3. Molding conditions
The most important molding conditions for TPU are temperature, pressure, and time affecting plasticization flow and cooling. These parameters will affect the appearance and performance of TPU parts. Good processing conditions should be able to obtain uniform white to beige parts.
(1) Temperature The temperature that needs to be controlled in the TPU process of molding includes the temperature of the feed barrel, the temperature of the nozzle and the temperature of the mold. The first two temperatures mainly affect the plasticization and flow of TPU, while the latter one affects the flow and cooling of TPU. A. Cartridge temperature The selection of cartridge temperature is related to the hardness of TPU. TPU with high hardness has high melting temperature and the maximum temperature at the end of the barrel is also high. The temperature range of the barrel used for processing TPU is 177~232℃. Generally, the temperature distribution of the barrel is gradually increased from one side of the hopper (the back end) to the nozzle (the front end), so that the TPU temperature can rise steadily to achieve the purpose of uniform plasticization. B. Nozzle temperature The nozzle temperature is usually slightly lower than the maximum temperature of the barrel to prevent the drooling that may occur in the straight-through nozzle. If a self-locking nozzle is used to eliminate salivation, the nozzle temperature can also be controlled within the maximum temperature range of the barrel. C. Mould temperature mould temperature has great influence on the intrinsic performance and apparent quality of TPU products. Its height depends on many factors such as TPU crystallinity and product size. Mold temperature is usually controlled by a constant temperature cooling medium such as water. TPU has high hardness, high crystallinity and mold temperature. For example, Texin, hardness 480A, mold temperature 20 ~ 30℃; Hardness 591A, mold temperature 30 ~ 50℃; Hardness 355D, mold temperature 40 ~ 65℃. The mold temperature of TPU products is generally 10 ~ 60℃. The mold temperature is low, the melt freezes prematurely and produces streamline, and is not conducive to the growth of spherulites, making the product crystallinity is low, there will be a late crystallization process, resulting in the product shrinkage and performance changes. B. Pressure injection process is pressure including plasticizing pressure (back pressure) and injection pressure. When the screw recedes, the pressure on the top of the melt is back pressure, which is regulated by the relief valve. Increasing the back pressure will increase the melt temperature, reduce the plasticizing speed, make the melt temperature uniform, color mixing uniform, and let out the melt gas, but will prolong the forming cycle. TPU usually has a back pressure of 0. 3 ~ 4 mpa. Injection pressure is the pressure exerted by the top of the screw on TPU. Its function is to overcome the flow resistance of TPU from the barrel to the cavity, to feed the mold filling rate of the molten material, and to compacted the molten material. TPU flow resistance and filling rate are closely related to melt viscosity, which in turn is directly related to TPU hardness and melt temperature. In other words, melt viscosity not only depends on temperature and pressure, but also depends on TPU hardness and deformation rate. The higher the shear rate is, the lower the viscosity is. The higher the TPU hardness, the greater the viscosity. Under the condition of constant shear rate, the viscosity decreases with the increase of temperature, but under high shear rate, the viscosity is not affected by temperature as much as that under low shear rate. The injection pressure of TPU is generally 20~110MPa. The holding pressure should be about half of the injection pressure, and the back pressure should be 1. Below 4MPa to make TPU plasticized evenly. C. The time required to complete an injection process is called the molding cycle. The forming cycle includes filling time, pressure holding time, cooling time and other time (mold opening, mold stripping, mold closing, etc.), which directly affects labor productivity and equipment utilization rate. The forming cycle of TPU is usually determined by hardness, workpiece thickness and configuration. TPU has a high hardness and a short cycle, while plastic parts have a long thick cycle and complex plastic parts have a long cycle. The forming cycle is also related to mold temperature. TPU molding cycle is generally between 20~60s. D. Injection speed Injection speed is mainly determined by the configuration of TPU products. The product with thick end face needs lower injection speed, while the product with thin end face needs faster injection speed. E. Low shear rate is usually required for processing TPU products by screw rotation, so a low screw rotation rate is advisable. The screw speed of TPU is generally 20~80r/min, and 20~40r/min is preferred. (4) Shutdown treatment Because degradation may occur after prolonged TPU at high temperature, after shutdown, PS, PE, acrylic plastic or ABS should be used for cleaning; Heat should be turned off if shutdown lasts more than 1 hour.