Due to its high hygroscopicity and susceptibility to hydrolysis, insufficiently dried TPU materials can result in processing difficulties and cause the injection-molded product to have a foamy or bubbled surface, or a non-smooth, non-glossy surface with visible bubbles. Therefore, to ensure defect-free production and product quality, it is recommended to fully dry TPU materials to a water content of less than 0.05% before processing.
If coloring is required, it is recommended to mix the TPU raw material with pigments or masterbatch and dry them together. Usually, a circulating air dryer or dehumidifying dryer is used to dry TPU, and depending on its hardness, the drying temperature should be between 80-110℃, and the drying time should be around 3 hours.
Except for certain special requirements, TPU is usually supplied in its natural color. Users can easily color TPU by adding pigments or masterbatch to achieve the desired color. The recommended usage amounts are as follows:
Pigments: 0.2%-0.5%; Masterbatch: 1%-4%.
The runner, purging, or scrap material can be crushed and mixed with new materials, but it must be dried again before use. In addition, for certain mechanical requirements, the recycled material should not exceed 30% of the overall raw material for injection molding. Extrusion materials must be processed separately or added to the injection molding new material. Also, recycled materials should not be stored for too long and are best dried and used immediately. Waste materials that cannot be recycled can be burned to generate heat energy.
A single-screw, three-stage injection molding machine is generally suitable for TPU injection molding. Compared with other plastics, TPU has high viscosity during melting and is prone to high shear forces in the screw barrel, which can damage its inherent properties. Therefore, a screw with a length-diameter ratio of 16-20 and a compression ratio of 2.0-25 is more appropriate for injection molding.
The adjustment and control of injection pressure, holding pressure, injection speed, and back pressure are very important for product size stability and demolding performance. Too low holding pressure can cause product sink marks, while too much can cause product demolding difficulties. The injection pressure and holding pressure should be adjustable within the range of 100-1200 bar, and the injection pressure (holding pressure) should be set to eliminate product sink marks. The holding pressure is usually set at 50% of the injection pressure to ensure even plasticization. Back pressure should be controlled within 10%-25% of injection pressure to ensure smooth material flow. The injection speed should be chosen based on the wall thickness of the product: slow for thick-walled products and fast for thin-walled products.
A single-screw, three-stage extruder with an L/D ratio of 20-30 and a compression ratio of 2-4 is suitable for TPU processing. The clearance between the screw and the barrel is about 0.1-0.2mm, and the optimal screw design should avoid excessive friction and overheating that could cause material degradation and reduce the mechanical properties of the product. The screw speed should be determined by the screw diameter. As TPU is sensitive to shear forces, excessive screw speed can lead to material degradation, while too low a screw speed can also lead to degradation due to excessive dwell time. Generally, the screw speed is set within the range of 25-60rpm.
To ensure smooth material flow and easy cleaning of the mold, dead corners should be avoided in the mold design. For pipe fittings or shaped extrusions, a shaped die should be used to reduce shear stress and obtain a stable extrusion volume. The length of the die should be 2-4 times the nozzle diameter.