Gas trapping is a common molding defect in the production of TPU smart wristbands, wearable-device components, and injection-molded TPU straps.
The problem may appear as white marks, gas streaks, burn marks, incomplete filling, or localized surface defects. When these defects occur, they should not immediately be attributed to changes in the TPU raw material.
Injection speed, mold venting, cavity differences, product structure, material drying, and processing parameters should all be evaluated before determining the actual cause.
The following two cases demonstrate how we helped customers solve gas-trap problems in TPU smart wristband injection molding.
Case 1: Fit*** Smart Wristband Project — Solving Gas Trapping by Adjusting Injection Speed
Project Background
The customer was using TPU injection molding material to produce components for a Fit*** smart wristband project.
During production, obvious gas-trap marks and surface defects appeared in certain areas of the product.
After reviewing the injection molding process, we found that the problem was mainly related to an excessively high injection speed.
When TPU melt enters the mold cavity too quickly, the air inside the cavity may not have enough time to escape. The trapped air is then compressed at the end of the flow path or in certain areas of the product, resulting in gas marks, burn marks, short shots, or other appearance defects.
Original Injection Parameters
The original first-stage, second-stage, and third-stage injection speed settings were:
Stage 1: 8 | Stage 2: 5 | Stage 3: 3
The overall filling speed was relatively high, leaving insufficient time for the air inside the mold cavity to escape through the venting system.
Adjustment
Based on the product structure, filling condition, and flow behavior of the TPU material, we adjusted the three injection speed stages to:
Stage 1: 7 | Stage 2: 4 | Stage 3: 2
By moderately reducing the injection speed at each stage, the TPU melt entered the mold cavity more smoothly and the air had more time to escape.
Result
After adjusting the injection speed, the gas-trap defect was eliminated.
The surface appearance returned to normal, and both the production stability and product yield were improved.
Case Analysis
For TPU smart wristbands with complex structures, thin walls, long flow paths, or limited mold-venting space, a higher injection speed does not always produce better results.
An excessively high injection speed may cause:
Air to remain trapped inside the mold cavity;
Compressed air at the melt-flow front;
Gas streaks or burn marks on the product surface;
Short shots or local filling defects;
Unstable appearance quality during continuous production.
Therefore, during TPU injection molding, the first-stage, second-stage, and third-stage injection speeds should be adjusted according to the product structure, mold design, and venting conditions.
Case 2: *Mi Band 2 Project — Identifying a Mold-Venting Problem Through Cavity-by-Cavity Analysis
Project Background
The customer was using TPU material to produce components for a *Mi Band 2 project.
At the beginning of production, only a small number of gas-trap defects occurred. However, as production continued, the defect rate increased significantly.
The customer initially suspected that the TPU raw material had changed or that there was a performance difference between different material batches.
Initial Investigation
The product was manufactured using a four-cavity mold.
The customer initially recorded only the total production quantity and the total number of defective parts. The production yield of cavities 1, 2, 3, and 4 was not recorded separately.
When only the overall defect rate is reviewed, it is easy to suspect changes in the TPU material, injection temperature, or general molding parameters.
However, for multi-cavity TPU injection-molded products, total production data can hide significant differences between individual cavities.
Analysis Method
After discussing the issue with the customer, we recommended recording the gas-trap defects and production yield for each individual mold cavity.
The cavity-by-cavity results showed that:
| Cavity | Production Yield |
|---|---|
| Cavity 1 | High production yield |
| Cavity 2 | High production yield |
| Cavity 3 | Significantly higher gas-trap defect rate |
| Cavity 4 | High production yield |
The data clearly showed that the problem was concentrated in cavity 3.
Root-Cause Evaluation
If the TPU raw material had been the main cause, similar defects would normally have appeared in all four cavities rather than remaining concentrated in one specific cavity.
The stable performance of cavities 1, 2, and 4 indicated that:
Material Performance Stable
The overall TPU material performance was stable.
No Flowability Abnormality
There was no obvious abnormality in material flowability.
Process Parameters Acceptable
The injection temperature and general process parameters were acceptable.
Cavity 3 Isolated Issue
The problem was more likely related to cavity 3 itself.
Based on the production data, we concluded that the venting condition or local mold structure of cavity 3 required further inspection.
Solution
The customer removed and inspected the mold, focusing particularly on cavity 3.
The following areas were checked and improved:
Vent depth and vent position;
Mold-parting clearances;
Runner and gate conditions;
Local carbon buildup or contamination;
Blockage and wear after long production runs;
TPU melt-flow behavior inside the cavity.
After confirming that the problem was isolated to cavity 3, the customer no longer attributed the gas-trap defect to the TPU raw material and instead focused on improving the mold venting.
Case Analysis
This case demonstrates that recording only the overall defect rate is insufficient for multi-cavity TPU injection molding.
When gas trapping, short shots, gas streaks, or burn marks occur, the production data for each cavity should be analyzed separately.
If similar defects occur in every cavity, the following factors should be checked first:
TPU material drying conditions;
Barrel and nozzle temperatures;
Injection speed;
Back pressure and screw speed;
TPU raw-material batch consistency;
Overall mold-venting conditions.
When the defect remains concentrated in one specific cavity, priority should be given to inspecting the vent, gate, runner, and local structure of that cavity.
Common Causes of Gas Trapping in TPU Smart Wristband Injection Molding
Gas-trap defects in TPU smart wristbands and TPU watch straps can be related to several processing and tooling factors.
1. Excessively High Injection Speed
When the TPU melt fills the cavity too quickly, the air inside the mold may not escape in time and can become trapped or compressed.
2. Insufficient Mold Venting
Vents that are too shallow, incorrectly positioned, contaminated, or blocked can reduce the mold’s ability to release air.
3. Insufficient TPU Drying
TPU is a moisture-sensitive material. If the TPU pellets are not dried properly before injection molding, absorbed moisture may turn into vapor at high processing temperatures. This can cause bubbles, silver streaks, gas marks, or surface defects.
4. Incorrect Processing Temperature
If the processing temperature is too low, the TPU material may not flow properly. If the temperature is too high, the material may degrade and generate gas, leading to burn marks, odor, color changes, or unstable product quality.
5. Differences Between Mold Cavities
Different cavities may have different runner lengths, gate conditions, mold temperatures, filling pressures, and venting performance. For this reason, the production yield of each cavity should be recorded separately.
6. Reduced Venting Performance After Continuous Production
During long production runs, carbon deposits, oil, dust, or material residue may accumulate in the vents. This gradually reduces the mold’s venting capacity and may explain why gas-trap defects are limited at the beginning of production but become more obvious later.
How to Select TPU Injection Molding Material for Smart Wristbands
Smart wristbands, TPU watch straps, and other wearable-device components generally require a combination of flexibility, abrasion resistance, elasticity, surface quality, and stable injection molding performance.
Depending on the product structure and performance requirements, customers can select a suitable polyester TPU material for injection molding with good flowability and processing stability.
When selecting TPU material for smart wristbands and watch straps, the following properties should be considered:
Mechanical Properties
Shore hardness;
Tensile strength;
Tear strength;
Elastic recovery.
Processing Properties
Melt flowability;
Demolding performance;
Compatibility with the mold and product structure.
Surface & Appearance
Abrasion resistance;
Surface feel;
Color stability.
Note: Even when two TPU grades have the same Shore hardness, they may perform differently in terms of flowability, molding cycle, demolding, surface appearance, and sensitivity to gas trapping. Therefore, TPU raw materials should not be selected based on hardness alone. The final product structure, mold design, injection molding machine, and processing conditions should also be considered.
Our Technical Support Approach
When gas trapping, gas streaks, burn marks, short shots, bubbles, or surface defects occur during TPU injection molding, the TPU raw material should not automatically be considered the only possible cause.
We normally evaluate the following factors:
| Evaluation Area | Specific Factors Reviewed |
|---|---|
| Material | TPU raw-material batch and physical-property data; TPU drying temperature and drying time |
| Temperature | Barrel and nozzle temperatures; Mold temperature |
| Injection Parameters | First-stage, second-stage, and third-stage injection speeds; Injection pressure and holding-pressure settings; Back pressure and screw speed |
| Mold & Cavity | Mold-venting conditions; Production yield of each individual cavity |
| Product Design | Product structure and TPU melt-flow path |
We provide not only TPU raw materials but also application and processing support based on the customer’s product design, molding equipment, and actual production defects.
Our goal is to help customers identify the real cause of the problem, reduce troubleshooting time, improve production yield, and achieve more stable TPU injection molding performance.
Conclusion
These two TPU smart wristband projects demonstrate that effective troubleshooting should be based on production data and actual molding conditions.
In the Fit*** smart wristband project, the gas-trap problem was solved by adjusting the three-stage injection speed from 8 / 5 / 3 to 7 / 4 / 2.
In the *Mi Band 2 project, cavity-by-cavity production analysis showed that the defect was concentrated in cavity 3. This allowed the customer to focus on improving the mold venting rather than replacing the TPU raw material.
When defects occur in TPU injection-molded products, a systematic evaluation of the TPU material, drying conditions, injection parameters, mold venting, and individual cavity data is often more effective than immediately changing the material.
Experiencing gas-trap defects in your TPU injection molding? Contact our technical team for application support and grade selection assistance.
