The tablet coating process is one of the most technical & critical processes which is performed in pharmaceutical industries after tablet compression depending on the specifications of the tablets whether it is film coating, sugar coating, enteric coating, or drug coating.
The coating is art so most pharmacists are not aware of its technicalities & even the operators are working on old trendy myths regarding Tablet bed temperature, and coating suspension spray rate & usually, there is no concept of the precise calculation of coating process time.
Here we will discuss & learn the following,
 How We Can Calculate Spray Rate?
 How Can We Validate Coating spray guns?
 How to calculate the Tablet coating process Time?
Importance Of Spray Rate & Gun Validation
If we are aware of the spray rate of a specific coating suspension we can easily Validate spray guns at various RPMs of the peristaltic pump.
Once we get the validation results of spray guns we can easily calculate the number of hours required to finish a specific volume of coating suspension so we can easily define the total number of hours for completion of a specific batch based on spray rate & validation calculations.
By defining the processing time we can easily plan our production in weekly or monthly production plans with great confidence.
How To Calculate Spray Rate?
To calculate the spray rate of the coating guns we must have following,
 Coating Pan
 Spray Guns
 Silicon Tubes
 Compressed air pipe
 Peristaltic pump
 Weighing Balance
 Polythene bags
 Tie Clips or Rubber band
Procedure
 First of all, install all the spray guns on the coating gun rod & connect all the guns with silicone tubing & compressed air pipe, connect the coating suspension pipe with the guns & place other end to the coating suspension container after passing through the peristaltic pump.
 Wrap a polythene bag around each gun with the help of a rubber band or tie clip & tare weight of each polythene bag must be known.
 Open the door of the coating pan & insert the gun rod inside with all the arrangements as we discussed previously.
 The spray rate Calculations mainly depend on the number of spray guns & speed of the peristaltic pump.
 Suppose we have 4 spray guns & peristaltic pump RPMs are 20.
 When the system is turned on & the spray process is started, the suspension from guns is collected in each polythene bag which was previously wrapped around each gun.
 After one minute stop the spraying process, remove each polythene bag & weigh it on the weighing Balance to determine the quantity of collected suspension.
 Suppose at 20 RPM following quantities are obtained from each gun,

 Coating Gun A = 50.2 gm
 Coating Gun B = 49.8 gm
 Coating Gun C = 50.4 gm
 Coating Gun D = 49.6 gm

To calculate the average spray rate use the following formula
A+B+C+D/4 = 200/4 = 50 gm/Min
So our average spray rate in one minute for 1 guns at 20 rmp of the peristaltic pump is 50 gm/Min.
Note
For spray rate Calculations of a specific coating suspension, the test is performed with the same coating suspension & if we just want to test it with a blank solution we can use only water.
Spray Gun Validation
In Spray Gun Validation we will learn to validate our spray rate at different RPMs of the peristaltic pump.
In previous portion we calculate the spray rate at 20 rpm which an average was 50 gm/min/ gun, suppose we now want to calculate the spray rate at 30 rpm so the calculation will be done by using the unit rule as,
At 20 rpm = 50 gm/min/gun
At 1 rpm = 50/20
At 30 rpm = 50/20 × 30 = 75 gm/min/gun
In the same way, we can validate our spray guns at different rpm of the peristaltic pump e.g at 50 rpm it will be 125 gm/min/gun
How To Calculate Tablet Coating Process Time
Our tablet coating process timing must be precisely calculated to ensure proper capacity utilization of coating machine & area.
Now the question is how can we calculate tablet coating spraying time?
The answer is very simple, we will learn to calculate the processing time of a specific coating suspension by using the abovementioned calculations.
Calculation
 Suppose we have 36 liters of a aqueous based coating suspension for a specific product & we want to calculate the time required to finish the spraying of 36 liters of suspension.
 We have 4 spray guns and at 30 rpm we have 75 gm/min/ gun spraying of coating suspension.First of all we will calculate the quantity of suspension sprayed in 1 hour at 30 rpm.
 As 75 gm is in one minute per gun so for 4 gun we have 75×4 = 300 ml/min
 Now simply multiply 300 by 60 to get the quantity in one hour.
300 gm/min × 60 = 18000 gm
Convert grams into kilograms as
18000/1000 = 18 kilogram
As a general rule for waterbased preparations, 1 kg is equivalent to 1 liter so we can calculate the Total time required as follow,
36/18 = 2 hours
Note
For organic solvents where 1 kg is not equivalent to 1 liter, the factor value is used to make adjustments.
Results
At 30 rpm of the peristaltic pump, the 36 liters of suspension will be finished in 2.0 Hours.
If we increase the peristaltic pump to 60 rmp by keeping in view that all other coating parameters are stable like tablet bed temperature, guntobed distance & physical appearance of the tablets, the same suspension will be finished in 1.0 hour.
Conclusion
The spray rate & coating gun validation calculations are very important & helpful to calculate the precise consumption of coating suspension over time & by using these calculations we can easily calculate the total time required to finish a specific coating suspension Volume.
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