High-speed filling of thin-walled injection molding requires high-speed injection by injection molding machine to fill the mold cavity without too thick solidified layer. High injection pressure is unnecessary. He just made up for the lack of injection speed and forced the melt into the unfilled mold cavity. This not only increases the required clamping force, but also causes high internal stress in the finished product and deformation after demoulding. The injection speed of general injection molding machine is about 100mm/s, which can't cope with thin-walled injection molding. Adding an oil pump can increase the injection speed by 25%. Double pump injection increased by 70%. Some manufacturers exchange regenerative injection for injection pressure and injection speed. Used when high injection pressure is not required for initial injection. The injection speed can be increased by more than 100%. Nitrogen cylinder can store the energy of oil pump in the form of pressure and release it during injection, which is a regular method to greatly improve the injection speed. Injection speed is divided into the following four categories: low speed 200-300 mm/s; Medium speed 300-600 mm/s; High speed 600-1000 mm/s; Ultra-high speed1000-2000 mm/s. Domestic injection molding machines can reach the medium speed level. Nitrogen cylinders are also called accumulators. High-pressure nitrogen is stored in a rubber bag, and the remaining space of the nitrogen bottle is filled with high-pressure oil. During the injection process, the pressure oil is released. Nitrogen cylinder is a kind of instantaneous large flow power source with basically constant pressure. Nitrogen cylinder can only provide instantaneous large flow, such as 0.5 second spectrum, but it is enough for high-speed thin-wall injection. The larger the nitrogen cylinder, the more constant the pressure, and the more pressure oil is stored. Fig. 3 Low inertia injection with nitrogen cylinder Only high-speed injection cannot meet all requirements of thin-wall injection. Also consider high acceleration and high deceleration. At the beginning of injection, the screw is stationary. From standstill to full speed, such as 400 mm/s, the screw should be accelerated. If the whole injection time is only 0.5s, it is expected to reach the full speed within 0.05s, and the acceleration rate exceeds 8 g; conversely, if the acceleration time needs 0.3s, it is not appropriate. The reason is that low acceleration reduces the average speed. Ignoring the melt viscous resistance, a = f/m. A is acceleration, f is thrust and m is mass. Therefore, thin-walled injection also needs large thrust and small mass. At present, the hydraulic injection molding machine mainly adopts the design of double injection cylinders. During injection, the injection back seat and the oil motor also move forward, and the quality is not low. By the way, in the common design of all-electric injection molding machine, the motor responsible for screw rotation also moves forward during injection. Single injection cylinder design, the oil motor does not move during injection, only the screw and the piston and piston rod of the injection cylinder move forward, and the quality drops a lot. High-rigidity engine oil pressure oil is elastic, which should be considered when paying attention to 0.05s acceleration. The cylinder piston has large area, short stroke and short oil pipe, which can reduce the influence of elasticity. When the hose can be replaced by a hard pipe, the rigidity of the oil circuit will also be improved. Servo valve The response of servo valve is faster than that of general proportional valve. Only when the cavity is full and the pressure is maintained can it play its greatest role. If the reaction is not timely, it will overflow and the finished product will produce burrs. Fig. 4 Full closed-loop control of servo valve In general, full closed-loop control can control injection speed, pressure keeping and back pressure. Full-closed-loop control monitors related variables (speed or pressure), and when there is deviation from the set value, it informs the servo valve to correct it. Simply put, the closed-loop control improves the stability (repeatability) of injection molding and reduces the rejection rate.

Controller A controller, usually called a computer, should issue a command to switch to pressure maintenance when the cavity is full. When the injection speed is 400mm/s and the allowable deviation of the electronic ruler is 0. 1mm, the controller can only have a deviation of 0.25 ms, and the controller is required to scan the electronic ruler every 0.1ms. If the controller adopts "real-time" control, the scanning is not adopted, but when the electronic ruler measures that the pressure holding point has arrived, an interrupt will occur and the controller will The injection cycle is short, and the ejection can save about one second. The mechanical structure should reach a period of four seconds, and the opening and closing of the formwork should be fast and stable (no vibration). Using proportional valve to open and close the mold has braking function. A highly rigid frame also helps. The deformation of the template directly affects the thickness of the mold cavity. When the wall thickness is 0.5mm, the formwork deformation should be controlled below 0.05 mm, so the rigidity of the formwork should be high (the reinforcement is appropriate and the formwork thickness is appropriate), and the spacing between four columns should not be too large. Plasticizing capacity: Plasticizing should be done in a four-second period, and the plasticizing capacity of the screw can be improved or the plasticizing time can be extended through pneumatic sealing. Double thread design can improve plasticizing ability. The long screw with the length-diameter ratio of 24-25 can increase the heat absorption area and effectively increase plasticization. Extremely high screw speed will increase the screw surface speed to above 1m/s, which has no negative effect on commonly used PP materials. Pneumatic nozzle seal allows continuous plasticization when opening and closing the mold, but the injection molding machine must have two power sources, such as two oil pumps. The mold is the same as the template of the injection molding machine, and the template of the mold should be thick to reduce deformation. High-speed injection molding needs good ventilation. Sufficient exhaust slots, the use of breathable die steel and vacuum pumping are all methods. The machining accuracy of the die is very high, so that the thickness of the circumference or four walls can be uniform. The requirements of multi-cavity mold are higher. The mold is equipped with ejection and blowing devices, so that the finished product after demoulding can fall quickly and close the mold immediately. Plastic raw materials should use high fluidity plastics. The melting index (MI) of PP plastics is as high as 60(g/ 10min), such as Moplen RP 1086 of Basell. Many finished products use PS/ABS because it requires both the toughness of PC and the fluidity of ABS, and is also suitable for thin-walled injection molding.