Coating viscosity has a crucial impact on product quality. If the coating solution is too viscous, imperfections such as bubbling, rough texture or orange peel may occur. On the other hand, if the coating solution is not sufficiently viscous, the coating on the product may not be thick enough to provide adequate protection. Also, if the coating is too thin, sagging, running or dripping may occur. Since any of these situations can lead to product rejection, it becomes very important to monitor the coating's viscosity, which can result in higher product quality and increased yields. In addition, viscosity management can produce significant savings in the use of coatings and coatings solvents, and it can minimize the downtime of the coating process. This article describes the design of a new automated in-line viscometer that can provide accurate management of a coating's viscosity and requires minimal maintenance. A number of coatings applications will be suggested that demonstrate the benefits of the system.
Principle Of OperationThe heart of the Cambridge Applied Systems' ViscoPro viscometer is a patented sensor (see Figure 1). The viscometer also includes a highly polished stainless-steel piston that operates in a measurement chamber through which a small fraction of the solution is diverted. Two electromagnetic coils move the piston back and forth in the chamber, and the viscous drag of the fluid resists the movement of the piston. Since the piston's motion is determined magnetically, there is no need for rotating or sliding seals.
The viscosity is directly related to the time required for the piston to travel through the measuring chamber and the absolute viscosity of the process fluid (in cP) is reported. A platinum resistance temperature detector, mounted at the base of the measurement chamber, continuously measures the temperature of the coating. At the end of each piston stroke, the electronics computes the rolling average for the number of cycles selected by the user (up to 50 cycles) and presents the viscosity, temperature, and temperature-corrected viscosity.
The sensor is calibrated at the factory with NIST certified viscosity standards; the accuracy of the system is in the order of ±1% and repeatability is ±0.5%. The system is also designed to be easily field calibrated.
In-Process MeasurementsThe sensor can be directly mounted into the tank of the coating system or in-line to provide in-process monitoring of the coating solutions' viscosity in all types of coating operations, including the following.
- Dip process (e.g. wire coatings)
- Spray processes (e.g. automotive paints)
- Roll/printing processes (e.g. coating of beverage cans)
- Flow processes (e.g. optical coatings)
A typical installation is shown in Figure 2.
The viscometer provides RS-232 and 4-20mA outputs to control other devices and to present a permanent record of the viscosity, temperature, and temperature-controlled viscosity of the coating solution. Additionally, the system can be used to automatically control solenoids that provide make-up water/solvent to the coating solution, and is typically used to control its concentration.
ApplicationsCambridge Applied Systems viscometers are currently in use in over 4,500 applications on an online basis. A majority of these are in the coatings area, and representative applications include the following.
- Wire enamel (dip coating). Magnet wire for the manufacture of electric motors, transformers and similar devices is coated with an electrically insulating enamel by a dip process (the coating process is repeated until the required thickness is obtained). In this application, the percent solids in the coating solution is the crucial parameter; if the coating solution contains a high percent-solids content, solvent costs can be reduced. Accurate management of the viscosity is crucial in controlling uniform film weight.
Steve LaPlant of Alcatel discussed his experience with Cambridge viscometers in the May 1999 issue of Wire & Cable Technology International. He noted in the article: "We needed a unit that could control viscosity at a desired temperature even though the ambient temperature was constantly changing (typically from 77 degrees F-113 degrees F)." Therefore, LaPlant needed a viscometer that sensed viscosity and temperature, and could use temperature-compensated viscosity as the key process variable.
He chose Cambridge's viscometer with a ViscoPro2000 processor and a SPC301 sensor and said it produced "impressive results." The performance of the viscometer is shown in Figure 3.
The figure makes clear that fluid temperature varies throughout the reporting period, and that viscosity varies with temperature. The figure also shows that controlling viscosity on a temperature-adjusted basis allows Alcatel to easily keep viscosity between their upper and lower control limits.
Summarizing, LaPlant reported that "the ViscoPro2000 system lowered the number of enamel resupplies, rejects and solvent usage, and also saved on manpower."
- Optical Coatings (flow coating and dip coating). Leader Industries, Plattsburg, NY, manufactures optical quality eye protection products for sports (swim and ski goggles, racquetball eyeguards, etc.). They have used the cup methods for many years and report the same shortcomings as the wire manufacturers. Leader Industries recently installed Cambridge systems on its coating lines. Leader reports that the precise control of the viscosity ensures that the product has the right amount of coating and has enabled them to save $100/day per line by reducing the amount of downtime required to measure the viscosity of the coating solution.
Beverage Can Coating (roll/printing process). Beverage cans and other cans are commonly printed and then coated with a thin layer of overvarnish to protect the decoration. Varnish thickness is crucial; if too much varnish is used, the can will appear bubbly, while if too little varnish is used the decoration will be easily scratched. The throughput of a can coating line is very high (>2,000 cans/min. per machine). In this application the viscosity of the overvarnish should be managed on an in-line basis because of the rapid throughput and the significant potential loss.
Cambridge Applied Systems' viscometers are being used to produce 28 billion cans in the United States alone on more than 50 can lines, and have been mandated for use on all can lines by one of the world's leading can manufacturers. In addition, John Liebenguth, director of Marketing at PPG Industries, recommends the viscometers for maintaining overvarnish quality to original viscosity specifications. United Can reports that since the system was installed there has been a dramatic improvement in quality and product consistency. A spokesperson said, "We're pleased with the results of the Cambridge system installed at our Hayward, CA, facility."
Automotive Paint (spray process). The paint lab in an automotive factory is a vital link in the finishing process. Variances in the paint job from vehicle to vehicle costs automakers millions of dollars a year. At a typical auto factory, 15-20% of the cars are not correctly painted the first time; and the repainting process may cost $5 million-$10 million per plant per year.
The predominant reason for the need to repaint cars is incorrect paint thickness. Managing the viscosity can help control the problem. Ted Silkey, manager of process engineering at Ford's Twin City Plant, St. Paul, MN, said, "If you can get a steady stream of paint to the operator then you can paint vehicles correctly the first time." A process engineering manager in an auto plant said, "If I can control viscosity, then I can deliver the paint."
The Ford Twin Cities Plant uses a Cambridge system that includes 34 viscometers that measure paint viscosity, temperature and temperature-compensated viscosity. Other instrument sensors monitor humidity, pressure drop across the filter, the flow rate and the tank level. A central computer monitors the various parameters and updates the viscosity every 20 seconds. The system reduces viscosity-related paint defects, which results in significant cost savings.
The Cambridge Applied System viscometer replaces a No. 4 Ford cup, which was highly susceptible to human error. Ted Silkey noted, "If five different people do a cup test, they will get five different readings."
- Paper Coatings (roll process). At Champion Paper Co., Hamilton, OH, 28 Cambridge viscometers are installed in coating lines to monitor and help control the viscosity of coatings for specialty papers such as pressure-sensitive labels, folders and postcards. Charles Wyatt of Champion reports that these viscometers provide more reliable viscosity measurements with a smaller degree of variability than rotational viscometers and require essentially no maintenance, thus minimizing the downtime of the process.
- Photographic manufacturing. Sterling Diagnostic Imaging uses Cambridge viscometers to measure (but not control) viscosity of product that will be coated onto diagnostic imaging film - X-ray, CT and MRI. Sterling installed two customized CAS inline viscometers three years ago to provide an online diagnostic for unplanned dilution or product degradation, for underdispensing of solutions or excess addition of viscous component. The viscometers allowed them, for the first time, to quantify the time and temperature dependence of the product viscosity.
"When one looks at in-process sensors," says Dave Bixby, Staff Process Engineer at Sterling Diagnostic Imaging, "issues like dependability, maintenance and calibration requirements, and cleanability are important. This is where the mechanical simplicity of the CAS electromagnetic design and its ability to self-clean made it the clear choice for our application. We haven't been disappointed."
Effective control of the viscosity of the coating solution leads to a significant improvement of the coating process by reducing product waste. The ViscoPro can reliably manage the viscosity of coatings used in a range of industrial processes. The system eliminates the difficulties inherent in the use of other systems (e.g., the problem of subjective data that is inherent in the cup method). Virtually all users of the viscometer report that the system provides more accurate, more precise and more reliable data than previously used cup methods. Since the system has only one moving part and no seals, it can be operated for long periods of time with minimum maintenance. The unit readily provides online real time measurements and is able to automatically control the composition of the coating to provide maximum product quality.
For more information on viscometers, contact Cambridge Applied Systems Inc., 196 Boston Ave., Medford, MA 02155; phone 781/393.6500; fax 781/393.6575; visit the Web site www.cambridgeapplied.com; e-mail info@cambridge applied.com.