A number of different handling processes are used to process continuous webs of elastic material into defined segments, such as discrete webs cut from a continuous web for subsequent processing. For example, such discrete webs of elastic material are commonly used as components in the assembly of diapers, training pants and other disposable absorbent garments. As a further example, the side panels of such absorbent garments are typically discrete webs of elastic material. In general, the absorbent garment manufacturing line in which the discrete webs are used comprise a pre-wound roll of the continuous elastic material that is unwound by a suitable drive mechanism and fed (often through various stations of the manufacturing line) to a cutting station at which the web is cut sequentially into discrete webs of the elastic material. Typically the continuous web is held in tension as it is transported from the wound roll to the cutting station so that the web is elongated to some extent. The discrete webs are then transported away from the cutting station to another station of the manufacturing line at which the discrete webs are placed on and assembled with other components of the absorbent garment being formed.
Research has shown that the stretchability (i.e., the ability of the web to elongate under tension) of a continuous web that is pre-wound on a wound roll of the web material may vary throughout the roll (i.e., from the beginning to the end of the roll, or throughout the radius of the roll), and from one roll to the next. For example, if a discrete web is cut from a segment of the beginning of a roll under a particular tension (e.g., at the cutting station), and another discrete web is later cut from a segment elsewhere (e.g., the middle or the end) on the roll under that same tension, the cut lengths may be different due to variant stretchability at different segments of the continuous web.
Accordingly, existing elastic web handling processes and, in particular, absorbent garment manufacturing lines often include some type of control system and/or method for detecting the cut length of the discrete webs and correcting the cut length when cut length errors are detected. For example, a feedback-type control system relies on a cut length error (i.e., the difference between a measured cut length of the discrete web and a target cut length) to determine the magnitude of a processing correction. Other known systems and methods utilize what is referred to as a feed-forward control system that predicts what the cut length will be based on conditions of the web prior to cutting, and when necessary adjusts the feed rate, web tension or other properties based on the predicted cut length. For example, in one such process registration marks on the continuous web are used to measure the distance therebetween just prior to cutting and to predict the cut length of the discrete webs based on this measurement. By comparing the predicted cut length to a target cut length, the manufacturing line is adjusted by the control system to change the predicted cut length so as to more closely match the target cut length.
However, when dealing with an elastic web that is under tension (and more particularly, stretched) upon being fed to the cutting station, the discrete webs will retract, or recover after being cut since they are no longer under tension. In particular, due to elastic effects the discrete webs will recover in some measure relatively quickly following cutting, and due to visco-elastic effects will further recover more slowly so that at a time t after being cut the length (i.e., the machine direction dimension) of the discrete web will have decreased. A similar effect is experienced in defined segments of the continuous web, such as between registration marks, where processing of the continuous web involves stretching the web and subsequently reducing or removing the stretching forces from the continuous web.
In prior web handling processes such as those described above, the “cut length” or “segment length” measured or predicted (and relied upon) by the control system manufacturing line is the length of continuous web that is actually cut at the cutting station, or other processed at a particular station in its stretched condition, not the length following recovery under reduced or zero tension some time t after cutting or processing. Thus, such a control system does not take into account the role that changes in elastic and visco-elastic effects have on the discrete webs or web segments as they are transported from the cutting or processing station to further processing stations. Instead, a particular uniform amount of recovery of the discrete web is presumed.
It is known, though, that the pre-wound continuous web of elastic material is subjected to both radial and circumferential stress that varies throughout the wound roll of web material, typically due to existing winding processes and the changing size of the roll from the first wind to the last. Because the continuous web may remain in this condition for a prolonged period of time before being used in the absorbent garment manufacturing line, the visco-elastic properties of the web can change (e.g., resulting in the web being partially or wholly set in a more elongated condition) and indeed may vary substantially throughout the radius of the roll of web material, or from one roll to the next. Thus, after a time t following cutting (or segment processing between registration marks), discrete webs cut from one segment of the wound continuous web may not recover as much as, or may recover more than, discrete webs cut from another segment of the wound continuous web. This can occur even if the cutting length at the cutting station is the same for the discrete webs because the permanent or partial set in the wound web varies throughout the web but the tension in the web at the time of cutting is generally uniform.
There is a need, therefore, for a system and method of controlling the length of a discrete web of elastic material at a time t following cutting of the discrete web from a wound roll of continuous web, and/or controlling the length of a segment of a continuous web of elastic material at a time t following a particular processing step, and more suitably such a system and method which takes into account both the visco-elastic and elastic conditions of the continuous web of material prior to cutting or prior to the particular processing step.