Solid silicone rubber can be formed by conventional thermosetting rubber processing techniques, including compression molding, extrusion molding, calendering and the like. Compared with other rubbers, silicone rubber is easier to process and does not necessarily require a secondary vulcanization process. For many specific applications (such as automotive connectors), silicone rubber is an ideal choice for fast vulcanization and relatively low odor. The following summarizes its processing and vulcanization process:
Compression molding is a widely used method in silicone rubber processing. It usually needs to be preformed according to the size and shape of the part, and then placed in the mold cavity to cure at a certain temperature and pressure. Compression molding is a labor-intensive process that requires an all-hands-on and de-flashing process, and requires minimal investment and equipment.
The transfer molding process is to press the unvulcanized rubber into the mold cavity through a fixed flow channel through the action of the hydraulic piston. Transfer molding is suitable for multi-cavity designs and can produce products with virtually no flash. The traditional hot runner design can lead to the retention of vulcanized waste in the runner, and the new cold runner design can better solve this problem. For some specific applications, transfer molding is even more suitable than injection molding.
Silicone rubber's relatively low viscosity and fast vulcanization make it ideal for injection molding. Although pre-formed silicone can also be transported well through the screw, the use of barrels ensures a more stable delivery rate and reduces the risk of manual handling of unvulcanized silicone. Depending on the size of the part, injection molding cycle times are typically 0.5-3 minutes. Due to the high injection pressure, the linear shrinkage of injection molding is also smaller than other molding processes. The rational design of the injection vent can avoid introducing air into the mold cavity.
Extrusion molding is a continuous molding process, such as pipe fittings, insulating sleeves for wires and cables, etc. Silicone extruders usually require water cooling and screw conveying devices. It is recommended to use high hardness, high wear-resistant steel to reduce wear, such as nitrided steel 4140. Typically, the screw has a compression ratio of 2:1 to 41 and an aspect ratio of 8:1 to 12:1. A 40 to 150 mesh stainless steel screen can help remove impurities, increase back pressure, reduce air bubbles and provide better dimensional control.
Calendering is suitable for processing continuous, uniform sheets of silicone under lined (or unlined) support. The standard calendering process generally consists of 3 to 4 calendering rolls at 0.5-3rpm. Silica gels with high Green strength and long-term remelting are more suitable for the calendering process. Therefore, it is recommended to place the silica gel for 24 hours before starting to use it. Unsupported calendering usually involves passing the silicone through heated rolls or hot sulphur tunnels before post-curing. Both lined and unsupported calendering processes can be vulcanized by high pressure steam.