The spray gun is a core part of the spray system in a pharmaceutical tablet coating machine. It converts a continuous flow of coating liquid into stable droplets and delivers them evenly onto the tumbling tablet bed.
Different atomization methods, gun structures, and control systems can produce very different spray conditions. They directly affect droplet size, spray coverage, tablet-bed wetting, and spray stability during long production runs.
1. What Type of Spray Gun Is Used for Pharmaceutical Tablet Coating?
Based on the basic atomization principle, spray guns used in tablet pan coating can mainly be divided into Pneumatic Spray Guns and Hydraulic Spray Guns.
Hydraulic / Airless Spray Gun
A hydraulic spray gun uses a high-pressure pump, often above 10 MPa, to force liquid through a small nozzle orifice. It does not rely on compressed air, so this method is commonly known as airless atomization.
The spray has high velocity and strong impact, which can damage tablets. For this reason, hydraulic spraying is not the main choice for modern high-efficiency tablet coating machines.
However, airless technology is well established in industrial coating. It is also used in some large-pellet and chemical applications.
Pneumatic / Air-Atomizing Spray Gun
A pneumatic spray gun uses high-speed compressed air to shear the coating liquid into fine, uniform droplets. In OSD applications, atomizing air commonly operates around 0.1–0.5 MPa.
This is the standard spray configuration used in modern pharmaceutical film coating machines. Pneumatic atomization is gentler on tablets, which helps reduce breakage, chipped edges, and surface damage. It also provides a wide adjustment range for air pressure, liquid flow rate, and spray width, making it suitable for film coating, enteric coating, sustained-release coating, and controlled-release coating.
These guns are often called two-fluid or two-substance spray guns. This is because coating liquid and compressed air both take part in atomization. Their main value is not simply “finer spray.” They can adjust droplet state based on formula viscosity, spray rate, tablet-bed size, and drying capacity.
2. Types of Pneumatic Spray Guns for Tablet Coating
Pneumatic air atomization remains the main technology used in modern tablet coating. Within this category, spray guns can be further compared by structure, air-control method, number of process media, and production functions.
A real production spray gun can be Air-Atomizing + External Mix + Independent Air Control + Anti-Bearding at the same time. These terms do not conflict because they describe different features of the same spray gun.
2.1 Classification by Mixing Location: External Mix vs. Internal Mix
Pneumatic spray guns can be divided into External Mix and Internal Mix designs according to where the coating liquid and atomizing air meet.
External Mix Spray Gun
Modern film-coating liquids, such as HPMC, PVA, and enteric acrylic resin systems, often have high solids content and relatively high viscosity. In an External Mix spray gun, the coating liquid and atomizing air travel through separate passages and only meet near or outside the nozzle outlet.
The high-speed air then shears the liquid stream into droplets at the gun tip.
A tablet coating pan normally operates with heated drying air at around 40°C–80°C. With an external-mix design, the polymer liquid does not meet the high-speed atomizing air inside the gun before leaving the nozzle.
This helps reduce premature solvent evaporation and film formation inside the gun, which can cause severe blockage. In practice, External Mix is also more suitable for viscous materials and can reduce the risk of nozzle clogging.
This design is well suited to modern production-scale tablet coating. Pharmaceutical coating spray systems also commonly use External Mix together with separate shaping or Pattern Air channels.
Internal Mix Spray Gun
In an Internal Mix spray gun, the coating liquid and compressed air meet inside the nozzle before leaving the gun. The two media interact inside the mixing chamber, which can produce strong atomization.
However, coating material can also build up inside the chamber and cause blockage. This type of design is mainly seen in some special enclosed research systems with high-frequency automatic in-line pneumatic cleaning or WIP functions, especially when highly volatile organic solvents are used.
For large-scale aqueous film coating, Internal Mix should not be selected without careful evaluation.
For commercial high-volume film coating, External Mix should normally be evaluated first. Internal Mix only needs to be considered as a main option when the supplier can provide clear information on spray range, formulation compatibility, cleaning method, and long-batch operating performance.
2.2 Classification by Spray Control: Atomizing Air and Pattern Air
One of the main differences between basic and advanced spray systems is how Atomizing Air and Pattern Air are controlled.
Atomizing Air
Atomizing Air, or AA, mainly breaks the continuous liquid stream into droplets. It has a clear effect on both droplet size and droplet velocity.
When other conditions remain the same, increasing Atomizing Air usually produces finer droplets.
Pattern Air
Pattern Air, or PA, is mainly used to adjust spray shape and spray width. Changing Pattern Air can also redistribute droplet size and spray density between the center and outer areas of the spray pattern.
In a multi-gun system, it directly affects spray-zone overlap.
Some systems use combined Atomizing Air and Pattern Air control, while more advanced production spray systems allow the two to be adjusted separately. Both configurations are used in pharmaceutical tablet-coating spray systems.
For large perforated drums, multi-gun systems, or projects that require precise process scale-up, Independent Atomizing Air + Pattern Air Control is worth prioritizing.
2.3 Classification by Number of Process Media: Two-Fluid vs. Multi-Substance Spray Systems
The most common configuration for tablet film coating is the Two-Fluid Spray Gun. The coating liquid and compressed air work together to create the spray.
For standard film coating with only one coating liquid, this design can already meet the basic requirements for droplet formation, spray-width control, and stable spraying.
If the process needs to atomize or mix two liquids at the same time, or requires a special multi-media atomization method, a Three-Substance or Multi-Substance Spray System can be used.
These systems are mainly intended for special multi-media processes and are not the default choice for standard pharmaceutical tablet coating. Three-Fluid is also not automatically more advanced than Two-Fluid.
For normal tablet film coating, Two-Fluid remains the main solution. Multi-Substance only becomes useful when the process truly requires additional media.
2.4 Functional Designs for Pharmaceutical Production
Good atomization does not automatically mean that a spray gun is suitable for continuous production. A short laboratory run and several hours of commercial spraying place very different demands on the spray system.
- Anti-Bearding
Film-coating liquids usually contain polymers, colorants, and other solid materials. During continuous operation, fine droplets and tablet dust may gradually collect around the nozzle and air cap.
This build-up is known as bearding. As bearding increases, the spray pattern can change. In severe cases, it may also affect the liquid and air openings.
Anti-Bearding designs reduce this build-up by changing the geometry and airflow around the air cap and gun tip. Professional pharmaceutical nozzle suppliers treat Anti-Bearding as a functional design for tablet coating, not as a separate atomization type.
- Clean-Out Needle and Shut-Off Needle
The main purpose of a Clean-Out Needle is to mechanically clear the liquid orifice and reduce blockage caused by material build-up around the nozzle.
A Shut-Off Needle, or liquid shut-off mechanism, quickly closes the liquid path. This helps reduce dripping after spraying stops and lowers the risk of local overwetting.
These two functions should be checked separately during equipment selection. Confirm whether the Clean-Out function can operate automatically or according to a programmed sequence.
Also check whether the Shut-Off function can stop liquid flow quickly and cleanly. For coating processes that tend to form deposits or require frequent spray start-stop cycles, having both functions gives more useful information than a general claim such as “anti-clogging needle.”
- Liquid Recirculation
If the coating liquid tends to settle, tank agitation alone may not keep the concentration uniform throughout the whole liquid circuit. When spraying stops, suspension left inside hoses, manifolds, and areas near the spray gun can also begin to settle.
For long batches or multi-gun systems, liquid recirculation should be considered together with agitation, pump type, and spray-stop logic.
It does not solve an atomization problem. Its purpose is to keep the coating liquid stable before it reaches the nozzle.
Repeatable Gun Positioning
Spray-gun position is also a process condition, not just a mechanical installation detail. A production machine should allow repeatable settings for gun-to-bed distance, spray angle, gun spacing, and spray-bar position.
After cleaning and reinstalling the spray guns, the validated geometry should be restored as closely as possible. Operators should not have to rely on visual adjustment for every batch.
Different batch sizes and coating formulations place different demands on the spray system. The right choice should start with working batch size, coating-liquid properties, and production time.
Common Tablet Coating Spray Gun Brands and Sources
Some large tablet coating machine manufacturers use their own spray-bar and nozzle systems
4. What Should You Check When Comparing Tablet Coating Machines?
When comparing two pharmaceutical tablet coaters, at least the following points should be confirmed.
5. Frequently Asked Questions About Pharmaceutical Tablet Coating Spray Guns
5.1 Common Spray-Gun Terms in Tablet Coating
- HVLP Spray Gun
HVLP stands for High Volume Low Pressure Spray Gun. It uses a high volume of air at relatively low pressure.
The design aims to reduce overspray and improve material transfer efficiency by limiting air-cap pressure, usually to ≤0.7 bar.
If the coating liquid is viscous or has a high solids content, this low pressure may not provide enough force to break up the liquid effectively. The droplets can become too large, which may lead to an orange-peel surface on the tablets.
For this reason, HVLP should be evaluated against the actual viscosity of the coating liquid and its practical value inside a closed, negative-pressure coating pan.
The actual atomization method, External or Internal Mix structure, Atomizing Air and Pattern Air control, spray-rate range, and spray-width range are more meaningful when evaluating a pharmaceutical coating spray gun.
- Three-Fluid
In tablet pan coating equipment, Three-Fluid is commonly used to describe a spray-gun configuration with one coating-liquid channel, one Atomizing Air channel, and one Pattern Air channel.
The practical value of this configuration is that Atomizing Air and Pattern Air can be adjusted separately. This provides more flexibility in controlling droplet atomization and spray coverage.
However, this terminology is not a universal spray-gun standard.
In other nozzle-engineering systems, Three-Fluid or Three-Substance may also mean two liquids plus one atomizing medium. That configuration is mainly used in spray drying or other special processes and is not the mainstream setup for normal tablet pan coating.
- Air-Assisted Airless
Air-Assisted Airless is a hybrid spray method between hydraulic and pneumatic atomization.
The liquid is mainly atomized by hydraulic pressure. Low-pressure air is then added around the nozzle outlet to further improve droplet atomization, spray shape, and edge distribution.
- Anti-Clogging
Anti-Clogging describes a spray gun’s ability to resist blockage. It is not a separate atomization principle.
It can be achieved through Anti-Bearding air caps, Clean-Out Needles, Shut-Off Needles, optimized internal flow paths, or liquid recirculation.
Active prevention — Anti-Bearding air cap: An external auxiliary high-pressure air curtain helps prevent coating liquid from sticking to the gun tip and forming a skin. This supports continuous production, reduces dripping, and is gentler on fragile tablets.
Mechanical cleaning — Clean-Out / Shut-Off Needle: A metal needle mechanically clears a blockage or closes the liquid path. It provides clean spray start-stop control, but during continuous spraying it may cause temporary spitting and tablet twinning.
Internal optimization — Flow path / recirculation: The coating suspension keeps circulating inside the gun. This mainly prevents solids from settling inside the liquid path and does not remove build-up on the outside of the air cap.
5.2 Why Can the Same Spray Gun Produce Completely Different Results After Adjustment?
The spray gun is only one part of the spray system. Spray rate, Atomizing Air, Pattern Air, and gun-to-bed distance all change the condition of the droplets when they reach the tablet bed.
In one pan-coating experiment, increasing the spray rate from 5.5 g/min to 7.5 g/min raised tablet-bed relative humidity from 26.3% to 33.5%.
In the same experiment, reducing the gun-to-bed distance from 5 in. to 3.5 in. increased tablet-bed relative humidity from 26.3% to 38.0%.
The same spray gun does not mean the same spray condition.
A higher spray rate increases the wet load entering the tablet bed, while a shorter gun-to-bed distance gives the droplets less time to lose moisture during flight.
Spray-gun settings must therefore be adjusted together with the drying capacity.
5.3 Are More Spray Guns Always Better?
No. Adding more spray guns can increase the total spray area, but gun quantity is only one variable in the spray-system design.
It must match the drum size, effective tablet-bed width, spray rate per gun, spray width, and gun spacing.
Too much overlap between adjacent spray patterns can make part of the tablet bed too wet. This may cause twinning, peeling, bridging over tablet markings, and local color variation.
On the other hand, too few spray guns or excessive gun spacing can leave dead zones in the spray coverage.
This can cause large differences in coating weight gain between tablets. Enteric or controlled-release films may become too thin in some areas and fail locally, while premature droplet drying can also leave the tablet surface rough and pitted.
5.4 Is Three-Fluid Always Better Than Two-Fluid?
No.
For normal tablet film coating, better spray performance does not come from simply increasing the number of fluids or channels. What matters is whether the spray gun can maintain stable control of droplet size, spray width, liquid flow rate, and spray overlap between multiple guns.
If the process uses only one coating liquid, a well-designed Two-Fluid Spray Gun can already meet the needs of standard film coating.
For large production equipment, it is more important to check whether Atomizing Air and Pattern Air can be adjusted independently. The spray gun should also have suitable Anti-Bearding, Clean-Out, Shut-Off, and stable liquid-feed functions.
A Multi-Substance Spray System only has real value when the process truly requires a second independent liquid or another additional medium.
Three-Fluid does not mean a higher technical level by itself. The key is whether the configuration fits the coating process.
5.5 Is There a Fixed Atomizing Air Pressure or Gun-to-Bed Distance for Pharmaceutical Tablet Coating?
No. Atomizing Air, spray rate, nozzle orifice, liquid viscosity, and gun-to-bed distance all affect each other.
For example, the same Atomizing Air pressure can produce different droplet sizes if liquid viscosity, spray rate, or nozzle diameter changes.
For this reason, the air pressure or gun distance used on another machine should not simply be copied as a standard setting.
The correct approach is to establish a process window for the actual formulation and equipment. The final settings should then be verified through the spray pattern, tablet-bed condition, and final coating quality.
5.6 What New Spray Technologies Are Being Developed for Tablet Coating?
Yes, new technologies are being developed. However, most of them cannot directly replace the traditional pneumatic spray gun.
Modern tablet coating is developing in two main directions: alternative coating methods and smarter control of conventional spray systems.
(1) Electrostatic Dry Powder Coating
Electrostatic dry powder coating is an alternative to traditional liquid film coating.
Instead of atomizing a water-based or solvent-based coating solution, an electrostatic spray gun charges fine coating powder and directs it onto the tablet surface. The charged particles are attracted to the tablets, and the deposited material is then heated or cured to form a continuous film.
Pharmaceutical studies have successfully applied this method to tablets using polymers such as Opadry AMB and Eudragit EPO. The process can produce a smooth, uniform film with good drug-release performance.
Its potential advantages include:
- less water or organic solvent;
- lower drying load;
- improved powder deposition through electrostatic attraction;
- an alternative for formulations that are sensitive to long wet-coating processes.
However, this method has not replaced pneumatic liquid spraying in mainstream pharmaceutical coating.
Powder charging, tablet conductivity, powder adhesion, and the final film-curing step all create new process requirements.
(2) Smarter and More Automatic Spray Control
Making pharmaceutical coating spray systems smarter and more automatic is another important development trend.
Traditional coating processes rely on preset values such as spray rate, Atomizing Air, Pattern Air, inlet-air temperature, and gun-to-bed distance. The next step is to combine these settings with real-time Process Analytical Technology (PAT).
A 2026 study used in-line near-infrared spectroscopy to predict coating thickness in real time and linked the measurement with PID feedback control.
The system could then adjust the coating process according to the measured film thickness instead of relying only on fixed process settings.
This means future spray systems may move from:
Preset Spray Parameters → Real-Time Measurement → Process Prediction → Automatic Adjustment
For pharmaceutical manufacturers, this can improve batch consistency and reduce dependence on manual adjustment.
Conclusion
For modern pharmaceutical tablet pan coating, the practical standard choice is the Pneumatic / Air-Atomizing Spray Gun. Hydraulic / Airless spraying is a real technology, but it is not the normal solution for modern tablet pan coating.
Within a pneumatic spray system, Two-Fluid / Two-Substance describes the two media used for atomization. External Mix and Internal Mix describe where the liquid and air meet, while Atomizing Air and Pattern Air describe the air-control method.
Anti-Bearding, Clean-Out, Shut-Off, and Liquid Return are used to solve practical problems during long pharmaceutical production runs.
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