Glue Skip on Corrugator: Causes and Fixes for Adhesive Application

Glue skip — areas on the corrugated board where adhesive was not applied — is a visible and costly defect. Skip bonding creates weak spots that fail during converting and shipping, and is one of the top three causes of customer quality complaints. This guide covers identification, root cause analysis, and corrective actions for glue skip on both single facer and double backer.

What Glue Skip Looks Like

Dry lines or patches on liner surface visible under backlight

Flute tips without adhesive shine — dull appearance vs glossy bonded areas

Board fails pin adhesion test in skip areas — values drop to zero

Converting problems: Box folds crack or separate at skip locations

Pattern types: Random spots, regular stripes (roll-related), edge-only, or machine-direction lines

The pattern of skip often reveals the cause — random skips suggest viscosity or contamination issues, while regular stripes indicate roll surface problems.

Causes on Single Facer

Applicator Roll Issues (Most Common — 35% of cases)

Doctor roll gap too wide — adhesive not transferred to applicator roll surface

Applicator roll surface worn, grooved, or chrome peeling

Roll not parallel to corrugating roll — gap wider at one end

Adhesive buildup on roll creating high spots that lift paper away from flute tips

Wrong roll hardness or diameter for current speed

Viscosity Problems (25% of cases)

Viscosity too high — adhesive too thick to transfer evenly at current speed

Viscosity too low — adhesive flows off roll before contacting flute tips

Viscosity changing during shift — batch aging or temperature drift

Foam in adhesive — air bubbles prevent contact at bubble locations

Speed and Gap Interaction

Line speed increased without adjusting doctor roll gap or viscosity

Rule of thumb: Higher speed requires lower viscosity or tighter doctor roll gap

Above 200 m/min: Viscosity should be 25-30 seconds; doctor roll gap 0.10-0.15 mm

Sudden speed changes cause temporary skip until adhesive system stabilizes

Corrugating Roll Condition

Worn flute profiles — paper does not contact applicator roll evenly

Roll contamination — oil, hardened glue, or paper dust on flute tips

Incorrect corrugating roll gap — paper lifts away from applicator roll

Causes on Double Backer

Liner applicator roll gap too wide or roll worn

Liner preheater over-drying surface — adhesive beads up instead of spreading

Hot plate temperature too high — adhesive sets before liner contacts single-face

Belt pressure insufficient — liner not pressed onto adhesive layer

Single-face assembly entering double backer with existing skip from single facer

Diagnostic Procedure

1. Identify skip pattern: Random, striped, edge, or full-width?

2. Check skip location: Single facer side, double backer side, or both?

3. Measure adhesive viscosity — compare to target for current speed

4. Inspect applicator roll and doctor roll surfaces under good lighting

5. Measure doctor roll gap with feeler gauge at both ends and center

6. Check adhesive temperature at applicator point

7. Review speed log for changes before skip appeared

8. Examine paper roll for edge damage or surface defects at skip locations

Corrective Actions

Immediate Fixes (During Production)

Reduce doctor roll gap by 0.02-0.05 mm incrementally until skip clears

Adjust viscosity: Add water to reduce 2-3 seconds, or add fresh batch to increase

Clean applicator roll surface with wet cloth during slowdown (never at full speed)

Reduce line speed 10-20 m/min until adhesive system stabilizes

Increase adhesive pump flow rate if system allows

Maintenance Fixes (Between Shifts)

Deep clean applicator rolls and doctor rolls — remove all hardened adhesive

Re-grind or replace worn applicator rolls (typical life: 2-5 years depending on speed)

Re-align applicator roll parallel to corrugating roll — tolerance within 0.05 mm

Calibrate doctor roll pneumatic or spring pressure system

Clean adhesive filters and check for pump cavitation or air entrainment

Verify adhesive batch preparation consistency — see adhesive preparation best practices

Systemic Improvements

Install automatic viscosity monitoring system with alerts

Standardize adhesive batch preparation SOP with fixed mix ratios

Train operators to recognize early skip signs before full defect develops

Maintain spare applicator rolls for quick changeover when surface degrades

Match adhesive formulation to line speed — instant-setting for high-speed lines

Prevention Schedule

Every shift: Viscosity check at start, middle, and end; visual roll inspection

Daily: Clean adhesive drips from applicator area; check doctor roll gap

Weekly: Deep roll cleaning; inspect roll surface with flashlight at low angle

Monthly: Measure roll diameter and check for out-of-round; calibrate gap settings

Quarterly: Evaluate roll regrinding or replacement based on skip frequency trends

Glue Skip vs Related Defects

Glue skip: No adhesive applied — dry area, zero pin adhesion

Weak bond: Adhesive applied but insufficient strength — low pin adhesion, not zero

Washboarding: Excessive adhesive causing corrugated washboard pattern on liner

Delamination: Bond formed but fails later — moisture, handling, or curing issue

Distinguishing these defects prevents misdiagnosis and wasted corrective effort.

Impact on Production Costs

Chronic glue skip at 2-3% of board area translates to

2-3% customer complaint rate on converted boxes

Rework and waste in die-cutting and printing stages

Potential line speed reduction to compensate — 5-10% capacity loss

Reputation damage with quality-sensitive customers

For viscosity control details, see our adhesive viscosity monitoring guide. Xuegong alkali-free adhesives are formulated for even application across a wide speed range. Contact us for applicator roll gap recommendations for your line speed and paper grades.