Liquid silicone rubber(LSR)is a two-component fast-curing material with platinum (Pt) as a catalyst. It can be formed by injection molding. The injection molding method can achieve mass production, rapid cross-linking and curing, and repeatable and stable production. .
Liquid silicone injection products have good thermal stability, cold resistance and superior electrical insulation, and will not produce toxic substances when burned. The application fields of liquid silicone are extremely wide, such as health products, automobiles, baby products, medical appliances, diving supplies, kitchen utensils, and sealing application products, etc. Liquid silicone rubber is an irreplaceable material in the current production design.
01 The forming process of liquid silicone rubber(LSR)
The molding process of liquid silicone rubber(LSR)requires only three steps: "measurement and mixing", "molding", and "curing and setting".
Liquid silicone rubber(LSR) has excellent fluidity, and it has firmness and flexibility after curing in the mold, so that liquid silicone injection products can be designed with unusual detailed structures and undercuts, which are other injection materials, such as plastics or thermoplastic elastomers etc can't do it.
Liquid silicone rubber(LSR)is usually injection-molded by an injection machine. The molding equipment is similar to that commonly used for thermoplastics, but the requirements for details are not exactly the same.
Liquid silica rubber(LSR) is a two-component or two-component liquid raw material, which is divided into A liquid component and B liquid component. The most commonly used commercially available packaging is 20 kg (about 5 gallons) pail or 200 kg (about 55 gallons) drum packaging . The pail or drum assembly is placed in a high-pressure pump equipped with a precise follower plate to push the viscous components out of the packaging drum through precise high-pressure control pumps, and use the mixing device to accurately mix A and B liquid components in 1 :1 ratio Mix well. In addition, because some products are designed with colors, it is possible to install a color-adding pump group and a color metering device equipped with color materials to fully mix A liquid, B liquid, additives, color materials, etc. in a static mixer before introducing them into plastics system. In the mixing system, the cooling medium must ensure that the mixture can be kept at a controlled cooling temperature (10~20°C) to inhibit the premature occurrence of crosslinking or curing reactions, and the liquid silicone mixture in the cooled state can remain stable for several days .
The plasticizing screw dedicated to liquid silicone has the functions of homogenization and mixing at the same time. The mixture is injected into the high-temperature molding mold through the screw. At the mold temperature of 170~200°C, the initial cross-linking reaction is triggered to cause the silicone material to undergo a curing reaction and shape the final product. When the liquid silicone mold uses a cold runner feeding system, it is worth noting that the runner temperature should be low and cold enough. In addition, in order to avoid material leakage or glue leakage, the needle valve device will be installed on the surface of the mold part, and the needle valve will immediately close the nozzle after the injection is completed.
02 Considerations for molding and processing equipment of liquid silicone rubber(LSR)
Key components of liquid silicone rubber injection molding machine
• Due to the low viscosity of liquid silicone rubber, material backflow and leakage must be considered during processing, so the sealing requirements for the injection screw are very important.
• In order to prevent the temperature-sensitive premature curing of liquid silicone rubber, the heat insulation design between the injection machine and the mold is very important. It is best to use a needle valve nozzle, and close the nozzle immediately after the injection is completed.
• Mixing and metering device for two components of liquid A and liquid B.
Quantitative feeding system
Quantitative feeding system has the following forms to choose from:
• The two-way pump can move up and down to feed materials, which can have a better pressure stabilization effect. The A liquid and B liquid pumps are connected and controlled synchronously by a hydraulic and pneumatic mechanism. Therefore, this type of feeding system can be more precisely controlled. Metering and high reliability.
• The one-way pump is a general type, and can only select one item to feed.
• Synchronous one-way pump with non-return valve.
• Metering cylinder systems are primarily used in conjunction with one-way pumps.
Mold design
In terms of design considerations, there are generally the following forms:
• The hot runner method wastes materials, is simple in design and low in cost, and is mostly used for large-scale products.
• The cold runner with needle valve can be automated and the molding cycle is short.
• For the cold runner system without needle valve, due to the high expansion coefficient of liquid silicone rubber, it will expand when heated, but will shrink slightly when cooled, so the finished product cannot maintain accurate side margins in the mold, so it is more recommended the needle valve cold runner system is used for processing.
• The liquid silicone should maintain a low temperature and fluidity. The cold runner adopts a closed system. During the injection processing cycle, the closed system uses a "sealing needle" or "needle valve" in each flow channel to control the liquid silicone rubber. accurate measurement. Due to the remarkable thermal expansion characteristics of silicone rubber, the shrinkage rate is 2 to 4% (the reaction curing temperature is 150°C), and silicone rubber has the characteristics of compression deformation. The analysis of liquid silicone flow/reaction curing is as follows:
• The reaction and curing chemical reaction of liquid silicone requires a certain reaction time.
• Ideal flow field (flow length ratio), in a flow channel with a diameter of 2mm and a length of 170cm, the flow distance of liquid silicone melt can exceed 100cm (L/t >500).
• Minimum thickness 1/1000mm.
• Excessive mold temperature will lead to premature curing of reactive crosslinking, which will cause flow restriction.
• Laminar flow avoids air bubbles.
• High velocity injection can result in turbulent flow.
• The viscosity of the material will change the flow pattern.
• Turbulent or turbulent flow patterns tend to cause white spots.
Based on the above analysis and description, the following key points should be paid attention to in the design of the mold:
• To avoid turbulence and jetting, use smaller pins or wing gates.
• Use T-shaped guide pins instead of cylindrical guide pins to increase mold positioning accuracy.
• Insulation or panels should be added.
• Do not use anti-rust agents/oils containing polymerization inhibitors. Generally, toluene, xylene, etc. can be used to wipe the mold. In addition, the cured liquid silicone is easy to stick to the metal surface, and the most commonly used demoulding techniques include thimble ejection and air ejection. Through the above information, it can be seen that the main problems to be solved by the silicone injection molding technology are the mixing and metering parts, as well as the sealing of the screw and the design of the mold.
03 Conclusion
The injection molding of liquid silicone rubber is a relatively unique processing technology, so it is necessary to have a basic understanding of liquid silicone rubber(LSR), and to give appropriate and correct design and planning in product design, mold design and production processing, and to fully understand liquid silicone injection. The principles of molding and production process design can avoid product problems and produce high-quality, high-efficiency liquid silicone products.
The injection molding processing of liquid silicone rubber is currently a market with good economic benefits, and there is an increasing trend in the industry for liquid silicone products, and there will be more and more new application products, which will inevitably lead to more More mold processing companies invest in the production of liquid silicone products, which will also make the liquid silicone injection molding industry chain and market more robust.