What will influence tablet film coating III
Writer： admin Time：2020-04-06 17:52 Browse：℃
1.How does film coating machine influence the coating quality
1)Factors influencing mist generation and forming
Some factors include spraying gun design, atomizing pressure generated by liquid/gas pressure, flowing rate, spraying diameter and shape, parameters of peristaltic pump, roller pressure and physical features of coating liquid. Spraying gun design influences atomizing distribution and uniformity. The second factor above affect flowing rate and volume of coating liquid through spraying nozzle. And the flowing rate will further have effect on coating liquid particle size. Any solution, driven by external momentum, can overcome surface tension and be distributed so that new surface and mist particles are formed. The following figure illustrates mist particle size range:
Figure 3
Coating liquid is pressurized and atomized through a single hole. Its growing process is shown in figure 4. When liquid flows through tube, the droplet at the tube end will increase its weight and overcome surface tension. When the flow in tube increases, liquid cylinder is formed. This status is called smooth flow. The radial acceleration lead to vertical vibration and split the cylinder. The length of liquid cylinder changes as liquid flowing rate increases to a certain degree. At that time, vertical vibration will shorten liquid cylinder length. Such an unstable status is migration flow. Afterwards, pressure and flowing rate both increase, the end of cylinder looks like wave. If the flowing rte increases continuously, spraying flow occurs. In practical application, surrounding air will generate relative speed between liquid and air. Marks “je” can be used to indicate atomizing degree.
je=va2*ρl*de/σ*{ρa/ρl}0.55
In the formula:
va: relative speed between liquid and air, m/s
ρa: air density, kg/m3
de: spraying nozzle diameter, mm
ρl: liquid density, kg/m3
σ: liquid surface tension, N/m
|
je<0.1
|
dropping
|
|
je≈0.1~10
|
Smooth flow
|
|
je≈10~400
|
Wavy flow
|
|
je>400
|
Spraying flow
|
For better mist, proper spraying gun and spraying pressure should be selected to suit coating liquid and spraying speed and volume of peristaltic pump adjusted to match tablet and drying parameters. Besides, if temperature of coating liquid is too low, the peristaltic pump should be stopped.
Factors influencing mist movement
Those factors are atomizing pressure, physical features of coating liquid, position and distance of spraying gun against tablet container and position of spraying gun in coating roller (generally toward the lower roller or internal wall). During mist movement, due to different viscosity and distance, aggregation and evaporation can affect mist particle size. In practical production, the position and spraying direction of spraying gun can be adjusted through corresponding mechanism of external electrical system. Therefore, aggregation and size of mist particles can be changed during their movement.
Factors influencing wetting and drying
In this process, mist contacts with and wets tablets, then water evaporates and film forms. After the first and second step of mist forming and moving, roller rotating speed adjustment can make sure that tablets turn effectively and will not be thrown out or broken. Appropriate air intake volume can guarantee air circulation during drying and drain timely. And generally, air exhaust volume is larger than its intake volume to form negative pressure in roller.
During production, atomization of coating liquid determine appearance, while it is also determined by atomizing pressure and atomizing system. When atomization starts, the control principle of atomizing speed and hot air temperature is: Wetting tablets slightly and avoiding tablet adhesion. The temperature cannot be too high or too low. For the former condition, drying is too quick and film is rough and uneven; while for the latter, humidity in roller will increase and tablet stick to each other. In addition, roller speed influences coating. Low speed will enhance film adhesion but high speed will not. During coating, tablets are easy to stick if drying temperature is too low, atomizing volume is too large or tablets don’t turn smoothly.
2.Conclusion
All in all, operators should be clear of well-knows about coating liquid, physical properties of tablets, physical parameters of coating environment and parameters of coating machine. Depending on these knowledge, they can optimize coating conditions and set operation parameters better.
