host posted on May 05, 2008 20:46
Static Electricity is the cause of many problems encountered in the transporting and converting of film webs in today’s modern machinery. As progressing technology allows faster production speeds, static related problems are also increasing, hampering production and affecting product quality.
Static charges are generated on the surface of the film as it unwinds from the roll and as it contacts and separates from surfaces such as idler rolls, nip rolls and printing or coating rolls. Charges exist in two polarities: positive and negative. Surfaces charged to the same polarity will repel each other. Opposite polarities will attract. A charged film of either polarity will attract to uncharged surfaces of insulators or conductors. These occurrences become especially evident in converting operations such as sheeting, bag making and die-cutting where the film is no longer under the control of the mechanical structure of the web and its transport system.
Film webs tend to develop high charges: 30 kV to 40 kV is common. The strong electrostatic fields associated with these charges attract dust particles, fibers, bugs and hair resulting in surface contamination. This causes obvious quality problems in printing, coating and laminating and cleanliness problems with food, medical and pharmaceutical packaging films. Uneven coatings and “wicking” of inks are often the result of static charges.
ESD (Electrostatic Discharge) of a charged conductor or a highly charged insulator can result in a sufficient energy to ignite hazardous vapors in coating heads and gravure printing operations. An ESD event can also disrupt logic in PLC’s and sensing equipment causing processing errors and costly down time. High charges, especially on a wind-up roll can result in uncomfortable electrical shocks to the operator when approaching the roll or touching the machine frame.
By having a good basic understanding of static electricity and how it can be controlled in film web processes, problems can be greatly reduced or eliminated resulting in more efficient web handling and improved product quality.
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