Created on 01.09
Prevent Blistering and Solvent Pop During IR Drying
Blistering and solvent pop are among the most costly coating defects on IR-assisted paint lines. They often appear suddenly—after a line speed increase, a coating change, or a heater adjustment—then trigger rework, scrap, and customer complaints. The frustrating part is that the coating may look fine right out of the flash-off zone and fail later in the oven or after cooling.
The good news: these defects are usually predictable. In most cases, they come from a mismatch between how fast the surface heats and how effectively solvents can escape. This article explains what causes blistering and solvent pop, how to diagnose them, and how to fix them with staged heating, airflow, and better control.Infrared Paint Drying Pillar
What are blistering and solvent pop?
Blistering
Blisters are raised bubbles or swollen areas in the cured film. They form when gases (often solvent vapor) accumulate under the coating and expand.
Solvent pop
Solvent pop is a more violent version of the same mechanism. Vapor pressure builds under the film until it ruptures, leaving craters, pinholes, or “popped” spots.
In short: both are driven by trapped vapor—the difference is severity and how the film fails.
Why IR drying can trigger these defects
IR heating is powerful because it can raise surface temperature quickly. That speed is also the risk.
If the coating surface heats too fast:
- the top layer can “set” early (surface skinning)
- solvents underneath cannot escape smoothly
- vapor pressure rises under the surface
- the film blisters or pops
This is why the same coating can run defect-free at one speed and fail when you push throughput or increase power.IR Flash-Off vs Convection
The most common root causes (ranked by frequency)
1) Too much energy too early (surface skinning)
High IR power at the first zone can form a skin before solvent release stabilizes.
Typical triggers
- increasing IR power to raise throughput
- reducing heater distance
- switching to a faster lamp or higher intensity setup
2) Insufficient flash-off time
Flash-off is not optional for many solvent-based systems. If the film enters higher heat before initial solvent release, defects rise sharply.
3) Poor vapor removal (airflow and ventilation)
Even with correct temperature, solvent cannot leave if the boundary-layer air above the coating becomes saturated. In that case, evaporation slows and vapor remains trapped.
4) Wet film too thick or uneven
Thicker wet films hold more solvent and require a gentler ramp. Variation in thickness creates localized popping.
5) Substrate thermal behavior (metal vs plastic)
- Metal conducts heat and often tolerates faster ramps
- Plastics and composites may overheat locally or deform, increasing defect risk
6) Line speed changes without recipe changes
Higher speed reduces dwell time. If power and airflow are not adjusted, the process becomes unstable.
How to diagnose the defect quickly
Step 1: Identify when the defect appears
- Appears immediately after IR zone → likely early overheating / skinning
- Appears later in the oven → flash-off insufficient or vapor removal weak
- Appears after cooling → trapped solvent/gas expands later
Step 2: Look for the “pattern”
- Edge-only defects → non-uniform heating, poor zoning, geometry effects
- Random scattered pops → film thickness variation, contamination, or unstable airflow
- Consistent across the part → recipe/ramp issue
Step 3: Confirm by controlled changes (one at a time)
Change only one variable per test:
- reduce early-zone power
- increase distance
- add airflow/exhaust
- slow line speed slightly
Then observe whether defect frequency drops.
Fix strategy: staged heating + airflow + control
1) Use staged IR heating (most effective fix)
A stable IR profile usually looks like:
- Zone 1: gentle pre-warm (avoid sudden surface peaks)
- Zone 2: controlled evaporation (steady solvent release)
- Zone 3: stabilization / equalization (prepare for curing)
What to avoid
- “Full power immediately” in the first zone
- Large power jumps between zones
- Extremely close distance at the start of the line
2) Improve vapor removal (don’t ignore airflow)
Even a perfect ramp can fail if vapors cannot leave.
Focus on:
- adequate air exchange above the coating
- removing saturated boundary-layer air
- avoiding dead zones (especially with complex parts)
If you use airflow, ensure it is controlled so it does not disturb the wet film.
3) Add zoning for geometry (reduce hotspots)
Edges, corners, and protrusions often receive more radiant energy. Zoning helps prevent:
- edge overheating
- center underheating
- shadowed areas staying wet
Zoning also improves repeatability between part variants.
4) Measure and control surface temperature
“Power setting” is not a temperature. For repeatability:
- monitor surface temperature consistently
- record stable recipes by coating + substrate
- document: zone powers, distance, line speed, airflow/exhaust settings
- IR Dryer Setup: Distance, Power, and Line Speed
Practical adjustments (what to change first)
If you’re troubleshooting on a live line, try this order:
- Reduce Zone 1 power (or increase heater distance slightly)
- Increase flash-off time (slower speed or longer flash-off zone)
- Increase ventilation/air exchange (remove vapors efficiently)
- Rebalance zoning (edges vs center)
- Then raise output carefully (small increments)
This sequence targets the most common causes without overcorrecting.
Prevention checklist (use this before ramping line speed)
- Confirm wet film thickness is consistent
- Use a staged ramp: low-to-moderate early power
- Verify flash-off is adequate before high heat
- Ensure air exchange prevents vapor saturation
- Add zoning for edges/complex geometry
- Lock recipes and change one variable at a time
- Re-test after coating, solvent, or substrate changes
FAQ
1) What is the fastest way to stop solvent pop right now?
Reduce early-zone IR intensity (or increase distance), and improve vapor removal. Stabilize first, then tune speed and power upward gradually.
2) Why does the coating look fine after flash-off but fail later?
A partially skinned surface can trap solvent. The defect may only appear when the part reaches higher temperatures downstream or during cooling.
3) Does more airflow always help?
Not always. Air exchange helps vapor removal, but uncontrolled airflow can disturb wet film leveling or introduce contamination. Use controlled, filtered flow patterns.
4) Do water-based coatings get solvent pop too?
They can. While the mechanism differs, rapid surface setting and trapped vapor can still produce blistering or pinhole-like defects if the ramp is too aggressive.
5) What process inputs do you need to recommend a stable IR recipe?
Coating type, wet film thickness, substrate, part size/geometry, line speed, target cure, installation space, and any current defect photos or descriptions.
Last modified: 2026-01-09
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