Silicone rubber extrusion and molding are the two most commonly used molding processes with very different principles.Extrusion is like "squeezing toothpaste”: the mud-like rubber preheated by the open mixer is continuously fed into the screw barrel, and after heating, shearing, and compression, it is forced through the mouth mold to obtain a tube, strip, or profile with a constant cross-section; then it is vulcanized through a hot air channel at about 200℃ to achieve cross-linking and shaping.The whole process is continuous and fast, suitable for products of infinite length or large length.Molding is “pressing": the pre-weighed rubber billet is placed in the cavity of the upper and lower heating mold, and a pressure of 150~180℃ and 5~20MPa is given after the mold is closed. The rubber material flows in the enclosed space and fills the cavity, and the finished product is obtained after short-term vulcanization; The process is intermittent, with a period of 30 seconds to a few minutes, which is suitable for parts with complex three-dimensional structures and high precision.
The differences between the two are reflected in four aspects: equipment, raw materials, performance and cost.Equipment: The extruder + vulcanizing tunnel occupies a long area, and the molding machine only needs a flat vulcanizing machine, and the mold cost is higher.Raw materials: Extruded rubber needs high plasticity and scorching safety, and white carbon black by precipitation method is commonly used; molded rubber can be filled with more staple fibers or conductive carbon black, which has low fluidity requirements.Product performance: The extruded parts have uniform density and good concentricity, but the parting line is obvious; the molded parts have thick flying edges and need to be trimmed twice, but they can obtain smooth surfaces, complex inverted buckles and metal inserts.Cost: Extrusion starts quickly and the waste is less than 2%, which is suitable for batches of ≥1000 m; the mold change is slow and the flying edge accounts for 5 to10%, but a short order of 100 pieces can be put into production economically.
Therefore, the selection process first depends on the shape and output: long size, simple cross-section, large demand-extrusion; three-dimensional, precision, small batches or inserts-molding.If the same product has both long strips and end joints, the straight section can be extruded first, and then the vulcanized joints can be molded to achieve complementary processes.
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IOTA-120 HTV phenyl silicone rubber - IOTA CORPORATION LTD.
