You've run the numbers. The coating suspension is prepared to the right solids content, the batch quantity is correct, and the theoretical weight gain per tablet is sitting right there in the calculation. Then the batch finishes, you weigh a sample of coated tablets, and the actual weight gain doesn't match what the math said it should be — sometimes higher, sometimes lower, occasionally inconsistent tablet to tablet. The calculation wasn't wrong. Something in the process between the calculation and the finished tablet was.
This is one of the more common points of frustration in the coating process, because the gap between "theoretical" and "actual" weight gain can come from several different places, and they don't all look the same when you're staring at an out-of-range result.
Start by confirming it's not the calculation itself
Before troubleshooting the process, it's worth re-checking the inputs to the calculation, since a wrong number here produces a result that looks exactly like a process problem.
- Core tablet weight — was the actual average core weight used, or a target/spec weight that's slightly off from what that batch is really running at?
- Solid content % — does this include pigments and plasticizers, or only the base polymer? Leaving out non-polymer solids understates the suspension's true solids content and throws off the suspension volume calculation.
- Batch quantity — a stale or rounded batch count here scales the whole calculation proportionally.
If those all check out, the gap is coming from the actual coating process.
The most common real-world causes
Spray losses (overspray). Not all of the suspension sprayed into the pan ends up on the tablets. Some is lost to atomization overspray that never reaches the bed, some evaporates before landing, and some deposits on the pan walls and baffles instead of the tablets. This is usually the single biggest reason actual weight gain runs lower than calculated, and it tends to get worse with higher spray rates, wider spray patterns, or a spray gun positioned too far from the tablet bed.
Pan wall and baffle buildup. Related to overspray, but worth calling out separately — coating material that sticks to the pan interior instead of the tablets is coating material you paid to make but never actually applied. On longer runs, this buildup can be substantial enough to visibly account for several percentage points of "missing" weight gain.
Drying rate mismatched to spray rate. If the inlet air temperature or airflow can't keep pace with the spray rate, tablets stay wetter than intended, which can cause sticking, picking, or twinning. Any tablets lost to picking/sticking defects are usually removed during inspection, which changes your sample's average weight gain even though the suspension applied was correct.
Uneven distribution across the batch. In a poorly performing pan (uneven baffle design, incorrect pan speed, or an undersized bed for the pan), some tablets pass through the spray zone far more often than others. The batch average might land close to target while individual tablets vary widely — which is why weight gain should always be checked against a proper sample size, not a handful of tablets grabbed at random.
Sampling and weighing technique. A surprisingly common source of apparent discrepancy: comparing coated tablet weight against a "target" core weight instead of the actual measured average core weight for that specific batch. Natural core-to-core weight variation means this comparison can look like a coating problem when it's really just normal core variability.
Suspension settling or inconsistent stirring. If the coating suspension isn't kept under continuous, adequate agitation, solids can settle out over the course of a long run, meaning the suspension sprayed at the end of the batch has a different actual solids content than what was sprayed at the start — even though the batch was prepared correctly.
A practical way to narrow it down
When the actual weight gain doesn't match the calculated value, it helps to work through these in order rather than guessing.
- Re-verify the calculation inputs (core weight, solids %, batch quantity) against the actual batch, not the spec sheet.
- Check whether the gap is consistent or highly variable across the sampled tablets. Consistent-but-off points toward a systematic cause (spray losses, wall buildup, suspension prep). Highly variable points toward pan performance or distribution.
- Look at process records for the run — spray rate, inlet/exhaust temperatures, pan speed, and total spray time versus what was planned. A mismatch between planned and actual process parameters is often the answer.
- Inspect the pan itself after the run for visible buildup on walls, baffles, and the spray gun area — this alone often explains a several-percent shortfall.
- Only after ruling out the above, consider whether the suspension itself was prepared or maintained correctly (solids content, agitation, settling).
Why this is worth tracking over time, not just per-batch
A single batch running slightly under target weight gain isn't necessarily a crisis, but if a particular pan, product, or operator consistently shows the same pattern batch after batch, that's a signal worth investigating properly rather than compensating for by simply spraying longer each time. Adjusting spray time to hit a weight gain target without understanding why the shortfall exists tends to mask the real issue rather than fix it, and it can quietly drift a validated process outside the parameters it was actually validated at.
If you're calculating coating material requirements regularly, the Coating Material calculator on PharmainformCalc handles the dry coating, suspension weight, and vehicle calculations automatically — useful as the starting reference point before comparing against what a batch actually achieves.