TL;DR
Board warp is caused by a moisture differential between the two liner facings. Check the moisture profiles of both liners, the double backer heat setting, and starch gel point consistency.
What you might see
- finished board warping concave on one face
- warp increasing as board cools on the stacker
- moisture imbalance between top and bottom liner on meter readings
- variable warp direction between liner grade changes
Likely causes
Top liner moisture higher than bottom liner creating differential shrinkage on cooling
Double backer heat set too high on the bottom face drying it faster than the top
Starch not fully gelled on one face due to insufficient heat, leaving wet adhesive
Incoming liner rolls with different moisture content by reel
Required tools
- Non-contact moisture meter
- Pin moisture meter for incoming reel check
- IR thermometer for heat plate verification
Safety first
- Double backer heat plates operate above 170 deg C. Use heat-resistant gloves when near heat plate areas.
- Web breaks on the corrugator release hot steam and adhesive. Maintain safe distance during break events.
Procedure
- 1
Measure moisture content of both liner faces at the corrugator exit with a non-contact moisture meter. Record top and bottom readings.[1]
- 2
A moisture differential greater than 1-2% between faces will cause warp on cooling. Identify the wet face.
- 3
If the bottom liner is wetter, increase the double backer heat plate temperature on the bottom section. If the top liner is wetter, check the preheater temperature for the top liner.[1]
- 4
Check the starch gel point temperature. Starch not at gel point on either face leaves residual moisture and causes adhesion and warp issues.
- 5
Verify incoming liner moisture content with a pin meter on the reel before loading. Segregate high-moisture reels and process them with increased heat.
- 6
Adjust preheat wrap angle on the wet liner preheater to increase heat exposure and reduce entry moisture.
- 7
After each adjustment, allow 10 minutes of steady state before re-measuring board moisture to confirm the direction of change.
Sources
BHS Corrugated BHS Modular Corrugator general technical documentation, BHS Corrugated
Corrugated board warp control and moisture balance, general corrugating practice (general)
More guides for BHS Corrugated BHS Modular
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