Following are the critical parameters in Coating operation
- Gun Geometry & calibration
- Atomizing/Pattern Air
- Pan Pressure
- Pan Speed
- Spray Rate
- Inlet/Outlet Air temperature
- Drying Capacity
- Conventional gun-Adhesion of Coating material
Gun Geometry and Calibration
Gun geometry and Calibration refers to the position of spray guns which includes ‘Gun Calibration’, ‘Gun to bed distance’, ‘Gun to gun distance’, ‘Gun to side of pan’, ‘choked guns’ and ‘anti bearing technology’.
Gun to bed distance:
- The most frequently utilized settings for aqueous coating are either 8 inches or 10 inches from the bed, contingent upon the spray rate employed.
- As the distance from the gun to the bed increases, the spread also increases. This distance is crucial when considering the drying rate and the spray patterns designed to adequately cover the tablet bed.
- The distance from the gun to the bed may vary based on the liquid used as the base. Water requires a greater distance compared to a rapidly evaporating solvent.
Gun to Bed Height:
- Tablet beds vary based on pan charge, dimensions, and configuration of the tablet.
- Firearms should be positioned at the base of the waterfall within the upper third of the bed.
- It is advisable to verify at the start of each campaign or whenever there is a change in pan charge.
Gun to Gun distance:
- Typically not mentioned as a specification or defined point on a worksheet.
- Setting is 5.5″ or 6.0″ from gun tip to gun tip between guns.
- When utilizing a single gun, position the gun at the center of the boom.
Gun to Side of Pan:
- The concept of setting is generally acknowledged; however, it is not often considered as a variable that can be evaluated.
- The gun ought to be positioned at the center of the pan.
Chocked guns:
- Firearms may become cocked due to movement in and out of the pan
- Mechanical adjustments
- Improper handling by the operator.
- Firearms can also become cocked as a result of loose fittings.
- Cocked side to side: in wet conditions.
- Cocked up or down: when the solution adheres to the pan.
- To verify if a firearm is cocked, observe directly down the boom.
Gun Calibration:
- Disparities in solution rates among guns are typical.
- A variation of no more than +/- 10 ml. between guns is permissible.
- In the event that the variation surpasses the advised limit, modify the needle stroke from the rear of the gun.
Gun-Anti-bearding Technology:
The recently introduced Schlick anti-bearding technology provides optimal operational security.
The Schlick ABC spray boasts an exceptionally uniform suspension distribution, characterized by a simultaneously very fine, consistent, and reproducible droplet size distribution.
- Anti-bearding cap (ABC) without „horns“
- No bearding, only fine coating
- Free air ducts
- Optimal spraying pattern
- A comparison of the flow patterns reveals that the dust particle flow (simulated here in the form of fog) stays away
Atomizing & Pattern Air
Atomizing Air
- Used to break the suspension stream into droplets
- Converts a “stream” of suspension into a “mist”
- The more the atomizing air, the smaller the droplets
- Too little atomizing air will lead to concentrated stream that will result in tablet picking and sticking very early in coating process and suspension is deposited on the surface of the pan
- Too much of Atomizing air and the suspension will evaporate and dry before reaching the tablets
Pattern Air
- Used to focus the flow pattern
- Used to form the suspension cone
- Pattern Air ensures that the air flow is Spread across the tablets
- Too much pattern air flattens the cone & may create an over spray condition, resulting in over wetting which causes “Picking”
- Too little pattern air concentrates the cone in an insufficient area of the tablet bed, resulting in tablet erosion or “picking”
- In some systems the pattern air is variable and is set by the operator while in some it is fixed
Coating Pan
- The pan is equipped with a perforated drum.
- There are baffles designed to facilitate this stirring action.
- Hot drying air enters the drum through the perforations located on the side opposite the tumbling tablet bed, and is drawn through the bed by a fan situated in the exhaust plenum.
- This plenum features an opening that closely conforms to the outer surface of the perforated curved drum.
- Typically, they share the same shape but vary in size for each model, and can be easily removed or substituted with baffles of different designs, depending on the physical properties of the tablet being coated.
- The operation of the SPRAY GUN unit is summarized as follows: Compressed air is directed to the air inlet manifold and subsequently distributed to three distinct internal supplies. One of these supplies passes through an air-pressure regulator to the liquid pump, with the pressure of this air indicated on the pump air-pressure gauge. This pressure regulates the liquid pressure.
- A second air supply goes through a regulator that manages the atomizing air-pressure gauge, and the air is then supplied directly to the atomizing air connection on the spray bar within the coating pan.
Anti-Slide Bars:
- Used to center the tablet bed in front of the outlet air plenum.
- Not a mixing device.
Baffles:
- Mixing device.
- More than one kind
Coating Pan Variables
Pan design/ baffling
– Free movement/ chipping / breakage
Speed
– Too slow leads to wetting/ sticking, too fast leads to improper drying
Pan load
– Excess leads to poor movement and hence poor coating, — Deficit leads to breakage/ chipping due to baffles
Process air
Air quality
Temperature
Air flow rate/ volume
Pan Pressure
- Negative pressure inside the pan
- Pan pressure is reduced when the seals in the pan weaken / break.
- Insufficient pan pressure may result in suspension drying on the gun boom, pan and the front door.
Pan Speed
There is no definitive setting. This is a relative configuration that depends on the size, shape, and load of the tablet.
Pan speed is defined as a range rather than a fixed value.
There are two fundamental pan speeds for each product:
– the initial speed required to attain a basic coating
– the steady state speed.
A slow pan speed may result in picking and cause tablets to adhere to one another.
Conversely, a faster pan speed can lead to rough coatings and physical damage to the tablets.
Spray Rate
Unit of measurement is grams/minute/gun or ml/minute/gun
Range specified in the Batch Record
Spray variables
1. Spray rate
2. Atomization
3. Spray pattern
4. Nozzle to bed distance
Temperature
Inlet air temperature is set by operator,
Outlet air temperature is a function of:
air flow
bed volume
spray rate,
bed temperature.
Broadly Outlet Temp can affected by Inlet air CFM, Inlet air Temperature, Spray Rate and Atomizing air
Drying Capacity
Drying capacity is a function of temperature, Quantity and initial humidity of incoming air
It becomes a critical parameter for products sensitive to heat or moisture
CFM is a factor in coating efficiency
CFM = Area (sq. ft.) X Velocity (ft. min.)
Conventional gun -Adhesion of Coating material
Adhesion of coating material happens due to high atomization, high pan speeds and air flow balance
Anti-bearding Nozzle -Schlick Pro ABC nozzle
- Anti-bearding cap (ABC) without “horns“
- No bearding, only fine coating
- Free air ducts
- Optimal spraying pattern
- An exceedingly homogenous spraying pattern
- Simple construction consisting of 7 individual parts O rings
- A pneumatically controlled cleaning needle
A comparison of the flow patterns reveals that the dust particle flow (simulated here in the form of fog) stays away