Introduction: Silcone Mold Making for Investment Casting Pendant
I work with Jim Kibler in his workshop. Together we have been making silicone molds for castings for decorative antique-style rifles. Most recently, I have been using silicone molds to make sterling silver necklace pendants out of ornate rococo thumb pieces.
Since I first learned about the artistic niche of antique style longrifle making, I wanted to try to find a way to participate. I can't exactly carry one of these rifles around with me to enjoy so I had to find another way. I think some of the rococo designs are perfect for conversation-starting, gorgeous silver jewelry.
Since I want to capture beautiful work that is already done, mold making for reproductions is the way to go. Obviously, this same process applies to reproducing other small metal parts or jewelry that you would like.
I will be showing the mold pouring part of the process and end just before the wax injection. If people are interested, I can go over making waxes and then investment casting in other instructables.
Jim has more insight into his workshop and other helpful info on his blog: www.jimkibler.net/blog
If you have any questions or requests for other Instructables that may be up our alley, please email us at firstname.lastname@example.org
All my best,
Step 1: Step 1: Prepare the Master
Silicone does a freakishly good job of capturing every detail and goes into every nook and cranny. Take time up front to clean and shape your master as much as possible before pouring the mold or else you will have to clean it up eventually anyway. And if you plan on making more than one casting, you will have will have to do the same work over, and over and over again.
The thumb piece from the Rifle Shoppe that we will be turning into a silver pendant is shown in it's raw cast state above.
For the flowery thumb piece I plan on casting, we (mainly Jim) had to do quite a bit to get it ready. It was too thin around the edges and too thick around the middle. He first removed additional metal with files. Then, the shape was a little off (there were some low spots on the front) so he hammered it until Jim's very finicky eye was satisfied that it was the right shape.
I used progressive sanding stones 100-400 grit to smooth the surface and Jim came in with the gravers to add the detail. To build up the thin edges, and fill the empty spots on the back, I piled some Bondo on the back and sanded it down.
For relatively thin, flat pieces like this, I add a piece of box tape to the back that goes beyond the edge because this makes it a million times easier to cut it out of the silicone and, as an added bonus for this jewelry application, it makes for a very slick surface on the back. Use a black Sharpie to mark the outer edge of the tape so you can see it more easily when you are cutting the silicone parting line later on.
This whole prep process took at least a couple of hours, but the silver castings should be in pretty good shape and require significantly less time.
Step 2: Step 2: Prepare the Mold Box
We use (and reuse) basic mold boxes that Jim made of plywood. They are surrounded on all sides, except for a hole big enough to pour silicone into and a sprue hole.
It's important to have enough screws all around to keep the two halves of the box tight and prevent flash when the wax is injected later.
For these thumb pieces I inject the wax into the middle of the back, so I put a dowel rod into the back of one of the sides of the box for the sprue and then super glue the piece in position.
I want to make sure there is ample silicone surrounding the thumb piece in order to hold up to the pressure of the wax, but I don't want to waste silicone. A rule of thumb is that I have at least 3/8" between the edges and the walls. Set depth of sprue to place thumb piece as close to the middle as possible.
When I am confident that the glue will hold, I close up the box, and get my silicone ready to pour.
Step 3: Step 3: Prepare the Silicone for Pouring
We use the Freeman V-3040 for our molds. It's transparent which makes it easier to cut them open and it captures a wild amount of detail. It's two part with a 10A:1B ratio.
To quickly estimate the amount of silicone that I will need to pour, I weigh another mold made with the same mold box. You can always to old fashioned conversion of volume to mass using the density of your silicone. It's worth the time to mix the right amount. (add 10% for residue sticking to the sides of your mixing container). We have been known to try to eyeball it before and skip this step and we usually end up mixing up way too much and wasting it.
This stuff needs to be degassed or else you will have gobs of bubbles in your silicone which will add warts to your wax.
Have your vacuum chamber ready and all your tools out before you get started. You have quite a bit of work time before the silicone sets up, but it's always best not to push it. In the photo, you can see in the vacuum chamber during degassing, the silicone rises quite a bit before collapsing down again.
I cut my container shown above too small, so I put a couple of rows of tape around the top for a little additional height to prevent an overflowing mess in my vacuum chamber. I use the scale (accurate to the gram) for accurately measuring my 10 parts A and my 1 part B and then stir for a couple of minutes.
The hardener, part B, needs to be added last and shouldn't touch the sides of the container unless it is already mixed with part A. Otherwise, it may get absorbed in the plastic and not mix up with your part A which may prevent your silicone from curing. I use the tare function of the scale to keep it easy. I added 382 grams of A, tared the scale, then added 38 grams of B.
Step 4: Step 4: Vacuum Chamber
Put the container in the chamber and begin pulling a vacuum, being careful to keep your hand on the release valve and your eye on the bubbles. A mess can happen very quickly as the mixture rises and can be prevented by letting a little bit of air into the chamber as the bubbles get close to the top. This air just keeps the mixture under control a bit but this won't hurt the final product.
Once the bubbles collapse, feel free to fully close the valve (if it's not already) and draw a full vacuum for 2 minutes. The mixture won't look like it's doing anything during this time, but this ensures there are no evil little bubbles left to mess with your hard work. Mixture during full pull after collapse. I'm glad Jim told me to add the tape or else I would have overflowed.
Step 5: Step 5: Pour the Silicone
The key during the pouring is to be careful and controlled to keep from adding air. Pour relatively slowly (about 1/2-3/4" stream works best) and into the bottom of the mold if possible. You don't want to pour directly on the part because it may entrap air as it falls from the part to the bottom of the box.
Don't stir as you are pouring (adds air) but you may find it helpful to use your stir stick to gently guide the silicone that is very thick and sometimes reluctant to pour neatly. Pouring a (different) mold. This requires lots of patience.
Step 6: Step 6: Curing
The curing process can be sped up with heat-- otherwise it takes about 12-24 hours. We just aren't that patient. We put it in a hot box that is about 100 degrees F for about 6 hours, turning halfway so that both sides get access to the heat source. We once made the mistake of letting it air cure near a cold window and it wasn't cured even after 24 hours. It's very temperature sensitive and the range of acceptable temperatures can be found on the manufacturers data sheet.
Step 7: Step 7: Finishing Mold and Getting Ready for Wax
Once the mold is done cooking, we unscrew the box and proceed to wrestle the silicone away from the plywood box. The silicone will fill all the little microscopic holes in the plywood and all those tiny tentacles need to be broken. There is a better way to do this with a box made out of something other than plywood, I'm sure, I just haven't taken the time to make it. I use a wide metal scraper to separate the box from the silicone and try to be careful not to slice into the edge of the silicone loaf.
Once I manage to get the silicone out of the box, then I need to get the part out the silicone. If I taped and marked my tape, this shouldn't be too bad. The parting line will be easy to find and cut. I use one of Jim's chisels to slice into the silicone, making sure to keep the two parts pulled open as I slice. This will keep me from accidentally cutting the same section twice or cutting out chunks. Just follow the black tape line, pull the piece and the dowel out and you have your mold! I rinse the mold, dry it with compressed air, and spray it with silicone to help the wax release.
Any questions, please ask! I would be happy to help.