- September 01, 2000, William E. Hawkins, Contributing Editor
There are many terms used to describe similar things in the converting industry. These vocabulary differences can present communication problems around the globe. This column is the first installment of a working glossary that is my interpretation of the terms I use when I describe technology connected with web handling.
Air Entrapment: The process of capturing boundary air between web wraps during the winding process.
Alignment: Making the axis of all rolls in the machine parallel to one reference roll.
Asperity: The roughness of the film surface, usually expressed in microns (one micron is one millionth of a meter) or micro inches (one micro inch is one millionth of an inch).
Baggy edges: Webs with longer film length on each edge than in the middle for any span.
Boundary air: Atmospheric air that stays on the film web surface until it is displaced, either mechanically or by a vacuum pump.
Bowed roll: A roll that has a curved axis. The roll covering is flexible and stretches during one half of a revolution and compresses during the other half. This type of roll is more difficult to turn than a straight roll, because energy is required to compress or stretch the covering during each revolution. The energy to turn the roll must be supplied by the film web when the bowed roll is not driven by another means.
Caliper variation: Thickness variation from point to point in the web. These variations may be orientated so that the thicker areas form machine-direction (MD) bands in the wound roll. Lanes of thicker web are often called transverse-direction (TD) gauge or standing gauge bands, because they are observed to stay generally at the same axial area while the roll is being wound. The thickness variation may also be oriented transversely across the web and form lanes. Thicker lanes in this direction often are called MD gauge variations because of the alternating thickness that is observed in the axial direction as the web is wound on a roll.
Chicken tracks, snail trails, etc.: Wrinkles that are seen in a winding roll or that develop after the roll is doffed. These wrinkles are usually caused by non-uniform web tensions and entrapped boundary air. When the entrapped boundary air escapes, the wrinkles orient themselves along the lines of forces produced by the non-uniform tensions in that area of the roll.
Core strength: The capability of the core to withstand the radial pressure of the film wraps that are wound under tension.
- Conductor: A material that can carry electrical current and/or transmit heat energy by conduction to another surface.
Contact roll: Referred to as a lay-on roll in certain configurations. This roll is usually used during winding to limit the amount of boundary air that is entrapped in the roll. This roll is also used to tighten the wraps on the roll. The roll may be stationary in the machine frame while the winding roll pivots away to accommodate roll buildup, or it may pivot into the winding roll when the winding roll axis is stationary.
Constant tension: The film tension does not change in the span between the last roll of the winder stand and the windup roll as the roll builds from core to full roll when winding in this mode. When thin, stretchable webs are wound in this mode, excessive radial pressure often builds as the roll diameter increases, to the point where the core compression strength is overcome and the core fails toward the axis.
Constant torque: The film tension reduces in the span between the last roll of the winder stand and the windup roll as the roll diameter builds when winding in this mode. This mode is sometimes called constant current winding when the winder chuck is driven by a separate electric motor, because the motor drive amperage remains the same as the roll diameter builds from core to full roll. Sometimes the outside wraps become loose and telescope (called "telescoping") before the required footage is wound on the roll when winding in this mode.
Counterbalance pressure: Fluid pressure applied to actuators used to offset the gravity force acting on a dancer roll, nip roll, or contact roll.
Creases: Film web fold-over wrinkles ironed into the web, permanent web defects that usually render the web reject for production.