Flexible Jewelry Molds

A friend asked how to cut a dome ring mold. Here is a very rough approximation of his model. I made the ring just so I could make the mold. :-)

Sprue former on the right end of the picture, silver model on the left. Note that the dome is very hollow.

The video I am working on will demostrate how to cut this type of plug mold. It is good for very thin objects with large inside spaces that need to be removed from the wax.

Note that the plug has an outer ring that carries the impression of the inside of the ring shank. There is an 'S' shaped bar that fills the center of the plug.

This shows how the plug is pulled out of the mold. Note how the pink handle of the plug pulls on the 'S' shaped bar and finally the end of the bar pulls the plug out of the mold.

I know. It is really difficult to picture how this happens. I plan for the video to make it clear how to cut a mold like this. 

Here is an image of the heat-cured silicone rubbers from Nicem (http://nicem.it) that I use for this mold. I prefer this rubber for this type mold because the shrinkage is the same across all the different rubbers from hard to supple.

How and Why I choose a mold rubber.

The best way to choose a mold rubber is to first consider the model that you are molding.
Answer these questions:

1. How delicate is the model - of what material is the model made?
    a> is it wax, plant, or animal? (therefore fragile and heat sensitive)
    b> is it wood, stone, or metal? (more robust and not sensitive to heats up to 350F (177C)
2. How quickly do you need a few waxes from the model?
    a> less than an hour?
    b> in a few hours?
    c> next day?
3. Will you accept some shrinkage from the model to the production item?
4. What sort of mold making equipment do you have?
    a> Vulcanizer?
    b> C-clamps, aluminum plates and aluminum mold frame?
    c> Nothing? ie, no way of heat curing a mold?
5. What shape is the model?
    a> light weight lady's ring (average thickness of less than 1 mm)
    b> heavy gent's ring (average thickness of around 1 mm)
    c> Thick belt buckle (average thickness of 3 mm and large-3 cm x 5 cm)

If the student doesn't know or doesn't care about these questions and has a vulcanizer and mold frame, let them use natural rubber to make their molds. Both silicone and natural rubbers produce good molds.

If the model is delicate, I tend to use a two part, room temperature vulcanizing (RTV ) compound. There are many good compounds on the market, RTV-RP from Zero-D, Freeman Jewel-Sil, Castaldo Liqui-Fast, and Rio Ditto RTV compound.
These compounds require the use of a vacuum chamber to remove bubbles from the mixture. They can be ready in 1 to 24 hours. These compounds usually give a low shrinkage.

The quickest mold compound to use is a two part compound like Castaldo Quiksil or Belicone Cold Molding Silicone Putty. They cure in about 20 minutes. Equipment needed is a mold frame, 2 or 3 mm thick mold plates, and a C-clamp or two. These compounds are said to be zero shrinkage.

Heat cured silicone mold compounds offer the most versatility. There are a few temperature ranges available. The silicones that cure between 160F (71C) and 190F (88C) are great for making molds of the higher melting temperature carving waxes. Higher temperature curing silicones are excellent for molding metal models and heat resistant models. You will need a vulcanizer, mold plates and a mold frame to cure these types of molds. A few benefits of heat cured silicones are no silicone release spray is needed to remove waxes from the molds; molds are durable enough to make many thousands of waxes; inside of mold can be engineered to include very soft to very hard areas allowing excellent injections of very thin (0.5mm) waxes. Shrinkages from 0% to 4% are available. See my mold compound comparison chart at http://www.flexiblejewelrymolds.com/material-comparisons.html to find the mold compound that best suits your job.

Finally, mold compounds come in many durometers - softness and hardness. Rubber bands are soft and about Shore 28 durometer. Car tires are hard and about Shore 60 durometer. Pack your model with soft silicone rubber around it, then surround that soft rubber with harder rubber. When you extract the wax injection, the soft rubber will 'let go' of the injection more easily without warping it. The surrounding of hard rubber will allow you to inject the wax at a higher pressure by containing that pressure. 

I was told at the beginning of my career to build in 8 to 12% shrinkage in my wax carved models to make the finished jewelry turn out the right size. How do we use the 4.0 to 0.0 % shrinkage figures we are given by the mold compound suppliers? Is the shrinkage consistent from a small mold to a large mold, a small model to a large model, from one red mold compound to the next red mold compound?

How do we compensate for this shrinkage? What is the conversion factor from model to mold for a given compound? Never mind how much the wax shrinks, the investment, nor the cast metal, nor the abrasive finishing processes. This is about shrinkage in mold compounds.

Method:

I test and compare two methods for determining shrinkage in the materials. One is using a single linear dimension in the mold, the other was an average of two dimensions at right angles to each other on the mold. I test compounds cured at the recommended vulcanizing temperature, and then at higher, or 'push cure' temperatures. I will also be testing RTV (room temperature curing) molds and compare them to the same RTV compounds cured at the manufacturers' recommended 'push-cure' temperatures. 

I expect to answer the question: "Is the shrinkage in a mold material different when it is 'push cured'?

First up is Rio Ditto 2. It is a great, firm, RTV compound that push cures in a couple of hours at 300F. Or, you could wait 16 to 24 hours for it to reach full hardness at room temperature. Here is an image of the ruler I embedded in my mold:

We are looking down into the top of the mold. The ruler is taped to the side of the mold frame. Green wax model is in the center of the mold frame. I like to use masking tape to hold the glass sides on the aluminum mold frame. 

Results on this test will be available on the new shrinkage data spreadsheet soon.

Here is an image of my first test of a heat cured silicone compound. I used Zero-D Products 'Zeroshrink'. It shows a section of stainless steel ruler on the mold compound. I'll vulcanize the mold and compare the shrinkage percentage I get off the length of ruler inside the mold to the shrinkage percentage I measure on the outside of the mold. I've always wondered how Castaldo and Zero-D measurement methods compare. :-)

I use RTV mold compound in my studio in a very frugal way. I learned this method from an attendee at the Santa Fe Symposium on Jewelry Manufacturing Technology (SFS) in the ’90’s. I’m very grateful to him for his suggestion. Sorry that I’ve forgotten his name. He has saved me quite a bit of money over the years and improved my wax injections of wax masters with this technique.

The first step is to take a grocery bag to your local camera and film processing store. Ask them if you can help them recycle their plastic film canisters. They usually say yes and give you a grocery bag full of empty canisters. The ones without internal ribbing are the best, though any of them will work.

Tools are: 

a gram scale accurate to 0.1 gram, your favorite RTV compound, spatula, mixing bowl, a carving wax master, a bit of sprue wax, wax pen, masking tape and a film canister. 

The wax should fit into the canister with a  minimum of 7mm clearance all around. I’ve used larger diameter PVC pipe to make larger cylinder molds. It is easier to use the U shaped aluminum mold frames  for larger waxes rather than use PVC pipe. Film canisters are frustrums - one end is a slightly larger diameter than the other. Great clamping pressure can be achieved by pushing the finished mold a little deeper into the canister prior to wax injection. The canister pushes in on the cylindrical mold supporting it against higher wax injection pressure.

Figure 1:  Blue wax square bezel with red sprue wax at bottom is the master. Round bottom of light gray film canister at left middle shows drilled hole into dome that makes a sprue former. Upper right shows film canister with  blue wax master in place. Right side of image shows yellow sticky wax normally used to connect master sprue with bottom of canister.

Mold making procedure: 

Drill a hole the diameter of the sprue wax in the little dome in the bottom of the canister. I wonder why they so conveniently pre-placed a sprue former in the film canister? Attach a 10 cm (2.5 inches) length of wax wire to the master. Thread the wax wire through the hole until the master is centered at the correct level in the canister. The correct level leaves at least a centimeter of space between the sprue former and the master. The correct level allows at least a centimeter of space between the master and the top of the film canister. Center the master in the available space for best results. From the outside of the canister, melt the sprue onto the edges of the hole in the bottom of the canister.

Figure 2 Wax sprue wire melted onto the edges of the hole in the canister.

While the sprue is still warm, turn the canister over and adjust the master so that it is centered in the canister. Add a line of wax on the insides of the canister to mark where the parting marks should go. This shows where to place your cut in case you are using an opaque RTV compound.

Figure 3 Two film canisters with blue wax masters positioned and almost ready for filling with RTV compound. Pink vertical lines inside canisters show where outside edges of molds should be cut.

Wrap a collar of masking tape around the top of the canister. This prevents overflow of the RTV compound when it is vacuumed in the canister. If the RTV is going to be “pushed”, that is, cured at a temperature higher than room temp, be sure to use sprue and attachment wax that melts at a higher temperature than the cure temperature!

Figure 4 Canister with collar of masking tape around it.

Although I chose Freeman Clear silicone RTV mold compound for this demonstration, this next image shows the cream colored base and bright green catalyst of Smooth-On Mold Max 40 before mixing. 

Figure 6. Note much larger mixing bowl to catch the foaming RTV compound as it expands during the vacuum de-airing.

Weigh and mix according the manufacturer’s instructions about 40 grams of RTV compound for one film canister mold. Vacuum de-air it in the mixing bowl for a couple of minutes after it rises and falls. Be sure to use a mixing bowl 5 times the volume of the mix. It rises a lot when first vacuumed. Fill the film canister to the plastic rim, covering the wax master. Re-vacuum the compound in the canister.

It is much easier to cut the canister off the outside of the cured mold cylinder than to try and save the canister. Remember, you have a whole grocery bag full of canisters.

Figure 7 Transparent RTV compound with side view of blue wax master.

Figure 8 Transparent RTV compound with bottom view of blue wax master. Notice the green carving wax sprue on this master because mold was ‘pushed’ to 200F in order to cure it more quickly.

Cut the mold like you would any other mold, following the wax marks on the outside of the cured cylinder. 

This video shows how I cut a cylindrical mold containing a similar master with prongs.

Cut 3 mm off the bottom of another canister identical to the one you just cut off your mold. Now you have your cylindrical ‘mold plate’ to hold the mold together as you inject wax into it. You can see how this works in the preceding video.

Figure 9 Two halves of the mold on the left. Film canister on the right with bottom cut out of it. The mold is supported inside the canister during wax injection.

Figure 10 Two halves of the mold with pink wax injection below them.

 When you are ready to inject the mold, powder the outside of the mold and the inside of the canister so that the mold will slide in and out easily. Inject some wax, add vents where needed, and have fun casting.