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RTV Moldmaking Tips Alpha


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

Addition Cure Systems.

Inhibition in addition cure systems (platinum cure) such as the Silastic® brand silicone moldmaking product line can range from tackiness to complete lack of cure. Among materials found to cause inhibition are sulfur-containing modeling clays, natural rubber such as latex and rubber gloves, masking tape, amine or sulfur-containing materials, and condensation cure (tin-catalyzed) silicone RTVs. Surfaces previously in contact with any of the materials mentioned may also be inhibited. Water, when present on the part to be molded, can also cause inhibition.

Condensation Cure Systems.

Inhibition in condensation cure systems (tin catalyzed) such as Dow Corning® 3100 series and Dow Corning HS series RTV silicone rubbers is not common. However, there are some sulfur-containing modeling clays that can retard the cure of some of the condensation cured RTVs. Complete cure may take days (with severe inhibition), but unlike inhibited addition cure materials, the condensation cure RTVs will cure when the clays are removed.

In the Dow Corning HS series, Dow Corning HS II 10-1 clear and 10-1 clay catalyst and Dow Corning HS-III 10-1 clear catalyst, when used with their respective bases, have been formulated to cure against inhibition-prone clays.

Techniques to Prevent Inhibition. A standard practice to prevent inhibition is the use of a “barrier coating” to prevent the inhibiting agent from contacting the uncured RTV material. A thin layer of clear acrylic lacquer such as Krylon®1 Crystal Clear Acrylic Spray Coating sprayed directly onto the pattern is an effective barrier coating in most instances.

Tips for Successful Moldmaking


1. 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 become trapped in the mixture and, if not removed by de-airing, can interfere with exact surface reproduction. One exception is Dow Corning® 3110 RTV Silicone Rubber. Due to its low viscosity, this material can be used in many applications without de-airing. Because the mixture of base and catalyst or base and curing agent will expand during de-airing, it is important to use a container that is between three to five times
the volume of the material itself. The higher viscosity silicone moldmaking rubbers will expand the most. The mixture can be quickly and easily de-aired in a vacuum chamber. This important step usually takes just a few minutes. Entrapped air may be removed by applying a vacuum of 26 to 29 inches of mercury. Dow Corning® HS series RTV silicone rubbers should not be de-aired for more than 5 minutes. Any more de-airing will change physical properties. In all cases, the material will expand and then recede to its original level. For approximate time to complete de-airing, consult the individual product data sheets.

2. Inhibition
Cure of Dow Corning® brand RTV materials may be inhibited by certain contaminants in or on the pattern to be molded. The inhibition reaction will impair or stop the crosslinking necessary for the RTV material to cure, resulting in tackiness at the pattern interface or a total lack of cure throughout the mold. If in doubt as to possible surface inhibition, a “patch
test” is recommended. Brush or pour a small amount of the catalyzed RTV selected onto a noncritical area of the pattern. Inhibition has occurred if the rubber is gummy or uncured after the recommended cure time has passed. Polyvinyl alcohol (PVA) is another effective barrier coat. This water-based solution can be applied by brushing, wiping, or spraying onto the master. It is important the film be completely dry before molding. After the mold is cast, the film of PVA can be removed from the pattern by placing it underwater and rubbing briskly. The film will redissolve.

3. Thinners
200 ® Fluid serves as a thinner and can be used with all Dow Corning silicone moldmaking rubbers. A wide variety of viscosities is available: 20, 50, 100 and 350 centistokes, with 50 cSt the most common. Using a thinner to lower the viscosity of the RTV bases requires only 1 to 3 percent of 200 Fluid and does not affect the physical properties of the rubber when using a thinner to reduce durometer.

4. Release Agents
pattern/master. For molds cured at room temperature, For Patterns/Masters. A release agent should be used to ensure easy removal of the cured rubber from the a simple mixture of 5 parts petroleum jelly to 95 parts solvent is recommended. Suitable solvents include VM&P naphtha or mineral spirits. The petroleum jelly-solvent mixture can easily be prepared by putting the items together in a suitable container and setting aside overnight. With slight agitation before use, the mixture is ready to apply. To ensure complete coverage, brush the mold liberally with the petroleum jelly-solvent mixture; then hit with a spray of air from an air gun. The air will aid in evaporating the solvent, eliminate puddling and the resultant loss of detail. This process will leave a thin film of petroleum jelly over the entire pattern/master. When casting a two-part mold, it is extremely important to completely cover the cured half of the mold with a good release agent to prevent the two halves from bonding together. Straight petroleum jelly can be used, as well as a solution of 30 to 50 percent petroleum jelly. Dusting with talc or baby powder has been found to be effective, as well as the use of Teflon®2 aerosols. For Silicone Molds. When first cast, silicone rubber molds exhibit natural release characteristics. Over time, however, the reactant agents in most casting resins will deplete mold lubricity and parts will begin to stick in the mold. A release agent should be used at the first sign of sticking and reapplied only when sticking
reoccurs. When using a silicone release agent, it is necessary to burnish in the release agent only on the area(s) where
sticking occurs. If using a silicone release agent in an aerosol container, there are cautions to be taken. Silicone can cause nonwetting spots (fisheyes) in nearby areas where painting is being performed. It is best to use straight dimethyl
fluid (200 Fluid) and rub it into the mold where sticking is occurring. Wiping off any excess will prevent nonwetting areas on the piece(s) cast from the mold.

5. Amount of Rubber Needed to Cast a Mold
• Dow Corning® HS II RTV high strength moldmaking rubber: 20 grams/cubic inch
• Dow Corning® HS III and HS IV RTV high strength moldmaking rubbers: 18.8 grams/cubic inch
• Dow Corning® 3110 RTV silicone rubber: 19 grams/ cubic inch
• Dow Corning® 3112 RTV silicone rubber: 23 grams/ cubic inch
• Dow Corning® 3120 RTV silicone rubber: 25 grams/ cubic inch
• Silastic® E and T RTV silicone rubbers: 18 grams/ cubic inch
• Silastic® J, K, L, and M RTV silicone rubbers: 21 grams/cubic inch

6. Patching Torn Molds
Using a steel brush, abrade the area to be patched, then clean the tear with a good grease cutting solvent such as naphtha or mineral spirits. Be sure the solvent has completely evaporated before proceeding. Because silicone rubber sticks so well to itself, for the strongest patch, it is recommended that you use the same silicone rubber that was used to make the mold. Silastic® 732 RTV Multipurpose Sealant can also be used to repair torn molds.

7. Compression Casting
When using silicone rubber materials to do compression casting, holes must be drilled to permit venting. One hole for every 25 square inches of surface area should be sufficient. Maximum size is 1/16 inch.

8. Oak Wood Patterns
When using new oak wood patterns, a microsized porosity often occurs at the open grain of the wood. To avoid this, apply some petroleum jelly to a clean cloth and gently rub the surface in the direction of the grain. Teflon is a registered trademark of E.I. du Pont de Nemours and Company.

9. Mold Box Release
A coating of Dow Corning® 236 RTV Dispersion on wooden mold boxes prevents resins, especially polyurethanes, from sticking to the mold boxes.

10. Mold Life Extension
Barrier Coating. The use of a barrier coat when casting polyurethanes can greatly extend mold life, in some cases up to 200 percent. The barrier coat should be sprayed into the silicone mold prior to each casting. When the cast part is removed from the mold, the barrier coat becomes the outer skin of the casting. The barrier coat can then be stained or painted, an important feature. Note that this type of barrier coat is different than the barrier coat mentioned in “Techniques to Prevent Inhibition.”
Reconditioning. Reconditioning can be accomplished by burnishing a low viscosity 200 Fluid into the surface. When the mold is to be put back in use, any excess fluid should be removed from the surface. This is necessary to ensure the cast parts will be paintable. Non-wetting or fisheyes can occur on the surface of the cast parts if all excess 200 Fluid is not removed.
Bake-Out. A bake-out is recommended to remove the hardeners, plasticizers and other materials that leech out of the casting materials and are gradually absorbed into silicone molds. A slow, gradual bake-out at 93°C (200°F) for eight hours or a rapid bake-out at 204°C (400°F) for two hours can be used.

11. Library Life
To extend the library life (shelf life) of a cured silicone rubber mold, it is important to thoroughly clean the mold before storage. If possible, a bake-out (see “Bake-Out,” in “Mold Life Extension”), followed by wiping the mold with a solvent, is best. If a bake-out is not possible, wiping the mold out with an aggressive solvent such as toluene will still help considerably. After cleaning the mold, apply a thin film of 200 Fluid (low viscosities of 20, 50, or 100 centistokes are best). Placing a master of wax, plaster or wood in the mold will help retain the mold’s shape.




Always provide adequate ventilation when using any solvent. In addition, all solvents should be completely evaporated before catalyzed RTV rubber is applied to the master pattern. When using any solvent, avoid heat, sparks and open flame. Follow the manufacturer’s directions on container labels, including precautionary handling statements.

Dow Corning believes that the information in this publication is an accurate description of the typical characteristics and/or uses of the product or products, but it is your responsibility to thoroughly test the product in your specific application to determine its performance, efficacy and safety. Suggestions of uses should not be taken as inducements to infringe any particular patent. Unless Dow Corning provides you with a specific written warranty of fitness for a particular use, Dow Corning’s sole warranty is that the product or products, as supplied, will meet Dow Corning’s then
current sales specifications. DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND OF FITNESS FOR USE. Your exclusive remedy and Dow Corning’s sole liability for breach of warranty is limited to refund of the purchase price or replacement of any product shown to be other than as warranted, and Dow Corning expressly disclaims any liability for incidental or consequential damages.
Dow Corning Corporation
Midland, Michigan 48686-0994
Dow Corning, Silastic and 200 are registered trademarks of
Dow Corning Corporation.

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