What Factors Affect the Quality of Flexible PU Foam in Batch Foaming?

The final quality of batch foaming products is not determined by a single factor, but by the complex interaction of internal chemical reactions and external physical conditions. A stable reaction process and effective heat management are key to obtaining an ideal foam structure. This article systematically analyzes the critical factors influencing reaction stability and thermal balance, from raw material preparation to curing and molding.

Synergy of Raw Materials and Equipment

Batch foaming begins with precise raw material preparation. In the formulation, silicone oil acts as a vital foam stabilizer and is typically premixed with water. Its chemical stability is crucial, as unstable silicone oil (e.g., hydrolyzed) can disrupt cell structure and cause product defects.

Equipment performance directly determines process operability. Based on the mechanical principle:
P = F·V (Power = Force × Velocity),
the motor must provide sufficient power to meet the minimum mixing strength, which limits the maximum batch size. Additionally, the motor’s dynamic response—such as high starting torque and precise braking for controlled stopping—is essential to ensure accurate mixing time.

Mixing Dynamics

The success of foaming largely depends on mixing precision. It is not just a simple combination of raw materials, but achieving microscopic uniform dispersion to provide ideal interfaces for chemical reactions. Similar to shear rate concepts in chemical engineering, sufficient shear force ensures a homogeneous dispersion system.

A critical parameter is the relative velocity between the stirring blades and liquid. If the liquid level is too low, the blades’ effective immersion depth decreases, reducing actual mixing efficiency despite vigorous surface motion. This also introduces excess air, harming product quality. Therefore, mixing efficiency depends on several geometric factors, including:

Stirrer design (blade projection area, angle, immersion depth)

Reactor dimensions (diameter-to-height ratio)

Internal structures (e.g., baffles)

Liquid level height determined by batch size

Impact of External Physical Factors

External conditions such as cover plate weight and mold geometry significantly affect foam quality.

Cover Plate Pressure: In covered foaming, the foam must withstand pressure from the plate during its weakest rising stage. This often requires formulation adjustments—such as increasing graft polyether content and reducing foaming agent use—to enhance early skeleton strength. Even a 10 kg cover over 3 m² exerts only ~33 Pa, much lower than atmospheric pressure, but its timing during the fragile growth phase can critically hinder bubble expansion.

Mold Geometry: Heat management varies greatly with mold shape. Cylindrical molds usually offer narrower process tolerance compared to square molds. This is because in cylinders, foam stress, reaction heat, and gases concentrate along the central axis, creating a strong thermal core effect with limited heat dissipation, often causing central overheating (“red heart”). Square molds, with corner structures, disrupt this symmetry and allow multidirectional heat/stress dissipation, offering a wider process window.

Thermal Inertia: For the same rigid foam recipe, foaming at 19:00 in summer may be more prone to “red heart” than at 15:00, despite lower ambient temperature. This is due to thermal inertia: molds absorb daytime heat and release it slowly in the evening. As a result, the foam’s exothermic reaction retains more heat inside, raising and prolonging the core temperature peak.

Cross-Batch Contamination and Mitigation

In real production, foam buckets are rarely fully cleaned, so residues from previous batches inevitably mix into fresh raw materials. These residues consist of partially chain-extended urea and urethane segments, essentially prepolymers with terminal hydroxyl groups (-OH), which alter the initial reaction pathway of the new system.

More critically, residues may contain unreacted TDI monomers, whose free NCO groups consume fresh catalyst and reduce activity. Common mitigation strategies include optimizing mixing steps to shorten the contact window—for example, dispersing TDI immediately after adding tin catalysts.

Notably, batch foaming allows relatively long cream times, giving high flexibility in formulation design and making it particularly suitable for producing specialized foams.