The Critical Parameters For Bead Coating - Gun To Bed Distance
Source : Yenchen Technical Process Consultant, Dr. Fred Rowley
- Usually stated as a specification on the worksheet.
- Most common settings are 8" or 10" from the bed, depending on the spray rate used.
- Always fixed, never use a range.
If you notice that your worksheet has a range for gun to bed distance, you may want to eliminate this setting. Obviously if one operator uses one setting on Monday and another uses a second different setting on Tuesday the size of the micro particles hitting the tablets will vary from day to day. They may be too dry if the distance is too great or too wet if the distance is too short.
Increasing gun-to-bed distance decreases drop velocity and increases drop size for twin-fluid atomizer produced sprays. This most probably caused by evaporation of small drops and/or droplet coalescence.
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| Too close | Correct |
Gun to bed distances may differ according to the liquid used as the base. Water demands a longer distance than a quickly evaporating solvent.
Gun to bed distance influences how wet or dry the tablet bed becomes. If the guns are placed too close the tablet bed will be overly wet causing both picking and twinning. If the guns are too far away the spray pattern will be too dry causing dry particles that will result in a rough coating or even tablet erosion due to insufficient depositing. In some cases the spray will dry right after it leaves the gun, pass through the entire bed or tablets and travel to the dust collector.
A decrease in gun-to-bed distance decreases film surface roughness. This effect is observed regardless of other operational parameters, spray gun design, and formulation properties.
If you have any technical questions, please contact with Grace Chiang Email:grace.chiang@yenchen.com.tw
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