We are facing problem in Metallized in edge to edge. As soon as metal start deposit on drum edge, it's giving heavy sparking near edge which is resulting bunch of holes in the film edges. We are applying slight graphite paint ( Release agent) on film edge to remove deposited aluminium after completion of cycle. Even if do not apply release agent then also we are facing the same problem.
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ANSWER
Most commonly electricity is the cause of sparking. The source of the electricity can vary. One possible source of sparking is if the system is not properly earthed and differential charges build up. The metallized film is, in effect, a large capacitor. The source of charge can be from triboelectric charging of the polymer web running over metal rolls. The amount of charge is dependent upon the polymer conductivity, moisture content, polymer type, roller type and speed.
One method of removing any triboelectric charge is to plasma treat the polymer. The plasma will spread through the chamber and provides a mechanism for neutralising the polymer surface. If the polymer is not plasma cleaned this may allow high surface charges to build up and result in t he discharge at the edges.
Another possibility is if the system is not well earthed and there is a plasma treater included. The plasma will require a cathode and anode. Generally the system is at earth potential and is used as the anode. If the system is not well earthed it can be that various components within the vacuum system end up at some floating potential. Where there is a potential difference this can also lead to sparking.
The use of the graphite paint may also contribute to the problem. If the carrier for the graphite is not completely dried there may be outgassing causing localised high pressure regions that would enable sparking to occur at lower voltages.
If you wish to check out the triboelectric charging there are meters that will measure the voltage. Although they are not designed for use in vacuum I have used one in a vacuum to check the level of charging under real conditions rather than at atmosphere. Often the surface charge can rise to several thousand volts which is more than enough to create arcs (sparks).
Checking out the system earth requires measuring any potential difference between the system and a good reliable earth. If there is a difference it is possible that there could possibly be charges induced from the high currents used to power the resistance heated boats.
Hopefully this answer gives you enough ideas to uncover the source of the sparking.
When metallising thin films it is normal practice to edge mask the film edges to prevent build up of Aluminium on the drum surface. If you are not edge shielding the film and metallising edge to edge,a quantity of Aluminium will build up on the face of the drum next to the unshielded edge.During longer runs the Aluminium build up on the drum will become thick enough for it become in contact with the zone seal seperating the evaporation zone from the winding zone. When this happens sparks can occur which result in holes and edge tears on the film edges. The Aluminium build up on the drum can also be a cause of pinholes as it is wound into the reel. For higher barrier film it would therefore be desirable to mask the film edges.
Posted by: Andrew Mitchell | March 30, 2007 at 04:13 PM