General Things You Should Know

1.) Gloves: You're always going to want to have latex gloves on while working with rubber, resins, and pouring molds.

2.) Be Carful, don't spill: Of course, no brainer, right? Well, this is just a heads up because none of us want resin to cure on our garage floors or the wife's extra table that you're using to pour your molds. :)

3.) Ventilation: Remember that we are working with chemicals and inhalation of emmited fumes in an enclosed area is not good for you. Open your garage or shed doors, and if you really want to be safe have a fan going.

Tips for Successful Moldmaking

Once you’ve selected the correct material and determined how to properly use it, you’re on the way to making good flexible molds with silicone rubber. The following recommendations will further insure your success.

De-Airing

De-airing is recommended for all silicone moldmaking rubbers when not using automatic dispensing equipment. The small air bubbles that result from hand mixing can become trapped in the mixture and, if not removed by de-airing, can interfere with exact surface reproduction.

Because the mixture of base and catalyst will expand during de-airing, it is important to use a container that is between 3 and 5 times the volume of the material itself. The lower-viscosity silicone moldmaking rubbers such as Dow Corning HS III RTV high strength silicone rubber and Dow Corning 3110 will expand the most, up to five times. The higher viscosity silicone rubbers such as Silastic E, J, M-2, and L will expand the least, approximately 2-3 times the volume.

The mixture can be quickly and easily de-aired in a standard vacuum chamber. This important step usually takes just a few minutes. Place the mixed silicone under vacuum at 29 inches of Hg and hold it there until it completely expands and recedes to it original level. Time required to complete de-airing depends on the size of equipment being used.

Thinners

Thinners can be used with all Dow Corning silicone moldmaking rubbers. Dow Corning 200 Fluid acts as a thinner of viscosity for the unmixed silicone. It will have a tendency to slightly lower the hardness of your cured silicone system a little bit. The 200 Fluid can bleed from the cured rubber if too much is added to the silicone, therefore we do not recommend exceeding 10%. The 200 Fluid will decrease the viscosity of the rubber and make it easier to pour.

Mold Life Extension

Reconditioning. The 200 Fluid can also be used to rejuvenate the mold surface to extend the mold life of the rubber. Casting resins will eventually dry out the oil that is within the silicone moldmaking rubber and by wiping a thin coat of the 200 Fluid into the mold prior to it completely breaking down, it is possible to rejuvenate the mold surface and extend the life of the mold. Rub the mold surface with the 200 Fluid and then wipe the mold surface dry with a clean cloth to remove any excess. Let the mold sit overnight.

Bake Out. A “bake out” is recommended to remove the hardeners, plasticizers, and other materials that leach out of the casting materials and are gradually absorbed into the silicone molds. A slow, gradual bake out at 200F for eight hours or a rapid bake out at 400F for two hours can be used. This will draw silicone oil from within the mold back to the surface extending the life of the mold.

Release Agents

Release agents are sometimes required for easy removal of some types of pattern materials that may occasionally stick. Silicones typically only stick to other silicones and will not typically adhere to any other materials. Dow Corning recommends using a thin layer of melted Vaseline to prevent adhesion between silicones and other difficult to remove pattern materials. Do not use a silicone mold release for it will act as a primer and promote adhesion to another silicone or your pattern.

Tips for Successful Casting

As with all chemicals, you should always read and follow all of the safety precautions prior to working with the materials. Read all the safety precautions found on the Material Safety Data Sheets, printed on the bottles, and in this catalog before working with the materials.

Keep Alumilite out of the reach of children, do not take internally, and do not use in any way other than it’s intended use. Even though it has very little odor, we still recommend using Alumilite in a well ventilated area. Safety first!

Mixing

Before mixing make sure you know the proper mix ratio of the material you are using. All of the casting resins are 1:1 by weight and/or by volume.

You must mix at least a half an ounce of each side to ensure you have a proper mix ratio. If you measure out 1/4 of an ounce of A in one cup and 1/4 ounce of B in another and dump the A into the B you will be off ratio due to the residue left in the A side cup. In larger amounts of resin batches, this will not be enough to throw off the mix and cause an issue with the resin setting up. But with small amounts of resin (1/4 oz of each side) this will be enough to effect the mix ratio and will typically result in parts that appear darker in color and remain soft (never harden).

Once the materials have been measured out in separate cups, the preferred method of pouring one into another, to decrease the amount of air introduced, is to pour the A side into the B side.

After the materials have been poured together, mix vigorously (keeping the stir stick in contact with the bottom of the cup - reduces air from being introduced into your resin) for approximately 15-25 seconds. Make sure to scrape the sides and the bottom of the mixing cup.

Pouring

Once the material is thoroughly mixed, pour the resin slowly down the side of your mold cavity. Tilting your mold will prevent the resin from splashing in the bottom of your mold and creating unwanted air bubbles that would then need to find their way to the top of the mold. Similar to tilting your glass as you pour a beverage rather than letting it splash off the bottom creating air bubbles.

Squeeze the brim of the cup to form a point. This will allows you to pour a smaller stream of resin into your mold controlling the flow and reducing the chance of unwanted air bubble entrapment against the surface of the part.

If your mold has undercuts, pour enough resin into the mold to fill it half way. Then, tilt and rotate the mold in the opposite direction of the undercut to allow the air to escape up the side of the mold. Squeezing or burping the mold at the same time will also help relieve the air trapped in the undercut and allow the bubbles to release from the mold surface. Once you see air bubbles come to the surface of the resin and you can be confident you have removed the air from the undercut, simply top off the mold by pouring the remaining resin into the mold.

Open Time

To increase the open time of Alumilite resins, simply place the “A” side in the refrigerator for approximately 30 min. before pouring. This will increase your open time by 30-60 seconds. When cooling your resin, you must preheat your mold to ensure a proper cure.

Miscellaneous Tips For Moldmaking & Casting

Demolding Silicone & Resin.

To aid in the release of silicone rubber from your mold box or your original, use a small amount of rubbing alcohol. The rubbing alcohol will make the cured silicone rubber very slippery and will help separate the silicone from the other surface. After you remove the original, dry out the excess rubbing alcohol with a paper towel or dry cloth. Be sure to warm and dry out the mold completely before pouring resin into the mold to avoid the alcohol from contaminating and effecting the resin.

You can also use rubbing alcohol to assist you in removing resin pieces out of a silicone rubber mold. Use the same process as mentioned above for removing your mold from the original and the mold box.

Release Agents.

When pouring RTV silicone moldmaking rubber against any non silicone surface, mold release is not required. If you are pouring RTV silicone moldmaking rubber against itself, you must use a mold release. Use Alumilite’s Rubber to Rubber release or smear a thin layer of Vaseline anywhere the silicone will come in contact with the already cured RTV silicone rubber. If no release is used the RTV silicone will bond to itself and you will have a solid chunk of silicone. You will then have to cut the silicone to remove your original.

Dyes.

Alumilite’s coloring dyes can be added to Alumilite’s casting resins to achieve any color you desire.

The dyes should be added into the “A” side of Alumilite’s resin for compatibility reasons. The dyes are reactive which means they will crosslink and become part of the cast piece. The dyes will crosslink with components on the “B” side and that is why we recommend for shelf life reasons to add the dye to the “A” side.

As a rule of thumb, the dyes can be added up to 5% of the weight of the “A” side to reach a desired color. Adding less dye will achieve a lighter color.

Fillers.

Alumilite casting resins can be filled with any dry filler of your choice. Dry fillers are used to reduce the amount of resin needed to cast a part making it more economic, add characteristics such as weight, feel, or texture, and also to add physical properties such as strength, heat resistance, and durability.

If your filler contains moisture your part will foam due to the reaction with the moisture in the system. Use dry fillers if you wish to avoid foaming in your part.

Shelf Life.

The Shelf Life of Alumilite casting resins are 1 year in a sealed unopened container. The material will remain useable as long as moisture does not contaminate the resin. For extended storage periods, store in a cool dry place.

Adhesion.

Bonding Alumilite is best when the material is still curing shortly after demolding. The best adhesives for bonding Alumilite are ones that promote a chemical bond. Adhesives that work include but are not limited to: MMAs, epoxies, CAs (super glues), one part silicones, one part urethanes, and hot melts.