TPU Injection Molding
TPU is a high-quality TPU precision component made by injection molding using thermoplastic polyurethane material, which has good toughness and elasticity, and can maintain good performance in high and low temperature environments.
Among the manufacturing processes of thermoplastic polyurethane (TPU) molds, the most common process is TPU injection molding. When a factory uses TPU as a material to complete the entire injection molding process, it needs to transport the dry and preheated TPU pellets from the hopper of the injection molding machine to the heated barrel and keep it in a molten state. Through the advancement of the screw (or plunger) and the action of the nozzle at the front end of the barrel, it is injected into a closed mold with a relatively low temperature, so that it fills the mold cavity, and is cooled, solidified, and molded under pressure. Finally, the same TPU product as the model cavity is obtained.
When plastic injection molding companies produce TPU plastic products, they need to strictly follow their process requirements. The most important factors mainly include problems affecting the plasticization flow, cooling temperature, pressure, and time. To ensure the products produced, companies There will be no deviations such as ripples or cracks, and they are often tested repeatedly during the manufacturing process.
The process of TPU injection molding is mainly affected by the processing temperature. The main problem is that the fluctuation range of the processing temperature is very narrow. Therefore, the factory requires that the processing equipment must have a cooling system or a temperature adjustment system. Through long-term experiments and manufacturing experience, it can The problem of failure in the injection molding process mainly comes from the raw materials, molds, injection molding machines, and injection conditions.
When TPU injection molding is in actual operation, to avoid the above problems, the general factory will proceed in the following manner:
The Injection Molding Process of TPU Material
- Barrel Temperature
If TPU materials want to be melted into a fluid state, general injection molding factories need to control their processing at a melting temperature between 190 and 220°C. Of course, if the hardness and melting point of some materials are too high, engineers may also need to increase the temperature to 240°C.
- Mold Temperature
If you want to control the quality of the final product, mold temperature is an issue that must be paid attention to. It mainly affects the surface quality and demoulding behavior of the product, and also affects the shrinkage and internal stress of the final structure (when exposed to cold). Usually, factories will control the mold temperature between 20-40℃ during production and manufacturing, but the performance of many TPU raw materials will be somewhat different. This is mainly because when purchasing raw materials, many raw material factories have upgraded these materials, such as using some improvements. For TPU grades and the use of glass fiber-filled reinforced TPU, the mold temperature should be increased to 60°C to ensure the best surface quality. For thick-wall product cooling, a reduction of approximately 5°C can reduce cycle times.
- Plasticization
For the plasticizing speed, the peripheral speed should be selected not to exceed 0.3m/s. Judging from the experience of BOOLING injection molding, it is the best state to utilize 30%-75% of the barrel’s capacity. If the barrel capacity is in a very low state corresponding to the injection volume, the molten material will stay in the plasticizing unit for too long, resulting in the problem of thermal decomposition of the molten material.
- Injection Pressure, Holding Pressure, Back Pressure, Injection Speed
The problem of injection speed mainly comes from the wall thickness of the product. Usually, for thick-walled product molds, engineers need to fill them slowly, while for thin-walled products, they need to be filled quickly. During the injection molding process, even if the mold is vented, it helps to prevent the so-called mold caused by trapped air. “Burn marks”.
Therefore, excessive injection pressure or holding pressure will also affect the demoulding state of the product and the finished product. Holding the pressure too low creates dents. Use a lower holding pressure than the injection pressure. Generally, the holding pressure can be 50% of the injection pressure, which is appropriate.
- Cycle Time
What determines cycle time is the shape of the product, wall thickness, cooling of the mold, and the material itself.
Key Points for Adjustment of the TPU Injection Molding Process
- The control of processing temperature has a crucial impact on the final size, shape, and deformation of TPU products. Therefore, factories need to pay special attention during manufacturing, and the processing temperature depends on the TPU grade and the specific conditions of the mold design.
- If the pressure holding phenomenon occurs at a slow speed and for a long time during the injection molding process, it will easily lead to molecular orientation, so that the size of the product obtained does not meet the actual requirements, and the product will be deformed, larger or smaller, and There is a big difference between horizontal and vertical shrinkage. Excessive holding pressure will also cause the product to be over-compressed in the mold. After the product is molded, the size will be larger than the size of the mold cavity.
- The injection speed and holding pressure switching position must be set accurately.
- The choice of injection molding machine model must be appropriate. For small-sized injection molding products, it is necessary to choose a small injection molding machine as much as possible to increase the injection molding requirements and facilitate position control. The factory can easily convert the injection speed and pressure settings.
- When the moisture content of TPU exceeds 0.2%, it not only affects the appearance of the product but also has obvious problems with its mechanical properties. The injection molded product has poor elasticity and low strength. Therefore, before injection molding, the factory needs to dry the TPU material at a temperature of 80℃/110℃ for 2-3 hours.
- The barrel of the injection molding machine needs to be cleaned before and after each use to prevent the mixing of other raw materials, which will lead to a reduction in the mechanical strength of the product. When the injection molding factory uses ABS, PMMA, or PE to clean the barrel, it is best to clean it again with TPU nozzle material before injection molding and use TPU nozzle material to remove the residual material in the barrel. When cleaning the hopper, pay special attention to the connection between the hopper and the base of the injection molding machine to see if other raw materials remain. This is an area that most technical workers tend to overlook during production.
- Release from the mold
Release agents may be used for mold release, such as silane-based release agents.
- Use of waste materials (crushed materials)
Crushed materials made from waste products, gates, defective products, etc. can be made into pellets for recycling if they are cleaned and dried. For injection molding, less than 30% of crushed materials in pure materials will not affect the performance of the product. If you crush the material yourself, the molded product must be tested as required to see if it meets the performance requirements.
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