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Process and Characteristics Analysis of Liquid Injection Molding


Ⅰ. Process technology of silicone injection molding


In the silicone injection molding process, liquid injection molding technology was the earliest application. Liquid injection molding is to accurately measure the A and B glues (components such as 1:1 or other ratios), transfer them to a static mixer, mix them and then transfer them to the injection device, and then remix them by the injection device and inject them into the hot mold. Inside. After the rubber compound in the mold undergoes a rapid vulcanization reaction, a silicone product with certain strength and elasticity is formed.


In the composition of liquid silicone, the role of catalyst and inhibitor is particularly important. Since liquid silicone needs to undergo a vulcanization reaction, a catalyst is added to accelerate the vulcanization reaction.


When the temperature of the liquid silicone reaches the vulcanization temperature, it has a very high vulcanization speed (at 200°C, the vulcanization speed reaches only 3 to 5s per mm of wall thickness). Moreover, liquid silicone cannot exist at a temperature of 40 to 50 ° C for a long time (at 50°C, even if it does not reach the vulcanization temperature, it will react slowly within 3 to 4 minutes).


Therefore, in order to prevent the vulcanization reaction of the liquid silicone before reaching the vulcanization temperature, an inhibitor needs to be added to its components. When the temperature of the liquid silicone reaches the vulcanization temperature, the inhibitor fails, and the liquid silicone reacts rapidly.


Ⅱ. Characteristics of silicone injection molding


(1) Compared with solid rubber molding, liquid silicone injection molding does not require plasticizing, mixing, pre-forming and other operating procedures, saving manpower, material resources and energy, and reducing equipment investment and floor space.


(2) Liquid silicone injection molding can realize the automatic processing process under fully airtight conditions. Silicone rubber molding company eliminates manual operation deviation, reduces the influence and pollution of various variable factors in the processing process, and ensures the size, precision and intrinsic quality of the product. This is conducive to the application of liquid silicone in medical applications.


(3) Due to the low viscosity of liquid silicone (generally in the range of 10-1000Pas), good fluidity and processability, its injection pressure is much lower than that of solid rubber and plastic injection molding. The general injection pressure is 20MPa, and in some cases it can be lower than 1MPa. Therefore, silicone injection molding can produce no burr plastic product and mould design, reduce material waste, and reduce wear on equipment and molds.


In addition, due to the good fluidity of liquid silicone, it is especially suitable for large products, products with extremely complex shapes or ultra-thin products, such as ultra-thin products with a molding thickness of 0.5mm and a length of 100mm. For solid compounds, this shape is difficult to form.


(4) The vulcanization speed of liquid silicone is very fast. Silicone injection molding is generally at 160-220, and the vulcanization reaction can be completed after tens of seconds to minutes, so the molding cycle is short and the production efficiency is high.


(5) The back pressure required for liquid silicone in screw metering is very small, generally less than 1.5MPa. For some very low viscosity liquid silicone compounds, back pressure may not even be set. This is mainly due to the good fluidity of low-viscosity silicone and the low gas content of liquid silicone during normal operation. On the contrary, the large back pressure will cause the density of the unvulcanized silicone to increase, destroying the accurate metering effect of the metering device.


(6) There is no shrinkage during the silicone injection molding process. However, due to the high thermal expansion coefficient of silicone, it usually shrinks by 2% to 3% after demolding and cooling. The exact shrinkage data depends on the material formula. From a processing point of view, the designer should anticipate some factors that affect shrinkage.


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