Post by Lee on Aug 22, 2004 0:25:12 GMT 1
In looking at the website of www.silpak.com, I do not see the pages of tips and tricks that I was told was there. So I will copy here the two pages that are in their catalog for the rest of you:
Tips, Techniques & Processes
Silicone: "How To" Instructions:
Silicone is a two part liquid which, when mixed, can be poured or brushed. An inexpensive brush will be satisfactory. Clay is frequently employed to partially embed the original model in order to establish parting lines for multi-piece molds and is practical for use in "voiding out" areas inside the mold box in order to conserve the RTV mold material. It can also serve as an emergency plugging material if the silicone rubber unexpectedly leaks from the mold box. Some silicone mold making rubbers are totally incompatible with the sulfer contained in most clays. When purchasing clay be sure to specify sulfur-free clay or Silpak's Klean Clay.
Solvent are used for cleaning up uncured material. Several solvents are suitable: Xylene, isopropyl alcohol, toluene, and paint thinner. These solvents should be used with respect to their vapor or flammability hazards. Acetone and methyl ethyl ketone commonly used to clean up uncured resins are less effective as solvents for silicone RTV.
When mixing batches of rubber an electric drill motor is recommended for large size batches. The drill motor should be fitte with a Jiffy type mixer or compatible impeller blade. The motor should operate at under 1,000 rpm. Mixing containers should be clean and made of cardboard, metal or plastic. When de-airing the RTV silicone, the container must be of sufficient size to allow for material expansion, approximately four times the RTV volume. Use a flat blade of wood, metal or plastic, which is capable of scraping the bottom and sides of the mixing container.
Wood, cardboard or plastic strips and masking tape are all useful to construct dams to contain the rubber in the desired shape around the models. Use Silpak's large or small plastic bubbles for alignment keys.
Melted wax applied quickly with a brush is an excellent medium to seal flat backed models to the mold box substrate or to seal up narrow gaps in the mold dams. Heat Silwax BD with caution due to its flammabliity.
Rarely does RTV silicone adhere to other materials, nor do many casting materials stick to it. Consequently, the need for mold release is limited to a few situations. For masters MR-1500 or MR-1500-50 may be used to give extra releasing action especially on porous surfaces.
A scale is necessary in order to properly portion the RTV base rubber with the catalyst, or by volume when a scale is not available. Triple beam gram scales are most commonly used, although any weighing device will suffice, as long as reasonable accuracy is assured.
Most Silicone RTV will bond fast to itself unless separated by a parting agent. Parting agent application is especially critical when silicone RTV parts are cast into RTV molds. A parting agent MR-1500 must be used when constructing multi-piece molds or the sections will bond together (use a hair dryer to melt to a gloss finish). Certain surfaces contain so much porosity, (i.e. open grain woods, porous stone, etc.) that the silicone physically locks onto the surface. Applying MR-1500 on these surfaces tends to seal up enough of the porosity without affecting visual detail on the mold surface. This permits release of the mold from the model without tearing away surface detail. In severe cases of surface porosity, MR-1500-50 or pure vaseline can be rubbed into the pattern, wiping away the excess with a clean non-lint cloth. Prepared releases may also be purchased.
Casting plaster is commonly used to construct rigid cases to support the flexible walls of RTV silicone molds. Tooling type plaster or cast shell plaster is recommended because it maintains flow control and the best dimensional accuracy. Art casting plaster will suffice in less critical situations. Some types of fiberous reinforcement such as Silpak's fiber-hemp or chopped fiberglass may be employed to strengthen the plaster. This would apply in situations where the plaster is troweled on as a mold backup, rather than poured into a contained mold box.
Since RTV silicone rubbers do not bond to other surfaces, a special primer must be employed when adhesion is desired. Such is the case when the mold box is to remain permanently attached to the RTV mold. The primer (P1409 or P1408) is brushed onto the surface and allowed to dry for one hour before the RTV is cast against it. Adhesion will result upon cure of the RTV.
When it is necessary to construct particularly strong molds that withstand rough handling or repeated flexing, the mold may be backed with a reinforcement fabric such as fiber veil or open weave cloth. Nylon hose mesh has been used effectively. Woven fiberglass cloth may also be used, however, it will not stretch. The fabric should not be incorporated in the RTV silicone pattern surface, unless the weave pattern is desired in the face of the finished mold. A face coat of approximately 1/16" of unreinforced rubber should be applied and allowed to partially cure before fabric reinforced coats are applied.
Proper de-airing of the mixed components to remove entrapped air is always recommended because removing air bubbles assures uniformly dense molds which better resist distortion and surface voids. The vacuum pump must be able to develop a minimum of 29 inches of mercury pressure and the chamber (bell jar) should accommodate a mixing container which holds four times the volume of catalyzed rubber to be de-aired. When a vacuum pump is not available, suspend the container with the mixed material above the pour area, pierce a small hole in the bottom of the container or plastic bag, and allow the this stream of RTV to fill the pattern box. Material dispensed in this manner will retain less entrapped air. Also a ramp can be used to flow rubber into a thin section of pattern area. Always pour in one corner allowing the RTV to flow across mold cavity pushing out the air.
Please call (Silpak, Inc.) for any further information or assistance.
USA (909) 625-0056
Tips, Techniques & Processes
Silicone: "How To" Instructions:
Silicone is a two part liquid which, when mixed, can be poured or brushed. An inexpensive brush will be satisfactory. Clay is frequently employed to partially embed the original model in order to establish parting lines for multi-piece molds and is practical for use in "voiding out" areas inside the mold box in order to conserve the RTV mold material. It can also serve as an emergency plugging material if the silicone rubber unexpectedly leaks from the mold box. Some silicone mold making rubbers are totally incompatible with the sulfer contained in most clays. When purchasing clay be sure to specify sulfur-free clay or Silpak's Klean Clay.
Solvent are used for cleaning up uncured material. Several solvents are suitable: Xylene, isopropyl alcohol, toluene, and paint thinner. These solvents should be used with respect to their vapor or flammability hazards. Acetone and methyl ethyl ketone commonly used to clean up uncured resins are less effective as solvents for silicone RTV.
When mixing batches of rubber an electric drill motor is recommended for large size batches. The drill motor should be fitte with a Jiffy type mixer or compatible impeller blade. The motor should operate at under 1,000 rpm. Mixing containers should be clean and made of cardboard, metal or plastic. When de-airing the RTV silicone, the container must be of sufficient size to allow for material expansion, approximately four times the RTV volume. Use a flat blade of wood, metal or plastic, which is capable of scraping the bottom and sides of the mixing container.
Wood, cardboard or plastic strips and masking tape are all useful to construct dams to contain the rubber in the desired shape around the models. Use Silpak's large or small plastic bubbles for alignment keys.
Melted wax applied quickly with a brush is an excellent medium to seal flat backed models to the mold box substrate or to seal up narrow gaps in the mold dams. Heat Silwax BD with caution due to its flammabliity.
Rarely does RTV silicone adhere to other materials, nor do many casting materials stick to it. Consequently, the need for mold release is limited to a few situations. For masters MR-1500 or MR-1500-50 may be used to give extra releasing action especially on porous surfaces.
A scale is necessary in order to properly portion the RTV base rubber with the catalyst, or by volume when a scale is not available. Triple beam gram scales are most commonly used, although any weighing device will suffice, as long as reasonable accuracy is assured.
Most Silicone RTV will bond fast to itself unless separated by a parting agent. Parting agent application is especially critical when silicone RTV parts are cast into RTV molds. A parting agent MR-1500 must be used when constructing multi-piece molds or the sections will bond together (use a hair dryer to melt to a gloss finish). Certain surfaces contain so much porosity, (i.e. open grain woods, porous stone, etc.) that the silicone physically locks onto the surface. Applying MR-1500 on these surfaces tends to seal up enough of the porosity without affecting visual detail on the mold surface. This permits release of the mold from the model without tearing away surface detail. In severe cases of surface porosity, MR-1500-50 or pure vaseline can be rubbed into the pattern, wiping away the excess with a clean non-lint cloth. Prepared releases may also be purchased.
Casting plaster is commonly used to construct rigid cases to support the flexible walls of RTV silicone molds. Tooling type plaster or cast shell plaster is recommended because it maintains flow control and the best dimensional accuracy. Art casting plaster will suffice in less critical situations. Some types of fiberous reinforcement such as Silpak's fiber-hemp or chopped fiberglass may be employed to strengthen the plaster. This would apply in situations where the plaster is troweled on as a mold backup, rather than poured into a contained mold box.
Since RTV silicone rubbers do not bond to other surfaces, a special primer must be employed when adhesion is desired. Such is the case when the mold box is to remain permanently attached to the RTV mold. The primer (P1409 or P1408) is brushed onto the surface and allowed to dry for one hour before the RTV is cast against it. Adhesion will result upon cure of the RTV.
When it is necessary to construct particularly strong molds that withstand rough handling or repeated flexing, the mold may be backed with a reinforcement fabric such as fiber veil or open weave cloth. Nylon hose mesh has been used effectively. Woven fiberglass cloth may also be used, however, it will not stretch. The fabric should not be incorporated in the RTV silicone pattern surface, unless the weave pattern is desired in the face of the finished mold. A face coat of approximately 1/16" of unreinforced rubber should be applied and allowed to partially cure before fabric reinforced coats are applied.
Proper de-airing of the mixed components to remove entrapped air is always recommended because removing air bubbles assures uniformly dense molds which better resist distortion and surface voids. The vacuum pump must be able to develop a minimum of 29 inches of mercury pressure and the chamber (bell jar) should accommodate a mixing container which holds four times the volume of catalyzed rubber to be de-aired. When a vacuum pump is not available, suspend the container with the mixed material above the pour area, pierce a small hole in the bottom of the container or plastic bag, and allow the this stream of RTV to fill the pattern box. Material dispensed in this manner will retain less entrapped air. Also a ramp can be used to flow rubber into a thin section of pattern area. Always pour in one corner allowing the RTV to flow across mold cavity pushing out the air.
Please call (Silpak, Inc.) for any further information or assistance.
USA (909) 625-0056