Film coating is one of the most critical steps in tablet and pellet production. A well-coated product looks uniform, tastes better, protects the active ingredient, and releases it exactly as intended. But coating is also one of the easiest steps to get wrong.
In this article, we break down the most common problems encountered during film coating — and more importantly, how to prevent and solve them. Whether you are running a lab-scale coater or a large production unit, these practical insights will help you achieve a stable, repeatable coating process.
Before Coating: Prepare the Tablets, the Solution, and the Machine
Coating does not start when you turn on the machine. It starts with the core tablets, the coating liquid, and the equipment.
1. Check the Tablet Cores
The quality of the core tablet directly affects the coating result.
- Fragile or brittle cores will break, chip, or generate dust as they tumble inside the pan.
- Uneven or rough cores will produce an uneven film, no matter how good your coating parameters are.
Before coating, confirm that the cores have sufficient hardness, a smooth surface, and acceptable friability. If the cores are not ready, the coating process cannot fix them.
2. Prepare the Coating Solution or Powder
The coating liquid must be:
- Fully dissolved — no lumps, no undissolved particles
- Uniform in concentration — consistent solid content from start to finish
- Used within the right time window — some solutions settle or degrade if left too long
If the solution has been sitting, stir it again before use. Some coating systems must be prepared fresh and used immediately, so production scheduling matters.
3. Inspect the Equipment
Before every batch, check:
- Pan cleanliness — residual coating from the previous batch can contaminate the next one
- Spray gun condition — a clogged nozzle causes uneven spray and uneven film
- Air supply — stable atomization pressure is essential for fine, consistent droplets
A clogged spray gun is one of the most common causes of coating defects. It is also one of the easiest problems to prevent with proper cleaning and inspection.
During Operation: Speed, Spray, and Temperature Must Work Together
Inside the coating pan, three things happen at the same time:
- Tablets tumble in the rotating pan.
- The spray gun atomizes the coating liquid onto the tablet surface.
- Hot air flows through, evaporating the solvent and forming a film.
The key parameters — pan speed, spray rate, and temperature — are not independent. Changing one usually requires adjusting the others.
Pan Speed
- Too slow: Tablets do not tumble enough. Some tablets get sprayed, others do not. The coating is uneven.
- Too high: Tablets are thrown against the pan wall. Friction increases, and tablets may break or chip.
Different tablet shapes and hardness levels require different speeds. The goal is a smooth, continuous tumbling motion across the spray zone.
Spray Parameters
This includes spray rate, atomization pressure, and spray gun angle.
- Spray rate too high: Tablets cannot dry fast enough. They stick together or to the pan.
- Spray rate too low: Coating efficiency drops, and the process takes too long.
- Poor atomization: Droplets are too large, producing a rough, uneven film.
- Wrong spray angle: Coating concentrates in one area, causing color variation and uneven thickness.
Temperature Control
- Inlet air temperature too low: Drying is slow. Tablets become sticky.
- Inlet air temperature too high: The surface film forms too quickly, trapping solvent inside. This is known as "false drying" and can cause defects later.
Outlet air temperature often reflects the actual condition of the tablet bed better than inlet temperature. Experienced operators monitor both.
The Three Factors Are Linked
If you increase the spray rate, you may need to increase temperature or reduce pan speed to allow enough drying time. If you increase pan speed, you may need to adjust the spray rate accordingly.
Coating is a dynamic balance. When the balance is right, the film is uniform. When it is off, problems appear.
A Simple Rule
When a problem appears, change only one parameter at a time. Watch the result. Then adjust again if needed.
Changing multiple parameters at once makes it impossible to know what actually solved the problem — or what made it worse.
After Coating: Drying and Cleaning
Continue Drying
After spraying is complete, tablets should continue to dry for a period of time to remove residual solvent. Follow the process requirements. Do not shorten this step just to save time.
Cleaning Is Not Optional
Cleaning is often underestimated, but it directly affects the next batch.
Clean:
- The coating pan
- Spray guns
- Liquid lines
- Filters
Dried coating residue is difficult to remove and can clog spray guns or contaminate future batches. After cleaning, inspect the equipment. Use the appropriate solvent when necessary.
The Core Principle of Film Coating
The goal of film coating is simple to state but hard to master:
Create a stable cycle of tablets, coating liquid, and hot air inside the pan.
When pan speed, spray rate, and temperature work together, the film is uniform. When they do not, problems appear.
Experienced operators are not just reacting to problems — they are anticipating them and making small adjustments before issues develop.
Looking for a Reliable Film Coating Machine?
Whether you are coating tablets, pellets, or granules, the right equipment makes a stable process possible.
We manufacture film coating machines designed for consistent performance, easy cleaning, and precise control of speed, spray, and temperature. If you are planning a new line, upgrading existing equipment, or looking for a reliable supplier, we are happy to help.
Contact us today to discuss your coating application and get a tailored solution.
