Dremel comes in. Use the edge of the cutoff disk to carve key dimples into the parting face of the first half of the mold. Much easier than forming them using lumps of clay on the molding board.

Another thing I do with the Dremel is use the disk to cut a channel about 1/8" wide and 1/8" or a bit more deep up both sides and across the bottom end of the parting face of the first half. If the mold is small with little clearance between the cavity and sides of the mold, I'll cut the channel right along the outer edges.

The reason for this channel is it provides very good alignment keying and helps seal the mold to prevent air blowing in through the parting line when pressure casting.

After making the second half of the mold (I use mold release products from Smooth On), out comes the Dremel again. This time it gets used to carve vent channels. The thickness of the cutoff wheel is perfect for vents. Cut a vent from the very bottom of the cavity, around it and up to the top edge where the fill sprue is. Cut other vents as needed, especially where there's any upward turned section that could trap air or small protrusions along the parting line.

Finally, cut a vent close to the fill sprue on either side. Many times bubbles will come right up to the base of the fill sprue and for some reason stop there instead of coming on out. Putting a vent into where they catch will let them out.

The powdered silicone from using the Dremel is some very sticky stuff. Wear a dust mask! After you're done with the cutting and carving you'll have to wash the mold with soap and a nail brush under hot, running water to get all the silicone dust off. Even then some will usually be hiding in nooks and crannies somewhere. Make the first pouring a junk one, it'll get out any leftover dust and junk in the cavity. It'll also reveal places you may need more vents or enlarged vents.

When assembling RTV molds for casting, don't just wrap rubber bands or straps around the mold! Cut a pair of boards a bit larger than the mold, then place one board against each half of the mold and put the bands or straps around the boards. That produces even pressure and doesn't distort the mold.

You only need just enough clamping pressure to hold the mold together so it won't leak plastic out.

Ceramic mold straps like these http://www.boothemold.com/classroom/Strap/index.html are ideal for holding RTV molds together (with clamping boards). It takes a few tries each time to get the tension right, not so tight you're mashing the mold, not so loose you'll be making plastic puddles.

The final item you'll need is a rubber mallet. Use this to give the mold a few thumps to jar loose bubbles so they'll rise up to the top and out the fill sprue.

I cast some pretty large parts, nearly 200 grams, in Smooth On's 200 series crystal clear urethane. I also cast several smaller items. The above are some of the tricks I've learned to produce good casts.

I also spin-cast pewter and zinc alloy in disk shaped hot vulcanized silicone molds. Unlike RTV silicone, spin casting metal uses some expensive equipment.