Procedure for Making a Silicone Molds

The neatness about silicone molding is that is simple and accurate, and will forgive for the mistakes that might have been made when a undercut surface wasn't accounted for. The idea behind this type of tooling is to encapsulate a master that we wish to generate a tool for into the silicone rubber material. This can be easily done buy supporting the master on spacers and enclosing it in some kind of frame.

Before starting the process of making a silicone rubber mold, it is necessary to gather all the required tools, materials, and safety equipment first. It is of particular importance to have everything ready and handy, since the working life of the polymers, i.e. while it can flow, is not very long.

The required tools for the process are a knife, a scalpel, a pair of cutting pliers, a file, a screw driver, and a spatula for mixing.

To set our master in position, we'll need a base plate, adjustable mold wall, thin wire, the master itself, and spacers.

additional equipment to use is contact paper, paper towels, 2 mixing beakers, a tube with a funnel, masking tape, plaster, and super glue.

The silicone rubber material comes in two components that mix by a volume ratio of 1:1. Propanol alcohol can be used to clean the polymers off before they cure. The casting material used to make parts, also mixes on a volume basis of 1:1. And finally, we need the release agent.

Since the polymers are toxic, a respirator mask must be worn during polymer handling. A lab coat, latex gloves and gaggles are necessary to prevent any body contact with the irritant polymers.

Procedure Steps

Before starting the silicone rubber mold, the surface of the master that we wish to replicate must be carefully examined for imperfections. This is a critical step because the silicone rubber mold and its polyurethane parts will exactly replicate the surface of the original master. To ensure a high-quality surface on the polyurethane cast part, one can use a smooth finish with #400 grit emery cloth. In our case, the master was very smooth and no polishing was required.

Elevate the master from the ground by attaching a support to it. The purpose of rising the master above the ground is to allow the silicone to form a mold wall that is at least 1/2 inch thick on the bottom. If the wall is thinner then 1/2 inch in any location, the mold might be weak enough to tore during de-molding. We elevated our master by gluing a screw with super glue as indicated. Since polymer from which new parts will be cast will be entering the silicone mold from the bottom, air in the mold must have a way to exit. This can be easily achieved by super gluing thin wires on the most top edges of the master, i.e. the edges where air will be trapped. We cut relatively thin wire with cutting pliers and sanded one edge flat to increase the wire bonding area.

Regardless of orientation, before securing the master to the plate, the plate should be first released. We used contact paper to avoid using any wax pastes. We secured the master to the plate by simply super gluing the bolt to the bottom plate surface. In some instances, modeling clay can be used to secure the master to the plate as well. If the master is not secured to the plate, separation could occur when pouring the silicone rubber into and around the master.

Next, apply a coat of wax release agent to all master surfaces. This will facilitate separation of the master from completed silicone rubber mold.

Place the adjustable mold wall around the master keeping in mind that it should be no closer then 1/2 inch, so that the silicone mold wall doesn't end up too thin. Fill with modeling clay any voids where silicone rubber could leak Make sure you take good care of this step. If the silicone starts leaking out, it is very hard to stop it since no material wetted with silicone can stick and stop a leakage.

Now we are ready to mix the silicone rubber materials and pour them into the mold.

Calculate approximately the volume of the silicon rubber material required to fill your mold arrangement. It is always good to prepare about 20% more to account for loses. Since the silicone material mixes on 1:1 ratio per volume, one half the required material should come from each component. Although we didn't have one, it is best to have disposable containers with a volume scale to mix the material in.

Make sure the material is thoroughly mixed keeping in mind that mixing should be done gently enough so that more air is not introduced into the material in form of bubbles. This is important, since the degassing process, which is commercially performed, will be skipped in our case for process simplicity. In case some bigger air bubbles are seen on the surface, use the end of a screw driver to poke them open. Keep also in mind that the working life of the silicone rubber material is 15-20 minutes, which means ones the components are in touch with each other, there is only that much time time to mix them and pour them.

It is best to pour the silicon slowly so that no air pockets are left around the master walls. The whole arrangement can be tilted at a slight ange if a large horizontal surface is present. The tilt should decrease the chances of getting air pockets trapped underneath large horizontal surfaces.

Once the mold arrangement is filled up to the necessary height, let the it cure for 24 hours before taking it apart.

To assure that the silicone materials cans will remain properly sealed for future material usage, spray some release agent around the cover of the Prepolymer silicone component. This components likes to form a crust and decrease the cover sealing tightness with time, if the release agent isn't used.

Once the curing time has elapsed, de-mold the arrangement. Since we made a one piece mold (one pouring only), we'll cut the obtained silicone rubber piece into two halves (cutting coud be avoided by making a two part mold)

Use the scalpel to slice the mold open . Several scalpel passes might be necessary before the master is reached. In any event, there is no need to worry about the smoothness of the cut. On the contrary, the more is the cut non-uniform, the less the chance of positioning the two mold parts incorrectly. However, do not exaggerate, after all a horizontal parting line on a vertical master surface gives best casting results. This is a two person job, where one person "pulls" the mold apart so that the scalpel travels easier during cutting while the other cuts.

Please keep in mind that this is the most dangerous step in the process of making silicone molding. Making sure that no risk of cutting yourself or your partner is present at any moment.

Once the cutting is complete, gently pull the two mold parts apart. Thereafter, pull the master out of the half mold. Silicone rubber is pretty flexible and can withstand a good amount of stretching. However, care should be taken so that the stretching is not too high to tore the rubber material apart

Now that we obtained a mold, we can cast parts.

Apply a cote of the release agent onto the silicone mold so that it will prevent the casting polymer from sticking to the silicone rubber.

Since best casting results are obtained by pouring the polymer from the bottom up, we used the supporting screw hole as the polymer intake cavity. The pouring is done by placing a tube into the silicone mold and a funnel on the other end of the tube. To prevent any leakage, the tube insertion hole should be sealed with plaster. Then, position the mold onto a stand so that the tube can nicely bent upward. Secure the two mold pieces so that they won't come apart when the polymer is poured in. We used tape to do so.

Mix the appropriate 1:1 volume ratio of the polymer necessary for replicating the master by accounting for the master's volume as well as loses that will occur in the tube and tube and container walls. Once again, the beaker is best to be disposable, since the cleaning process can be difficult. The tube, naturally cannot be cleaned. If funnels are scarce or expensive, they can be cleaned out with propanol alcohol before the polymer cures.

Once the casting polymer has been mixed, keep the funnel above the mold's top surface and slowly pour the polymer in. Remember to adjust your mixing and pouring time so that the polymer doesn't cure before it has fully penetrated into the mold.