PU sandwich panel delamination usually comes from four areas: off-ratio PU chemistry, cold or contaminated facings, poor belt pressure, or cutting before the foam has cured enough to hold the steel skin. Start by checking the failure face, because foam tearing inside the core points to weak foam while a clean steel surface points to adhesion or sheet preparation. If the panel looks fine at the saw but opens after stacking, treat it as a curing and heat-retention problem first.
PU Delamination Causes
PU sandwich panel delamination is a loss of bond between the steel facing and the polyurethane core. In production, the main triggers are off-ratio isocyanate/polyol dosing, low sheet or chemical temperature, contaminated coil surfaces, uneven laminator pressure, and short cure time before trimming or stacking. Start diagnosis at the failure face, then adjust one variable at a time.
ASTM International C297/C297M, reapproved in 2024, treats core-to-facing bond integrity as the condition needed for facing stability and load transfer in sandwich panels. The Occupational Safety and Health Administration isocyanates overview, citing its March 2014 fact sheet, explains that isocyanates react with hydroxyl groups to produce polyurethane polymers. That chemistry sounds simple on paper. On a moving panel line, timing decides whether the foam wets the steel before it gels.
Read the broken panel like a process log:
- Foam residue stuck to steel: adhesion happened, then the core failed or cured weak.
- Clean painted steel: wetting failed, or oil, moisture, passivation, or low sheet temperature blocked adhesion.
- Paint or primer lifting from steel: the coil coating failed, so PU adjustments won’t solve the batch.
- Edge-only opening: belt edge pressure, side sealing, or foam distribution is suspect.
Chemical Ratio First
A sustained 2-3% ratio drift can change cell structure, tack, and core strength before operators see a dramatic color or rise-height change. Check the chemical supplier’s target isocyanate index for your exact system, whether you’re running Wanhua WANNATE PM-200 polymeric MDI with a local blended polyol or a BASF Elastopor-type PU/PIR package. Don’t copy another factory’s ratio because their steel thickness, line speed, and ambient humidity may be different.
On a custom PU sandwich panel production line, stable metering matters more than a larger mixing head. Ask for metering pumps that can be cup-tested by component, pressure gauges before the mixing head, and production records that show pump speed, chemical temperature, and line speed for every shift.
Daily ratio checks should be boring:
| Check | Practical Method | Red Flag |
|---|---|---|
| Component output | 60-second cup test from each pump | Output differs from set ratio by more than your chemical supplier allows |
| Chemical temperature | Inline sensor plus handheld confirmation | Polyol tank warm, hose cold, or return line much hotter than feed |
| Mixing quality | Cut fresh foam and inspect cells | Streaks, coarse cells, powdery zones, or dark wet patches |
| Pour pattern | Watch the first 2 meters after the head | Foam reaches one edge late or leaves dry bands |
If the foam is brittle and dark, the isocyanate side may be high. If the foam is soft, wet, or slow to tack, the polyol side, catalyst package, moisture, or low temperature needs attention. Stop there and test. Guessing during a live run wastes coil.
Temperature Hides Adhesion Trouble
Cold steel steals heat from the reaction. The foam starts to rise, touches the skin, and then gels before it has enough tack to grip the coating. You peel the sample and see a clean metal surface. The operator blames the formula. The sheet was 18°C.
For many rigid PU/PIR panel systems, a practical starting window is 20-25°C for chemical components and about 35-45°C for steel facings, unless the chemical supplier’s data sheet gives a different range. PIR recipes often need a hotter and slower process than standard PU because trimerization timing is less forgiving. If your factory switches between polyurethane and mineral wool orders, the PU and rockwool panel production line should store separate sheet-heating recipes instead of relying on one operator memory setting.
Use four checks before changing the formula:
| Temperature Check | Why It Matters |
|---|---|
| Top and bottom sheet surface temperature | A 5-8°C difference can create one-side delamination |
| Coil storage temperature | Overnight cold coils need preheating time, not faster line speed |
| Dew point near the decoiler | Condensation on steel blocks adhesion fast |
| Mixing-head and hose temperature | Warm tanks don’t help if hoses cool the chemicals before mixing |
Surface prep is part of temperature control. Oil, dust, cutting coolant mist, and wet protective film can all cause PU sandwich panel delamination that looks like a chemistry problem. Run a water-break test or dyne pen check on suspect coils, then compare adhesion on a cleaned strip and an untreated strip from the same coil.
Pressure And Cure Timing
Belt pressure needs to hold the sheet against the expanding foam without crushing the cell structure. Too little pressure leaves voids near ribs, edges, and lap zones. Too much pressure can push foam away from the edge and create a dense, stressed skin that opens after cooling. Flat at the saw doesn’t mean stable.
Start from the commissioned belt gap. For a 50 mm wall panel, verify the actual gap across left edge, center, and right edge with feeler gauges or dial indicators, then compare it with finished thickness after 30 minutes and again after 24 hours. If the panel grows after stacking, the foam is still reacting or trapped heat is driving post-expansion. Slowing the line by 0.5-1.0 m/min often works better than raising pressure because it gives the foam more time under control.
Fast correction order:
- Return line speed to the last known stable commissioning speed.
- Reset belt gap across the full width, not just the operator side.
- Match top and bottom belt temperatures before changing chemical ratio.
- Peel-test one marked panel after cutting, after 30 minutes, and after 24 hours.
Pressure changes won’t fix steel paint delamination, crushed panels from forklift handling, or corrosion caused by wet storage. Those failures need coil, packing, or handling corrections.
Delamination Diagnosis Table
The fastest way to find the cause is to cut the panel across the defect, label the sample, and stop arguing from memory. A 300 mm by 300 mm peel sample from the left edge, center, and right edge tells more than a full shift of opinions.
| Symptom | Likely Cause | First Plant-Floor Check | Process Adjustment |
|---|---|---|---|
| Full-width clean steel | Wrong ratio, cold sheet, oil, or moisture | Cup test plus sheet temperature map | Restore ratio, preheat coil, clean or change coil |
| Top skin only fails | Top sheet too cold or pour pattern delayed | Compare top and bottom surface temperature | Raise top preheat, adjust pour position |
| Edge strip opens | Weak edge pressure or poor side sealing | Measure edge thickness and foam fill | Reset side guides, belt gap, and foam distribution |
| Opens after 24 hours | Short cure or post-expansion | Repeat peel test after storage | Reduce line speed, extend cooling, lower stack height |
| Random patches | Coil contamination or coating issue | Water-break test and cleaned-strip comparison | Reject coil section or add surface cleaning |
One variable per trial. Change ratio, speed, pressure, and temperature together and you’ll get a good panel by accident, then lose the setting next Monday.
Commissioning QC Plan
For post-purchase troubleshooting, the best fix is a short control plan that operators can run every shift. At YZ Welding, we prefer a production log that connects defect photos to machine data: chemical batch number, component temperature, pump frequency, mixing pressure, sheet temperature, belt gap, belt temperature, line speed, panel thickness, and peel result.
For overseas buyers, this is also a supplier decision point. Before ordering a new line or major upgrade, ask whether the supplier can provide closed-loop metering, recipe storage for PU and PIR, multi-zone sheet preheating, belt-gap adjustment records, and remote support for commissioning data. A low-price line without process records leaves your engineer diagnosing delamination with a knife and a notebook.
Set this minimum QC routine:
- First panel after startup: cut, label, and peel top and bottom skins.
- Every 2 hours: record sheet temperature at left, center, and right.
- Every coil change: run a quick surface check before full-speed production.
- Every disputed batch: keep samples for 24-hour peel comparison.
- Every formula change: confirm cure time before raising line speed.
FAQ
What causes PU panel delamination?
The most common causes are wrong isocyanate-to-polyol ratio, cold steel sheet, oily or damp facings, weak belt pressure, and early cutting before the core has cured. Edge-only failure usually points to pressure or sealing, while full-width failure points to chemistry or sheet prep.
How do you test adhesion?
Use a daily hand peel for fast screening, then confirm disputed batches with flatwise tensile testing. ASTM C297/C297M is a common reference for core-to-facing bond strength in sandwich constructions.
Can pressure fix delamination?
Pressure helps only when the foam has already wet the facing and the chemistry is reacting on time. Excess pressure can collapse cells, push foam away from edges, and create delayed failure after curing.
When should panels be scrapped?
Scrap panels when the facing lifts by hand, the steel surface is clean after peeling, or delamination crosses load-bearing areas. Narrow edge faults may be cut back only if the buyer’s specification allows it.
For the next production run, prepare five panel samples: left edge, center, right edge, top skin, and bottom skin. Test them 30 minutes after cutting and 24 hours after storage, then send the ratio log, sheet-temperature map, belt-gap readings, and foam photos to YZ Welding so we can help identify whether the fix is recipe correction, line setting, or a raw-material check.
