Moldmaking with silicone rubbers
Once you have selected the correct product and determined how to use it properly, you are on the way to making good flexible molds with silicone rubbers.
Here are some useful tips for moldmaking using Xiameter
De-airing is recommended for all moldmaking that use silicone rubbers, particularly when you are not using automatic dispensing equipment. This prevents air bubbles forming and interference with surface reproduction. You can also use a product with a low viscosity such as the XIAMETER RTV-3112 Base. This can be used in many applications without de-airing because of the mixture of base and catalyst or base and curing agent that will expand during de-airing. It is important to use a container that is between three and five times the volume of the material because the higher viscosity silicone rubbers will expand. The mixture can be quickly and easily de-aired in a vacuum chamber. Entrapped air may be removed by applying a vacuum of 27 to 29 inches of mercury. XIAMETER HS Moldmaking series RTV silicone rubbers should not be de-aired for more than 5 minutes as this will change the physical properties. In all cases, the material will expand and then contract to its original level.
Cure of XIAMETER brand RTV materials may be inhibited by certain contaminants in or on the pattern to be molded. A patch test is recommended where you can brush or pour a small amount of the catalyzed RTV selected onto a noncritical area of the pattern. If the results show the rubber is gummy or uncured after the recommended cure time, then inhibition has occurred.
Inhibition in addition cure systems (platinum cure) such as the XIAMETER brand silicone moldmaking product line can range from tackiness to complete lack of cure. Some of the 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 cause inhibition.
Inhibition in condensation cure systems (tin catalyzed) such as XIAMETER RTV-31xx series and XIAMETER HS Moldmaking 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.
XIAMETER RTV-3081-F Curing Agent, when used with its recommended bases, has been formulated to cure against inhibition-prone clays. A standard industry practice to prevent inhibition, is to spray a thin layer of clear acrylic lacquer onto the pattern which will act as an effective barrier coating.
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 is completely dry before molding. Having casted the mold, the film of PVA can be removed from the pattern by placing it underwater and rubbing briskly.
Silicone oil (PDMS) serves as a thinner and can be used with all XIAMETER brand silicone moldmaking rubbers. A wide variety of viscosities is available: 20, 50, 100 and 350 centistokes, with 50 cSt the most common. Thinners can be used to reduce RTV base viscosity and cured rubber durometer. Reduction of viscosity and durometer can be achieved using 1 to 3 percent of PDMS fluid with minimal effect on physical properties. Further viscosity and durometer reduction can be achieved with increased PDMS levels; however, higher levels of PDMS will affect the mechanical properties, which will require end-user evaluation.
Release Agents for Patterns/Masters
A release agent should be used to ensure easy removal of the cured rubber from the pattern or master. For molds cured at room temperature, a simple mixture of 10 parts petroleum jelly to 90 parts solvent is recommended. Suitable solvents include VM&P naphtha or mineral spirits.
The petroleum jelly–solvent mixture can be easily 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, eliminating puddling and the resultant loss of detail. This process will leave a thin film of petroleum jelly over the entire pattern or master.
When casting a two-part mold, completely cover the cured half of the mold with a good release agent. This will prevent the two halves from bonding together. For example, use straight petroleum jelly, 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® aerosols.
On the first cast, you may find silicone rubber molds exhibit natural release characteristics. Over time the mold lubricity may be depleted and parts will begin to stick in the mold. A release agent should be used at the first sign of sticking and reapplied when sticking reoccurs.
When using a Silicone release agent in an aerosol container, it can cause nonwetting spots (fisheyes) around the areas where painting is being performed. It is recommended to rub silicone oil (PDMS) into the mold where sticking is occurring and to wipe off any excess.
Calculating Material Needs
A few simple calculations can help determine the amount of material needed to cast your mold:
- Find the specific gravity of the moldmaking material you have chosen.
(This data can be found in the product selection guide or in the product data sheets.)
- Calculate the approximate volume of the mold.
- Multiply the volume by the specific gravity.
- Add 10% to cover loss during mixing and handling.
- Product specific gravity = 1.21
- Mold volume = 1000 cm3
- 21 x 1000 = 1210
- 1210 + 10% = 1331 g of product should be prepared.
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. Suitable products from Dow Corning include Dow Corning 732 RTV Multipurpose Sealant can be used to repair torn molds.
When using silicone rubbers 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.
Oak Wood Patterns
When using new oak wood patterns, apply petroleum jelly to a clean cloth and gently rub the surface in the direction of the grain. This will help minimise microsized porosity at the open grain of the wood.
Mold Box Release
A coating of Dow Corning 236 Dispersion on wooden mold boxes prevents resins, especially polyurethanes, from sticking to the mold boxes.
Mold Life Extension
When casting polyurethanes, using a barrier coat can greatly extend mold life, where in some cases up to 200 percent. Prior to casting, the barrier coat should be sprayed into the silicone mold.
When removing the cast part from the mold, the barrier coat becomes the outer skin of the casting. Note that this type of barrier coat is different than the barrier coat mentioned in Techniques to Prevent Inhibition.
Reconditioning can be accomplished by burnishing a low viscosity PDMS fluid into the surface. When the mold is put back into use, any excess fluid should be removed from the surface. This is necessary to ensure that the cast parts will be paintable. Non-wetting or fisheyes can occur on the surface of the cast parts if all excess PDMS fluid is not removed. To remove the hardeners, plasticizers and other materials that leach out of the casting material, you can use a bake-out. Bake the molds for longer times at lower temperatures, such as 90°C (200°F) for six hours to overnight, or at higher temperatures, such as 120°C (250°F), for one to two hours.
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, bake-out as mentioned earlier and wipe the mold with a solvent. If a bake-out is not possible, wiping the mold out with an aggressive solvent such as toluene will help considerably. After cleaning, apply a thin film of PDMS fluid (low viscosities of 20, 50, or 100 centistokes). To retain the mold’s shape, you can place a master of wax, plaster or wood in the mold.