When we ask from a tablet coating operator what is the suitable batch size for a specific tablet coating pan mostly we get an answer regarding the batch size he is working with.
But the best load for a pan is not a fixed figure. It depends on the tablet coating machine, the shape and weight of the tablets used for coating, and how the tablets pack together in a pan. An overloaded tablet coating pan gives uneven coating and sticky tablets. An underloaded tablet coating pan gives chipped edges due to lack of smooth rotations and wastage of coating solution spray due to pan emptiness. Both overloaded and under loaded pan results in rejection, deviations and a poor coating yield.
This article explains how to work out the load for a tablet coating pan, how you can check a planned batch against pan load, and what to do when a batch does not fit as per requirement of pan load.
Why pan loading matters
A coating pan works smoothly during coating only when the tablet bed moves properly. Tablets must roll in a steady cascade in the pan instead of sliding, so that every tablet passes through the spray zone often and for about the same time, and then dries in the warm air. The depth of the tablet bed controls all of this during the process of tablet coating. It decides how long a tablet stays in the spray zone, how far it falls, and how far the spray guns sit from the bed surface. A bed that is too deep or too shallow breaks that pattern.
| Too little in the pan | Too much in the pan |
|---|---|
| If tablet bed is too shallow, so tablets fall a long way and hit the pan wall resulting in surface roughness, chipping. | If the tablet bed is too deep, the tablets at the bottom move slowly and mix poorly resulting in non uniform coating. |
| All the spray is not showered on tablet bed & most of the spray portion misses the bed and falls on the pan surface resulting in wastage of coating suspension. | Spray gun to tablet bed distance is reduced, giving local over-wetting during coating. |
| More chipping and edge erosion due to lack of smooth rotations of tablets in the pan. | More twinning, picking and sticking from tablets resting against each other during spraying process. |
| Uneven coating because few tablets pass through the spray zone at a time. | Uneven weight gain because the bed does not turn over properly. |
| Long coating spray time for the same coating quantity, with poor spray efficiency. | Dust and fines are harder to clear, and drying air does not reach the whole bed due to overloading. |
Coating Material Calculator, which gives dry coating, suspension and vehicle for a target weight gain.
Work from the supplier’s range, then your validated range
Every coater has a minimum and a maximum working load, set by the supplier and confirmed in your own qualification and process validation. Some are given as a volume of tablets, others as a weight for a reference product. Use the figures that are in your qualified documentation, because the spray guns, the baffles and the airflow have been set up and tested for that range. Running outside it means running a process that has not been shown to work.
Turn the pan volume into a batch weight
If the capacity is given as a volume, convert it into a weight for your tablet with the bulk density of the tablets themselves, not of the granules or blend.
- Fill a clean container of known volume with core tablets, such as a measuring cylinder or a calibrated bucket, using the same pouring method each time.
- Weigh the tablets and divide by the volume to get the bulk density in kg per litre for that specific Batch.
- Multiply the pan’s working volume by this density to get the nominal batch weight for that tablet.
- Calculate the minimum and maximum batch from the percentages in your qualified range.
A round tablet and a capsule-shaped tablet of the same weight pack differently, so the same pan holds a different weight of each.
Practical Example
A coater has a nominal working volume of 150 L, and an example qualified range of 50% to 100% of nominal. The tablets have a measured bulk density of 0.62 kg/L. A batch of 240,000 core tablets of 400 mg is planned, which weighs 96.0 kg.
| Step | Calculation | Result |
|---|---|---|
| Nominal capacity (weight) | 150 L × 0.62 kg/L | 93.0 kg |
| Planned batch as a share of nominal | 96.0 ÷ 93.0 × 100 | 103.2% (over the maximum) |
| Split into two equal lots | 96.0 ÷ 2 = 48.0 kg; 48.0 ÷ 93.0 × 100 | 51.6% (inside the example range) |
| Tablets per lot | 48.0 kg × 1,000,000 ÷ 400 mg | 120,000 tablets |
The planned single batch is 3.2% (103.2%) above the maximum, so it should not go into this pan in one lot. Splitting it into two lots of 48.0 kg each will be at 51.6% of nominal, which is inside the range. The coating material is then calculated for each lot: 120,000 tablets × 12 mg gain (3% of 400 mg) = 1.44 kg of dry coating, and at 15% solids that is 9.6 kg of suspension. Our Coating Material Calculator calculate it for you on a single click.
Use your data to put in calculator to get the results.
What to do when the batch does not fit
- Too large
Divide the batch into equal lots that each sit inside the range. Check that your registered batch size and your batch record allow it. - Too small
If batch size is very low then you can do 2 things,
Increase your batch size or use the pan of low capacity.
Checks after loading
- Run the pan and watch the bed. The tablets should cascade evenly, with no dead spots and no tablets dropping against the wall.
- Measure the gun-to-bed distance with the bed in motion, and compare it with the set-up in the process.
- Confirm that the spray pattern covers the width of the bed and does not reach the pan wall.
- Record the load in the batch record, with the bulk density used and the percentage of nominal.
If defects such as twinning, picking or edge erosion appear later, the load is one of the first things to review. See Tablet Coating Defects: Causes and Fixes for the full list of causes with solutions.
Common Mistakes
- Using the same batch weight for every product in the same pan (Not calculating bulk density for each product)
- Taking the density of the granules or the blend instead of the finished tablets for calculating the pan load.
- Measuring the density once and using it for years, even after a change of tooling or formula.
- Setting the gun distance with an empty or half-full pan.
- Pushing a batch just over the maximum to avoid splitting it.
Frequently asked questions
How full should a coating pan be?
It depends on the machine and the product. Use the minimum and maximum working load in your qualified documentation, and convert it to a weight for your tablet with its bulk density.
Why does the load change from one product to another?
Tablet size, shape and weight change how tablets pack in the pan, so the same pan volume holds a different weight of each product.
What happens if I overload the pan?
The bed turns over slowly and mixes poorly, so coating is uneven and tablets are more likely to stick or twin. The guns can also end up too close to the bed resulting in tablet sticking.
Does pan loading affect coating reconciliation?
Indirectly. Poor loading increases chipped and broken tablets, which lowers the count of good tablets at the end. See Coating Reconciliation.