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In choosing among the increasingly sophisticated methods of applying roll coatings, including Direct/Reverse/Differential Offset Gravure and Reverse Roll coaters, the converter needs to take into consideration the basic concepts of each method, why and when they are used, and the advantages and limitations for each. The selection of proper equipment components, including gravure roll-pattern selection, type of coating-feed system, gravure-roll doctoring and roll finishes/tolerances for reverse roll coaters, depends on a thorough knowledge of each method.

This is a method in which a web, passing around an elastomer-covered impression roll, is coated in a nip formed with an engraved applicator roll. As shown in Figure 1, the rolls rotate at the same speed, with the transfer of the coating to the web dependent upon its release from the engraved cells. Although gravure coating is used over a wide viscosity range, it is most popular for applying low-viscosity, thin-film coatings at high speeds.

The coating is applied to the applicator roll either from a pan in which the roll is immersed or from an enclosed applicator feed system. In most instances, excess coating is removed by a flexible doctor-blade pressing against the rotating roll.

Choice of feed system often depends on speed and process. At applicator or roll speeds up to 750 fpm, a pan design is often used for low-viscosity coatings. As speed and viscosity increase, coating must be forced into the engraved cells to minimize air entrapment and foaming, using an enclosed feed system. Enclosed systems are also employed to minimize evaporative losses when solvent coatings are applied.

Factors that affect coating weight and quality include:

• Applicator roll pattern, cell volume and shape
• Thickness and hardness (durometer) of the impression roll covering
• Nip pressure developed between the impression and applicator rolls
• Type, position and operating pressure of the doctor-blade system against the applicator roll
• Viscosity and flow properties of the coating
• Type and finish of the web being coated
• Speed of the operation
• Atmospheric conditions at the time of coating

Direct gravure coating continuously applies the same amount of coating to a web through a broad speed range, making it a straightforward concept as long as the doctor blade and nip settings remain the same.

One of the most important factors is the selection of the applicator-roll pattern and cell volume. For a given coat weight, selection is based upon the solids content of the coating, its viscosity and flow characteristics. Direct gravure coating is versatile, applying from low-solids/low-viscosity coatings to higher-viscosity coatings up to 100 percent solids.

Recognize that only a percentage (from 50-80 percent) of the volume of coating in the engraved cells is transferred. Pyramid cells often release the lowest amount, while tri-helix patterns release the highest. Release characteristics can be increased by applying an electrostatic-assist charge to the impression roll.

Cell volume is measured in billion cubic microns per sq in. (BCM/in².). It is possible to select the same pyramid or quad pattern number with a different depth, depositing more or less coating material. The choice of cell structure depends on the coating viscosity and flow parameters. One might select a pyramid or quad-cell engraving for a low-solid, low-viscosity coating, rather than a tri-helix pattern because the discrete cells retain the coating in cup-like fashion. Once the pattern is selected, the cell volume (BCM/in.²) can be calculated.

Applicator rolls are engraved a number of different ways. Originally, rolls were mill (tool) engraved or chemically etched. With today's technology, very fine, accurately developed cells are manufactured using electronic stylus and laser technology. Rolls usually have a protective covering of chromium. The use of ceramic-coated, laser-engraved rolls, however, allows converters to preserve cell volume. The release characteristics of such ceramic rolls are usually better than chrome-covered rolls.

The thickness and hardness (durometer) of the elastomer rubber covering on the impression roll also plays a role in coat-weight application.

Doctoring of the applicator roll is usually done by using a flexible steel blade. There are applications where a doctoring roll can be used, but the hydraulic force developed at the nip of the applicator and rubber-covered doctoring roll usually results in significant coating carryover onto the web. The thickness and length of a doctor blade, the force applied to it, and its position on the applicator roll determine the amount of coating wiped from the roll surface.

The doctor blade can be placed in a "chisel" mode, (Figure 2, top), where the blade tip opposes the direction of travel of the roll, or in the "trail" mode, (Figure 2, bottom), where the tip points in the direction of travel of the roll. Properly used, the "chisel" mode provides the maximum wipe of the applicator roll, since the probability of blade lifting is minimal and shearing force is high. In the "trail" mode, depending upon blade thickness and length, lifting can occur due to the hydraulic forces that develop from the action of the coating fluid against the underside of the blade. Improperly used, either mode of doctoring can result in coating carryover due to blade flexure.

Enclosed, chambered coating applicators feature a combination of powered coating feed and doctor-blade metering. Coating compound is pumped through the chamber enclosure via ports designed to optimize the coating distribution. Doctor blades are placed at the top and bottom of the applicator, affording total enclosure control of the system. Resilient end cheeks conform to the applicator roll surface. The unit is mechanically or pneumatically loaded against the roll.

In pneumatic systems, compensation for blade wear is automatic. By enclosing the coating compound in a feed applicator, evaporation of solvents is prevented, making this method a favorite for emission-controlled applications. Flushing systems, complete with waste management controls, are also available.

While the major advantage of direct gravure coating is the ability to consistently apply the same amount of coating to a web under constant operating conditions, there is little opportunity to make significant coat-weight changes without using a different applicator roll. Using some doctoring techniques, where excess coating is allowed to remain on the lands above the applicator roll engraved surface, some weight can be added without compromising process control.

This method employs the basic features of Direct Gravure Coating, except that a speed differential is created between the applicator roll and the web, with the applicator roll rotating opposite the direction of the web. The reverse gravure technique takes advantage of the film-splitting characteristics of most coatings.

The web is carried by the impression roll (see Figure 3), which may or may not be independently driven, depending upon web tension and the ability of the web to rotate the roll. The applicator roll is driven in the opposite direction to the web travel, thereby creating a coating which is both metered and smoothed. Depending upon the coating viscosity and flow properties, the rate of speed of the applicator roll will alter the amount of coating applied. This allows coat-weight variation while using the same engraved applicator roll.

While this technique offers finer coat-weight control, it is limited by the characteristics of the coating. Most coatings will split or separate when they are applied/transferred from one surface to another. It is important to operate a coater at a speed where the film splitting works to the converter's advantage in the coating process.

If the differential speed between the web and applicator roll is too small, "ribbing" (machine direction lines) may develop. This usually occurs if the applicator roll speed is too slow and the coating nip is starved of coating material. This is a function of the coating viscosity and its surface tension. Converters must therefore operate above that speed or change the coating-material flow properties. Conversely, if the speed differential is too great (i.e. the applicator roll turns too fast in the reverse direction), the phenomenon of "cascading" (seashore effect) can result, this time due to coating flow through the nip, resulting in surges on the downstream side of the nip.

Among the advantages of this coating technique is the ability to smooth a higher-viscosity gravure application that is deposited in a discrete pattern from a pyramid, quad, or tri-helix cell applicator roll. Further, low, dry coat weights can be controlled accurately by altering the applicator roll speed. This is more particularly true when using coatings that exhibit increased resistance to shear (Newtonian coatings) than when using shear-thinning (thixotropic) coatings, where little variation in coat weight is seen with applicator roll-speed change.

Reverse gravure coating usually employs gap-control devices to control the gap between the impression and applicator rolls. This is particularly important with heavier gauge materials. The thickness and hardness of the elastomer rubber covering on the impression roll plays a role in the coat-weight application. A thick rubber covering allows more coating to pass through the nip. For thin rubber coverings, the gap between the impression and applicator rolls affects coat weight more dramatically than the rubber covering thickness. These coaters cannot be operated with a tight nip because of the counter rotation of the applicator roll to the web.

This coating method (see Figure 4) applies material to a web through an intermediate smooth roll that is fed with coating material from an engraved applicator roll. The impression, offset, and applicator rolls are independently driven. The speed differentials among the rolls make this coating method particularly well suited to the application of viscous, high-solids coatings to a web, at low coat weights, by use of the film-splitting process. The applicator roll can rotate in either a forward or reverse direction.

The impression roll may be made of chrome-covered steel. The transfer roll is elastomer covered, with the covering thickness and durometer affecting coating transfer. The applicator roll remains an engraved roll. Gap adjusting devices between the various roll sets allow appropriate coating transfer to the web.

The addition of the third roll allows the coating transferred to the web to be the result of the action of the applicator/transfer rolls and the transfer/impression rolls. This promotes low coat weight products at even higher line speeds. The combination of the speed and gap-control features makes this coater the most versatile type for gravure coating applications.

Most differential offset gravure coaters are designed to operate in both the Direct and Reverse Gravure modes, by altering the web path.

For purposes of this presentation, a nip-fed Reverse Roll Coater will be used (Figure 5, left). It is considered the most versatile for applying precise coatings of varying viscosity and thickness, particularly on webs that do not have uniform caliper. Reverse roll coating takes advantage of gap control, supplemented by differential speed control of the applicator and metering rolls, causing the coating to be precisely metered before being applied to the web. Each roll is independently driven.

The substrate is carried on a resilient, rubber-covered carrier roll, operating at line speed. A steel applicator roll and a metering roll are gapped to each other to help determine the coating thickness applied. The steel rolls feature high tolerances to 50-millionths of an inch for concentricity and total indicator runout. The roll surface is chrome covered, with a finish in the 2-8 Ra range. Coating is fed into the gap of these steel rolls, contained by a trough and edge-dam system.

The applicator roll normally operates at speeds from 1.2 to 2.5 times faster than web speed, thereby depositing a greater amount of coating for a given gap between the metering and applicator rolls. The ratio of the speed of the metering roll to the applicator roll can be an important factor in determining the surface quality of the coating applied. It also has some effect on the thickness of the coating.

Coating is precisely premetered onto the surface of the applicator roll by the gap setting and by the action of the metering roll speed. Since both rolls are steel, they do not deform as does an elastomer-covered roll. Hence, for a given gap, the flow parameters of the coating (capillary number, surface tension) and the speed of metering to applicator rolls determines the coating thickness.

For coatings that exhibit increased shear resistance (Newtonian coatings), the ratio of metering to applicator roll speed will affect the quantity of the coating ultimately applied to the applicator roll. If the speed of the metering roll is too low, "ribbing" (machine direction lines) will occur. It is usually caused by air becoming entrained on the metering roll, resulting in competition between coating-flow pressure at the nip and the cross-machine coating-flow properties. The smaller the coating gap, the higher the frequency of the "ribbing." Increasing the metering roll speed stabilizes the coating on the applicator roll. If the ribs remain, you may have to increase the coating gap.

If, however, the speed of the metering roll is too high, "cascading" (seashoring) will occur. Cascading takes place when the dynamic contact line of the coating, normally located upstream of the gap, passes from the upstream side to the downstream side of the gap, entrapping air in the roll gap. This results in an increase in coating film thickness upstream due to surging. It is therefore important to find the combination of metering to applicator-roll gap and speed that allows coating in the stable range. The flow properties of the coating (capillary number, surface tension) become less of a factor as proper speed ratio and coating gap are established.

However, when applying coatings that exhibit a narrow stable operating range, gap control becomes extremely important. The ability to process a wide range of different coatings makes the reverse roll coater very popular.

For shear-thinning (thixotropic) coatings, there is very little change in the metered film thickness as a function of metering roll speed. This is due to relatively low capillary-number and surface-tension values of this family of coatings. The viscosity of the coating is important, however, and is defined as the process shear rate developed in the area of the gap where the shear-thinning takes place.

As in all coating processes, it is imperative to doctor excess material carried around the metering and applicator rolls. The use of composite-material doctor blades, acting in the chisel mode, is typical in most applications.

The gap between the metering and applicator rolls is controlled by a precision wedge block or rotary-screw system. While adjustment of the control units may be performed manually, most devices use servomotors to ensure accurate, repeatable settings. Weight and thickness measuring gauges that monitor the coating application can send a signal to activate the gap-set control motors to make adjustments. The gap adjustment system between the applicator rolls and carrier rolls controls the transfer force used when coating the web.

While the cost of a reverse roll coater is higher than that of other roll coaters, it offers the greatest flexibility and precision.