How to Solve Shrinkage Problems in flexible PU foam Production?

Introduction — Why shrinkage requires a practical action guide

Shrinkage in flexible PU foam is not caused by a single factor but is the result of multiple linked steps across the production chain. In practice, when this defect appears we most need an efficient, intuitive troubleshooting workflow rather than reinventing theoretical derivations from scratch. This article aims to provide a practice-based action guide to help you quickly locate the problem and take targeted measures.

First step: Emergency diagnosis — Perform a “post-mortem” on the product to trace root causes

When shrinkage occurs, the first thing to do is not to adjust the formulation, but to perform a “post-mortem” on the defective foam. By observing its physical appearance we can rapidly narrow the scope of investigation.

If the foam body has uniform overall shrinkage but the cell structure is normal: this usually points to an imbalance between the foaming and gelation reactions. Imagine the “skeleton” forming too slowly or insufficiently strong to support the volume after gas expansion. In this case, focus on catalysts and polyols in the formulation.

If the foam surface shows depressions, collapses, or local shrinkage: this may indicate uneven raw-material mixing. There may be dead zones in the mixing equipment causing delayed reaction in some regions.

If the foam feels weak and undercured to the touch: this is very likely related to insufficient curing temperature or time. If the foam is demolded or leaves the curing area before internal reactions finish, its structure cannot support its own weight and will collapse.

If the foam expands rapidly during foaming but then rapidly contracts: this usually indicates excessive gas during the late foaming stage, exceeding the bearing capacity of the gel skeleton. Check for moisture in raw materials, especially free water in the polyol.

Second step: Data Review — Pinpoint abnormalities in key links
After the preliminary diagnosis, the next step is to immediately check production records and use data to verify your hypotheses. This is far more effective than blindly changing formulations.

Check raw-material batches: compare current materials to quality inspection reports, confirming whether the isocyanate and polyol NCO content, hydroxyl number, moisture content and other key indicators are within standard ranges. If abnormalities are found, replace the batch immediately and notify the supplier.

Review process parameters: compare stirring speed, mixing time, mold temperature, or curing temperature between the good batches and the problematic batch. Any small deviation can affect the final product. For example, a 5°C drop in mold temperature could double the curing time.

Monitor environmental conditions: review recorded shop floor temperature and humidity. Low ambient temperature slows reaction speed; high humidity can introduce extra moisture. Both can lead to shrinkage.

Third step: Treat the problem — Minimize adjustments and verify

Once the cause is identified, measures should follow the “minimal adjustment” principle to avoid introducing new variables.

If the problem points to catalysts: make small adjustments to the gel catalyst dose—use gradients of 0.05–0.1 phr on the existing formulation—then run small-scale trial productions to observe results.

If the problem points to curing: without changing the formulation, try increasing mold temperature by 5°C or extending the curing time to 48 hours to verify whether this resolves the issue.

If the problem points to uneven mixing: inspect the wear condition of mixing heads, verify the accuracy of metering pumps, or adjust stirring parameters to ensure all raw materials are uniformly mixed.

Summary: From troubleshooting to prevention

The ultimate goal in addressing shrinkage in flexible PU foam is to shift from “post-event troubleshooting” to “prevention.” By establishing a complete Process Quality Control (PQC) system and routinely monitoring raw materials, processes, and environment, most shrinkage issues can be prevented at the source. This is not only about fixing a defect, but about raising production quality management to a new level.