- March 01, 2001, William E. Hawkins, Film Handling Solutions Ltd.
Shaftless unwind/rewind stands: Basic pieces of equipment used in holding the supply roll(s) and production roll(s) on the converting machine. These stands usually are equipped with at least one fixed (braking or drive side) rotary chuck and one horizontal sliding (idler side) rotary chuck that will interface with the mandrel ends on cores that are dressed with expandable mandrels or that will interface directly with the roll core ends. The chuck faces usually are conical so that the mounted core or roll is centered when the sliding chuck is end-loaded against the core or mandrel ends.
During high-speed operation (above 200 fpm), one of the most important features that these stands can possess is the ability to accurately center the roll on the turning axis. Another important feature for the rewind stand during high-speed operation is rigidity. Rewind vibration due to roll eccentricity is often the cause of excessive boundary air inclusion during rewind at high speeds.
Stack compression: What happens to the wraps on the rewind roll when a contact (lay-on or rider) roll is used. Stack compression of film webs is made possible by two non-related phenomena. One is the height of the web surface asperity, and the other is the amount of entrapped boundary air between the wraps of the roll.
Surface asperity promotes limited stack compression by permitting adjacent web surfaces to deform around the asperity in such a way that the centerlines of the adjacent webs are closer together under contact roll pressure than they would be if no contact roll were used.
Entrapped boundary air is compressed by the pressure of the contact roll within the footprint of that roll. This compression results in closer wrap centerlines than if no contact roll were used.
One asset of stack compression is that it reduces the radius differences on the rewind roll during the rewind process and works to improve roll formation.
Static charge and static charge reduction: Related terms in the converter field. The amount of electrostatic charges on the web surface that can be measured with a static meter is usually referred to as static charge. These charges are sometimes called “triboelectric” charges. They result when a dielectric surface is separated from another surface quickly.
The charge buildup will occur whether the other surface is conductive (grounded or not) or is another dielectric material. Static charge may be removed by producing clouds of both positive and negative ions (charged air molecules) close to the web surface.
These ions flow from the point of generation to the film surface by the force of the electric fields generated by the static charges. The air molecules give up or absorb electrons at the web surface. This action reduces the static charge on the web surface. The optimum frequency for generating the charge-reducing ions depends on the web speed, with higher-speed processes requiring higher frequencies. You can use 60 Hz as a frequency benchmark for a web handling process running at 700 fpm when a 6-mil piano wire is used as the generating source.
Many processes reduce static charge in their machines with grounded tinsel or “static string” because of simplicity and economics. These methods are not as effective as powered ion generators mentioned above. They depend on ion generation between the electric field on the web and the very small grounded points on the tinsel or static string. As a result, static charges may not be entirely removed from the web using these methods.
One point to remember when using tinsel or static string: For optimum efficiency, the charge removal device should be very close but not touch the running web.
William E. Hawkins has 30-plus years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/474-5840; e-mail: email@example.com.