TPU injection molding process and mold details (Part 1)

1 Injection molding
The most suitable processing method for TPU is screw injection molding machine. With a single thread of normal length, the three-stage screw can produce a good plasticized uniform melt. If high plasticizing capacity (production capacity) is required, a longer screw can be used.
Figure 1.1 Injection screw for processing TPU
Screw in compression zone is not applicable due to high shear force. TPU plasticization requires high energy and high torque drive of screw. Insufficient torque will lead to the fluctuation of screw speed and uneven plasticization. Within the limits, higher barrel temperature will produce good results, although it will risk overheating of materials. The path between the nozzle and the barrel head shall be designed to have no dead angle, so that the material can be injected without thermal damage. Accurate temperature control of the heating system of the screw barrel and nozzle is the key point. Take care to ensure that the nozzle is evenly heated throughout its length. Beware of local overheating and possible cold of the molten material on the path. The molten TPU is neither corrosive nor abrasive. For this reason, the screw does not need any special alloy steel or reinforced coating.
Fig. 1.2 Injection molding of shift handle
2 Temperature setting of screw barrel and die
Injection unit: TPU shall be processed at melting temperature between 190 and 220 ° C. For some hard grades, the melting temperature may increase to 240 ° C. The melting temperature range of the specific level TPU can be found in the relevant product information table. Fig. 2.1 shows the guiding values for heating the screw barrel and nozzle under different Shore hardness.
Fig. 2.1 Temperature setting for different hardness
Mold: The main influence of mold temperature is surface quality and demoulding behavior. It also affects the shrinkage and internal stress of the final structure (when cold). Normal mold temperature shall be 20 - 40 ° C. However, with some improved TPU grades and glass fiber filling to strengthen TPU, the mold temperature should be raised to 60 ° C to ensure the best surface quality. For the cooling of thick wall articles, a reduction of about 5 ° C can reduce the cycle time.
Plasticizing: For the plasticizing speed, the circumferential speed shall not exceed 0.3 m/s. The metering stroke should be between 1 D and 4 D. The following figure shows the maximum speed under different screw diameters.
Fig. 2.2 Functional relationship between screw speed and screw diameter
The practical experience is that it is the best to use 30 - 75% capacity of the screw barrel. If the injection volume corresponding to the screw barrel capacity is very low, the melt will stay in the plasticizing unit for too long. It will cause thermal damage of molten substance.
Figure 2.3 Decomposition of TPU melt in screw barrel processing
Injection pressure, holding pressure, back pressure, injection speed
For ideal processing, the key point is to continuously control the pressure and injection speed. It shall be able to control the injection and maintain the pressure within the range of 100 to 1200 bar. In order to homogenize, there must be a back pressure, which is usually set between 1 and 2.5% of the injection pressure. The injection speed is mainly based on the wall thickness. Generally, the mold for thick-walled articles needs to be filled slowly, while thin-walled articles need to be filled quickly.
In addition to wall thickness and cavity type, mold exhaust plays an important role in injection speed, which helps to avoid so-called "burn marks" caused by high-pressure hot air. The injection pressure/holding pressure exerts a great influence on the space stability and demoulding.
Too high injection pressure during molding, while too low holding pressure produces dents. It is more difficult to form overload and demould. It is advisable to work with a staggered pressure, that is, a lower holding pressure than the injection pressure. According to the rule, 50% of the injection pressure is appropriate. This can produce goods with the minimum internal pressure.
Figure 2 Typical cycle steps of 4TPU processing
Cycle time
The cycle time is determined by the shape of the object, the wall thickness, the cooling of the mold and the material itself. The following figure shows the effect of wall thickness on the injection cycle. TPU can be divided into three levels: hard, medium and soft.
Fig. 5.5 Relation between cycle time and wall thickness and hardness
Demoulding
TPU replication mold needs to be described in detail. The soft-level TPU is very strange, and it can produce very unusual wall items. This must be considered in the design of the mold. Release agent can be used for release. Silane-based release agents such as Baysilon M have good effects. Silane-free release agent can also be used, but it must be used frequently.
3 Waste recycling
Crushed materials made from waste materials, gates and unqualified products can be made into particles for recycling if they are clean and dry. For injection molding, adding less than 30% crushed material to the pure material will not affect the performance of the product. If you process crushed materials by yourself, the molded articles must be tested as required to determine whether they can meet the performance requirements.