Controlling static on an unwinding roll

Abstract

Excessive static charges on insulating webs are the root cause for a number of problems in roll-to-roll manufacturing operations. Static charges attract contaminants and cause discharges that can ignite flammable vapors, shock operators, damage machine control systems, and change the surface chemistry of carefully formulated products. Webs are commonly unwound and wound several times in a manufacturing operation. For example, webs formed by an extrusion process are wound. The roll may be unwound in a coating, slitting, or converting process and wound a second time in a customer roll. Finally, the roll may be unwound a third time in a customer application such as printing or in a label application process.

Static charges on the web are stored in each wound roll. Each time a roll is unwound is a unique opportunity to neutralize static. Proposed here is a new method for neutralizing static on unwinding rolls. The static control method has three key elements. First, a high performance static bar must be located to neutralize the outside surface of the unwinding roll. Second, a static bar must be located downstream of the first conveyance roller to neutralize the inside surface of the web. Third, the first conveyance roller after the unwinding roll must contact the inside surface of the web.

The method to neutralize static on an unwinding roll is analyzed to show that static charge separated at the unwinding nip by tribocharging may be substantially reduced. The same method also reduces static charges wound into the roll from previous operations.

Finally, the effect of the first conveyance roller on static control is discussed. The web exiting the unwinding roll is likely to have a high level of static that will cause a static discharge known as pre-nip ionization that occurs in the gap between the charged web and the surface of the first roller prior to contact. Pre-nip ionization requires that the first conveyance roller contact the inside surface of the web. Otherwise, the charge neutralization performance of this method is compromised and the web will remain highly charged through the production operation resulting in high static in the winding roll.