A Glossary of Descriptive Terms - Fifth Installment

Here is the fifth in this series of glossary columns, intended to improve the technical communication between new and seasoned members of the converting industry.

- Nip rolls: Normally a set of two rolls (the set may be made with more) that have parallel axes, with at least one roll mounted on movable supports so that the roll faces can be pressed together (nipped) in order to give the rolls sufficient traction to pull the web through the machine at the desired web speed. One roll should be covered with an elastomer, and the other should have a metal surface. The elastomer-covered roll must be crowned in order to produce a uniform nip footprint across the full roll width. Sometimes these rolls are used to isolate web tensions in the two adjacent zones. There are many web handling processes in which there is no satisfactory substitute for nip rolls. These include applications such as collapsing the bubble on blown film lines, some types of cast web stretchers, web laminators, squeegee applications for liquid removal, etc.

- Neck-in: The amount of width reduction that takes place in the film web when that web is subjected to tension in the machine. Permanent width reduction takes place when tension exceeds the yield strength of the web material. Permanent width reduction distorts the web thickness profile by making the edges thicker than the rest of the web. This leads to winding problems as the thicker edges build diameter much faster than the rest of the web. Web tension must be controlled carefully in any process zone when the ambient temperature is elevated, since the yield strength tends to be very temperature-sensitive.

- Parallel roll alignment: The mechanical adjustment of each of the machine rolls so that the axis of each roll is aligned with the axis of a selected master roll on the machine. Parallel alignment is done so that the web may be pulled evenly over the process rolls as it moves through the machine.

- Permanent deformation: The condition of an area of film web that has been stretched beyond its elastic limit and remains deformed after the tension has been removed. Permanent deformation may occur in very small areas, such as the damaged area surrounding slip pimples, or larger areas, such as when MD or TD wrinkles occur in the wound roll, or even larger areas, such as described in "neck-in" item above.

- Roll profiles: The possible shapes of the surfaces of film guide rolls. There are two acceptable shapes for film web handling: the straight cylinder and the concave surface cylinder. The crowned roll is a special case involving nip rolls where the crowned roll surface is designed to conform to the deflection of the straight cylinder roll that is resisting the nipping load. Crowned rolls should never be used as web guide rolls.

- Runout: The axial turning eccentricity of the circumference of the roll surface from the point on the roll axis where it is being measured. During runout, all points on the roll surface do not rotate in the same axial circle. Total indicated runout (TIR) is the largest value of the eccentricity variation when the measurement instrument has been incrementally traversed the total length of the roll face. Runout is present to some degree in all turning rolls. An acceptable magnitude of runout depends on the web process.

- "S" wrapped rolls: Rolls that are usually installed in pairs so that the thread path forms an "S" when each roll is wrapped 180 deg. They are installed this way to provide more pulling or braking traction on the web. Sometimes several pairs of these type of rolls are installed to provide the amount of tension isolation needed in the process.


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