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The Right Products, The Right Expertise: Tips for Successful Moldmaking with Silicone Rubbers

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 . The following recommendations will further ensure your success.

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 by applying a vacuum of 27 to 29 inches RTV selected onto a noncritical area of mixture and, if not removed by de- of mercury. SILASTIC™ HS Moldmaking the pattern. Inhibition has occurred if the airing, can interfere with exact surface series RTV silicone rubbers should not be de- rubber is gummy or uncured after the reproduction. One exception is SILASTIC™ aired for more than 5 minutes. Any more recommended cure time has passed. RTV-3112 Mould-Making Base. Due to its de-airing will change physical properties. low viscosity, this material can be used in In all cases, the material will expand and Addition Cure Systems many applications without de-airing. then contract to its original level. For Inhibition in addition cure systems Because the mixture of base and catalyst approximate time to complete de-airing, ( cure) such as the SILASTIC™ or base and curing agent will expand consult the individual product data sheets. silicone moldmaking product line during de-airing, it is important to use a can range from tackiness to complete container that is between three and five 2. Inhibition lack of cure. Among materials found to times the volume of the material itself. Cure of SILASTIC™ RTV materials may be cause inhibition are -containing The higher viscosity silicone moldmaking inhibited by certain contaminants in or modeling clays, natural rubber such as rubbers will expand the most. on the pattern to be molded. latex and rubber gloves, masking tape, amine- or sulfur-containing materials, The mixture can be quickly and easily If in doubt as to possible surface inhibition, and condensation cure (-catalyzed) de-aired in a vacuum chamber. This a “patch test” is recommended. Brush silicone RTVs. important step usually takes just a few or pour a small amount of the catalyzed minutes. Entrapped air may be removed Surfaces previously in contact with any Thinners can be used to reduce RTV base For Silicone Molds. When first cast, of the materials mentioned may also be viscosity and cured rubber durometer. silicone rubber molds exhibit natural inhibited. Water, when present on the part Reduction of viscosity and durometer release characteristics. Over time, to be molded, can also cause inhibition. can be achieved using 1 to 3 percent however, the reactant agents in most Condensation Cure Systems of PDMS fluid with minimal effect on resins will deplete mold lubricity Inhibition in condensation cure systems physical properties. Further viscosity and and parts will begin to stick in the mold. (tin catalyzed) such as SILASTIC™ durometer reduction can be achieved A release agent should be used at the first RTV-31xx series and SILASTIC™ HS with increased PDMS levels; however, sign of sticking and reapplied only when Moldmaking series RTV silicone rubbers higher levels of PDMS will affect the sticking reoccurs. is not common. However, there are mechanical properties, which will When using a silicone release agent, it some sulfur-containing modeling clays require end-user evaluation. is necessary to burnish the release agent that can retard the cure of some of the condensation cured RTVs. Complete cure 4. Release Agents for Patterns/ only on the area(s) where sticking occurs. may take days (with severe inhibition), Masters If using a silicone release agent in an but unlike inhibited addition cure aerosol container, there are cautions to materials, the condensation cure RTVs A release agent should be used to ensure be taken. Silicone can cause nonwetting will cure when the clays are removed. easy removal of the cured rubber from spots (fisheyes) in nearby areas where the pattern/master. For molds cured at SILASTIC™ RTV-3081-F Mould-Making painting is being performed. It is best to room temperature, a simple mixture Curing Agent, when used with its use silicone oil (PDMS) and rub it into the recommended bases, has been of 10 parts petroleum jelly to 90 parts mold where sticking is occurring. Wiping formulated to cure against inhibition- solvent is recommended. Suitable off any excess will prevent non-wetting prone clays. solvents include VM&P naphtha or areas on the piece(s) cast from the mold. Techniques to Prevent Inhibition mineral spirits. A standard practice to prevent inhibition The petroleum jelly–solvent mixture can 5. Calculating Material Needs easily be prepared by putting the items is the use of a “barrier coating” to A few simple calculations can help prevent the inhibiting agent from together in a suitable container and setting determine the amount of material contacting the uncured RTV material. A aside overnight. With slight agitation you’ll need to cast your mold: thin layer of clear acrylic lacquer sprayed before use, the mixture is ready to apply. directly onto the pattern is an effective • Find the specific gravity of the To ensure complete coverage, brush the barrier coating in most instances. moldmaking material you have chosen. mold liberally with the petroleum Polyvinyl alcohol (PVA) is another (This data can be found in the product jelly– solvent mixture, then hit with effective barrier coat. This water-based selection guide or in the product data a spray of air from an air gun. The solution can be applied by brushing, sheets.) wiping or spraying onto the master. It air will aid in evaporating the solvent, is important the film be completely dry eliminating puddling and the resultant • Calculate the approximate volume of before molding. loss of detail. This process will leave a the mold. After the mold is cast, the film of PVA can thin film of petroleum jelly over the • Multiply the volume by the specific be removed from the pattern by placing entire pattern/master. gravity. it underwater and rubbing briskly. The When casting a two-part mold, it is • Add 10% to cover loss during mixing film will dissolve. extremely important to completely and handling. cover the cured half of the mold with a 3. Thinners Example: good release agent to prevent the two Silicone oil (PDMS) serves as a thinner halves from bonding together. Straight • Product specific gravity = 1.21 and can be used with all SILASTIC™ petroleum jelly can be used, as well as a • Mold volume = 1000 cm3 silicone moldmaking rubbers. A wide solution of 30 to 50 percent petroleum variety of viscosities are available: 20, 50, jelly. Dusting with talc or baby powder • 1.21 x 1000 = 1210 100 and 350 centistokes, with 50 cSt the has been found to be effective, as well as • 1210 + 10% = 1331 g of product should most common. the use of Teflon aerosols. be prepared

pg 2 6. Patching Torn Molds When the cast part is removed from the viscosities of 20, 50, or 100 centistokes mold, the barrier coat becomes the outer are best). Placing a master of wax, plaster Using a steel brush, abrade the area to be skin of the casting. or wood in the mold will help retain the patched, then clean the tear with a good mold’s shape. grease-cutting solvent such as naphtha or The barrier coat can then be stained or mineral spirits. Be sure the solvent has painted, an important feature. Molds made with condensation cure (tin catalyzed) systems may revert (soften) if completely evaporated before proceeding. Note that this type of barrier coat is placed in airtight storage. For maximum Because silicone rubber sticks so well different than the barrier coat mentioned library life, Dow recommends platinum to itself, for the strongest patch, it is in Techniques to Prevent Inhibition. cured molds. recommended that you use the same Reconditioning silicone rubber that was used to make Caution Reconditioning can be accomplished by the mold. DOWSIL™ 732 RTV Adhesive/ Always provide adequate ventilation burnishing a low viscosity PDMS fluid Sealant can also be used to repair torn when using any solvent. In addition, into the surface. When the mold is to be molds. all solvents should be completely put back in use, any excess fluid should evaporated before catalyzed RTV rubber 7. Compression Casting be removed from the surface. This is is applied to the master pattern. When necessary to ensure that the cast parts When using silicone rubber materials to using any solvent, avoid heat, sparks and will be paintable. Non-wetting or fisheyes do compression casting, holes must be open flame. Follow the manufacturer’s can occur on the surface of the cast parts drilled to permit venting. One hole for directions on container labels, including if all excess PDMS fluid is not removed. every 25 square inches of surface area precautionary handling statements. should be sufficient. Maximum size is Bake-Out Health and Environmental 1/16 inch. A bake-out is recommended to remove Information the hardeners, plasticizers and other 8. Oak Wood Patterns To support customers in their product materials that leach out of the casting safety needs, Dow has an extensive When using new oak wood patterns, a materials and are gradually absorbed into Product Stewardship organization and a microsized porosity often occurs at the silicone molds. open grain of the wood. To avoid this, team of Product Safety and Regulatory Bake molds for longer times at lower apply some petroleum jelly to a clean Compliance (PS&RC) specialists available in temperatures, such as 90°C (200°F) for cloth and gently rub the surface in the each area. For further information, please six hours to overnight, or at higher direction of the grain. see our website, consumer.dow.com, or temperatures, such as 120°C (250°F), consult your local Dow representative. 9. Mold Box Release for one to two hours. Contact Us A coating of DOWSIL™ 236 Dispersion 11. Library Life Visit www.silastic.com to learn more on wooden mold boxes prevents resins, To extend the library life (shelf life) about the many product options available especially , from sticking of a cured silicone rubber mold, it is to you from SILASTIC™. to the mold boxes. important to thoroughly clean the mold 10. Mold Life Extension before storage. If possible, a bake-out (see Bake-Out, in Mold Life Extension), followed Barrier Coating by wiping the mold with a solvent, The use of a barrier coat when casting is best. If a bake-out is not possible, polyurethanes can greatly extend mold wiping the mold out with an aggressive life, in some cases up to 200 percent. solvent such as toluene will still help The barrier coat should be sprayed into considerably. After cleaning the mold, the silicone mold prior to each casting. apply a thin film of PDMS fluid (low

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