How to Solve Cracking of Flexible PU Foam in Batch Foam Machine Processes?

When producing flexible PU Foam for furniture and mattresses using a batch foam machine, cracking is a typical cause of yield loss. Its root lies in an imbalance during the foaming process — specifically, a mismatch in the rates of the foaming reaction (gas generation) and the gelation reaction (skeleton formation).

I. Problem Diagnosis: Locate the Production Stage from Crack Morphology

The foam rises freely inside the foaming box; the cracking morphology directly points to which production stage has the specific problem.

1. Large Internal Voids and Structural Delamination

Phenomenon: After cutting, the foam shows irregular large internal voids, horizontal layered separations, or an overall loose structure.

Diagnosis: This indicates that gelation is generally slower than foaming. The formation of the polymer skeleton cannot keep up with gas generation, causing fragile cell walls to rupture or fuse.

Key causes:

 Catalytic system imbalance: gel catalysts (e.g., stannous octoate) have insufficient activity or are under-dosed.

 Raw material temperature too low: leads to an overall slow reaction rate.

 Poor raw material mixing: creates locally weak zones.

2. Longitudinal Cracks at the Top

Phenomenon: One or several through longitudinal cracks appear at the center top of the foam.

Diagnosis: The top center completes foaming and gelation last and is the weakest part of the polymer network. When internal pressure is too high or the top environment is too cold, this weakest point cannot bear the internal stress and tears.

Key causes:

 Ambient temperature too low or excessive ventilation: drafts quickly carry away reaction heat at the top, causing premature stiffening and loss of elasticity.

 Excess foaming agent: excessive water generates too much gas, producing abnormally high internal pressure.

3. Bottom Settlement and Surface Cracking

Phenomenon: The foam bottom shows overall sagging or unevenness, sometimes accompanied by transverse cracks.

Diagnosis: The core cause is collapse of the bottom cell structure. In early foaming the bottom bears the most weight; if gel strength there is insufficient to support the upper foam, cells will rupture and fuse.

Key causes:

 Bottom gelation rate too slow: low catalyst efficiency at the bottom is the main reason.

 Poor overall load-bearing formula: early strength of the foam is inadequate.

II. Systematic Solutions

To solve cracking in batch foam machine processes, focus on adjusting the formulation and key process parameters.

Formulation Adjustments (Core Measures)

Strengthen the skeleton: moderately increase the amount of gel catalyst (e.g., stannous octoate). This is the most effective method to strengthen internal and bottom skeletons and raise early foam strength; it can simultaneously improve internal voids and settlement issues.

Control gas pressure: appropriately reduce the amount of water to lower gas generation, control internal pressure, and prevent top cracking.

Improve flowability: preheat raw materials to 25±2°C, or use low-viscosity polyether polyols to help achieve more uniform mixing.

Process Optimization (Create a Stable Environment)

Environmental control: maintain the workshop ambient temperature steadily between 22–28°C and avoid through-drafts to provide the foam with a uniform, mild reaction environment.

Mixing assurance: ensure the foaming machine has sufficiently high stirring speed and adequate mixing time to guarantee uniform catalyst distribution.

Summary and Reflection

Solving cracking in batch foam machine flexible PU Foam centers on understanding its “free-rise” character; all measures should aim to synchronize overall gas generation and skeleton formation. Increasing the catalyst to raise early gel strength is the core lever to address a series of defects from internal voids to bottom settlement.

When multiple defects appear together in your production practice, how do you determine the primary breakthrough to address first, and how do you balance formulation adjustments against their impacts on other foam physical properties (such as hardness and density)?