Controlling A Fluidized Bed Level

Q. I am having problems finding out the parameters for my fluid bed powder. I’m using vibration to achieve better surface but haven’t really been successful yet. The main problem is to get straight surface in a tolerance of ±2 mm. I am air drying.

Perhaps I have already found out where my problem...I am using bad material as fluidizing plate. Fluidizing plate is a extremely fine porous polyethylene plate.

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Can you help? P. K.

A. After reading your question, I am struck by the complication of your approach to your particular application. As I understand it, you are trying to coat electrical connectors using a fluidized bed. The powder you are using is probably an epoxy. This method of powder coating electrical components to apply an insulation barrier is very common. However, your approach is very uncommon.

For instance, the tolerance you are trying to maintain for the fluidized bed level is ±2 mm (approximately ±0.08 inches). This is totally unrealistic. Between the small air bubbles escaping the surface of the powder in the bed and air currents within the bed caused by the fluidizing air, you would be lucky to control the surface of the bed to within ±13 mm (approximately 0.5 inches). I have no doubt that you must control the coating on your part within your stated tolerance. However, this is normally done by masking the part using a separate mask, using the part fixture to mask the part or removing the coating from the desired area using a precision vacuum apparatus.

As for your design, the fluidizing plate and vibrator are very appropriate for designing a fluidized bed. Designing the air chamber under the fluidize plate to ensure that you create an appropriate amount of backpressure is equally as important. This will guarantee the fluidizing air is equally distributed across the entire fluidizing plate—a must for even fluidization. The vibrator adds sonic waves through the bed, adding motion to further even out the fluidizing of the powder. If you have achieved an even simmering surface with your design, then you have succeeded in building a fluidizing bed to industry standards.

Finally, my recommendation to you is to forget about controlling the fluidized bed level to within ±2 mm and concentrate on masking your part or removing the excess coating using a precision vacuum to achieve your desired coating tolerance.