The Processing Precautions for the TPU High-Performance

Sealing Material Series

I. Drying

1. Prior to molding, TPU material must be dried regardless of whether the original packaging is intact or has been opened.

TPU is inherently hygroscopic. Moisture absorbed by the resin will cause material degradation during extrusion, leading to changes in molecular weight and a decline in mechanical properties. When processing, open the package and load the entire contents into the hopper at one time. Each batch should fill the hopper as completely as possible, and the hopper should be sealed tightly. Drying is best performed in a dehumidifying dryer.

2. Because TPU is highly hygroscopic and prone to hydrolysis, insufficiently dried TPU will cause processing difficulties, surface defects on injection-molded parts, and a reduction in the physical properties of finished products. Therefore, TPU material must be thoroughly dried before molding until the moisture content is below 200 ppm (0.02%).

3. If pigments or color masterbatch are used for coloring, they must be thoroughly mixed with the natural-color TPU pellets and dried together.

Drying Conditions:

50. Hot-air drying: 110 °C for 4 hours
50. Dehumidifying drying: 100 °C for 3 hours

(If only a small sample is available, an oven may be used. Spread the pellets evenly in the oven so that every pellet is fully exposed to the heat.) Processing must be carried out immediately after drying is complete.

II. Injection Molding Conditions

1. Screw Requirements

A conventional single-screw, three-zone injection molding machine is generally suitable for processing TPU. Compared with other plastics, TPU exhibits high viscosity in the molten state, which easily generates high shear forces in the barrel and may damage the inherent properties of TPU. Therefore, a screw with an L/D ratio of approximately 16–20 and a compression ratio of approximately 2.0–2.5 is generally regarded as the most suitable for TPU injection molding.

To prevent degradation caused by overheating, a general-purpose screw can be divided into three functional zones, each with a distinct purpose:

Fig. 1 — General-purpose screw for TPU injection molding  (Diameter Ø45 mm / Compression ratio 1:2 / Length 20D)

a) Feed Zone: Located directly below the hopper. When processing TPU, this zone should be 30%–40% of the total screw length. The material is heated by the barrel wall as it is conveyed forward, and by the time it reaches the front end of this zone, its temperature should be close to its softening point. The flight depth in this zone remains constant.

b) Compression Zone: TPU is melted in this zone. The flight depth decreases gradually to increase the pressure inside the barrel and ensure full contact between the melt and the barrel wall. The compression ratio in this zone should be between 2.5:1 and 3.5:1.

c) Metering Zone: This zone ensures the melt is homogeneously mixed, and the flight depth remains constant. This zone should be 30%–40% of the total screw length.

2. Processing Temperature

In injection molding, a temperature profile increasing gradually from the feed zone to the metering zone is generally used, while the nozzle temperature should be slightly lower than that of the metering zone to prevent drooling. (Please refer to the processing temperature data in the property sheet of each grade.)

3. Injection Speed

Injection speed is related to the screw size and the wall thickness of the part. It is generally in the range of 30–60 rpm, and should never exceed 100 rpm; otherwise, excessive shear will cause melt degradation. Larger screws should be operated at lower injection speeds to reduce shear heating.

(Recommended injection speed: medium to slightly slow.)

4. Injection Pressure

a) Injection pressure is related to the melt temperature, injection speed, mold temperature, and mold design, and is generally in the range of 7–10 MPa. To prevent flash, a low-then-high injection pressure profile is recommended.

b) Hold pressure: For thin-walled products, the hold pressure should be 50%–70% of the injection pressure; for thick-walled products, it should be 80%–100% of the injection pressure. Excessive hold pressure will generate internal stresses that impair part performance and may cause demolding difficulties; insufficient hold pressure will lead to shrinkage and sink marks on the part.

c) Screw back pressure: Screw back pressure is generally in the range of 0.3–0.7 MPa. Screws with a lower compression ratio require higher back pressure.

5. Mold Temperature

Generally, the mold temperature should be kept between 25 °C and 45 °C (room temperature range).

III. Mold Shrinkage

Because TPU is synthesized from monomers in fixed stoichiometric proportions and behaves as a semi-viscous gel in the molten state, its shrinkage rate is easily affected by the design of the finished part (such as gate location, wall thickness, and geometry) and by the molding conditions (tmperature, hold pressure, injection pressure, and mold temperature). It is therefore rather difficult to predict the shrinkage rate of an injection-molded part with very high precision.

Reference mold shrinkage rate: 1.0% – 1.2%

IV. Mold Design

Schematic of the mold is shown below.

Fig. 2 — Schematic of the injection mold

a) Sprue: The sprue should be as short as possible and have a draft angle of approximately 2.5°. The diameter at the sprue entrance should be slightly larger than the diameter of the injection-machine nozzle. To keep the pressure balanced between the runner and the cavity, the diameter at the sprue exit should also be slightly larger than the runner entrance diameter.

b) unner: The runner should be as short as possible, with a diameter of 6–10 mm being preferable. A runner that is too small in diameter makes melt flow difficult and requires higher injection pressure; a runner that is too large results in a longer injection cycle and requires a larger shot volume.

c) Gate: The gate should be as short as possible in order to reduce shear and pressure drop, allowing the melt to enter the cavity more easily. Because stress concentration is often severe at the gate location, the gate should be placed at a non-critical (secondary) position on the molded part. At the same time, the gate location should ensure that the last-filled region of the cavity can be properly vented.

d) Venting Groove: Venting grooves should be located at the end of the plastic flow, typically on the cavity side of the parting line. A groove depth of 0.03–0.1 mm and a groove width of 5–10 mm is recommended. For small parts, the venting requirement is limited; if the venting point lies directly on the parting line, the small gap formed when the mold closes can usually provide sufficient venting, and a dedicated venting groove is not required.

V. Annealing Treatment

Annealing the injection-molded part at 90–110 °C for 10–20 hours can effectively release internal stresses within the part. After annealing, the compression set of the seal can be significantly reduced.Post-Curing Conditions

Because the TPU material in this series contains a special chain extender, in order to achieve the optimum physical properties, the injection-molded products must be placed in an oven at 110 °C within 2 hours after molding for curing of 20 hours, and then removed and allowed to stand naturally at room temperature for at least 20 hours. Curing at room temperature alone cannot achieve the same effect.

VI. Color Treatment

Use color masterbatches with TPU as the carrier wherever possible, or select a masterbatch whose carrier is compatible with TPU and whose melting point and melt flow index are similar to those of TPU.

If pigments must be used, do not add oil-based pigment dispersants.

VII. Recycled Material Treatment

Sprues, runners, and defective parts may be ground and blended with virgin material for reuse, but they must be dried once again before use. To meet certain mechanical property requirements, the proportion of recycled material used in injection molding should not exceed 30 wt% of the total feedstock. In addition, recycled material should not be stored for too long — it is best to dry and use it immediately.

The physical properties of TPU will deteriorate after repeated drying, so dried material should be used up in a single run. Recycled material is also preferably used on its own and should not be blended with virgin material.

If sprue/regrind must be added, the addition level should not exceed 5%.

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