This article provide you the information about coating tablet process step by step complete with the parameters influenced and equipment for coating tablet.

The coating process is usually a batch driven task consisting of the following phases:
  • Batch identification and Recipe selection (film or sugar coating)
  • Loading/Dispensing (accurate dosing of all required raw materials)
  • Warming/Preheating
  • Spraying (application and rolling are carried out simultaneously)
  • Drying
  • Cooling
  • Unloading
 Modern Machine Coating Tablet

Tablet coating equipment may include :
  • spray guns
  • coating pan
  • polishing pan
  • solution tanks
  • blenders and mixers
  • homogenizers
  • mills
  • peristaltic pumps
  • fans
  • steam jackets                                                                                           Spray Gun in Coating Pan
  • exhaust
  • heating pipes
  • scales
  • filters


 Coating Pan

1. Tablet coating takes place in a controlled atmosphere inside a perforated rotating drum. Once you load a batch of tablets into the coating pan, you need to preheat the tablets and allow time for dust and tablet flash to exit the pan. Angled baffles fitted into the drum and air flow inside the drum provide means of mixing the tablet bed. As a result, the tablets are lifted and turned from the sides into the center of the drum, exposing each tablet surface to an even amount of deposited/sprayed coating. Once the temperature of the outlet air reaches 42° to 46°C, usually within 15 minutes, spraying can begin.

2. The spray guns create a fine mist of coating solution that dries just after it contacts the tablet. The liquid spray coating dried onto the tablets by heated air drawn through the tablet bed from an inlet fan. The air flow is regulated for temperature and volume to provide controlled drying and extracting rates, and at the same time, maintaining the drum pressure slightly negative relative to the room in order to provide a completely isolated process atmosphere for the operator.

3. As the water evaporates, it leaves the solids behind to form a thin film on the tablet. The key to tablet coating is to get the surface slightly wet and immediately dry. Remember: apply the coating in many short, fast exposures, not in long, slow exposures.

4. Once the base coating is applied, you can increase the rate of solution addition and the pan speed proportionately. Typically, it takes about 20 minutes before increasing the spray rate and pan speed significantly.

Soft tablets and tablets that are very porous may require an initial spray rate that is slower than the average of 100 milliliters per minute per gun. Be sure to monitor spraying to see whether the spray pattern changes. If it does, there is likely a buildup of solids on the gun tips. You can correct this only by cleaning the tips, which means stopping the spray and the pan.

5. The film coating solution dries on the tablet surface because there is a constant supply of hot air entering the drum and passing through the drum’s perforations into the bed of tablets. Over time, the film builds layer after layer of solids.

How long it takes to form the final film varies from dozens of minutes to a few hours. It depends on
  • tablet quality : tough enough to tumble as they’re coated and dried, consistent in porosity and hardness, free of dust. You can’t coat a bad or marginal tablet and expect a good tablet when you’re done. Tablets must not break apart during the preheat cycle at the start of the coating process or during the first few minutes of exposure to the atomized solution. Consistent hardness of the tablet surface enables the coating to “lock” into the surface. If the surface is too soft, the impingement of the solution can erode the tablet. Too hard a surface will not allow the solution to impinge and adhere, and the coating will peel away. Both of these coating defects can also occur by over- or under-applying the coating solution or by applying the coating with too much or too little force. A combination of these factors could also be at work.
  • the coating solution type (solvent-based coatings dry faster)
  • the percentage of solids in the coating
  • the rate of coating addition
  • the air volume
  • air temperature
  • the air pressure within the coating cabinet.

6. After you’ve finished applying the solution and drying it, the tablets must cool.

For coatings to adhere properly, the tablets must remain at a specific temperature, the solution must be applied at a consistent rate, and the motion of the tablets must be active yet tranquil. Disrupt any of these conditions, and you will produce a defective tablet.