An Overview of Roll Coating | New Era Converting

The following is the fourth and final part in a series of 4 blogs that will go through a white paper written by New Era Converting Machinery&#;s Bob Pasquale. The white paper is titled &#;An Overview of Roll Coating Methods, Capabilities and Limitations.&#;

Litong supply professional and honest service.

Part 1 offered an overview and introduction to the blog series. Part 2 discussed Kiss Coaters and Squeeze Roll Coaters. Part 3 went through Gravure Coating, Offset Gravure Coating, and Reverse Roll Coaters. Part 4 will wrap the series up, finishing with Transfer Roll Coaters and Other Roll Coaters, and then offer a brief Conclusion.

&#;&#;

TRANSFER ROLL COATERS

Transfer roll coaters are typically used for applying thin coating layers, particularly as required for 100% solid coatings. These coaters typically use 4, 5 or 6 rolls, with each consecutive roll running in the opposite direction of the previous one. The rolls in the stack alternate from rubber covered to steel surface rolls (typically chrome plated). The web is supported by the last roll in the series, which is referred to as the backing roll and runs at web speed. The web passes between the backing roll and the next to last roll, referred to as the applicator roll, which also typically runs at web speed. The coating is typically placed in the gap between the first two rolls, with the first roll, referred to as the metering roll and the second roll both rotating away from the coating puddle (Figure 6 and Photo 6).

Each roll from the metering roll through the applicator roll runs faster than the previous one, with the coating continuing to be reduced in thickness by the combination of the gap between the rolls and the increase in speed. The coating is transferred from the applicator roll to the web as it passes through the gap with the backing roll.

Table 6 (below) shows the typical operating speeds, coating application rates and coating viscosity for a transfer roll coater.

The transfer roll coater is an expensive coater to purchase. It has a very good speed range and has fairly wide coating thickness and viscosity ranges, though it does not work well with low viscosities. In particular it is very good for thin coating laydowns, particularly at high speeds. It is good for changing coating laydown rates. Because it does offer the ability to change the coating laydown rate, it can be a somewhat difficult to operate and can have issues with repeatability of coating rates. It typically requires a fair to high amount of maintenance. Because of the number of rolls involved in the coating application change over from one coating to another/clean up can be time consuming.

OTHER TYPES OF ROLL COATERS

There are many other roll coater configurations that have been developed to meet a wide variety of coating applications. Many of these are either hybrids, variations or combinations of the previously described coaters.

One such example is a dip coater which in its simplest form is similar in design to the kiss coater except that the web passes beneath the roll where it is dipped into the coating being held in the pan. Like the kiss coater, it is typical that a metering device/system follows the application of the coating.

Another example is a coater that uses a die to feed a coating to the surface of an applicator roll, which then transfers the coating to the web.

Still another example is commonly referred to as a Levelon Coater (Figure 7) which can be considered a combination of kiss and reverse roll coating, where the coating is applied to the web by a kiss roll and is then metered off as the web passes between a backing roll and a reverse running metering roll.

A final example is as shown below (Photo 7). This three roll pan fed coater combines many of the features of a squeeze roll coater, an offset gravure coater and a transfer roll coater to obtain the desired coating laydown.

CONCLUSIONS

As illustrated above, there is a wide variety of roll coaters that has been developed through the years. Each of these coaters present certain strengths and weaknesses. It becomes important to select a coater that that best matches the coating being applied as well as the parameters of the operation.

BIOGRAPHY:

Bob Pasquale is one of the founders and principals of New Era Converting Machinery, where he serves as President. He holds a degree in Mechanical Engineering from Stevens Institute of Technology and has worked in the web converting industry since . He is the holder of several patents in the industry. Bob can be reached at .

REFERENCES:

Booth, George L., Coating Equipment and Processes, New York, New York: Lockwood Publishing Company,

Satas, Donatas, Web Processing and Converting Technology and Equipment, New York, New York: Van Nostrand Reinhold Company,

###

*That concludes our 4 part blog series, &#;An overview of Roll Coating Methods, Capabilities and Limitations&#; &#; to go back and read part 1 click here, or for part 2 click here and for part 3, click here! Thanks for reading!*

In this Tech-File, we will be discussing primarily about the roll coating application technique. Roll coating is one of the many types of coating techniques (few coating techniques to name: spray coating, powder coating, screen coating, curtain coating, knife coating, transfer coating, and dip coating). Roll coating itself consists of subcategories which we will be discussing in this literature.

For more roll coating machinesinformation, please contact us. We will provide professional answers.

Roll coating is widely used in printing and packaging industry for applying coatings and adhesives. Knowing about the different components of roll coating and its working mechanism will give coating and adhesive formulators a better understanding for developing their products as per application technique.

.

Roll coating is and efficient method applicable only to uniform flat or cylindrical surfaces. Relatively slow evaporating solvents must be used to avoid viscosity buildup on the rolls of the coater.

..

.

Kiss Roll Coating

This is the simplest kind of roll coating. In this setup a single rotating roller acts as both a metering device and an application device. Half of the roller is immersed in the coating bath, and as it rotates, it carries a film on its surface which is then transferred on the substrate as the roller rotates. The coating thickness is optimized by hydrodynamics, by varying the rotating speed of the roller, coating viscosity and surface tension.

.

Direct Roll Coating

In direct roll coating, the substrate passes between two rollers &#; the applicator roller and a backup roller, which are rotated in opposite directions. The rollers pull the material being coated between them as shown in figure below. Direct roll coating is used for coating sheets and coils.

.

.

The pickup roll runs partially immersed in a tray, containing the coating. It transfers the coating to the applicator roll rotating in the same direction. A simultaneous metering roll rotates in the opposite direction of the applicator roll. The metering roll helps to maintain the coating thickness with the help of a doctor blade, by optimizing the clearance between the blade and the roll.

Such type of an arrangement can be used for high viscosity coatings.

Failure to clean the metering roller surface will result in coating defects, such as streaks or film roughness.

.

Reverse Roll Coating

In reverse roll coating, both the rollers (application roller and support roller) rotate in the same direction, with the substrate being pulled between the two rolls. This process is widely used for coating coils and is not much feasible for sheets.

Reverse roll coating has the advantage that the coating is applied by wiping rather than by film splitting. A smoother film is formed, and the problems of leveling are minimized.

It is the flow in the metered gap, which is the crux of all reverse roll coaters, determining the coating thickness and uniformity.

..

Gravure Roll Coating

Gravure roll coating is the process where engraving on the roller acts as a metering device. The engraved roller is partly submerged in the coating bath and as it rotates, coating liquid fills the engraved pattern and excess coating liquid forms a film on the roller&#;s surface. A doctor blade removes the excess surface film from the gravure roller surface, and it then presses against the substrate to transfer the coating material in the engraved pattern to the substrate.

Gravure roll coating is a technique that has evolved from the printing industry and used to apply very thin coatings of low viscosity liquids at high speed.

Gravure rolls are characterized by the number of cells per unit area of the roll surface.

.

.

Direct gravure coating: In direct gravure coating the moving web is held against the roll by either a backing roll or by applying a high tension and wrap angle to the web. The web moves in the opposite direction to the surface of the roll at roughly the same speed.

Offset gravure coating: An offset gravure system makes use of two rolls: a gravure roll and rigid roll covered with a thick layer of a deformable material (rubber). These are nipped together such that the gravure roll is impressed into the adjacent deformable roll surface, producing what is referred to as a negative gap. Both the rolls rotate in the same direction but at a different speed which controls the coating thickness. Offset printing results in a clear and sharper image.

.

.

Are you interested in learning more about aluminum roller surface coating process? Contact us today to secure an expert consultation!

References 

• Organic Coatings &#; Science and Technology. 3rd Edition. Wiley .
• Smart Textile Coating and Laminates. 2nd Edition. Coating and laminating processes and techniques for textiles. E. Shim. North Carolina State University, Raleigh, NC, United States. .
• Permeability properties of Plastics and Elastomers. 4th Edition. Production of films, containers, and membranes. Lawrence Mckeen. .