The most important molding conditions of TPU are temperature, pressure and time that affect the flow and cooling of plasticization. These parameters will affect the appearance and performance of TPU parts. Good processing conditions should be able to obtain uniform white to beige parts.
The temperature to be controlled in the molding TPU process is the barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of TPU, and the latter two temperatures affect the flow and cooling of TPU.
a. Barrel temperature The choice of barrel temperature is related to the hardness of the TPU. TPU with high hardness has a 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 ℃. The temperature distribution of the barrel is generally from the side of the hopper (rear end) to the nozzle (front end), gradually increasing, so that the temperature of the TPU rises smoothly 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 molten material from drooling in the straight-through nozzle. If self-locking nozzles are used to prevent drooling, the nozzle temperature can also be controlled within the maximum temperature range of the barrel.
c. Mold temperature Mold temperature has a great influence on the internal performance and apparent quality of TPU products. Its height depends on many factors such as the crystallinity of TPU and the size of the product. The mold temperature is usually controlled by a constant-temperature cooling medium such as water. The TPU has high hardness, high crystallinity, and high mold temperature. For example, Texin, hardness 480A, mold temperature 20 ～ 30 ℃; hardness 591A, mold temperature 30 ～ 50 ℃; hardness 355D, mold temperature 40 ～ 65 ℃. TPU product mold temperature is generally 10 ~ 60 ℃. The mold temperature is low, the melt is frozen prematurely and a streamline is produced, and it is not conducive to the growth of spherulites, so that the crystallinity of the product is low, and the later crystallization process will occur, which will cause the product to shrink and change in performance.
The injection molding process is based on pressure including plasticizing pressure (back pressure) and injection pressure. When the screw retreats, the pressure on the top melt is the back pressure, which is adjusted by the overflow valve. Increasing the back pressure will increase the melt temperature, reduce the plasticizing speed, make the melt temperature uniform, the color materials mix evenly, and discharge the melt gas, but it will prolong the molding cycle. The back pressure of TPU is usually 0.3 to 4MPa.
The injection pressure is the pressure exerted on the TPU by the top of the screw. Its function is to overcome the flow resistance of the TPU from the barrel to the cavity, fill the mold of the melt, and compact the melt. TPU flow resistance and mold filling rate are closely related to melt viscosity, and melt viscosity is directly related to TPU hardness and melt temperature, that is, melt viscosity is not only determined by temperature and pressure, but also by TPU hardness and deformation rate. The higher the shear rate, the lower the viscosity; the constant shear rate, the higher the TPU hardness, the greater the viscosity.
Under the condition of constant shear rate, viscosity decreases with increasing temperature, but under high shear rate, the viscosity is not affected by temperature as much as low shear rate. The injection pressure of TPU is generally 20 ~ 110MPa. The holding pressure is about half of the injection pressure, and the back pressure should be below 1.4 MPa to make the TPU plasticize evenly.
The time required to complete an injection process is called the molding cycle. The molding cycle includes mold filling time, pressure holding time, cooling time and other times (opening mold, demolding, closing mold, etc.), which directly affects labor productivity and equipment utilization. The molding cycle of TPU usually depends on the hardness, the thickness of the part and the configuration. The high cycle of TPU hardness is short, the thick cycle of the plastic part is long, and the complex configuration of the plastic part is long. TPU molding cycle is generally between 20 ~ 60s.
d. Injection speed
The injection speed mainly depends on the configuration of the TPU product. Products with thick end faces require lower injection speed, while thin end faces provide faster injection speed.
e. Screw speed
Processing TPU products usually requires a low shear rate, so a lower screw speed is appropriate. The screw speed of TPU is generally 20 to 80 r / min, preferably 20 to 40 r / min.
(4) Stop processing
Since TPU may degrade for a prolonged time under high temperature, after shutdown, it should be cleaned with PS, PE, acrylic plastic or ABS; if it is shut down for more than 1 hour, the heating should be turned off.
(5) Post-processing of products
Due to uneven plasticization in the barrel or different cooling rates in the mold cavity, TPU often produces uneven crystallization, orientation, and shrinkage, which results in internal stress in products. This applies to thick-walled products or products with metal inserts. Is more prominent. Products with internal stress often suffer from a decrease in mechanical properties during storage and use, and the surface has silver streaks or even deformation and cracking. The way to solve these problems in production is to anneal the products. The annealing temperature depends on the hardness of the TPU product. The annealing temperature of the product with high hardness is also higher, and the temperature of the hardness is lower. The temperature is too high may cause the product to warp or deform, too low to achieve the purpose of eliminating internal stress. The annealing of TPU should use low temperature for a long time, and the products with lower hardness can reach the best performance after being placed at room temperature for several weeks. When the hardness is below Shore A85, it is annealed at 80 ℃ × 20h, and above A85 at 100 ℃ × 20h. Annealing can be carried out in a hot air oven, taking care not to overheat locally and deform the product.
Annealing can not only eliminate internal stress, but also improve mechanical properties. Because TPU is a two-phase form, phase mixing occurs during TPU thermal processing. During rapid cooling, due to the high viscosity of TPU, the phase separation is very slow, and there must be sufficient time for it to separate and form micro-domains to obtain the best performance.
(6) Mosaic injection
In order to meet the needs of assembly and use strength, metal inserts need to be embedded in TPU parts. The metal insert is first placed in a predetermined position in the mold, and then injected into a whole product. For TPU products with inserts, due to the large difference in thermal performance and shrinkage between the metal insert and the TPU, the insert and the TPU are not bonded well. The solution is to pre-heat the metal insert, because the insert reduces the temperature difference of the melt after preheating, so that the melt around the insert can be cooled slowly during the injection process, and the shrinkage is more uniform. The hot material shrinkage function prevents excessive internal stress around the insert. TPU insert molding is relatively easy, and the shape of the insert is not limited. As long as the insert is degreased, it is heated at 200 to 230 ° C for 1.5 to 2 minutes, and the peel strength can reach 6 to 9 kg / 25 mm. For a stronger bond, apply adhesive to the insert, then heat at 120 ° C, and then inject. In addition, it should be noted that the TPU used must not contain lubricants.
(7) Reuse of recycled materials
In the process of TPU processing, waste materials such as main channels, diverter channels, and substandard products can be recycled. From the experimental results, 100% recycled materials are not blended with new materials, and the mechanical properties decline is not too serious. It can be fully used, but in order to maintain the physical and mechanical properties and injection conditions at the optimal level, the recommended recycled material ratio is 25% ~ 30 % Is better. It should be noted that the specifications of the reclaimed material and the new material are preferably the same. Contaminated or annealed reclaimed material should be avoided. The reclaimed material should not be stored for too long. It is best to granulate and dry it immediately. The melt viscosity of recycled materials is generally reduced, and the molding conditions are adjusted.