The most suitable method for processing TPU is a screw-type injection molding machine. A single screw with a typical length and a three-section screw can produce well-plasticized and evenly melted materials. If high plasticizing capacity (production volume) is required, a longer screw can be used.
Short compression zone screws are not suitable due to high shear forces. TPU plasticization requires high energy and high torque to drive the screw. Insufficient torque can cause fluctuation in screw speed and uneven plasticization. Within limits, higher screw temperatures produce good results, although there is a risk of material overheating.
The nozzle and barrel pathway should be designed without dead corners, allowing materials to be injected without being damaged by heat. Precise temperature control of the heating system for the barrel and nozzle is key.
Ensure that the nozzle is uniformly heated along the entire length. Beware of local overheating and potential cooling on the pathway. Molten TPU is non-corrosive and non-abrasive. For this reason, the screw does not require any special alloy steel or strengthening plating.
TPU should be processed at a melting temperature between 190 and 220°C. For some hard grades, the melting temperature may be increased to 240°C. The melting temperature range of specific TPU grades can be found in the relevant product information table.
It also affects the shrinkage and internal stress of the final structure (when cooling). Normal mold temperature is applied at 20-40°C. However, for some improved TPU grades and TPU reinforced with glass fibers, the mold temperature should be increased to 60°C to ensure optimum surface quality. For thick-walled items that need cooling, reducing the temperature by about 5°C can reduce the cycle time.
A circumferential velocity of no more than 0.3 m/s should be selected for plasticization speed. The metering stroke should be between 1D and 4D. The maximum speed at different screw diameters is shown in the figure. The actual experience is that using 30-75% of the screw capacity is best. If the screw capacity corresponds to a very low injection volume, the molten material will stay too long in the plasticization unit, which can cause thermal damage to the molten material.
For ideal processing, the key is continuous control of pressure and injection speed. It should be able to control injection and holding pressure within the range of 100-1200 bar. To ensure uniformity, back pressure is required, usually set between 1 and 2.5% of the injection pressure. The injection speed is mainly based on wall thickness. Usually, for molds of thick-walled items, slow filling is required, while for thin-walled items, fast filling is required.
Reasonable wall thickness and cavity type are essential, but the importance of exhaust should not be ignored. When the exhaust is poor, the high-speed and high-pressure melt will cause an "internal combustion engine effect" in the mold cavity, which may cause burning, yellowing, or even underfilling of the product. When setting the injection speed and holding pressure, the cooling and shaping of the TPE part and demolding should also be considered.
High orientation and excessive pressure caused by too fast injection speed can affect the cooling and shaping of the part, and serious deformation may occur. When injection is completed, sufficient holding pressure is needed to support the shaping of the part and prevent the cooling shrinkage from affecting the demolding. In practical experience, the holding pressure for TPE molding is about 50% of the injection pressure.
