TPU (thermoplastic polyurethane) is widely processed by injection molding for shoe soles, wheels, seals, protective parts and industrial components.

However, TPU is sensitive to moisture and processing conditions. Problems such as bubbles, short shots, flow marks, sticking and yellowing can occur during injection molding.

Here are some common TPU injection molding problems and what to check first.

TPU Injection Molding Troubleshooting Overview

Problem First Factors to Check
Bubbles or voids Material moisture, melt temperature, trapped air and degradation
Short shot Melt and mold temperatures, injection settings, gate and runner
Flow marks Flow uniformity, temperatures, injection speed and gate position
Mold sticking Cooling, mold surface, draft angle and TPU hardness
Yellowing Heat exposure, residence time, contamination and degradation
Flash Injection pressure, melt temperature, clamping and mold condition

1. Bubbles or Voids in TPU Parts

Possible causes:

  • TPU pellets contain too much moisture
  • Melt temperature is too high
  • Air is trapped during injection
  • Material has started to degrade

What to check: Dry the TPU properly before processing and avoid leaving material at high temperature for too long.

If bubbles suddenly appear, material moisture should be one of the first things you check.

For preparation guidance, see our TPU Drying Guide.

2. Short Shot or Incomplete Filling

A short shot occurs when the TPU does not completely fill the mold cavity.

Possible causes:

  • Melt temperature too low
  • Mold temperature too low
  • Injection pressure or speed too low
  • Gate or runner too small
  • Poor material flow

Solution: Gradually adjust melt temperature, injection pressure and speed. Also check whether the gate and runner design are suitable for the part.

3. Flow Marks

Flow marks can affect the appearance of transparent, colored or high-gloss TPU parts.

Possible causes:

  • Uneven material flow
  • Low melt or mold temperature
  • Improper injection speed
  • Poor gate position

Solution: Increasing mold temperature or optimizing injection speed can sometimes improve surface appearance. Review gate position if the defect repeatedly appears in the same area.

4. TPU Sticking to the Mold

TPU parts may sometimes be difficult to demold, especially with softer TPU grades.

Possible causes:

  • Mold temperature too high
  • Insufficient cooling
  • Poor mold surface
  • Inadequate draft angle
  • TPU grade is too soft for the mold design

Solution: Improve cooling, check the mold surface and review the draft angle.

Avoid relying excessively on mold-release agents, as they may affect later printing, bonding or coating.

5. TPU Turns Yellow During Molding

If transparent or light-colored TPU becomes yellow during processing, possible causes include:

  • Melt temperature too high
  • Excessive residence time
  • Material degradation
  • Contamination
  • Repeated use of degraded material

Solution: Reducing unnecessary heat exposure and residence time can help minimize processing-related discoloration. Check the machine and material-handling system for contamination.

For applications requiring long-term UV color stability, the choice between aromatic TPU and aliphatic TPU should also be considered.

6. Flash Around the Mold

Flash occurs when excess TPU flows outside the intended mold cavity.

Possible causes:

  • Injection pressure too high
  • Melt temperature too high
  • Excessive material flow
  • Mold clamping problems
  • Worn or damaged mold surfaces

Solution: Check processing pressure first, then inspect the mold parting surface, clamping condition and wear if the problem continues.

Quick TPU Injection Molding Checklist

When a molding problem occurs, check these factors first:

1. Drying
Is the TPU sufficiently dry?

2. Temperature
Are the barrel, nozzle and mold temperatures appropriate?

3. Injection conditions
Check injection speed, pressure and holding pressure.

4. Cooling
Is the cooling time sufficient?

5. Mold design
Check the gate, runner, venting and draft angle.

6. TPU grade
Are the selected hardness and flow behavior suitable for the part?

Change one processing parameter at a time and record the result. Changing several settings together can make the true cause of a defect harder to identify.

Conclusion

Many TPU injection molding problems are related to moisture, temperature, injection conditions or mold design.

Instead of changing several parameters at the same time, troubleshoot the process step by step, starting with TPU drying and material condition.

Always use the processing recommendations for the specific TPU grade as the starting point, then optimize the conditions according to the part and mold.

For TPU grade selection or processing support, contact Longyixing Chemicals with your part design, hardness, molding equipment and current processing conditions.