- January 01, 2001, William E. Hawkins, Contributing Editor
I have had very positive feedback from the glossary of descriptive terms that I began in the September 2000 issue (see October and December issues for installments two and three). The fourth installment follows.
- lateral shifting: Sidewise movement of the web as it moves through the converting machine. If the web-shifting movements are not corrected, the web tracks off center of the winding roll(s) during that period. A shifted web is usually the result of tension changes in the moving web. These changes can result from speed changes in the machine, or they can be due to ambient temperature changes that cause the web tension members to grow at a non-uniform rate side to side. Lateral shifting also may be due to uneven drying and/or shrinkage forces acting on the web surface. Web steering is essential to maintain web tracking on the machine centerline when ambient temperature is causing the lateral shifting.
- lay-on roll: The contact roll that is used to limit the amount of boundary air that is entrapped between the wraps on the roll as it is being wound. This roll is sometimes referred to as a rider roll or simply a contact roll. The lay-on roll also is used to spread the web when there is substantial wrap around the roll. And the roll is used to tighten the wraps of the winding roll by utilizing the stack compression available in the winding wraps. The magnitude of contact area pressure that is exerted in the lay-on roll nip determines the amount of boundary air entrapped and the winding roll hardness.
- load cell roll: A web guide roll that has strain gauge or other type sensors that sense the amount of force (web tension) that the web is exerting on the roll. Usually there is a sensor for each end of the sensor roll. The signals from each end normally are combined and compared with a reference signal in the control panel of the device that maintains web tension in the area of the machine that is being controlled. When there is a difference between the reference signal and the signals from the load cell roll, the controlling program of the panel will signal the tension-producing control panel to change the web tension in the direction to null (or match) the reference and load cell signals.
- MD wrinkles: Ridges in wound rolls that resemble the strengthening, undulating bends in a tin can. They often are referred to as tin can wrinkles. They usually are formed under the surface of the winding roll during the winding process although you may be able to see these wrinkles as the roll is being wound. When the MD wrinkles form under the surface, they will appear on the roll surface after some lag time has elapsed, because the entrapped boundary air that has been supporting the web bleeds from between the wraps out the roll ends. MD wrinkles usually are formed when a lay-on roll is used, and they always appear between two standing gauge bands. There may be several standing gauge bands, and MD wrinkles may appear between all of these bands, or they may span between the most prominent ones that may include some bands of lesser diameter. MD wrinkles may be reduced significantly by randomizing the gauge variation across the web width, reducing gauge variation, and exerting lateral stress on the web as it is laid onto the winding roll.
- modulus of elasticity: The value of the amount of stress divided by the amount of strain in the elastic region of the web material. You calculate this ratio by dividing the amount of force acting on a cross-sectional area of that web by the amount the web has elongated. This ratio is very helpful in calculating the correct tensions to apply in any zone of the machine that prevents permanent deformation of the web.