How to Precisely Control the Foaming Height of Flexible PU Foam in a Continuous Foaming Machine?

Controlling the foaming height of flexible PU foam is a key process to ensure product specifications and density consistency. The core principle lies in balancing total raw material supply, adjusting mechanical parameters, and optimizing chemical formulations, allowing the final foam volume and target density to match precisely. In real production, all parameters must be dynamically and finely adjusted based on on-site conditions.

I. Primary Mechanical Parameter Control: Fast Adjustment Without Changing Core Formula

Without altering the chemical ratio of materials, adjusting the machine’s physical operation parameters can quickly and effectively change the foaming height.

1. Management of Raw Material Feed Rate (Total Flow Rate)

If a higher foam block is required, operators should increase the total volumetric flow rate of main components such as polyether polyol and isocyanate (e.g., TDI). More raw material input means more mass in the final product. With a fixed forming width and conveyor speed, the accumulation thickness per unit length increases, thus increasing foam volume. Conversely, to reduce foam height, the total feed rate should be decreased.

Key Requirement: When adjusting the total flow rate, maintain consistent mass or molar ratios among all components to ensure chemical reaction balance and prevent foam quality or density deviations.

2. Correction of Conveyor Belt Speed

The conveyor belt speed directly determines the accumulation of material per unit time or area—one of the key physical parameters controlling foam height.

Slower belt speed: For a fixed total flow rate, more material accumulates per area, resulting in a thicker and higher foam block.

Faster belt speed: The raw material spreads thinner, reducing height.

When adjusting belt speed, curing and stabilization time on the conveyor must be considered. Excessive speed may cause the foam to exit the tunnel before stabilizing (under-cured), while a too-slow speed may cause over-curing.

3. Adaptive Adjustment of Forming Width (Material Confinement Range)

In a continuous foaming machine, the foam block’s lateral dimension is determined by the forming sideboards.

Narrower width: Reducing the distance between sideboards confines the same material flow into a smaller cross-section. The constrained material accumulates thicker, effectively increasing foam height.

Wider width: Expanding the side boundaries allows material to spread wider, reducing height. This method is suitable when both height and width need adjustment simultaneously.

II. Fine Optimization of Chemical Formulation: Ensuring Long-Term Stability

Slight formulation adjustments can fundamentally alter foam characteristics, suitable for cases requiring consistent density and long-term performance.

1. Adjustment of Blowing Agent Dosage (Gas Generation Volume)

Increasing water (chemical blowing agent reacting with isocyanate to produce CO₂) or physical blowing agents (e.g., methylene chloride, MC) directly drives foam volume expansion. However, blowing agent adjustment must be accompanied by proportional changes in amine and tin catalyst dosages to rebalance the foaming (gas formation) and gelling (polymer framework formation) reaction rates. This ensures foam structure stability and prevents over-foaming, collapse, or coarse cell formation.

2. Setting and Adjustment of Target Density

When total material input remains constant, lowering the target density (e.g., from 28 kg/m³ to 25 kg/m³) means the foam must expand to a larger volume for the same mass, thus increasing foaming height. Conversely, increasing the target density reduces volume and height. Density is the fundamental mathematical basis for engineers to adjust height precisely.

3. Role of Foam Stabilizer (Surfactant)

Appropriately increasing silicone surfactant dosage improves bubble nucleation and enhances film stability, reducing bubble rupture during foaming. Stable bubble structures retain more generated gas, indirectly and slightly increasing final foam height while improving pore uniformity.

III. Response Strategies for Production Abnormalities

Experienced operators select optimal adjustments based on actual foam conditions:

Insufficient height, excessively high density: Indicates material volume is too small. First, increase total flow rate or reduce conveyor speed. If density remains too high, slightly increase blowing agent dosage.

Excessive height, foam surface collapse: Indicates excessive volume with insufficient skeleton strength. Reduce total flow rate or increase conveyor speed. Check and fine-tune gelling catalyst dosage to ensure the polymer structure forms in sync with foaming speed.

Uneven local height: Often caused by uneven spreading or poor equipment leveling. Adjust the swing speed and angle of the mixing head (if equipped), check conveyor levelness, and ensure the mixing head outlet is unobstructed.

Continuous height fluctuation: Typically results from unstable metering or temperature variations. Regularly calibrate metering pumps and maintain raw material temperature between 20°C–25°C. Temperature fluctuation directly affects viscosity, metering precision, and chemical reactivity.

IV. Key Principles for Professional Operation

Single-variable adjustment: Always modify one parameter at a time (e.g., adjust flow rate first, then conveyor speed after stabilization). Changing multiple variables simultaneously makes cause-and-effect untraceable.

Quality over height: Foam quality (density uniformity, compression set, tensile strength, etc.) must always take precedence over height. Never sacrifice performance for height.

Parameter recording and archiving: Log all parameter adjustments and resulting foam data (height, density, performance). This production database enables faster parameter matching and setup for future runs.