Air Entrainment

Chatter is a repetitive crossweb barring

  • Periodic variation of flow rate
  • Mechanical vibration
  • Oscillation of coating bead

CROSSWEB CHATTER – this set crossweb band pattern is associated with a mechanical vibration. The mechanical vibration could be associated with line speed oscillation, backing roll runout variation, fluid delivery system instability, or vacuum chamber resonance.

  • Eliminate vibration/oscillation

CROSSWEB CHATTER – containing this variable calls for studying the vibration frequency of the proposed variables.

Ribbing is a continuous downweb coating defect

  • Coating bead instability

DOWNWEB BANDS – this set band pattern running downweb is associated with a poor pressure gradient at the coating head. 

  • Decrease coating gap

DOWNWEB BANDS – this can be reduced by improve the flow dynamics of the coating head, which is typically physical adjustments in the equipment variables.

Air entrainment is a dynamic wetting line bead development and maintenance issue causing bubbles under coating. This defect is more difficult to overcome as line speed increases.

  • Air pulled in at dynamic wetting line

UNSTABLE CROSSWEB BANDS – this variable crossweb band pattern is associated with oscillation or instability in the coating bead. (one example of air issues)

Air Entrainment
Air that is introduced into the fluid prior to the coating head is entrained. I remember the term by thinking about the fact that the bubble is already on the “train track” before it enters the coating head. You are probably already thinking this, but it is easier to keep air out than to remove it once it is in the system.

So how does air enter the system prior to the coating head? One way is through excessive agitation of the liquids in the mixing tank. This can be reduced through proper mixing techniques and preparing batches ahead of time to allow settling prior to the coating run.

It is also possible to place a nitrogen blanket over the fluid to remove air. If the liquid moves from one vessel into another, before the coating head, both vessels need to have air reduction measures.

The fluid moves, right? So when the fluid passes a seal, make sure the seal does not leak and cause air entrainment. Lastly, make sure the fluid moves uphill as much as possible. Any bends in the pipe/tubing will allow air to settle. This curved settlement area for the air will slowly leak into the coating head, showing up as a random and difficult to diagnose bubble problem. If a curve in the tube or pipe is necessary, a relief valve will help.

Temperature and pressure also can be your friend. If the process and materials allow, you can heat the fluid up and pull a vacuum. These process conditions will help reduce bubble formation prior to the fluid traveling to the coating head. Ultrasonic measures also have been utilized to reduce bubbles, but these have had mixed results.

  • Vacuum systems
  • Electrostatic field
  • Surface energy compatibility with surface tension (corona, plasma, flame, and primer)

UNSTABLE CROSSWEB BANDS – this can be improved by helping pin the coating bead to the substrate mechanically or chemically. (one example of air issues)

Air Entrapment



Air entrapment includes bubbles in coating.

  • Air in fluid prior to pump

Air Entrapment
Air that is trapped between a fluid and a solid (substrate) is entrapped. This can occur for many reasons, but if you read my article on “What the Coating Bead Can Tell You,” you would understand that the surface energy of the fluid and the substrate play a major role in the reduction of air entrapment.

Vacuum again can be used to reduce bubble formation. At the point of interaction of the fluid and substrate, the vacuum can remove air from the interface, allowing the fluid to adhere more substantially to the substrate.

In an extreme case of air interfering with coating, a coating defect appears that is referred to as “herringbone” (where there is a periodic cross-web defect that resembles the angled look of the v-shaped weaving pattern). This coating defect usually means that the coating head and substrate are not in proper position to seal the fluid to the substrate.

As the coating head is positioned for a better seal, the air introduced becomes more of a minor defect that may be noticeable only on the final coated product. Even with full coating, however, the improper seal of the fluid to the substrate may lead to fluid flow changes in the coating bead or build-up and streaking of the coating fluid on the substrate.

The interaction of fluid, substrate, and vacuum all are affected by the speed of the line. The faster you go, the more difficult it is to avoid air.

  • Time
  • Cone flow
  • Dampening chamber
  • Sonic attack

Dewetting presents as a surface pattern where the layer of fluid retracts from the substrate.

  • Surface tension gradient
  • Vacuum system
  • Electrostatic field
  • Surface energy compatibility with surface tension

Streaks are prolonged downweb defects.

  • Particles
  • Bubbles
  • Non-uniform wetting of slot exit

UNSTABLE DOWNWEB BANDS – this shifting set of bands running downweb is associated with microvortices created at the coating head. 

  • Clean
  • Simulate and adjust

UNSTABLE DOWNWEB BANDS – this can be an individual streak caused by dirt or poor physical setup of the coating equipment, or interaction of the substrate to the fluid.




Particles are a point defect occurring downweb. If it is repetitive it could be an issue with the roll diameter. In this case measure frequency, then clean and replace as necessary.

  • Contamination
  • Gel
  • Clean
  • Reduce recirculation zones

A chevron defect presents as an upside-down V shape on the coated surface.

  • Acoustic wave in vacuum chamber
  • Proximity to substrate

CHEVRONS – large inverted “V”s in the coating is associated with periodic oscillation in coating. 

  • Change vacuum level
  • Move closer to substrate

CHEVRONS – this can be improved through closer physical proximity of the coating head or reduction in acoustic waves in the vacuum chamber.

A hashmark defect presents as a V shape on the coated surface.

  • Gravity-driven coating defect in curtain coating

HASHMARKS – small “V”s in the coating is associated with gravity driven instability in the fluid attachment to the substrate. 

  • Alter flow rate and line speed to impinge fluid

HASHMARKS – this multi-point defect can be improved with viscosity change, chemical attachment phenomenon, or curing variables.

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