| SRM (SILICONE RUBBER MOLDING) |
         SRM(Silicone Rubber Molding) are usually used to replicate parts for prototyping purposes or final products purposes. SRM(Silicone Rubber Molding) by VC(Vacuum Casting) are suitable for the projects with quantity of prototypes/products less than 50 pcs, high quality demands, and designs with complex structures, such as undercut, which is hard to be demoulded from metal mold. RTV Silicone Rubber Molding Process Description Room Temperature Vulcanization (RTV) Silicone Rubber Molding are the least expensive and fastest way to create about a dozen prototype parts. As shown in the figure below, even with undercuts, the part is able to be removed through distortion of the mold. The figure illustrates a small toy locomotive molded with polyurethane.  Polyurethane casting material is commonly used for product prototypes. Polyurethane can be formulated to have material properties similar to elastomers or ABS. Polyurethane can be machined, sanded, glued, and painted. Polyurethane can have material properties, color, and surface texture very similar to that of production materials. (a) An RP pattern is used to create a master male pattern as shown below. The RP technique most often used is the SLA process. (b) The SLA part is made slightly larger (e.g. 0.003 in/in) than the final product since this is what shrinkage the final part will see. (c) The SLA part is sanded to a suitable cosmetic finish and can be sealed. (d) The master pattern is fitted with a sprue and gate and then surrounded by a parting surface which establishes the parting line for the mold (see the illustration). Alternatively, the cured mold can be cut carefully to form the parting line.  (e) The assembly is fixtured in a vat, and liquid RTV is then poured over the pattern and parting surface combination as shown below.  (f) Once cured, the RTV is removed from the vat and separated from the pattern and parting line surface to yield the two halves of the molding tool. RTV air-cures, so that the cure time depends on the geometry, the RTV type, and the environment. Cure time can range from 0.5 to 40 hours. Aging of the mold after cure for up to three days can improve mold life. (g) Thermoset resin such as polyurethane is poured or injected into the mold, as shown below.  (h) Finally, the finished part is removed from the mold. Undercuts are overcome by distorting the mold, which springs back to its original shape as long as distortion is not too severe.  (i) The part must then be post-processed by trimming any flash, as shown below, and possibly sanding. The gate and sprue must also be removed.  VC(Vacuum Casting) - replication process for SRM Vacuum casting, gravity casting and thin wall RIM(Reaction Injection Molding) are all used in SRM, but vacuum casting is most common used. Reasons as below: * Parts are of higher quality than with Gravity casting or thin wall RIM, with thinner walls and fewer defects. * Polyurethane materials can simulate ABS, Nylon, Polycarbonate, etc. VC involves using a vacuum jacket on the mold and injection pressure to fill thin-wall areas. The figure below illustrates the vacuum configuration, which is similar to the vacuum jacket used for injection or transfer molds.  Polyurethane - materials for SRM A polyurethane, IUPAC abbreviation "PUR", but commonly abbreviated "PU", is any polymer consisting of a chain of organic units joined by urethane (carbamate) links. Polyurethane polymers are formed through step-growth polymerization by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl (alcohol) groups in the presence of a catalyst. Polyurethane formulations cover an extremely wide range of stiffness, hardness, and densities. These materials include: * Low-density flexible foam used in upholstery, bedding, and automotive and truck seating * Low-density rigid foam used for thermal insulation and RTM cores * Soft solid elastomers used for gel pads and print rollers * Low density elastomers used in footwear * Hard solid plastics used as electronic instrument bezels and structural parts Polyurethanes are widely used in high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives and sealants, Spandex fibers, seals, gaskets, carpet underlay, and hard plastic parts. Polyurethane products are often called "urethanes". They should not be confused with the specific substance urethane, also known as ethyl carbamate. Polyurethanes are neither produced from ethyl carbamate, nor do they contain it. |
