What should I do if there are uneven bubbles during the polyurethane foaming process?

The problem of uneven bubbles during the polyurethane foaming process directly affects key indicators such as insulation, cushioning, and mechanical properties of the material. The following is a systematic solution that covers root cause analysis, process adjustments, and optimization in multiple regions:

1、 Cause analysis and corresponding measures

1. Uneven mixing of raw materials

Performance: Local bubbles are too large or too small, with significant density fluctuations.

reason:

Insufficient mixing of A/B components (isocyanates/polyols).

Uneven distribution of foaming agent (physical or chemical).

solve:

Increase the mixing head pressure (recommended 15-20MPa) and use dynamic or static mixers to enhance turbulence.

Check the viscosity difference of raw materials (the viscosity difference between polyols and isocyanates should be less than 500 mPa · s), and preheat (40-50 ℃) if necessary to reduce viscosity.

2. Imbalance of foaming reaction rate

Performance: The size of the pores varies, and some areas collapse or expand excessively.

reason:

The catalyst (amine/organic (based on the actual report) (based on the actual report) tin) ratio is improper, and the gel reaction and foaming reaction are not synchronized.

The ambient temperature or mold temperature is unstable.

solve:

Delayed amine catalysts (such as Dabco TMR) delay gelation, allowing bubbles to fully grow.

Organic (based on actual reports) tin catalysts (such as T-12) control the crosslinking rate, typically in a ratio of amine: tin=2:1~3:1.

Adjust the catalyst system:

Control the mold temperature between 40-60 ℃ (hard foam) or 25-40 ℃ (soft foam), with a temperature difference of less than ± 3 ℃.

3. Foaming agent issue

Performance: Bubble merging (opening) or shrinkage (low closure rate).

reason:

The volatilization rate of physical foaming agents (such as cyclopentane, HFCs) is too fast.

Excessive use of chemical foaming agents (such as water) leads to concentrated release of CO ₂.

solve:

Physical foaming agent: Use a foaming agent with a higher boiling point (such as HFO-1233zd, boiling point 19 ℃), or lower the injection temperature.

Chemical foaming agent: Control the amount of water (usually 0.5-2% for hard foam and 2-4% for soft foam), and use silicone oil stabilizer (such as L-580, dosage 0.5-1.5%) to stabilize the foam pores.

4. Insufficient nucleation

Performance: The number of bubbles is small and the size is too large.

Reason: Lack of nucleation points for bubbles, resulting in gas accumulation and the formation of large bubbles.

solve:

Add nucleating agents: nano silica (0.1-0.5%), talc powder (1-3%) or finer calcium carbonate.

Mechanical stirring introduces microbubbles (such as high-speed stirring for 2-3 minutes during the pre mixing stage).

5. Improper process parameters

Performance: There is a significant difference between the edge and the central bubble.

reason:

Excessive injection speed can cause turbulence or poor mold exhaust.

The curing pressure is unstable (especially sensitive during low-pressure foaming).

solve:

Adopting segmented injection: Fill the mold cavity at low speed by 50% and switch to high speed to reduce eddy currents.

Add exhaust slots (width 0.1-0.2mm) or microporous exhaust membranes to the mold.

Control the curing pressure (recommended 0.1-0.5 MPa for hard foam and atmospheric pressure for soft foam).