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Mar 1, 2006 12:00 AM
Static electricity is a common occurrence in virtually all converting operations that handle moving webs. It occurs from the friction generated by a web as it passes over surfaces designed to guide or manipulate it. This is especially true for a plastic web.
Everyone is familiar with static electricity. It’s the jolt or shock you receive from touching a metal door knob, another person’s hand, etc., in very dry environments. Another example is the trick small children enjoy when someone rubs an inflated balloon on their hair and then “adheres” the balloon to a ceiling.
Why is static electricity a concern to converters of moving webs? It can cause four problems. One is that static electricity can attract dust and dirt to a web. Such a contaminated web obviously would detract from the appearance and perhaps the functionality of a finished product.
A second problem that arises from static electricity is difficulty in handling sheeted or die-cut items. Static electricity will cause these items to repel one another, making stacking an extremely difficult chore.
A third is the jolt or shock a worker in a converting plant may get. While this is usually not sufficient to cause bodily harm itself, a worker who receives such a jolt could be startled to the point where he might receive an injury from falling or hurt himself by contacting moving machinery.
Tinsels, misters and more
A particularly dangerous problem with static electricity is that it can cause a spark. In certain environments, such as those where flammable vapors from the use of solvents are present, this can cause an explosion or fire.
Four common techniques are available to control static electricity in a converting operation. They are metallic tinsel, misters or humidifiers, antistatic agents, and static eliminator bars. Obviously, each possibility has advantages and disadvantages.
Anti-static metallic tinsel looks very similar to the material people use to decorate Christmas trees. A strand of the tinsel drapes across a moving web so it contacts the web. This is a simple and relatively inexpensive method to reduce static. It has no level control—it either works or it doesn’t. One disadvantage is that the tinsel or pieces from it can be caught at nip points.
An alternate approach to static elimination is a humidification system. These are turnkey units that add a certain amount of water per unit of time into the atmosphere. Through automatic sensors that are part of the system, the unit can increase or decrease the output level depending on the relative humidity level in the confines of the converting operation. This allows automatic control of the relative humidity to the desired level. Many converters use a level of 40 to 60 percent at normal room temperature. Humidification systems actually can contribute to improved health and well-being of workers by reducing respiratory ailments and increasing total comfort at work.
Anti-static and ionizing bar
Anti-static agents are another way to control static. These are amines or amides. They can be incorporated into a polymer that will form a particular plastic web, or they can be added to coatings applied to a plastic web. They work by blooming to the surface of a web and attracting moisture to the surface. Care must be taken to assure the blooming of these materials does not decrease the functionality of a web.
Finally, there is an ionizing bar, probably the alternative with the highest degree of technology. Many converters use these units with great success. Detailed information on them is available from their suppliers, including data on installing them at the optimum location or locations.
David J. Bentley is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at firstname.lastname@example.org