Thermoplastic polyurethane elastomer, also known as thermoplastic polyurethane rubber (TPU), is a (AB)n-type segmented linear polymer. A is a high molecular weight (1000-6000) polyester or polyether, B is a diol with 2-12 linear carbon atoms, chemical structure between AB segments is a diisocyanate. Thermoplastic polyurethane rubber is crosslinked by intermolecular hydrogen bonds or by light crosslinking between macromolecular chains. Both crosslinking structures are reversible with increasing or decreasing temperature. In molten state or solution, intermolecular forces weaken. However, after cooling or solvent evaporation, strong intermolecular forces re-establish bonds, restoring original solid properties. Typical TPUs include spandex.

 

The most suitable processing method for thermoplastic polyurethane (TPU) is injection molding. During TPU injection molding, dried, preheated TPU pellets are fed from injection molding machine's hopper into a heated barrel. Once molten, they are propelled by screw (or plunger) and nozzle at the front of barrel into a relatively cool, closed mold, filling mold cavity. Material then cool and solidify under pressure. After demolding, a TPU product identical to original mold cavity is obtained.

Using a common three-stage screw for TPU injection molding ensures excellent and uniform plasticization of melt. If higher plasticization capacity (production throughput) is required, a longer screw can be used (Figure 1.1).

 

Figure 1.1 Injection Molding Screw for TPU Processing
TPU plasticization requires high energy and a high torque drive. Insufficient torque can lead to fluctuating screw speeds and uneven plasticization. Within limits, higher barrel temperatures can produce good results, although there is a risk of overheating.
Injection molding machine design: Barrel is lined with a copper-aluminum alloy, and screw is chrome-plated to prevent wear.
Screw length-to-diameter ratio (L/D) is preferably 16-20, with a minimum of 15. Compression ratio is 2.5/1-3.0/1. Length of feeding section is 0.5L, compression section is 0.3L, and metering section is 0.2L.
A check ring should be installed near screw tip to prevent backflow and maintain maximum pressure. For TPU processing, a self-flowing nozzle with an inverted cone outlet and a nozzle diameter of at least 4 mm should be used, smaller than 0.68 mm inlet of sprue collar. Nozzle should be equipped with a controllable heating ring to prevent material solidification.
Passage between nozzle and flange should be designed to eliminate dead corners and prevent material residue. Precise temperature control of barrel and nozzle heating system is crucial to ensure uniform heating of material in barrel.

TPU should be processed at a melt temperature between 190 and 220℃. For some hard grades, melt temperature may be increased to 240℃. Figure 2.1 shows guideline values for barrel and nozzle heating settings for different Shore hardnesses.

 

Figure 2.1 Temperature Settings for Different Hardnesses

Mold temperature primarily affects part surface quality and demolding behavior, as well as shrinkage and internal stresses in final structure (when cooled). Normal mold temperatures should be between 20℃ and 40℃. However, with some modified TPU grades and glass-filled reinforced TPU, mold temperature should be increased to 60℃ to ensure optimal surface quality. For cooling thick-walled parts, a reduction of approximately 5℃ can reduce cycle time.

For plasticizing, select a peripheral speed not exceeding 0.3 m/s. Figure below shows maximum speed for different screw diameters.

 

Figure 2.2 Screw Speed as a Function of Screw Diameter
A practical rule of thumb is to utilize 30%-75% of barrel capacity for optimal performance. If barrel capacity is too low for injection volume, melt will remain in plasticizing unit for too long, leading to thermal decomposition of melt.

 

Figure 2.3 Decomposition of TPU Melt in Screw Processing

Injection speed is primarily based on wall thickness. Generally, molds for thick-walled products require slow filling, while molds for thin-walled products require fast filling. In addition to wall thickness and cavity type, mold venting is also crucial to injection speed, helping to prevent air entrapment and so-called "burn marks."
Excessive injection/holding pressure can affect product demolding. Excessively low holding pressure can cause sink marks. Using a holding pressure lower than injection pressure—generally, 50% of injection pressure is appropriate.

 

Figure 2.4 Typical Cycle Steps in TPU Processing

Cycle time is determined by product shape, wall thickness, mold cooling, and material itself. Figure below illustrates effect of wall thickness on injection cycle, quantifying TPU into three grades: hard, medium, and soft.

 

Figure 2.5 Relationship between cycle time and wall thickness and hardness

3.1 Controlling processing temperature has a crucial impact on final product size, shape, and deformation. Processing temperature should be determined based on TPU brand and mold design. General trend is that to minimize shrinkage, processing temperature should be increased.
3.2 Slow speeds and prolonged holding times can lead to molecular orientation, potentially resulting in smaller product dimensions, but also greater product deformation and significant differences in lateral and longitudinal shrinkage. High holding pressures can also cause over-compression of product within mold, resulting in product being larger than mold cavity dimensions after demolding.
3.3 Injection speed and holding pressure switching positions must be accurately set.
3.4 Injection molding machine model must be appropriately selected. For small-sized injection molded products, a smaller machine should be selected as much as possible to allow for a longer injection stroke, facilitate position control, optimize injection speed and pressure switching settings.
3.5 When moisture content of TPU exceeds 0.2%, not only will product's appearance be affected, but mechanical properties will also deteriorate significantly, resulting in poor elasticity and low strength. Therefore, it should be dried at 80℃/110℃ for 2/3 hours before injection molding.
3.6 Injection molding machine barrel must be cleaned thoroughly. Even the slightest intrusion of other materials can reduce product's mechanical strength. Barrels cleaned with ABS, PMMA, or PE should ideally be cleaned again with TPU sprue compound before injection molding to remove any residual material. When cleaning hopper, pay special attention to any residual material remaining at connection between hopper and injection molding machine base, a point often overlooked by many technicians.
3.7 Mold Removal
Mold removal can be performed using a release agent. Silane-based release agents are effective. Non-silane release agents are also acceptable, but they must be used frequently.
3.8 Use of Waste (Pulverized Material)
Pulverized material from waste products, sprues, defective products can be recycled by pelletizing if cleaned and dried. For injection molding, less than 30% of crushed material in pure material will not affect performance of product. If you crush material yourself, molded product must be tested as required to ensure that it meets performance requirements.