Identifying Gravure Coaters
Gravure coating is an older, well known water and solvent-based coating technique. There are three major gravure application systems dominating the market: Pan-Feed, Open Applicator and Closed Applicator.
These applications systems face four major challenges in obtaining high-quality gravure coating. These challenges are:
- Coating flow around the gravure roll
- Air entrapment in the coating
- Gravure wear
- Coat weight adjustments on-the-fly
This Tech Tip will address advantages and disadvantages of the three above-mentioned coating techniques as well as offering solutions to the challenges.
Pan-Feed Applicator
The Pan-Feed gravure applicator is the simplest system available in the market. It is relatively easy to run and is supplied in two configurations; direct gravure or off-set gravure. (see fig. 1) The diagram shows the air entrapment problems with a typical pan fed system.
Fig. 1
Advantages of this system are:
- Easy to operate
- Minimal equipment set-up
- Simple coating supply system
- No special gravure pattern required
Disadvantages of this system are:
- No gravure wear compensation
- Large amounts of coating circulating in pan
- Increased open surface allows for solvent evaporation
- Uncontrolled coating applied to the roll
- Entrapment of air in coating substrate at speeds over 100fpm
- Maximum line speed for psa’s approximately 250fpm
Open Gravure Applicator
The Open Gravure Applicator ensures greater control of coating flow around the gravure roll.
Fig. 2
Coating head is located in the three or nine o’clock position and may be used in direct or offset gravure configurations (see fig.2).
By limiting the open coating surface solvent evaporation is reduced, although it does not eliminate the laminar air entering the coating through empty gravure cells. The advantage of the open applicator is that the flooded metering doctor blade will eliminate any air entrapment.
Advantages of this system are:
- Easy operation
- Excellent coating uniformity in CD machine direction
- Limited amount of coating circulating
- Minimal solvent evaporation due to small open surface area
Disadvantages of this system are:
- No gravure wear compensation
- Solvent may evaporate into atmosphere
- Line speeds limited to approximately 600fpm
- Air entrapment at speeds above 100fpm
- Long distance between coating metering and transfer due to doctor blade location
Closed Gravure Applicator
By adding the second blade to the top of the open applicator, the coating pond becomes enclosed, hence closed gravure applicator. This technique allows for higher line speeds (see fig. 3).
Fig. 3
Advantages of this system:
- Closed chamber- no solvent evaporation issue
- Flexibility placing coating head
- Speeds to 1200fpm for psa’s.
- Controlled coating flow in head
- Extremely easy to clean
- Short distance between coating metering and transfer
Disadvantages of this system:
- No gravure wear compensation
- No temperature control
- At high speeds, coating turbulence may lead to air entrapment
Black Clawson suggests a solution to these gravure coating disadvantages. The Pressurized Gravure Head (PGH) minimizes the amount of air injected into the coating while also providing a wider coatweight range. Direct and offset gravure models are available (see fig. 4).
Fig. 4
Advantages of this system include:
- Best available uniformity in CD direction
- Up to 40% coatweight change with same gravure roll
- In-line coating close loop control
- Closed chamber- solvent evaporation is a non-issue
- Speeds up to 3000fpm
- Easy to clean
- Coating head can be temperature controlled
- Coating head maintains same location for forward or reverse coating
Disadvantages of this system:
- Complicated coating supply system
- Complex operating mode, requires additional training
For more information on gravure coaters, coating components or systems, please contact Black Clawson Converting Machinery at 315-598-7121 or e-mail your questions to bc@bc-egan.com