One reason for the increased attractiveness of slot-die coating is that the intense competition generated by industry consolidation and globalization has increased pressure to maximize economy, productivity and quality assurance. Quality is especially critical in medical, electronic, photographic and other high-end markets. At the same time, there is a growing mandate for further reductions in emissions of volatile organic compounds (VOCs) from solventborne coating systems.
The slot-coating die differs from the roll coater in two critical respects:
1. It is a “pre-metered” system that applies an adhesive or coating to the web at a constant rate and permits precise control over coat weight and cross-web distribution; this reduces waste, allows higher line speeds, and increases product quality and uniformity.
2. It is an enclosed system, reducing emission of volatiles and preventing airborne contamination.
The Basic Elements The function of a slot-coating die is to maintain the fluid at the proper temperature for application, distribute it uniformly to the desired coating width and apply it to the web. In a typical construction, the body of one type of slot-die coating head (EDI’s Ultracoat) is about 9-in (230-mm) long in the machine direction and 5-in (127-mm) high; widths up to 140 in (3.5 m) are available, as against a typical maximum of 85 in (2.1 m) for roll coating. Slot dies can be designed for wet-coating thicknesses from 0.0005 in (12 microns) to 0.010 in (250 microns).
The die is split into top and bottom sections that are bolted together. Enclosed within these sections is a flow channel machined into one of the body sections. The key portion of this channel is the coat-hanger-shaped manifold, whose widest segment, at the exit slot, corresponds to the coating width. The manifold distributes the coating fluid that enters the die to its full target width and is designed to generate a uniform, streamlined flow of material through the exit slot of the die.
The schematic illustrates three configurations for slot-die coating:
• Direct coating, in which material is applied directly to a substrate supported by a backup roll, achieves coating thicknesses as low as 0.00075 in (18 microns) and can replace roll, gravure, Myer rod and some doctor-knife applications.
• Indirect coating, in which material is applied to a precision-metering roll and transferred to a substrate on a backup roll, can achieve thicknesses down to 0.0005 in (12 microns) or less in replacement of the same conventional methods as direct coating.
• Free-span coating applies material directly to a substrate without the assistance of a backup roll. Replacing many doctor-knife applications, this method is used with solutions that adhere to the substrate without pressure or, in the case of substrates that absorb material quickly, where penetration of the substrate is undesirable.
Constant Application Rate The superior control over coating application provided by the slot die is attributable to its constant application rate. An extruder or positive displacement pump feeds the adhesive or coating into the die at a pulse-free, uniform rate, and all of the fluid that goes into the die is applied to the web. In roll coating, only a portion (perhaps 50 to 80 percent) of the coating on the applicator roll is actually deposited on the web, the amount varying with such factors as the viscosity of the fluid, the speed of the web and the speed of the rolls.
An added disadvantage of roll coating attributable to this partial use of the coating on the applicator roll is the need to recirculate the remaining coating fluid. The contamination that can result damages product quality and leads to raw-material waste.
The constant application rate of slot-coating dies also enables them to coat efficiently at line speeds exceeding those of roll coaters. Because roll coating allows only a partial transfer of coating fluid from an applicator roll to an impression roll (or to an offset roll), product defects can develop at high speeds as a result of film splitting. Proper design of the die lips can prevent this film-splitting defect in slot-die coating. Achievable speeds vary widely with the coating being applied. In hot melt applications, slot dies typically coat at speeds of 300 to 1,000 feet (91 to 305 meters) per minute but have been successfully run as fast as 1,800 ft/min (550 m/min.). The corresponding high speed achieved by roll coating is 1,200 ft/min (365 m/min.)
• Coating-specific manifold design. To optimize flow of the coating fluid through the manifold and onto the substrate, EDI engineers each manifold in accordance with the rheology of the fluid to be applied. Rheology, the unique “fingerprint” of a fluid, is determined by plotting the viscosity of the fluid against its shear rate at a specific temperature. Using the resulting data, EDI employs computer-aided engineering and computerized machining to develop the complex internal contours of the manifold.
• Support carriage for die positioning. EDI’s Ultracoat Die Support System is an adjustable carriage that precisely positions the Ultracoat die at the optimum angle and proximity to the roll and isolates the die from vibrations that can affect coating application.
• Closed-loop control. In a fully automated coating line, downstream thickness- or weight-measuring devices automatically respond to variations from target by sending signals that either speed up or slow down the feeding of fluid into the die. Alternatively, they can signal a modification in line speed relative to application rate.
The computerized version of the lip-adjustment system enhances the operational precision of production lines with closed-loop control. In the case of EDI’s Autoflex system, gauge-profiling centers on a series of closely spaced, thermally actuated adjuster blocks arrayed along the adjustable lip of the die. These blocks operate in response to feedback from downstream thickness or coat-weight scanners. When a thicker-than-target area is detected in the coating, power to the cartridge heaters at corresponding points in the lip is automatically increased. This causes the blocks to thermally expand, which tightens the lip gap in the area. Conversely, thinner-than-target areas are addressed by a reduction in power.
Unlike manual and computerized systems for localized adjustment of slot-coating dies, methods of adjusting the cross-direction profile in roll coating are limited to average improvements over the whole width of the coating, and they seldom achieve tolerances better than about ±5%, regardless of target coat weight. The tolerances achievable using slot dies with Ultraflex and Autoflex systems are ±6 and ±3 percent, respectively, for coat weights of 20 grams per square meter (gsm). As coating weight increases, this accuracy improves considerably. At 125 gsm, for example, Ultraflex and Autoflex systems provide tolerances of 3.5 and 1.75 percent, respectively.
The smoothness and flatness of two surfaces at or near the exit slot of the die are critical for product quality. These surfaces are:
1. The lip land just inside the exit slot, and
2. The face of the wet lip (the lip passed by the moving web after it has passed the exit slot).
The improvements in dimensional tolerances made possible by precision machining of these surfaces may seem minuscule, but in applications involving high volumes or costly coating materials, the economic gains can be great. The effect of flatness variations becomes progressively greater as coating weights become smaller.
In order to stabilize the interaction at the gap between die lips and substrate, it is important to design the lips so that the “meniscus”— the fluid that flows up and down to fill the gap between the outer faces of the lips and the moving web — is “pinned” to the faces of the wet and dry lips. Failure to pin the meniscus results in chatter-type defects.
The proper application of most coating fluids requires a sharp and clean break from the die surface. The exiting surface can have a radius no larger than 50 microns. EDI routinely supplies dies with a 25-micron radius to this surface.
Under pressure as it exits the die, material is also forced to move somewhat “upstream,” along the face of the dry lip but not as far as its outer edge. The extent of this upstream meniscus has a decided effect on coating properties. If it extends too far toward the outer edge of the dry lip, upstream overspill can occur. If it stops too close to the exit slot, on the other hand, the result will be defects caused by air entrainment.
Also important for stabilizing this interaction between die and moving web are the angle of attack between die and substrate, the distance between the two and the degree of offset between the lips. These parameters can be adjusted to optimize the process.
Having supplied slot-die systems for many years, EDI is prepared to assist converters in making the transition from roll coating. The first step is typically to set up a trial to check on the adhesion, appearance and other requirements of a particular application and determine the need for changes in solution formulae. The next step is custom die manufacture — building a slot die unique to the converter’s application. Finally, the converter reaps the benefits of a system that provides a new dimension of control and quality assurance.
“When planning the operation, there was really no question about selecting a slot-die system rather than a roll coater because of the optically clear, glass-smooth surface requirements of our products,” he says, noting that roll coaters leave unacceptable ribbing lines in these applications. “Even though I had a high comfort level with EDI dies from lengthy experience with them at a large Midwest coater, we did evaluate competitive equipment. But in addition to a track record of reliable performance, the EDI Ultracoat die had the most precise and repeatable mounting system — a critical process factor.” The entire line was engineered by Dri-Tec, Inc., Milwaukee.
Venture Coatings had decided to locate the new plant “in the middle of a cornfield,” says Hammann, knowing full well that it would be difficult to hire trained operators in the immediate vicinity. “We decided to hire young people without any experience and provide hands-on training as soon as the facility opened. A key reason we knew this approach would succeed was EDI’s Jeff Seckora.” Seckora, who is EDI’s product manager for coating dies, started up the Ultracoat II slot die “almost instantaneously” on two separate occasions, according to Hammann.
“When we started up the line, Jeff spent a full day with our operators explaining slot-die use for waterborne materials, then the next day running the line while they watched, followed by yet another day supervising them as they operated the line,” says Hammann. He returned when the solvent operation could be started, spending another two days getting operators up to speed. “And he has stopped by twice since that time, just to make sure the equipment is running smoothly. Jeff adds new meaning to the word responsive," states Hammann.
For more information on Venture Coating Technologies, call 800-892-0273.