In recent years, the application of liquid siliconerubber is more and more widely, its molding technologies have also got rapid development. Liquid silicone is a kind of non-toxic, heat resistant, high resilience flexible thermosetting material. Its rheological behavior is mainly characterized by low viscosity, fast curing, high shear thinning, and the thermal expansion coefficient. Composed of liquid silicone vulcanization of products has strong adaptability to temperature, high purity, good transparency, less volatile matter content, oil resistant aging resistance, insulation and resistance to chemicals, etc. Liquid silicone injection molding normally is used to do silicone manufacturing for LSR parts for various industries, including automotive, construction, electronics, and healthcare.
How is Silicone Molding Done?
Among the liquid silicone moldings process, the Liquid Injection molding (Liquid Injection molding/LIM) technology is the first application. Liquid injection molding is A, B glue (composition as shown in table 1) by pumping system to the measurement system, Press 1:1 or another precise metering, conveying to the static mixer, mixing and then to the injection device, the injection device to mix after injected into the hot mold. Inside, the liquid silicone rubber will get a rapid curing reaction to form a silicone product with certain strength and elasticity. In the composition of liquid silicone, the effect of catalyst and inhibitor is particularly important. Due to the liquid silicone needing a vulcanization reaction, so adding the catalyst is a necessary step to accelerate the curing reaction. When the curing temperature is up to 200 ℃, the curing rate of each mm thickness just needs 3 ~ 5 s. Liquid silicone cannot long exist at the temperature of 40 ~ 50 ℃. If not reach the required vulcanizing temperature, liquid silicone rubber will also react slowly. Under this condition, inhibitors are needed. Inhibitors will be useless until liquid silicone rubber reaches vulcanizing temperature, and liquid silicone rubber will react quickly.
1) Compared with solid rubber molding, the process of liquid silicone injection molding excludes plasticate, mixing, and preforming process operation, which saves manpower, material resources, and energy as well as reduces equipment investment and area. 2) liquid silicone injection molding can be realized under the condition of fully closed automatically processing, which eliminates manual operation, and deviation reduces the influence of all kinds of variable factors in the process and pollution to ensure the dimensional accuracy and the inner quality of products. For that, liquid silicone rubber injection molding is good for silicone manufacturing of medical parts products. 3) Because the viscosity of liquid silicone rubber is very low (generally in the range of 10~ 1000PA-S), well-result in fluidity and processing, so the pressure of silicone injection molding is much lower than the injection molding pressure of solid rubber and plastic. The general injection pressure of LSR injection molding is 5~20MPa, or less than 1 MPa in some cases ensuring the production of fly-free products, reducing material waste while reducing equipment and mold wear. In addition, due to the good fluidity of liquid silicone rubber, it is especially suitable for forming large products, very complex shape products, or ultra-thin products, such as forming a thickness of 0.5mm, length of 100mm ultra-thin products, this shape is difficult to form by solid rubber. 4) The back pressure of measuring liquid silicone rubber in the screw is small, generally less than 1.5 MPa, for some low viscosity of liquid silicone rubber, set back pressure is not necessary for the reason that the low viscosity silicone own perfect liquidity. In normal working conditions, the gas content in liquid silicone is very low. Instead of strong back pressure can lead to unvulcanized silicon density increases, which may create a side effect on the precise measurement of the metering device. (5) Liquid silicone has no shrinkage during the LSR injection molding process, but due to silicone has a high thermal expansion coefficient, and therefore often has 2% ~ 3% shrinkage after moving from mold and cooling.
I.Vital accessories for Silicone Injection Molding 1) Due to the low viscosity of LSR, the reverse flow of materials and leak adhesive should take into consideration. So the screw seal is a must 2) To prevent LSR curing, a nozzle needle valve must be used. 3) The mixture device and measuring equipment for glue A.B are needed.
II. Design of Silicone Injection Molding Molds 1) Hot runner. The design typically has more waste of materials, design is simple but low cost. This design is suitable for more big-size products. 2) Cold runner with needle valve. This mold can realize automation, the cycle is short. 3)cold runner without needle valve. Due to LSR’s high expansion coefficient, expansion happens when heating while cooling but has small shrinkage, so parts can’t maintain accurate side from in the mold.
The best solution: (1) Use the cold runner (2) LSR maintains low liquidity and temperature (3) Cold runner system adopts a closed type in the injection cycle. The closed system use a “seal glue needle” or “needle valve” to control the accurate measurement of LSR materials.
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