Improvements in equipment, materials and control technologies have helped tape casting become an integral part of many high-tech manufacturing operations

Tape casting was invented shortly after World War II, when Glen Howatt used the first tape casting machine to produce ceramic capacitors. This forming method has evolved significantly since then, and the past decade in particular has seen rapid advances in tape casting technology. Tape casting is routinely used today in the manufacture of products such as multilayer ceramic capacitors (MLCCs), low temperature co-fired ceramics (LTCCs), lithium batteries, fuel cells, oxygen separators and thermistors. In addition, progress in nanotechnology is leading to new and exciting applications for tape casting very thin films (less than 5 microns [0.0002 in.]) in the development of products such as ceramic body armor, optical films, medical films used as drug delivery systems, and microporous membranes.

Because products made by tape casting form the basis of many important industries, it can be helpful to periodically review recent trends and advances to gain a better understanding of this innovative technology.

High-Precision Equipment

Tape casting is similar to web coating and can be described as a process that uses controlled fluid flows to produce a uniform layer or coating of one material on another. Web coating usually pertains to thin protective, decorative or functional coatings, while tape cast products are generally thicker and most often removed from the substrate or web for further processing.

In general, tape casting systems include fluid flow control units or coating heads, dryers and web/carrier conveyors, as well as take-up and unwind equipment. The process relies on free surface flows of viscous or solids-loaded liquids to form a layer of film that is continuously deposited on a moving substrate.

Tape cast films are usually thicker than 20 microns (0.0008 in.), but can be much thinner when nanotechnology processing methods are used. High-precision knife or doctor blade systems that are available today produce very precise films as thin as 12 microns (0.0005 in.), while slot die or lip coating systems are used for films as thin as 5 microns (0.0002 in.). Other methods that are being adapted to apply even thinner films (1 micron) to substrates include micro gravure rolls, as well as forward and reverse roll coaters.

The knife/doctor blade is a coating application in which the film is formed on a continuous web or carrier from fluid in a reservoir, which is metered through a gap between the knife and the carrier substrate to control film thickness. This is by far the most widely used, simple, versatile and inexpensive method, and it operates over a broad range of thicknesses, fluid viscosities and casting speeds. The knife/doctor blade system is used with high fluid viscosity in the range of 1000 to 30,000 mPa·s to cast thicknesses from 12 to 1000 microns (0.0005 to 0.040 in.) with coating speeds up to 100 meters/minute (330 feet/minute).

Slot die/lip coatings are applications in which metering of the fluid flow occurs through a slot positioned across the web/carrier. The fluid flow is proportioned by the viscosity of the fluid and the fluid reservoir height or pressure gradient imposed at the slot. The film thickness is controlled by carrier speed settings. Uses include a wide range of fluid viscosities up to 50,000 mPa·s to casting film thicknesses in the range of 4 to 400 microns (0.00015 to 0.015 in.), and casting speeds of 10-200 meters/minute (30-650 feet/minute).

Enhanced Film Properties

The fluids used in tape casting can be simple or complex and usually consist of either a resin or polymer with various additives, such as functional powders, pigments, plasticizers, binders, dispersants, or a solution in which water or solvent is added to these constituents. The fluid is made homogeneous by proper mixing, or milling when necessary, so that the additives are completely dissolved and adequately dispersed. Where particle packing and density in the end product are major concerns, nanoparticles provide an even greater range of properties, especially when an ultra-thin film is required. In any case, today's control systems ensure that the viscosity and consistency of the fluid is uniform and suitable for the intended coating or casting operation.

Nanomaterials, in the size range of about 100 nm (0.000004 in.), have been found to significantly improve film properties. For example, adding as little as 2% by volume of nanoparticles to a resin matrix increases strength by 100%, improves thermal stability by 100°C and enhances thermal resistance. The use of polymer-based nanocomposites is being evaluated in an effort to develop better electrical properties in products such as thin film sensors, photoluminescents and other optical devices.

Improved Process Control

The diversity of tape casting applications for both ceramic and non-ceramic products has become the driving force for many of the recent advances in the technology. The tape casting process has become more automated, product quality has improved, and fabrication of the final shape of the product has become easier.

The production of a steady stream of high-quality tape or coated products requires perfect control of the casting material, casting rate, casting speed, curing or drying method, and product take-up. The newest generation of tape casting systems uses the latest computer control technology to provide this level of control. Human machine interface (HMI) control systems provide a simplified operator interface and graphic displays that make process control easier, leaving almost nothing to chance.

These "what you see is what you get" (WYSIWYG) HMI systems are easy to use, shorten operator training time and dramatically increase productivity. For instance, overview display screens show the entire casting/coating operation at a glance, and interlock display screens show the status of system conditions that are necessary to operate safely. Intuitive menu bars provide a simple, fast way to access specific operator screens while providing a high level of automation. Control of process parameters such as drying temperatures, carrier drive speed, air velocity, atmosphere, fluid flow and reservoir level are also integrated into the HMI package. In addition, cast film thickness is continuously scanned and controlled automatically through a closed loop feedback system.

A significant advantage of the modern HMI system is the ability to manage production through the use of historical trending, alarm logging and statistical process control. The collection and interpretation of process data provides the ability to easily design recipes that can be stored and downloaded at a later date to repeat the desired operating conditions.

Many tape casting systems available today offer HMI packages as standard, but existing equipment can also be upgraded to take advantage of this technology.

Future Advances

Many dramatic breakthroughs in this technology have occurred since the evolution of tape casting from web coating began with the seminal work and equipment used by Glenn Howatt to produce ceramic capacitors in the 1940s. As industry, government and university researchers work to further improve and find new uses for tape casting, this forming method will no doubt continue to be an integral part of many ceramic industry developments.

For more information about tape casting technologies, contact HED International, Inc., Unique/Pereny, Route 31, Box 246, Ringoes, NJ 08551; (800) 433-5456 or (609) 466-1900; fax (609) 466-3608; e-mail info@hed.com ; or visit http://www.hed.com .

For Further Reading

• Satas, C. and Tracton, A., Coating Technology Handbook, Marcel Dekker Inc., 2001.
• Mistler, R. and Twiname, E., Tape Casting, Theory and Practice, American Ceramic Society, 2000.
• Thayer, A., "Nanomaterials," Chemical and Engineering News, Vol. 81, No. 35, 2003.
• Hickman, K., "Nanomaterials: It's a Small, Small World," Cambridge Scientific Abstracts, 2002.
• Miyamura, H., "Recent Trends in Functional Coatings in Japan," The Technical Progress, 1996.
• Shanefield, D., Organic Additives and Ceramic Processing, 2nd Edition, Kluwer Academic Publishers, 1995.
• Gutoff, E. and Cohen, E., Coating and Drying Defects, John Wiley and Sons, 1995.
• Hirano Tecseed Co. Ltd., Technical Report (II), Comma Coater and Lip Coater, Tokyo, Japan, 1995.