In recent years, the application of liquid silicone has become more and more extensive, and its molding technology has also developed rapidly. Liquid silicone is a non-toxic, heat-resistant, highly resilient flexible thermosetting material. Its rheological behavior is mainly manifested in low viscosity, rapid solidification, shear thinning and high thermal expansion coefficient. Products made of liquid silicone vulcanization have strong temperature adaptability, high purity, good transparency, low volatile matter content, oil resistance, aging resistance, chemical resistance and outstanding insulation. It is widely used in the fields of automobiles, construction, electronics industry, health care, mechanical engineering, food industry, etc.
In the liquid silicone molding process, liquid injection molding (LiquidInjectionmolding/LIM) technology was first applied. Liquid injection molding is to accurately measure the A and B rubbers (components such as 1:1 or other ratios), transport them to the static mixer, mix them, and then transport them to the injection device. The injection device mixes them and injects them into the hot mold. After the rubber 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 catalysts and inhibitors 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 liquid silicone reaches the vulcanization temperature, it has an extremely high vulcanization speed (at 200°C, the vulcanization speed reaches only 10 ... 3~5s), and liquid silicone cannot exist in a temperature of 40~50℃ for a long time (at 50℃, even if the vulcanization temperature is not reached, it will slowly react within 3~4 minutes). Therefore, in order to prevent the liquid silicone from vulcanizing before the vulcanization temperature reaches the vulcanization temperature, an inhibitor must be added to its components. When the vulcanization temperature is reached, the inhibitor fails and the liquid silicone reacts quickly.
1. Molding characteristics
1) Compared with solid rubber compression molding, liquid silicone injection molding does not require plasticizing, mixing, pre-molding and other operating procedures, saving manpower, material resources and energy, reducing equipment investment and floor space.
2) Liquid silicone injection molding can realize the automatic processing process under fully enclosed conditions, eliminate manual operation deviations, reduce the influence and pollution of various variable factors in the processing process, and ensure the dimensional accuracy and intrinsic quality of the product, which is conducive to the application of liquid silicone in medical treatment.
3) Since liquid silicone has a very low viscosity (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, it can produce flash-free products, reduce material waste, and reduce the wear of equipment and molds. In addition, due to the good fluidity of liquid silicone, it is particularly suitable for molding 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. This shape is difficult to mold for solid rubber.
4) Liquid silicone has a very fast vulcanization speed. During injection molding, the vulcanization reaction can be completed in tens of seconds to minutes at 160-220 degrees, so its molding cycle is short and production efficiency is high.
5) The back pressure required for liquid silicone during screw metering is very small, generally less than 1.5MPa. For some liquid silicone rubbers with extremely low viscosity, back pressure may not even be set. This is mainly because low-viscosity silicone has good fluidity and the gas content in liquid silicone is very low during normal operation. On the contrary, large back pressure will increase the density of uncured silicone and destroy the precise metering effect of the metering device.
6) Liquid silicone does not shrink during injection molding, but because silicone has a high thermal expansion coefficient, it usually shrinks by 2% to 3% after demolding and cooling. The exact shrinkage data depends on the material formula. From the processing point of view, the designer should consider some factors that affect shrinkage in advance.
2. Design points
1) Liquid temperature control technology is used to control the barrel and the nozzle temperature is controlled between 5 and 25. Cooling water is usually used as the cooling medium.
2) In order to prevent the liquid silicone from solidifying, a hydraulically driven needle valve sealing nozzle is used. When the injection is completed, the needle valve immediately closes the nozzle.
3) During the molding process, the mixing uniformity of A and B rubber materials directly affects the quality of the vulcanized product. In some cases where the product quality requirements are high, the mixing degree of the rubber materials after passing through the static mixer cannot fully meet the requirements, and the supplementary mixing effect of the screw can better meet the requirements. Since the rubber material is liquid and does not need to undergo plasticization, the screw is designed to be a screw with no compression or low compression and weak shearing effect, and does not require a large aspect ratio. It is generally not suitable to directly apply the screw structure on the injection molding machine, because its screw structure usually has a strong shearing effect, which is easy to cause local vulcanization and affect the performance of the product. Therefore, it is of great significance to design a screw specifically for liquid silicone injection machines. If the product quality requirements are not high, you can consider using a plunger instead of a screw to complete the injection.
4) Liquid silicone has low viscosity, and a lower injection pressure can produce a higher injection speed. The air trapped in the mold is easily involved in the product. In order to obtain high-quality products, you can consider vacuuming the mold before injection. The purpose of driving away the air can also be achieved by adjusting the clamping force, that is, when the clamping force is low, the clamping force is increased after the liquid silicone is filled to 90% to 95% of the cavity. This can effectively exhaust the air and prevent the liquid silicone from overflowing and producing flash.
5) The viscosity of liquid silicone is low, and it is easy to form backflow during injection. Therefore, the sealing effect of the check ring must be good. Generally, it is not suitable to directly apply the check structure on the injection molding machine. Its working process: When the screw is metered, the rubber material pushes the check ring through the hole on the plug rubber ring under the push force of the screw and enters the screw head.
6) The cold runner technology is required in the liquid silicone injection molding process, and the runner must be cold enough. There needs to be a good enough insulation effect between the runner and the mold to ensure that the liquid silicone will not vulcanize in the runner and block the nozzle, which helps to reduce waste and save costs.
7) If the molded product is extremely soft, sensitive, and high-demand, you can consider adding a pneumatic brush to clean the mold and use a robot to eject the product.
Most of the composite insulators in the power industry are formed by liquid silicone, and the volume of insulators used for high voltage is usually large, and the mold volume of the molded insulator is also large. When using liquid silicone to produce insulators, because its vulcanization speed is much faster than that of solid silicone, if the ordinary injection device is used to inject the rubber, it is easy to cause the liquid silicone to fill the mold before the mold is filled, and the flow channel is blocked by the vulcanized silicone, so that the performance of the molded product cannot meet the requirements, and even the complete product cannot be formed. Based on these problems, the casting molding process was developed on the basis of injection molding.
The basic molding principle of casting molding is: A and B are pumped to the metering system, and after metering, they enter the static mixer, and after being fully mixed in the static mixer, they enter the mold directly. Before the A and B mixed glue enters the mold, the mold is first kept warm at a lower temperature by the mold temperature controller. After the rubber fills the mold, the mold is heated to a set temperature, and then kept warm, vulcanized, cooled, and opened to complete the molding process. This molding method is suitable for occasions where the product volume is large and the appearance quality requirements are not high.
This molding method eliminates the injection device, saving costs and machine space. However, due to the lack of a screw to remix and homogenize the rubber material coming out of the static mixer, and the uneven and discontinuous pressure provided by the metering and mixing system, the molding cycle is usually relatively long, so it is not suitable for high-speed, high-yield and high-quality product occasions.
The unique characteristics of liquid silicone itself make liquid silicone molding technology diverse. In addition to the liquid injection molding and casting molding processes introduced in this article, there are also co-injection molding, two-component molding, inlay molding, extrusion molding, physical foaming molding and other processes. Mastering and applying liquid silicone molding technology can not only produce high-quality products, but also save energy and reduce waste. Therefore, the development and application of liquid silicone molding technology is of great significance. At present, the research on liquid silicone molding technology abroad is becoming more and more in-depth, the products developed are diverse, and the application fields are becoming increasingly extensive. However, my country's liquid silicone molding technology is still in its infancy, and there is still a lot of research and development work to be done. However, I believe that in the near future, through the joint efforts of peers, liquid silicone injection molding technology will develop rapidly in my country.
