The process of eliminating most of the boundary air between winding roll wraps on other than very narrow rolls is complex. The lay-on roller must
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116 The Plastic Film and Foil Web Handling Guide stay in intimate contact with the web within the entire footprint of the nip at all times. For optimum boundary air exclusion, the contact pressure must remain constant in the footprint at all times. Eccentricity in the winding roll generates forces that work to negate constant nipping pressure across the roll width. This happens because the lay-on roller must move in a pivotal or linear fashion to follow the run-out in the winding roll. The inertia of the lay-on roller and the associated equipment delay that movement and prevent the actuators from keeping constant contact pressure with the winding roll.
Therefore, the optimum design for the lay-on roller is one that will follow winding roll run-out with the least amount of variance in contact pressure.
Roll eccentricity usually does not occur at the same time in a locus of points across the full width of the winding roll. The true axis of the winding roll is often skewed with respect to the axis of rotation. In other words, during rotation, one side of the roll surface will be closer to a horizontal TD reference line than the other side. This movement appears as a wobble to the observer. Wobble in the winding roll forces the lay-on roller axis to be skewed with respect to the true rotation axis, which is parallel to the true axis of the rotating chucks holding the winding roll.
Usually, the lay-on rollers are mounted at the end of arms that pivot into and out of operating position. These arms may be short when the lay-on roller assembly is mounted on a linear traveling carriage that adjusts for winding-roll buildup, or they may be long with stationary pivot points that are anchored to the machine. When one lay-on roller arm is rotated without the other, the lay-on roll axis is skewed by the parameters governing the rotation arc of the arms.
Because of the nonaligned tracking forces between the lay-on roller surface and the winding-roll surface, an unstable condition can develop and the lay-on roller may bounce against the winding roll. Sometimes there is a torque shaft connecting the arms through the pivot axis. The torque shaft is used there to make sure both arms rotate together when they are moved into or retracted from operating position. The shaft is necessary because air cylinders do not extend uniformly, even when very accurate flow control valves are used on the exhaust side of the actuators.
A stiff torque arm will lift the side of the lay-on roll not being rotated out of position by the winding roll eccentricity, out of contact (or greatly reduce its contact pressure) with the winding surface. Competent designers will call for splitting the torque shaft and installing a split-shaft coupling that has a small amount of rotational clearance. The rotational clearance in the torque-shaft coupling allows the two arms to be at slightly different degrees of rotation, so that both ends of the lay-on roller can stay in contact with the winding surface during winding-roll eccentricity.
Initial alignment of the lay-on roller axis to a precision core is essential to have near uniform contact on the winding roll. The preferred method of obtaining this alignment is to install a device that uses harmonic gears for very fine and accurate adjustment on the split-torque shaves in addition to the coupling that provides a small amount of rotational freedom to the torque
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Chapter eight: Winding technology 117 shaves. The device is easily locked in place once the lay-on roller surface has been matched to a precision core surface. A precision core is a must for lay-on roller aligning. This core should never be used for anything but setup after installation or maintenance.
Pivoting lay-on roller systems often operate with different pressures on the actuators. This pressure offset usually is used to compensate for the nonaligned initial setup or after a maintenance outage. Sometimes the offset is an attempt to compensate for tapered caliper or gage. Operators faced with tapered gage or bad alignment will argue with veracious zeal that the ability to operate each side of the lay-on roller at different pressures is necessary to wind quality rolls. Such people should carefully read the fol-lowing section.
Linear traveling lay-on roller systems also must compensate for the skewed winding roll axis to keep contact pressure nearly uniform across the winding roll surface when eccentricity is present. However, the moving carriage of the linear system must be rigid to prevent binding the carriage as it is moved into and out of operating position on its side rails. One method to resolve this problem is to install shortpivot arms that hold the lay-on roller. Shortpivot arms on the linear carriage provide flexibility for lay-on roller contact. Each of these arms has its own actuator to maintain contact pressure. An infrared beam (or similar type of device) on the carriage senses buildup on the winding roll and signals a servo unit to move the linear carriage to compensate appropriately for roll buildup.
The best lay-on roller configuration for flexibility is one that uses a combination center-pivoted metal-surface backup roller with an indepen-dently suspended, free-swinging elastomer-covered lay-on roller. This com-bination gives near uniform TD contact during wobble type movements of the winding roll. Figure 8.10 shows one of the most stable lay-on roller configurations I ever tested on eccentric winding rolls. The short pendulum arms supporting the lay-on roller provide uninhibited freedom of movement to keep the lay-down nip contact at near constant pressure during the wobble movements of the winding roll. The center-pivoted backup roller adjusts to the changing axial movements of the lay-on roller with low inertia. There is one very important rule that must be followed with this design: The axis of the pivot pin in the center of the pivoted backup roller must be perpendicular to the plane between its axis and the axis of the winding roll. In Figure 8.10 the centerline of the stationary shaft that holds backup roller is parallel to and located at the elevation of the winding roll axis. The pivoting support arms are very stiff but light in weight. A servo unit moves the carriage when very small deflections of the pivot arms are detected.
One key to stability with this system was the control-circuit design for the servo unit. The servo control circuit was programmed to wait until the oscillation movements of the assembly arms were outside a predetermined operating zone before signaling the servo to move the carriage. A small laser detected the assembly-arm rotation. The program was divided into five zones when the carriage assembly was put in operating position. The
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118 The Plastic Film and Foil Web Handling Guide
forward zone allowed the carriage to advance rapidly until contact was made with the winding roll. The second zone slowed the servo speed until the center or operating zone was reached. The carriage was stationary in the operating zone. As buildup on the winding roll occurred and the lay-on assembly arms were rotated backward into the third zlay-one or the carriage retreat zone, the servo was commanded to retreat slowly, but only after five full winding roll rotations were completed without any intrusions detected in the operating zone. The carriage stopped retreating when the operating zone was again reached as the assembly arms rotated forward during the retreating movement of the carriage. The last zone was an emergency retreat speed for the servo. There was also an end-of-rotation travel switch, hard wired, that bypassed the control circuit to prevent catastrophic damage.
Figure 8.11 shows the center-pivoted backup roller in more detail. The assembly arms were tied together with a split-torque shaft coupled with Figure 8.10 Small-diameter lay-on roller for high-speed processes.
Alternate to Keep Assembly Arm Near Vertical
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Chapter eight: Winding technology 119
a split-shaft device that had harmonic gears. Once the lay-on assembly arms were aligned with a precision core on the winder, no more adjust-ments were needed.
Stack compression is another complexity of winding with a lay-on roller.
Under normal winding conditions, the lay-on roller will compress several layers of web wraps and bring their surfaces closer to each other under the nip than they are around the rest of the winding roll circumference. This stack compression is made possible primarily by two phenomena: (1) the amount of entrapped boundary air between the winding wraps and (2) the web can be made to deform around the surface asperity.
Surface asperity is necessary to prevent the webs from blocking (sticking together) as they are pressed together under the lay-on roll nip or any other nipping process. Webs with high surface asperity (A > 0.25 μRa) will usually deform sufficiently around the asperity under the nip to be a significant variable in the winding process. A web with high surface asperity is much easier to wind than webs with low surface asperity (A > 0/0.10μ) because deformation around the surface asperity reduces the effect of caliper or gage band buildup. Also, higher surface asperity gives the web good slip prop-erties (low coefficient of friction). Good slip reduces slip dimple defect gen-eration in the nipping zone.
The largest amount of stack compression comes from boundary air that is entrapped between the wraps. Figure 8.12 shows the nipping mechanics of a lay-on roller operating on a winding roll.
The highest nipping force is in the low-slip zone on a line between the centers of the lay-on roller and winding roll. This zone also has the highest frictional force between the incoming web and the last wrap on the winding roll. Essentially, the velocities of the incoming web, the lay-on roller surface and the last wrap on the winding roll are the same in the low-slip zone.
Because of the radius difference between the two rolls, the outside wrap Figure 8.11 Cross-section of center-pivoted backup roller.
Sealed
Anodized (Black) AI Outer Shell
AI Non-Rotating Inner Shell
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120 The Plastic Film and Foil Web Handling Guide
of the winding roll is moving at a higher velocity before the nip than in the low-slip zone. Thus, it must slow to the speed in the low-slip zone as it approaches that zone. And there is film-to-film slip in this region. Also, the larger amount of stack compression, the larger amount of slip is nec-essary to make the velocities equal in the low-slip zone. There is film-to-roller slip on the exit side of the lay-on film-to-roller nip as the radius of the winding roll expands after the nip. The lay-on roller surface must allow this slip to freely occur or abrasion of the roll surface will occur. Debris is usually generated when the lay-on roller surface is abraded. Debris adds to the slip-pimple generation.
Smooth web surfaces are vulnerable to slip-pimple defect generation by debris particles in the film-to-film slip zone. Slip-pimple generation from debris appears to occur when debris particles lock the webs together in the film-to-film slip zone, causing a very small amount of web stretching to occur on the incoming web around the particle. This stretching reduces web thick-ness in front of the leading edge of the particle and increases thickthick-ness around and on the receding edge. Because of the increased thickness in the localized area around the debris particle, the relative velocity is increased in the film-to-film slip zone, and a slightly greater amount of stretching occurs around the particle on the next and each succeeding pass, as the area goes under the lay-on roller nip. Thus, there is a continual buildup around the particle area and the area becomes an objectionable defect when it is visible Figure 8.12 Stack compression of winding roll by lay-on roller.
Film to Film Slip Incoming
Web
Film to Roll Slip
Low Slip Zone
R
r
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Chapter eight: Winding technology 121 in the web. Slip-pimple defects may also occur without the particle seeds described above. Smooth surface webs without enough surface asperity to prevent localized blocking (web adherence to web) under the lay-on nip generate significant slip pimples immediately when winding is commenced, whether a debris is present or not.
Very smooth surface webs cannot be successfully wound with a lay-on roller without the introduction of some form of slip agent. Sometimes it is possible to meter the right amount of boundary air between the webs that will provide a lubricant for slip but still allows the webs to interlock on the surface asperity high points. This must be very carefully done with a textured lay-on roller surface. Web stiffness is a significant variable in the metering process.
Stiffer webs will require less metered air between the wraps than very flexible ones, because the entrapped boundary air is better dispersed between the wraps of the stiffer webs. Each product has its own characteristics that deter-mine the amount of boundary air to meter into the roll, so there is no one lay-on roller surface texture that is optimum for all products. Sometimes an inter-leaf material is used to perform this function on very valuable end-use mate-rials, but this process is limited because it is expensive.
A special air lay-down device was used to wind very good-looking rolls of very smooth webs at high speeds (up to 1000 ft/min) without producing slip pimples, but the rolls would uncoil unless they were securely taped while tension was still being applied to the last wrap. Lagging the rolls to stop the uncoiling did not solve the uncoiling problem. The lagged rolls would uncoil a few days later when the holding tape was removed. They also could be easily telescoped when the outside wraps were pushed in the transverse direction.
Extended experiments with this device indicate that web interlocking is a necessary and not an expedient part of the winding process in most wind-ing applications. Surface asperity or some other means of lockwind-ing the webs together, such as applying edge-thickening techniques at the roll edges, is required for commercial winding processes on very smooth films. Being able to wind without a web interlocking function is of little practical value if the roll does not keep its integrity after it was doffed.
The special air lay-down device, shown in Figure 8.13, lays the web down on the winding roll by a pressure bubble. Air pressure in the inlet channel ranges between 3 and 4 psig. Flow was between 300 and 400 cfm.
Inlet air pressure was not sensitive to speed in preventing excessive bound-ary air from being ingested between the wraps. The bubble ends were open to the atmosphere and most of the bubble air escaped out the ends. The bubble was very stable during operation. The upper seal was a very smooth rounded bar that was held away from the winding roll surface by air pressure in the bubble working against the projected frontal area of the air lay-down device. An insignificant amount of air escaped under the bar. The lower seal contained a full-width nozzle that was perpendicular to web. The web was held away from the seal surface by bubble air escaping from each side of the nozzle. The device applied pressure against the bubble to exclude
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122 The Plastic Film and Foil Web Handling Guide
ary air. The loading pressure ranged from 0.5 to 1.5 PLI. Air actuators were used to apply this pressure.
The air lay-down device was mounted on pivot arms and these arms were mounted on a horizontal linear moving carriage. A small movement of the pivot arms activated the servo that moved the carriage, so that the pivot arms remained almost vertical during winding roll buildup. The nozzle was deckled to keep the jet flow inboard of the web edges. The main channel of the device was deckled so that both outlets were adjusted for web width at the same time. The deckles seemed to be placed optimally when set in about 1/2 in. from the web edge on each side. Winding speeds up to and including 1000 ft/min were demonstrated on a full range of surface rough-ness, and on film thicknesses from 2.5μ to 175μ films without excessive boundary air entrapment.
There are two major limitations to commercial use of this device for winding. One limitation is noise, especially on webs > 12μ thick. The noise is produced because the nozzles tend to vibrate the thicker webs at high frequency. The other is the energy consumption for the process. A 150-hp motor is used to drive a 12-stage compressor for the air supply. A cooler is needed after the compressor. A good filter after the cooler is necessary to prevent debris from getting on the web.
There was another interesting test result. Debris was thrown on the web upstream of the device while it was winding clear, smooth webs. This debris did not cause slip pimples in the wound roll. Inspection of the device after the test showed debris had collected on the bottom seal and had blown out the ends of the lay-down bubble. The device had static neutralizer wires operating during this test. It was not tested with the bars turned off.
Figure 8.13 Air lay-down device.
Upper Seal
Winding Roll
Nozzles Bubble
Lower Seal Incoming Web
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Chapter eight: Winding technology 123