A tablet coating defect case study is most useful when it traces the defect back to the interaction between core properties, coating formulation, equipment settings, and drying conditions. In commercial tablet production, visible defects are rarely caused by one variable alone. A film that peels may begin with a weak tablet surface, but inappropriate spray rate or insufficient polymer adhesion can turn that weakness into a batch-level rejection.
For formulators and procurement teams, the practical lesson is clear: coating performance depends on consistent inputs as much as it depends on operating discipline. The following case examines a common aqueous film-coating failure and the corrective approach used to restore an acceptable finish without sacrificing throughput or tablet quality.
Tablet Coating Defect Case Study: Peeling and Picking
A pharmaceutical manufacturer was coating immediate-release tablets using an aqueous, cellulose ether-based film-coating system. The target was a uniform, opaque colored film that improved appearance, supported product identification, and protected the tablet from handling abrasion. Initial pilot batches performed acceptably. During scale-up, however, the production team observed picking, localized peeling, and occasional rough areas on tablet faces and edges.
The defects appeared after the first portion of the coating suspension had been applied. Tablets began sticking intermittently to the pan surface and to one another. When separated by the motion of the coating pan, small sections of the film and, in some cases, portions of the tablet surface were removed. The result was an unacceptable visual appearance, increased rejects, and a risk of weight variation if the process continued.
The first reaction was to increase inlet air temperature and accelerate drying. That reduced visible sticking for a short period, but it introduced a second problem: the film became less uniform, and edge cracking increased after tablets cooled. The issue was not simply that the batch was too wet. It was that wetting, drying, core strength, and coating formulation were out of balance.
Defining the Failure Mechanism
Picking occurs when a coating surface remains tacky long enough for tablets to adhere to equipment or to each other. Peeling can follow when the dried film lacks sufficient adhesion to the tablet core. These failures may be related, but they do not always have the same root cause.
In this case, the investigation began with four questions: Was the tablet core sufficiently hard and low in friability? Was the coating suspension prepared to the intended solids level and viscosity? Was atomization producing droplets that could dry appropriately before tablet-to-tablet contact? And was the pan receiving enough drying air without causing overly rapid film formation?
The team found that core tablets met the general hardness specification, but friability was near the upper internal limit. Fine surface particles were also present because the dedusting step was not operating consistently. This created a weak boundary layer between the core and the film. A coating can adhere well to a clean, cohesive tablet surface, yet fail when the surface contains loosely bound fines.
Testing of the coating suspension identified a second concern. The actual viscosity was higher than the validated target range. The polymer solution had been mixed under conditions that allowed partial agglomeration, and the final dispersion was held longer than planned before use. Higher viscosity changed spray behavior at the selected gun settings, creating larger droplets and a wetter tablet bed.
Finally, process records showed that exhaust air conditions varied significantly between shifts. High humidity in the coating room reduced the system’s drying capacity. Operators responded by changing inlet air temperature, but atomization air pressure and spray rate were not adjusted in a coordinated manner.
Corrective Action: Start With the Core and Coating Suspension
The manufacturer first tightened control of core tablet preparation. The dedusting unit was inspected and recalibrated, and tablets were screened before coating to remove loose fines. Compression settings were reviewed to improve surface integrity while maintaining required disintegration behavior. This is an essential trade-off: increasing tablet hardness can improve coating resistance, but excessive compaction may slow disintegration or alter dissolution. The correct target depends on the dosage form and finished-product specification.
The coating formulation was then reassessed. The film-forming polymer grade had to provide adequate adhesion, controlled viscosity, and acceptable film flexibility at the working solids level. Hydroxypropyl methylcellulose and other pharmaceutical cellulose ethers are widely used for aqueous film coating because their substitution profile, particle properties, and viscosity grade can be tailored to the process and desired film characteristics.
However, selecting a higher-viscosity grade is not automatically a solution for stronger film formation. Excessive viscosity can impair atomization and increase the probability of overwetting. A lower-viscosity grade may spray more easily, but the coating solids level, pigment load, plasticizer level, and final film strength must still be considered together.
For this batch, the manufacturer established a narrower incoming-material control plan. The coating polymer was checked against agreed viscosity and moisture specifications, while the finished suspension was tested for viscosity, appearance, and absence of undispersed particles before transfer to the coating unit. Standardizing hydration time and mixing sequence prevented the batch-to-batch variation that had contributed to the original problem.
Process Changes That Restored Film Quality
Once the core and suspension were controlled, the process team adjusted the coating parameters as a connected system rather than treating each setting independently. The spray rate was reduced during the initial application phase, when cores are most vulnerable to overwetting. Atomization was optimized to produce a finer, more consistent droplet pattern. Inlet air temperature and airflow were set to maintain a stable product-bed temperature rather than pushing for the highest possible drying rate.
This distinction matters. Excessive drying can cause spray-drying before droplets coalesce on the tablet surface. That may lead to roughness, poor color development, and weak intercoat bonding. Insufficient drying leaves the tablet bed tacky and encourages picking. The acceptable operating window is determined by the equipment design, pan load, tablet geometry, coating solids, and room conditions.
The team also introduced a controlled preheating step and verified spray-gun alignment before each run. Poor gun-to-bed distance or uneven spray overlap can create localized wet zones even when average process values appear correct. Nozzle wear and partial blockage were included in routine checks because they can change droplet size without an obvious alarm.
After these changes, the manufacturer completed three confirmation batches. The coated tablets showed uniform color, acceptable surface smoothness, and no visible picking or peeling. Weight gain remained within target, and dissolution testing confirmed that the modified coating process did not negatively affect product release.
What Procurement Teams Should Require From Coating Polymer Suppliers
A coating defect investigation often exposes a supply-chain issue as well as a process issue. For pharmaceutical applications, buyers should not evaluate a cellulose ether only by nominal viscosity. They should request a consistent technical package that addresses identity, viscosity range, moisture, particle characteristics, microbial controls where applicable, and documentation aligned with the intended regulatory market.
Consistency between lots is particularly valuable during scale-up and transfer between coating lines. Even small shifts in polymer behavior can require changes to spray settings, solution preparation time, or solids content. A supplier with controlled production systems, application knowledge, and reliable export support can help manufacturers reduce these adjustment cycles.
Kima Chemical supplies pharmaceutical cellulose ether grades for customers that need controlled film-forming, binding, and viscosity-modifying performance. Requirement discussions should cover the tablet core, coating formula, process equipment, target solids level, and commercial volume, not only the requested product name.
Preventing Repeat Defects
The most effective preventive action is a process-control plan that connects raw-material release to finished-tablet inspection. Core friability, coating-suspension viscosity, product-bed temperature, exhaust conditions, spray rate, and atomization should be trended by batch. Visual inspection remains valuable, but it should confirm process control rather than serve as the first warning of a failure.
When a defect appears, changing one parameter aggressively can conceal the actual cause and create a different defect. A controlled investigation, supported by representative samples and documented operating data, provides a faster path to a stable process. The best coating line is not the one run at the highest temperature or spray rate. It is the one operating consistently within a validated window, using a polymer system matched to the tablet and the equipment.