In recent years, the application of liquid silica gel has become wider and wider, and its molding technology has also developed rapidly. Liquid silicone is a non-toxic, heat-resistant, high-recovery flexible thermosetting material. Its rheological behavior is mainly characterized by low viscosity, rapid curing, shear thinning and high thermal expansion coefficient. It has the advantages of strong temperature adaptability, high purity, good transparency, low content of volatile substances, oil resistance and aging resistance, chemical resistance and outstanding insulation, etc. widely used.

Liquid injection molding
In the liquid silicone molding process, the liquid injection molding (Liquid Injection molding/LIM) technology was first applied. 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. , After the compound in the mold undergoes a rapid vulcanization reaction, a silicone product with certain strength and elasticity is formed.

In the composition of liquid silica gel, the role of catalyst and inhibitor is particularly important. Since liquid silica gel needs to undergo a vulcanization reaction, a catalyst is added to accelerate the vulcanization reaction. When the temperature of the liquid silica gel 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), and the liquid silica gel cannot exist at a temperature of 40 to 50 °C for a long time (50 °C Even if the vulcanization temperature is not reached, the reaction will slowly occur within 3 to 4 minutes). Therefore, in order to prevent the vulcanization reaction from occurring before the temperature of the liquid silica gel reaches the vulcanization temperature, an inhibitor needs to be added to its components. When the curing temperature is reached, the inhibitor fails and the liquid silica reacts rapidly.

1. Molding characteristics

1) Compared with solid rubber molding, liquid silicone injection molding does not require operations such as mastication, mixing, and preforming, which saves manpower, material resources and energy, and reduces equipment investment and floor space.

2) Liquid silicone injection molding can realize the automatic processing process under fully airtight conditions, eliminate manual operation deviation, reduce the influence and pollution of various variable factors in the processing process, and ensure the dimensional accuracy and intrinsic quality of the product. It is beneficial to the application of liquid silicone in medical treatment.

3) Due to the low viscosity of liquid silica gel (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. In some cases, it can be lower than 1MPa, so it is possible to produce flash-free products, reduce material waste, and reduce equipment and mold wear. In addition, due to the good fluidity of liquid silicone, it is especially suitable for molding large products, products with extremely complex shapes or ultra-thin products, such as ultra-thin products with a thickness of 0.5mm and a length of 100mm. This shape is difficult for solid rubber compounds. shaped.

4) The vulcanization speed of liquid silica gel is very fast. During injection molding, the vulcanization reaction is generally completed at 160 to 220 degrees Celsius after tens of seconds to several minutes, so the molding cycle is short and the production efficiency is high.

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

6) Liquid silica gel does not shrink during injection molding, but because silica gel 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 a processing point of view, the designer should anticipate some factors that affect shrinkage.

2. Design points

1) Adopt liquid temperature control technology 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 solidification of the liquid silicone, a hydraulically driven needle valve type 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 the A and B compounds directly affects the quality of the vulcanized products. In the case of high quality requirements for some products, the mixing degree of the compounds after passing through the static mixer cannot be fully satisfied. Requirements, the supplementary mixing action of the screw can better meet the requirements. And because the rubber compound is liquid and does not need to undergo plasticization, the screw is designed as a screw with no compression or low compression and weak shearing action, and does not require a large aspect ratio. Generally, it is not suitable to directly apply the screw structure on the injection molding machine, because the 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 the screw specially used for the liquid silicone injection machine. If the product quality requirements are not high, you can consider using the plunger instead of the screw to complete the injection.

4) The viscosity of liquid silica gel is low, lower injection pressure can produce higher injection speed, and the air trapped in the mold is easily involved in the product. In order to produce high quality parts, consider vacuuming the mold before injection. It is also possible to adjust the clamping force to drive out the air, that is, when the clamping force is low, the liquid silicone is filled to 90% to 95% of the cavity, and then the clamping force is increased, which can effectively exhaust air. , and can avoid the overflow of liquid silica gel, resulting in flash.

5) The viscosity of liquid silica gel 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 will push the non-return ring through the hole on the rubber ring and enter the screw head under the action of the screw pushing force.

6) The liquid silicone injection molding process requires cold runner technology, and the runner must be cold enough. There needs to be sufficient thermal insulation between the runner and the mold to ensure that the liquid silicone will not be vulcanized in the runner and block the nozzle, which will help reduce waste generation and save costs.

7) If the molding is extremely soft, sensitive and high-demand products, consider adding a pneumatic brush to clean the mold, and use a robot to eject the product.