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Reprinted From Tablets & Capsules, May 2004 1

tablet coating

Optimizing spray gun performance

Rudolf J. Schick Spray Analysis and Research Services

Tablet coating systems are designed and calibrated for specific applications. When there are changes to the application, you must re-evaluate the coating system, including its spray guns. This article describes how to evaluate spray gun performance and how to improve and maintain that performance.


here are many reasons to coat tablets:

· to improve their appearance and facilitate identification; · to disguise an odor or an off-taste; · to help the consumer ingest the tablet;


· to help the ingredients withstand exposure to air, light, and moisture; · to control dust; and · to facilitate handling and packaging. Whatever the reason for coating, when you apply aqueous-film, solvent, or sugar solutions to tablets, there is one goal: even, consistent coating. An uneven, inconsistent coating carries significant consequences, including rejected product, production downtime, prolonged maintenance, and high operating expenses. Spray guns. Most spray guns perform very reliably because equipment manufacturers tailor the coating systems to the application and configure the spray guns to apply the coating solution efficiently. This high level of performance leads many people to perceive spray guns as simple devices: So long as the guns keep spraying, people assume they are working properly. As a result, it's possible to overlook spray gun performance when you troubleshoot problems with coating quality. That's a mistake. Spray guns are precision components built to yield a very specific performance under very specific conditions. Even a small change in the process-- such as using a different coating formulation--can quickly lead to substandard product. By understanding spray gun performance and learning how to optimize it, you can make a big impact on your operation's profitability. The sidebar on page 5 gives an example.

product quality. If product quality is good, you know that the spray guns are getting the job done. However, the difference between adequate spray gun performance and optimal spray gun performance can be quite large. That is, even with satisfactory quality, there may be room for improvement. For example, you may be able to reduce the amount of coating solution, energy, labor, and maintenance the system requires. Let's look at the most critical factors in the coating process, and then we'll look at ways to optimize performance.

Drop size and size distribution

Once you understand that correctly forming the drops is integral to the success of the coating process, you will understand the importance of controlling drop size to maintain coating uniformity. Keeping drop size and drop

Figure 1

When drops of coating solution reach the tablet surface, they penetrate it, spread, and coalesce. Heat and airflow within the coater remove moisture from the coating.

Evaporation Coating drops

Core penetration

The coating process

In coating systems that use spray guns, coating begins with the formation of drops, a process called atomization. In tablet coating, most spray guns atomize the coating solution using pneumatic "external-mix" atomizers. External mix means that the liquid and air streams are mixed outside the nozzle, and thus you control the air and liquid flow independently. Internal-mix atomizers, on the other hand, mix the air and liquid streams within the atomizer, and thus you cannot control the air and liquid flow independently. With internal-mix atomizers, changing the air flow will also change the liquid flow. Tabletcoating spray guns typically create a spray pattern that is flat, but it is sometimes conical. As the finely atomized drops exit the nozzle, they contact the solid tablet surface, then spread and coalesce with neighboring drops. As more drops reach the surface, they form layers. With exposure to heat and airflow, the moisture in the coating solution evaporates, leaving a dry, uniform film around the tablet. See Figure 1. In standard industry practice, several spray guns are mounted on a manifold within a coating machine. Each gun forms a flat spray pattern, and you can adjust the pattern by changing the airflow, air pressure, the coating flow rate, and other parameters. Still, how do you know whether your spray guns are performing optimally? There is no simple test or standard method to measure spray gun performance. Surely the first and fastest way to gauge performance is to evaluate

Contact spread

size uniformity steady is a challenge, because both vary according to the nature of the coating (type of solids), the solution viscosity, the spray gun design, the amount of solution flowing through the spray gun, and the pressure of the atomizing air. Evaluating drop size is complicated. It requires highly specialized equipment and an environment that replicates the tablet coating machine environment, the coating solution itself, and the spray guns. That's why most people conduct spray performance tests at labs that specialize in particle and drop size research and analysis. Even so, it is important that you grasp the fundamentals of drop size so that you can understand and interpret the data you receive from outside experts. Spray pattern. In addition to controlling drop size, you must control the spray coverage precisely. That means positioning the spray guns so that the spray patterns overlap properly. The overlap must not create uneven distribution or heavy edges.

Measuring drop size

Not all drops in the spray pattern are the same size, so instead of focusing on a single drop size, we look at


the distribution of the drop sizes within a given spray pattern. However, we still must measure the size of individual spray drops within the spray pattern. In general, tablet coating works best when the drop size distribution falls within a narrow range. Furthermore, it is important to limit the number of small drops in the spray because they dry quickly and tend to create dust. We express drop size in microns, or micrometers, which are denoted as m. A micron is very small. See Figure 2. One micron equals 1/25,400 inch or 0.001 millimeter, and it takes 3,175 microns to equal 1/8 inch.

Figure 2

Comparison of three drop sizes, in microns ( m)


500 m


1,200 m


5,500 m

The two methods for measuring drop size are the spatial technique and the flux technique. See Figure 3. With the spatial technique, an instrument instantaneously samples the drops present within a specific volume of the spray pattern. The spatial technique generally collects the data using holography, high-speed photography, or light scattering. Whatever the mechanism used, the measurement senses the number density of drops in each size class, as well as the number of drops per unit volume. The flux technique measures individual drops as they pass through a sampling region, typically a cross section of part of the spray pattern. The flux technique generally

collects data using optical means, such as laser-imaging instruments and phase Doppler analyzers that can sense individual drops. The technique is sensitive to the particle flux, which is the number of particles passing through the sampling area. Results are expressed in terms of unit volume and time (cm3 / cm cm/s). In practice, a phase Doppler particle analyzer is the most common instrument used for measuring drop size of tablet coating sprays. In this application, the spray guns usually generate a flat spray pattern, in which the volume of drops is heavily concentrated at the center of the spray and lightly concentrated toward the edges of the spray. See Figure 4. To acquire a comprehensive picture of the drop size distribution throughout the spray pattern, the phase Doppler analyzer collects data at several radial locations within the spray pattern and combines that data. Figure 5 shows the results of a phase Doppler analysis of the drop size distribution of an entire spray when the coating flows at 30 milliliters per minute and at 60 milliliters per minute. Notice how the liquid flow rate affects drop size distribution: The higher the flow rate, the larger the drop size. There is also a wider range of drop sizes at the higher flow rate.

Putting test results into practice

When you buy a new tablet coater, the spray guns are selected and positioned to deliver the proper drop size, and coating quality should be high. If it isn't, work with the people who supplied the tablet coater until you resolve all the problems. However, because the spray guns were selected specifically for your current coating application, even a slight change in a process condition can affect drop size and drop size distribution. So if you start finding quality problems, document what has changed since you installed the coating system and ask the supplier of the coating system or the spray gun manufacturer for assistance. Of course, you can also use the trial-and-error approach and adjust the spray gun settings yourself. But be prepared: You may not get the desired results. In fact, in my experience, the only guarantees of trial and error are decreased production and wasted product as you attempt to fine-tune the spray gun settings. It is usually more expedient and cost-effective to seek expert help. The expert will likely have reference data about the drop size and drop size distribution that your spray guns generate. That's important information to have before adjusting the spray guns. It's also wise to consult the equipment manufacturers or a spray technology consultant if you are changing your application or if you think that your operating costs are too high. The manufacturer's rep or the consultant can conduct specialized tests to pinpoint the spray gun settings required to meet performance targets. The rep or consultant can also help you locate the spray guns you need and help develop a custom solution to your problem.

Figure 3

Two common methods of analyzing tablet coating sprays

a. Spatial sampling

Technique's features: 1. Average of finite volume 2. Instantaneous sample 3. Sensitive to number density

. . .. ° . . . . . . .° . . °. . ° .° ° . . .. . . . ° ° . .° ° ° ° . . ° °

Measurement volume

b. Flux sampling

Technique's features: 1. Time averaged 2. Sensitive to particle flux

.. .

° ° °



Measurement cross section

. ..


° °





Figure 4

Drop size distribution of a single spray gun

20 Volume (millimeters) 15

pattern away from the face of the spray gun's air cap. Tests of these new spray guns show that they can operate eight times longer than standard guns before any bearding occurs. Figure 6 shows two air caps. One is a traditional design, while the other has an anti-bearding design. If bearding is giving you trouble, look at these spray guns. Not only do they eliminate bearding under most conditions, they extend the intervals between

Figure 5


Drop size distribution of a spray pattern at two flow rates


100 90


16 08 -0.00 -5.08 0.16 15.24 0.32 25.40 24 .40 .32 -25 -20 -15. -10. -5. -1 -2

80 70

30 ml 60 ml


Distance from center (centimeters)

60 50 40 30 20 10 0 10 100

Watching for wear

If your process hasn't changed but you start seeing quality problems, you should check the spray guns for wear. Like any component, spray guns are subject to normal wear, and their performance will deteriorate over time. Hour after hour of spraying a coating solution under pressure gradually removes nozzle material, enlarging or otherwise distorting the nozzle orifice and its internal passages. The result is a deteriorated spray pattern or a larger drop size as the liquid flow rate increases or the spraying pressure decreases. Either way, the coating becomes uneven. Because wear is gradual, you probably won't notice any signs of change. In fact, wear of 10 to 30 percent is not detectable by the naked eye. Thus preventive maintenance is the best strategy to ensure trouble-free performance. Check the spray guns regularly and replace them after they've operated beyond a specified number of hours.

Drop size (microns)

maintenance periods, thus increasing production time. Whatever your problem, if you're not satisfied with the performance of your spraying system, seek a second opinion. Maybe another type of spray gun will improve performance or give you equal results at a lower cost with less maintenance. Check with a consultant or a spray system manufacturer. They have the expertise to test the spray guns in a laboratory to evaluate drop size and drop size distribution.

New spray guns

Spray technology continues to improve, and you can find several new spray guns that integrate easily into your existing tablet coater. The high-efficiency spray set-ups on the market today can reduce air and energy consumption, because they provide the same spray performance at lower air pressures. In fact, some people using these new spray guns report a 50 percent reduction in air consumption. Bearding. A common problem with spray guns used in tablet coating is buildup on the perimeter of the nozzle orifice. This phenomenon, known as bearding, blocks the liquid and air orifices, distorting the distribution of the spray. In such cases, you must clean the guns often to maintain coating quality. (Gun design has the most influence on bearding, but coating operations that produce a large number of fines also contribute to bearding. A turbulent coating environment is another factor that leads to bearding.) However, there are now "anti-bearding" spray guns on the market that atomize the spray and create the spray

Figure 6

Comparison of air caps. A traditional spray gun air cap is at right. An anti-bearding spray gun air cap is at left.


Do the math: the cost of bearding

An inefficient tablet coating system can cost you several hundred thousand dollars annually. Here's an example. Assumptions · The tablet coating operation runs 16 hours a day, 5 days a week, and 52 weeks a year. · Two technicians are involved in the operation, and total labor cost is $40 an hour. · The value of each hour of production is $200. In other words, 1 hour of downtime costs $200. If your tablet coater isn't optimized, tablet quality may be unacceptable, and you may need to dispose of a batch from time to time. We've helped customers reduce waste by as much as 90 percent. But let's assume that optimizing the spray reduces waste by 50 percent. Let's further assume that bearding is a problem, and that you must clean the nozzles every 60 minutes and that the total downtime associated with each cleaning is 10 minutes. Changes Adding anti-bearding nozzles reduces downtime, and allows you to coat tablets by an additional 4, 6, or more hours. But let's be conservative and assume that the new nozzles reduce labor and production downtime by 3 hours a day. Results As a result of the changes, you save $666 a day, $3,330 a week, and $173,160 a year.1 Actual savings will vary based on actual labor costs, the cost of wasted product, and the value of production time. Regardless, this example shows you the high price of tolerating tablet coating inefficiency.[1] T&C

1. Calculations performed using the spray optimization calculator at


Tablet coating can be challenging because it doesn't tolerate even slight process changes. Nonetheless, reliable and efficient performance is certainly achievable, because hundreds of pharmaceutical companies do it every day. Even so, most coating operations have room for improvement. Use the information in this article to consider basic improvements that can significantly improve quality and efficiency while reducing operating costs. T&C

Rudolf J. Schick is industrial division director of Spray Analysis and Research Services, PO Box 7900 Wheaton, IL 60189. Tel. 630 665 5000, fax 630 260 0842. Website: He is responsible for spray consulting, testing, and research services for industrial spray applications. He has more than 15 years' experience in spray characterization and is active in the international drop-size community. He serves on the board of directors for the Institute of Liquid Atomization and Spray Systems and participates in the American Society of Testing and Materials' subcommittee E29 on particle size measurement.

ARCSAR003 Printed in the U.S.A.

©Spraying Systems Co. 2004


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