The symptoms that identify backside treatment are problems associated with web handling. The most common is blocking. The term blocking refers to the attraction of a treated surface adhering to the backside of the material when wound onto a roll. This becomes evident in unwinding, resulting in a separation or delamination of the web.
Backside treatment occurs when air is trapped between the backside of the web and the ground roll. The air beneath the web becomes ionized and corona treatment occurs. In some cases, backside treatment may not be of a level that is easily indentify.
When corona treatment occurs, the electrical energy discharged in the air gap in the form of corona will be distributed between the top and bottom of the web, wherever the conditions for corona exist. With a portion of the total energy applied to the undesired bottom surface, the amount of energy applied to the desired top side will be reduced. Generally, the treatment on the top surface of the material will decrease.
The primary causes of backside treatment can be attributed to a lack of station cleanliness, specifically a dirty ground roll. To conceptualize the cause, imagine the web contacting the surface of the ground roll. Any high point or buildup on the roll surface potentially lifts the material off the ground roll face. The immediate area surrounding the point is no longer contacting the roll face, resulting in a void or an air gap. The air gap beneath the material allows corona to be generated. Reducing the accumulation of dirt or residue on the surface can substantially reduce the conditions for backside treatment to occur.
Other causes of backside treatment are wrinkling, improper web wrap, or air bubbles trapped beneath the surface of the film. If wrinkling is the cause, the material handling problems must be addressed prior to the station. Slippage of materials on the ground roll results in a laminar airflow layer underneath the web. In most situations, proper station design and installation on the line reduces these causes. However, corrective action may require driving the ground roll or applying a nip roll to ensure the material positively contacts the roll face.
A few simple troubleshooting tips can help identify backside treatment. Check the actual ground roll speed and compare it to the web speed. A variance indicates slippage. This test may be difficult to detect slight amounts of speed differentials.
To determine if a nip roll is required, a paint roller test can be performed. Apply a standard paint roller to the ground roll at an accessible location where the web enters the treating station. Mark the test area and check the material for backside treatment. If the results improve, a nip roll most likely will correct the problem.
In applications where low tension or thin materials are involved, nipping may cause other material problems. In these cases, it may be necessary to replace the ground roll bearings or convert the ground roll to a low inertia roll core.
For further suggestions or assistance in analysis of backside treatment or other issues, please contact Enercon Industries Service Department for assistance.
|Dirty treater roll
||Clean ground roll
Implement cleaning program
|Low web tension
||Ensure proper web threading
Check web tension
Use nip roll
|Wrinkles in film
||Flatten film on ground roll
Extend web wrap
Increase web tension
Add entry nip roll
Add drive system to ground roll
Originally appeared in eNews: Maintenance 2nd Quarter 2002.