Foam tells you what went wrong. It just tells you in the morning, after the crew has left. Learn to read it on the job, in the first test patch and the first cavity, and the fix costs a few minutes instead of a return trip.
The four causes behind almost everything
- Temperature. Drums, heater, hose, substrate, air. Cold slows the reaction and kills the bond; heat cooks it.
- Ratio. The rig must deliver A and B at 1 to 1 by volume. Anything that starves one side, a screen, a check valve, a cold drum, a pump losing stroke, throws it off.
- Moisture. Water in the reaction, on the substrate, or in the A drum.
- Pass thickness and timing. Too thick a lift, or the next lift too soon.
The table
| What you see | Looks like | Usual cause | Check first |
|---|---|---|---|
| Swiss cheese, voids, open pockets through the foam | Holes in a cut core, foam feels light and inconsistent | Cold substrate, cold material, gun too close or too fast, moisture, ratio off | Substrate temperature, drum temperature, hose temperature at the gun |
| Shrink-back | Foam pulled off the studs or sheathing overnight, gaps at the edges, a sheet that lifts | Cold substrate, summer grade in cold conditions, a thick hot lift that cooled stressed, dust or frost under the foam | Substrate temperature and dew point the day it was sprayed; the grade on the drum |
| Dark or brown core, burnt smell | Cut core is dark in the middle, light at the faces; surface may crack | Overheating: lift too thick, heater too high, winter grade in warm weather | Lift thickness; heater and hose set points; grade |
| Glassy, brittle, crumbles, dark and hard | Snaps rather than compresses; sometimes very friable | ISO-rich: B-side starved | B screen, B pump, B drum level, B drum temperature |
| Soft, spongy, slow to cure, sticky, fishy smell | Stays tacky, dents and stays dented, smells of amine | Resin-rich: A-side starved | A screen, A pump, A drum level; an A drum with crystals |
| Blisters, delamination between lifts | Hollow spots, a lift that peels from the one under it | Next lift sprayed before tack-free, or moisture between lifts | Timing; humidity and surface condensation |
| Peels off in a sheet, substrate clean underneath | No bond at all | Dust, oil, rust, frost, or a substrate under the dew point | Substrate prep |
| Rough, popcorn surface, pattern breaking up | Pebbled face, poor cell structure | Pressure low for the chamber, cold material, worn chamber | Pressure, chamber size, chamber and side seals |
| Slow rise, flat, low yield everywhere | Foam rises lazily, every set comes up short | Cold drums, old B-side, rig under temperature | Drum age and storage; heater and hose |
| Color far off the normal for that product | Lighter, darker or streaked | Off-ratio or overheated | Pressures A versus B; heat |
Foam color varies by product and by day, so judge color only against the same product sprayed right.
The five-minute diagnosis
When the test patch or the first cavity is wrong, stop and go through this in order. It finds the cause nine times in ten.
- Drums. Temperature of each with an infrared gun. Date on each. Level in each. A-side bung: any crystals.
- Heat. Heater and hose readings, and the hose temperature at the gun end on a cold day.
- Pressure. A and B at the machine while spraying. A gap that opens up is a restriction on the low side.
- Substrate. Temperature, dew point, moisture, dust, frost, oil.
- Gun. Chamber face, side seals, screens, air cap.
Change one thing, spray another patch, read it again.
Off-ratio is the one that costs the most
Foam that is off ratio can look nearly right at the wall and fail over months: shrinking, smelling, never fully curing. The two tells are the ones in the table, glassy and brittle for ISO-rich, soft and fishy for resin-rich. The fix is never at the gun; it is upstream, where one side is starved. Stop the job until the pressures hold and a test patch cures normally.
When the foam is already on the wall
Bad foam does not get better with a lift over it. Cut it back to sound, bonded material, clean the substrate, and re-spray in proper lifts at the right temperatures. Photograph the cut cores and the gauge readings; the manufacturer and the customer will both want to see them.