CORRUGATED APPLICATIONS
Winding Education and Information

Step 3 – Identify Additional Needs of Your Winding Equipment

Cores Materials

Paper composite core –Paper composites are the most common material used for winding cores. Paper cores are a product of winding paper and epoxy to create a stiff, low cost cylinder. There are many options in paper cores to meet specific needs, including special surface coating or laminated layers on the inner, outer, or side surface. Some frequency reused paper cores will have metal ends to reduce damage in handling.

Plastic or composite fiber core – Plastic and composite cores are used in place of paper cores in clean processes to eliminate the concern for core dust or debris, as a more durable option, or for improved dimensional tolerances and reduced deflection.

Aluminum or steel core – Metal cores are most common in large diameter uses, especially diameter over 12 inches, and for in-house reuse, such as between a coating and slitting operation. Metal cores are also common in extremely wide operations, such as the large rolls wound at the end of a paper or film maker and for products that create extremely high internal roll pressures.

No core – To reduce the cost of core recycling or waste, some products (such as wallpaper and wrapping paper) and trim winders will wind on a collapsible shaft without a core.

Whether paper, plastic, composite, or metal, the core shaft or chuck must be able to grab the web with enough torque capacity to create the required winding tension.

CORE GEOMETRY

Core and Buildup Ratio: For many processes, the core dimensions and material are pre-determined. Much of the converting industry (especially widths from 6 to 72 inches wide) has standardized on 3” and 6” inner diameter paper composite cores with wall thickness ranging from 1/8 to 3/4 of an inch. Large diameter cores, whether metal or paper, should always be considered for in-house rolls, since larger cores will require less buildup ratio (roll final diameter divided by core outer diameter) and create less winding problems or waste.

Core Width: Some wound rolls use cores that are 6 to 12 inches longer than the web width to allow roll handling from the extended core stubs. Slit rolls commonly use core equal to or slightly less than the slit width to allow rolls to be stacked on end or packed side by side.

Spools: Some rolls, especially rolls with a strong tendency to telescope or are easily damaged, are wound on spools (cores with flanges on either side). Spools are useful in limiting telescoping and, if strong enough, can support the roll during storage and shipping. However, spool flanges also make it difficult to use nip and gap rollers in winding, which may lead to more misalignment and wound-in wrinkles.

BUILD-UP RATIO AND THICKNESS VARIATIONS

A roll’s buildup ratio is its final diameter divided by its core diameter. Rolls with buildup ratios of 3:1 or less are usually easier to wind. Radially compressible materials, such as many paper, non-wovens, and textiles, can wind to large buildup ratios (sometimes over 10:1) without problems, but film and foils may have trouble trying to reach a 4:1 buildup ratio.

Larger buildup ratios create three problems. In center winding, higher buildup ratios will require higher torque capacity with the winding roll, leading to possible slip defects such as telescoping. Large buildup ratios means more layers are pressing inward towards the core, creating more high pressure defects such as core crushing and starring. Large buildup ratios let long-term crossweb thickness variations magnify into larger crossweb roll diameter variations and associate defects such as wrinkling, shifted layers, and baggy web.

Products that are relatively incompressible in the thickness direction are more prone to thickness variations directly creating diameter variations. Diameter variations can be forgiven to a certain point, but once diameter variations grow to a similar order of magnitude as winding web strain, problems such as wrinkling, web shifting, and baggy web creation will increase.

OSCILLATION
To reduce the negative effects of crossweb thickness and diameter variations, many winding processes include oscillation, where the web thickness bands are shifted to during winding. In paper and film making, it is common to laterally shift the trim knives (usually in a triangle wave), then guide the web to a straight wound roll. The long term angle in the web will shift the web 1 to 3 inches over 100 feet more is relative imperceptible in the sheet, but may create significant benefits by reducing roll diameter variations and associated defects. (Equipment needs for oscillation: oscillating trim knives system, sidelay or displacement guiding)

In some coating processes, the edge sensor target point is intentionally shifted laterally, winding a roll with staggered edges (again in a triangle wave). This is more common, like larger cores, when the customer in a subsequent in-house process and the stagger or oscillated wound roll is easily straightened out by the unwind guiding of the next process. (Equipment needs for staggered winding: mechanical or electronic edge position oscillator, sidelay or displacement guiding)

In blown or tubular film making processes, where crossweb thickness variations are often quite high, the bubble collapsing process is rotated over 180 or more degrees, shifting the thickness variations across the entire width of the winding roll. This total randomization of thickness variations greatly reduces roll diameter variations and related defects.


DEFLECTION -Excessive deflection in a core or shaft may cause product waste from misalignment or wrinkling, or at high speeds create safety problems from roll bounce or whipping.
Core-shaft or core-chuck systems should be designed for deflections of less than 2 mils per foot of width. To minimize deflection, journals (the distance from the shaft end to the side support should be kept as short as possible.

The deflection of a core-shaft system is dependent on both the core and shaft design. Increasing diameter will always be the strongest variable to reduce deflection for a given length and load.

Full width cores will distribute their load and create less deflection that the same weight roll on a narrow width core. Multiple slit rolls on a shaft will create more deflection that the same weight of a single wide roll. The material and cross-sectional area of a shaft will determine its rigidity. Solid or thick-walled shafts will have less deflection than expanding leaf shafts.

Alignment -Winding rolls and shafts should be aligned to the same specification as rollers. The default specification should be less than 2 mils per foot of width.

Winders often have alignment problems. Winders may be well-aligned when there is no load, but deflect unevenly under the weight and tension of a large roll. Cantilevered winding shafts aide in easy loading and unloading, but often need a free-side support to minimize deflection under load.

Turret winders are also prone to alignment problems. To align a turret winder, like any pivoting element, first align the pivot axis or shaft. If a turret winder has a solid pivot shaft, make sure to machine a section for level and tram measurement. Once the pivot axis is aligned, then the turret winder shaft arm or structure can translate than alignment to the winding or roll shaft. Many turret winders are aligned in the winding position, but have difficulty holding alignment during the indexing cycle as the roll moves to the unloading position.

Trimming -Trimming is a slitting process where the two edges are removed before winding. Trimming may be required to cut off thick, ragged, or uncoated edges or to cut the web down to an accurate width. Successful trimming depends on 1) creating a continuous trim cut and 2) taking the trim away.

Trimming fails to create a continuous strand if the width variation or web shifting leads to a trimmed strand too narrow to slit or handle or the slitting fails to make a continuous cut.
Whether your trim is waste or to be recycled, trim creates two new webs that can be a handling challenge. Trim collection system may be pneumatic or mechanical. Some trims are wound along side the primary web or on separate trim winders (often level winders). Pneumatic trim handling systems are popular since they directly transport the web to a hopper and eliminate roll handling operations.

Slitting-Many winding processes are downstream of slitting where a wide web is converted into two or more narrower rolls. Winding downstream of slitting is like juggling several balls at once with each slit strand leading to an individual winding roll.
Slitting just upstream of winding should lead to good roll edge alignment. The slit edge is created by the knife position, which should be accurate to a couple mils or less. Good tensioning and tracking from slitting to wind up should maintain this accurate edge position in the winding roll. A short, well aligned web path from slitting to winding will ensure the best roll edges.

Spreading-Spreading is often included just upstream of winding to prevent wrinkles entering the winding roll. A spread may be used as the gap roller in gap winding or as the roller immediately upstream of the nip roller in nip or surface winding.
Spreading is also common before slitting to ensure the web is taut and flat as it is cut. Any buckling of the web between slitting knives will create slit strand width variation.
If slit rolls are all wound on a common shaft of surface winding position, then spreading may be required to create a gap between the rolls to prevent shuffled layers. However, spreading slit strands introduces the possibility of a tracking error and will usually create less precise edge alignment in the slit rolls.

Duplex Winding-Duplex winders have two winding positions. The two positions may be ‘in’ and ‘out’, such as with a duplex turret winder, alternating between the winding position and the roll loading / unloading position. In a slitter, duplex means two winding positions where alternating odd and even number slit strands are routed to one of two winding shafts.

Turret Winding-Turret winders are used to aide in aligning the web of a finished roll to a new core. Two winding spindles are mounted parallel to each other, indexing from the winding position to a roll unloading / core loading position. When a winding roll is complete, the turret rotates the new core into the winding position, maintaining tension on the web. With manual or automatic roll transfer, the web can then be attached to the new core in the proper lateral position.

Turret winders are commonly used at the end of extrusion and coating lines to allow uninterrupted continuous operation. Turret winders are also used on some high end slitter-rewinders to reduce roll change-over time and improve alignment and tensioning of new slit rolls.

A duplex turret system is an advanced option for slitter-rewinders using four winding shafts and two turrets.

Roll Transfers-There are three methods to transfer the web from a finished roll to a new core: manual, automatic zero-speed, and automatic at-speed splicing.
Manual splicing is completed entirely by the operator, sometimes with alignment help of a turret system. Manual splice may be a serious safety hazard if attempted on a moving web.
Zero-speed splicer will automatically sever, align, and attach the expiring web to a new core while the web is stopped. Zero-speed splicers are often paired with web accumulators to collect and dispense the web between the stopped and running process. The length of web accumulated is equal to total time required to decelerate and splice times your line speed.

At-speed splicing is required when line speeds and splice time make accumulator size unreasonable. The four steps of at-speed roll transfers are: speed match the new core to the expiring web, paste the web to the new core, sever the web from the expiring roll, and switch over the tension control to the new core.

Interleave Winding-An interleave web is a secondary web added to a winding roll to provide a buffer or separator between the layers of the primary web. Interleaves may be used to prevent scratching, provide good release at unwinding, or change the winding characteristics of the winding roll (such as reducing the stack modulus, absorbing the entrained air layer, or reducing crossweb diameter variations). A winder with an interleaving option includes an interleave unwind and tension control system.

Level Winding-Level winding is used in winding narrow web. Like winding up a fishing line, thread, or garden hose, the narrow web is wound onto a core several times its width. The narrow web is wound across the core in a spiral path then back onto of itself spiraling in the other direction. Level winding creates an extremely long length of narrow web on a single core, reducing the frequency of roll change-overs in the next process.

Temperature and Winding-To avoid problem from roll variations caused by thermal expansion and contraction, it is best ensure the product is at room temperature before winding. If your winding process is downstream of a hot process, avoid thermal effects in your wound rolls by including proper cooling prior to winding.

Special topic: Winding after slitting

Differential vs. Lock-Core Slit Roll Winding - The simplest method for winding rolls after slitting is lock-core winding. Multiple cores are locked to a common shaft (often in a duplex arrangement with half the slit rolls on one lock-core shaft and half on another). Winding torque is applied to the shaft, usually controlled by a friction or magnetic clutch. The torque is distributed between the slit rolls proportional to their diameter and initial tension.

If all the locked cores are equal diameter and start with the same tension (taking care not to have any loose strands), then all the slit rolls will wind with the same tension. However, slit strands will always have some amount of thickness variation between strands which often leads to diameter variations. As the slit rolls buildup, the diameter variations will increase, changing the amount of material pulled in by each roll per revolution. The larger than average diameter rolls will pull in more material and have above average tension. The smaller rolls will pull in less material and have below average tension. These variations in wind tensioning from slit roll to roll often leads to some percentage of too tight or too soft roll defects.

Differential winding, also called slip-core winding, is used to improve winding uniformity between slit rolls wound on a common shaft. In differential winding, each core slips independently on a common shaft with the torque controlled at each individual roll.

Differential winding shafts have two main design factors that set them apart:
1) Is the differential winding core-dependent or core-independent?
2) Is the differential shaft loaded from the end axially or from the inside radially?

Differential Winding: Core Dependency - Is the differential winding core-dependent or core-independent?
Core-dependent differential winding uses the inside or side surface of the core as part of a frictional clutch, so the core material properties will affect slit roll winding tension.
Core-independent differential winding uses a core grabbing element to lock onto the core and provide the core-side slipping frictional surface. Using a core-grabbing element allows for a better engineered friction surface or magnetic clutch to control the torque transfer, improving the roll-to-roll tension uniformity, but the additional components between the core and shaft will lower the shaft bending stiffness.

Differential Winding: Side vs. Radial Loading - Is the differential shaft loaded from the end axially or from the inside radially?
Side-loading differential shafts create a slipping clutch at each core by pressing together a stack alternating slipping cores (with or without core grabbing elements) with keyed, non-slipping spacers. Side-loading differential shafts create nominally the same load per core, so they are not a good choice for winding narrow and wide slit rolls on the same shaft. The sliding contact between core and spacer may also be a heat and debris generator, but larger side contact area (relative to radially loaded systems) can improve lateral wobble and slit edge alignment. Side-loaded differential shafts are simple and can have a nearly solid shaft so they are less prone to deflection problems than radially loaded shafts.

Radially loading differential shafts create a slipping clutch between the shaft and the core’s (or core grabbing element’s) inner diameter. The radially slipping elements have some finite width with each element creating nominally the same torque transfer. Radially loading differential shafts have no problem with wide and narrow rolls winding on the same shaft since each core receives torque proportional to its width. Radially loaded differential winding shafts will have a minimum width so that no two cores are contacting the same slipping element. Due to the internal complexity of radially loaded differential shafts, they will typically have more shaft deflection than an end-loaded shaft in the same geometry and load.

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