Why Measuring Treatment in kW is Dangerous
A common misconception is that the output number on your corona treater power supply is equal to the amount of treatment you are applying to your film. This is not the case. The output number represents the amount of power you are instructing the power supply to provide your system’s electrodes.
The actual amount of surface treatment applied to the film is measured in Watt Density and takes into consideration the following:
- Power Supply Setting
- Electrode width
- Line speed
- Number of sides being treated
(Note that the calculation calls for the electrode width, not the width of the film you are treating. If you are using a metal electrode which enables segmented treating you should calculate the combined width of active segments.)
The formula to calculate watt density is:
Power Supply Output (watts)
Electrode Width (ft) * Line Speed(ft/min) * Number of Treat Sides
So, why is this a more accurate way to calculate treatment? Consider a line that normally runs at 300 ft minute, has a fixed electrode width of 3.5 ft treating one side of the film, and the power supply is set to 2kw. Watt density is calculated as follows:
2 kw Power Supply Output (watts)
Electrode Width 3.5 ft * Line Speed 300ft/min * 1 Treated Side
The watt density is 1.9 watts/ft2 per minute.
Changes in line speed
If the line speed is increased to 400 feet per minute and the power supply output is left at 2kW the amount of applied treatment reduces to 1.43 watt/ft which may not be enough power to reach the desired dyne levels.
Changes in treatment width
Or consider the success you’ve had on your current 3.5 ft wide line treating your film. You invest in a new 5 ft line and presume if 2kW were sufficient on the 3.5ft line it will be appropriate on your new line. In this case a 2kW output will only provide a watt density of 1.33 watt/ft2 per minute.
Calculating Watt Density
There are two ways to calculate and run your lines with consistent watt density. The first is to educate your operators on the watt density calculation and have them manually adjust the power supply output when running different line speeds.
If this is impractical or your line speed ramps up and down over a course of a run then you may want to take advantage of the Watt Density and Proportional Speed Controllers included with Enercon’s Compak™ Pro Power Supplies.
