Tablet coating defects are resulted due to mismatch between various tablet coating process parameters like how fast liquid goes onto the tablet bed, and how fast that liquid dries. If the coating suspension lands faster than the warm air can dry it, tablets turn sticky and stick together or to the pan. If it dries before it lands, the droplets cannot spread out, and the surface comes out rough.
This article covers the defects observed most often on a coating pan, with the common causes and solutions.
Check the basics before you change any settings
A lot of coating defects are blamed on spray settings when the real cause is somewhere else. Before touching the process, check three things.
- Tablet cores. Weak, friable or very dusty cores chip and erode in the pan, and cores that are too moist give poor film adhesion. Check hardness, friability and moisture against the specification before starting the spraying process.
- Coating suspension
Confirm that the solids content, the mixing time and the viscosity are as per BMR, and that the suspension is stirred gently all through the spraying. Settled pigment gives colour variation, and a suspension that is too thick gives a rough surface. - The equipment. Check the spray nozzles for partial blockage, the gun-to-bed distance and angle, the spray pattern and the tubing for leaks.Also check the heads of the peristaltic pump are working fine.One worn nozzle can ruin a batch.
Coating Material Calculator, which gives dry coating, suspension and vehicle for a target weight gain
Coating Reconciliation, which compares core with coated tablets on a count basis.
Picking and sticking
Picking is when small pieces of film are pulled off a tablet, usually leaving a bare spot on tablet and core is visible, because tablets stuck to each other or to the pan wall and then separated. It is almost always a sign of over-wetting or under heating.
Reasons
Following are common reasons
- Too High Spray rate.
- Low inlet air temperature
- Slow pan speed
- Less gun to bed distance
- Low atomization pressure
- Less Airflow volume
- Poor exhaust opening
Adjustments
- Reduce the spray rate
- Increase inlet air temperature
- Increase pan speed
- Increase airflow volume
- Increase atomization pressure
- Increase gun to bed distance
- Increase exhaust opening
Twinning
Twinning is when two tablets stick together by their flat faces and dry in that position. It is most common with flat, oval or capsule-shaped tablets, which tend to lie against each other.
Reasons
Reasons are usually the same as for sticking and picking but it is related to the specific tablet shape as mentioned above.
Adjustments
As per my practical observation the increase in pan speed is the best possible solution to avoid tablet twinning during the tablet coating process.Remaining solutions are the same as for sticking and picking.
Check the load against the pan capacity (see Coating Pan Loading) and make sure the baffles are doing their job in turning the bed. A shape change is the last resort, because it means a product change.
Orange peel (rough surface)
An orange-peel surface looks like a peel of orange (dimpled and dull instead of smooth). It occurs when suspension droplets dry before they can spread and merge into a continuous film.
Reasons
- Too high inlet air temperature
- High tablet bed temperature
- Low spray rate
- High atomization pressure
- High gun to bed distance
- Too thick coating suspension (highly viscous)
Solution
The best possible solution is to reduce the inlet temperature along with reducing the gun to bed distance.Revise the suspension formulation of it is highly thick and reduce the atomization air pressure.
Logo bridging and infilling
When the coating fills in a debossed logo or score line, or bridges across it, the marking becomes unclear. This is a cosmetic defect, but it can be a quality complaint, and on scored tablets it affects splitting of tablet.
Reasons
Viscous suspension, an excess spray rate that wets the recesses, poor adhesion of the film in the logo area, and a shallow or narrow logo design.
Solution
- Reduce the viscosity within the specification, lower the spray rate and check the film plasticiser level.
- A recurring problem may need a change in tooling, such as a wider and deeper logo, which is a decision for development rather than for production.
Cracking, peeling and blistering
These defects show a poor bond between the film and the core, or stress inside the film. Cracking often appears at the edges and at the tablet crown, and blistering appears when trapped moisture or air pushes the film up.
Reasons
It occurs due to brittle film resulting from less plasticizer.A core that expands or loses moisture after coating is another contributor, drying that is too aggressive, a film that is too thick, and poor adhesion because of a dusty or very smooth core.
Solutions
Confirm the plasticiser level in the formulation, moderate the drying conditions and review the core moisture. Check the weight gain against the target as well, because an excess gain increases film stress. The Coating Material Calculator shows the theoretical gain and the dry coating needed, and the weight gain troubleshooting guide explains how to narrow down a gap between the calculation and the batch.
Colour variation and mottling
Uneven colour, either between tablets or across a single tablet, is common with pigmented coatings. It is a quality and appearance problem, and for some products a regulatory one.
Reasons
Pigment that has settled or not been dispersed well in suspension, an uneven spray pattern that leaves some tablets with less coating, a bed that does not mix well, and colour migration during drying.
Solutions
Check the suspension preparation and the stirring, run a spray pattern check, and improve the bed movement by correcting the pan speed and load. Applying a little more coating can hide a minor variation, but it is not a fix for poor mixing.
Edge erosion and chipping
When the edges of the tablets are worn away or the film chips at the edges, the cause is usually mechanical.
Reasons
Following are the reasons
- Weak core tablet
- High pan speed
- Long processing time
Solutions
Check the core hardness and friability, reduce the pan speed within the validated range and check that the total coating time is not longer than necessary. Edge erosion also shows up as lost tablets in the coating reconciliation, so a low yield together with fragments in the dust collector points here.
Quick reference table
| Defect | Most likely cause | First thing to try |
|---|---|---|
| Picking / sticking | Over-wetting | Lower spray rate, raise inlet temperature |
| Twinning | Over-wetting, low pan speed | Lower spray rate, raise pan speed |
| Orange peel | Droplets drying too early | Lower temperature, shorten gun distance |
| Logo bridging | High viscosity, wet recesses | Lower viscosity and spray rate |
| Cracking / peeling | Film stress, poor adhesion | Check plasticiser, core moisture, gain |
| Colour variation | Settled pigment, poor mixing | Check stirring, spray pattern, bed movement |
| Edge erosion | Weak cores, high pan speed | Check friability, reduce pan speed |
A simple order for troubleshooting
- Stop and look at the tablets. Describe the defect exactly, and note when in the process it started.
- Check the cores, the suspension and the equipment before changing a setting.
- Check the process readings against the validated ranges,inlet and exhaust temperature, airflow, spray rate, atomizing pressure and pan speed.
- Change one parameter at a time, by a small step, and give the bed time to respond before judging the result.
- Stay inside the validated ranges. If the fix needs a setting outside the range, raise a deviation or a change control, because the batch is then outside the validated process.
- Record every change and its effect in the batch record, so the next batch starts from what was learned.
Frequently asked questions
What is the most common cause of film coating defects?
An imbalance between the rate at which the coating is sprayed and the rate at which it dries. Too wet gives picking, sticking and twinning. Too dry gives a rough, orange-peel surface.
Can I fix a defective batch by recoating?
Only if the process and the approved procedures allow it. Recoating adds weight gain and can change the release profile, so it is normally a deviation with a quality assessment, not an operator decision.
Why does the same product coat well on one day and badly on another?
Ambient humidity, the inlet air condition, the suspension preparation and the core moisture all change from day to day. Record them with each batch, and the pattern usually shows up.
How do coating defects affect reconciliation?
Broken, chipped and rejected tablets reduce the count of good tablets. A coating yield below the limit with fragments in the dust collector often points to edge erosion.