How can water-control technology solve core scorching in low-density flexible PU foam?

In flexible PU foam production, “water control” is the core thread running through the entire foaming process. For low-density flexible PU foam in particular, it is a true lifeline for both safety and quality. Because low-density formulations require higher water content and generate extremely intense exothermic reactions, even minor mistakes can lead to core scorching, collapse, or even fire hazards.
In this article, we break down the underlying logic of water control in flexible PU foam foaming through 12 key Q&As, helping you fully understand the real mechanism behind safe and stable production.

• Q1: Why are low-density formulations always prone to core scorching?

The most direct way to reduce foam density is to increase water content. However, the reaction between water and isocyanate is highly exothermic.

The lower the density, the more concentrated the heat generated per unit volume—and the harder it is to dissipate. This inherent contradiction between low density requirements and high reaction heat makes temperature control the single most critical technical challenge when producing low-density flexible PU foam on a polyurethane foaming machine.

• Q2: What happens if the core temperature of the foam goes out of control?

165 °C is widely recognized as the safety red line for conventional flexible PU foam. Once the core temperature exceeds this threshold, a chain reaction occurs:

Surface quality deterioration: The most common issue is yellowing or darkening of the foam core.

Loss of physical properties: High temperatures damage polymer molecular chains, making the core brittle and reducing elasticity.

Severe safety risks: If heat cannot dissipate and reaches the auto-ignition point, serious fire hazards may occur during later curing stages.

• Q3: Why is MC commonly added as a cooling aid in flexible PU foam production?

Low-density formulations release large amounts of heat, requiring an efficient physical medium to offset it:

Physical temperature-control mechanism: MC has a boiling point of only 39.8 °C. As the reaction heats up, MC rapidly vaporizes.

Latent heat of vaporization: This phase change absorbs a large amount of heat, forcibly lowering the peak core temperature.

Mainstream solution: Compared with expensive liquid CO₂ systems, adding MC to sponge production line equipment is more cost-effective, simpler to operate, and technologically mature.

• Q4: If MC reduces temperature, why does excessive MC cause soft collapse and weak support?

MC is a double-edged sword, and its cooling effect comes with side effects:

Dilution effect: As a solvent, MC dilutes the reaction system and significantly slows down the gelation process that forms the foam skeleton.

Physical defects: If MC content is too high, cell walls lack sufficient strength during formation, leading to soft collapse, overly soft feel, or even foam collapse.

• Q5: What is “effective water” and what role does it play?

Effective water is a comprehensive technical parameter. It represents the true reaction driving force after accounting for MC’s inhibitory effect on gelation.

It serves as a critical indicator for engineers to judge whether a formulation is balanced and production-safe.

• Q6: How is effective water calculated?

You can apply the following industry-standard practical formula:

W=W1−MC/8.4​

W: Effective water

W₁: Actual water added in the formulation

MC: Amount of methylene chloride added

8.4: Empirical coefficient indicating that every 8.4 parts of MC offset approximately 1 part of water’s gelation effect

• Q7: How should effective water values be applied in production?

Based on accumulated production data, effective water follows two critical thresholds:

Lower limit: 2.5
If W<2.5W < 2.5W<2.5, reaction driving force is insufficient. Even with high actual water content, foam may fail to rise properly, resulting in severe closed cells or dead foam.

Upper limit: 4.5
If W>4.5W > 4.5W>4.5, reaction heat exceeds MC’s cooling capacity, signaling extremely high core-scorch risk.

• Q8: How can the effective water formula help prevent core scorching in practice?

When calculations show that effective water is approaching or exceeding 4.5, immediate intervention is required:

Increase MC dosage: Enhance cooling capacity to forcibly lower WWW and improve safety.

Reduce water content in tandem: If MC addition has reached equipment limits, water must be reduced to lower total heat generation—even at the cost of slightly higher density—to prioritize safety.

• Q9: What effective water value is recommended for low-density flexible PU foam below 25 kg/m³?

Such foams are commonly used in furniture cushions and lightweight applications:

Recommended value: W≈4.0W

Logic: This provides sufficient expansion power while reserving about 0.5 as a safety buffer for raw material variability and ambient temperature fluctuations.

• Q10: What about “medium-density ultra-soft” foam commonly used in mattresses?

Mattress comfort layers require both softness and structural integrity:

Recommended value: W=3.0∼3.5

Logic: This range utilizes MC’s softening effect while keeping effective water above 3.0 to ensure adequate support strength.

• Q11: How should effective water and temperature control be adjusted for slow-recovery foam?

Slow-recovery systems have poorer thermal stability and slower gelation:

Recommended value: W=3.0∼4.0

Temperature control: Core temperature must be strictly controlled below 160 °C, requiring precise MC regulation.

• Q12: How should the formula be adjusted for environmental changes and raw material batches?

Even with advanced automated foaming systems, dynamic adjustments are necessary:

Seasonal adjustment: In summer, higher ambient temperatures call for slightly lower effective water; in winter, the opposite applies.

Raw material batches: Different suppliers and batches vary in reactivity. After material changes, lab-scale trials using the formula are essential before full-scale production.

Mastering water-control technology ultimately means finding the optimal balance between chemical reaction heat and physical control methods. In practice, strictly keeping effective water WWW within the 2.5–4.5 safety window—while flexibly responding to environmental changes and emphasizing lab trials—is key to consistent high-quality output.

As a one-stop service provider in the flexible PU foam industry, we not only develop high-precision foaming machines and equipment but also offer comprehensive formulation consulting to help mattress factories, furniture manufacturers, and new investors overcome production challenges and achieve stable, efficient manufacturing.