Through the report on silicone injection molding made by Baisu Injection Molding Systems Co., Ltd. and Battenfeld, the important content includes the characteristics of the material, the introduction of the molding process, etc. 1． Characteristics of Liquid Silicone Liquid Silicone Rubber (Liquid Silicone Rubber) is a non-toxic, heat-resistant, highly resilient flexible thermosetting material. Its rheological behavior is mainly characterized by low viscosity, rapid solidification, shear thinning and a high thermal expansion coefficient. LSR is a dual-liquid rapid vulcanization material using platinum as a catalyst. It can be produced by injection molding, large quantities, rapid vulcanization, and repetitive mechanical production. Its products have good thermal stability, cold resistance, excellent electrical insulation properties, and will not produce toxic substances when burned. Therefore, it has become an irreplaceable material in the production and design of health products, automobiles, baby products, medical products, diving products, kitchen utensils, seals, etc. 2. Molding process LSR is a two-component liquid material, divided into component A and component B. The work of the mixer fully mixes component A and component B in a precise 1:1 ratio. And because some products are colored, they are equipped with a color pump set and a color measurement part. A+B components, additives, colors, etc. are fully mixed and then enter the plasticizing system. This plasticizing screw has the functions of homogenizing and mixing at the same time. The mixture is injected into the hot mold through the screw. At the mold temperature of 170 to 200°C, the silicone gel undergoes a curing reaction. When using a cold runner system, it is important to note that the runner must be cold enough. In order to avoid glue leakage, a needle valve is installed on the surface of the mold part. When the glue injection is completed, the needle valve immediately closes the nozzle. 2.1 The feeding system can choose the following forms: 1) The two-way pump can move up and down to feed, and can maintain the pressure very well. Since the A and B pumps are connected and controlled synchronously by hydraulic and pneumatic, this form of The feeding system is relatively reliable and accurate. 2) The one-way pump is a universal type and can only feed materials in a single direction 3) A synchronous single-phase pump with a check valve 4) The metering cylinder system is mainly used in conjunction with the single-phase pump 2.2 Key components of the liquid silicone rubber injection molding machine 1) Due to the low viscosity of LSR, material backflow and glue leakage must be considered during processing, so sealing of the screw is necessary. 2) In order to prevent LSR from solidifying, a needle valve nozzle should be used. 3) Mixing and metering components of components A and B 2.3 Mold design In mold design, there are generally the following forms: 1) Hot runner wastes materials, is simple in design, low in cost, and is mostly used for large products. 2) The cold runner with needle valve can be automated and the cycle is short. 3) Due to the high expansion coefficient of LSR, the cold runner without a needle valve will expand when heated and shrink slightly when cooled, so the parts cannot maintain accurate side margins in the mold. Therefore, cold runner processing can be used. LSR should maintain a lower temperature and fluidity. The cold runner adopts a closed system. In the injection cycle, the closed system uses a "sealing needle" or "needle valve" in each channel. ” to control the accurate measurement of LSR materials. Because silicone rubber has significant thermal expansion characteristics, the shrinkage rate is 2 to 4% (vulcanization temperature is 150°C), and silicone rubber has the characteristics of compression deformation. Analysis of LSR flow/vulcanization: 1) The vulcanization chemical reaction of liquid silicone rubber requires a certain reaction time. 2) Ideal flow, in diameter 2mm, 170cm, flow distance exceeds 100cm. 3) Minimum thickness 1/1000mm 4) Too high mold temperature will cause vulcanization, causing flow obstruction. 5) Laminar flow can avoid bubbles 6) High-speed injection will cause turbulence 7) The viscosity of the material will change the flow pattern 8) Turbulence will cause white spots. Therefore, the following aspects should be paid attention to when designing the mold: 1) In order to avoid turbulence and jetting, use a small injection port. 2) Use needle-shaped or wing-shaped glue injection ports. 3) Use T-shaped guide pins instead of cylindrical guide pins. Use an injection air gun when demoulding, and add a heat insulation layer/board. 4) Do not use anti-rust agents/oils containing polymerization inhibitors. Generally, toluene, xylene, etc. can be used. In addition, cured LSR is easy to stick to the metal surface. The most common demoulding techniques include ejection pin ejection and air ejection. . From the above information, we can see that the main problems solved by silicone injection molding are its mixing and metering parts, as well as screw sealing and mold design. Liquid silicone rubber (Liquid Silicone Rubber) is divided into A glue and B glue. The quantitative device is used to control the two. The ratio is 1:1, and then thoroughly mixed through a static mixer (StaticMixer), injected into the injection tube, and then injection molded.
Injecting liquid silicone into the hot runner mold to produce silicone products can achieve the advantages of one-time molding, no waste and automation.