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Consultant's Corner: Developing Roll Hardness on Center and Center-Surface Winders By R. Duane Smith, Product Manager - Specialty Winding

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CONSULTANT'S CORNER

Winding with the proper roll hardness is extremely important to be sure that high quality rolls are produced, handled and stored, shipped and then processed by your customer at maximum production speeds without defects.  In this Consultant’s Corner, we’ll summarize the winding principles used to develop roll hardness.  Included are suggested “rules of thumb” for starting values for different materials to assist in optimizing the roll structure that works best for consistently producing high quality rolls of materials to meet your customer’s requirements.

Roll hardness is developed in different ways on different winders. However, the basic principles of building roll hardness remain the same regardless of winder. These principles are best summarized in the acronym T.N.T.:  Tension – the winding web tension; Nip – the nip of the pressure roll or drum; and  Torque - from the center drive or torque drum.

Tension Principle of Winding

When winding elastic materials, web tension is the dominant principle used to control roll hardness. The greater the web tension, the more stretch put into the web and the harder the wound rolls.  A good rule of thumb for elastic webs is the amount of web tension should be between 10 to 25 percent of the web material’s elastic limit. For center winding, web tension is programmed to control roll density. For proper roll hardness, winding tension needs to be tapered as the roll diameter increases. For center-surface winding, the web’s conveyance tension coming into the winder is typically constant. This allows the web to be stretched equally from the start of the wind until the finished roll diameter. The roll’s hardness profile is built using programed torque for tapered winding tension with a constant web tension.

Nip Principle of Winding

When winding inelastic webs, nip is the dominant principle used to control roll hardness. The nip controls the roll hardness by removing the boundary layer of air following the web into the winding roll. The rolling nip also adds in-wound tension to the roll. The harder the nip, the harder the winding roll will be. The rule of thumb for Nip is use only sufficient nip loading to remove the air in order to wind hard, straight rolls without building too much in-wound tension. Winding rolls too tightly can cause problems and/or the deformation of the web over the high caliper areas.

Torque Principle of Winding

The amount of torque being supplied by the center drive affects the roll’s hardness by tightening the web layers as the roll diameter builds. When winding in the center/surface mode, winding tension typically starts at 25 to 50 percent greater than the incoming web tension. This is then tapered as the roll builds in diameter to a value at or less than the incoming web tension. The surface drive will regenerate or pull negative torque (braking) when the winding tension is greater than the incoming web tension. As the winding roll builds larger in diameter, the surface drive will gradually provide less braking until it reaches zero torque when the winding tension equals the web tension. If the winding tension is programmed to go below the incoming web tension, then the surface drive will pull positive torque to make up the difference between the lower winding tension and the higher web tension.

Controlling Roll Hardness

There are several factors to consider when determining TNT. These include:
    •    The type of web material being wound:
        -    Extensible/Non-Extensible
        -    Compressable/Non-Compressable
        -    Thick/Thin
        -    Slippery/Sticky
    •    The width of the rolls being wound
    •    The finished diameter of the wound roll
    •    The speed of the winding operation

    For a quality roll, we want to start winding with a good solid foundation.  This requires as much roll hardness as possible. Then, as the winding roll builds in diameter, we need to taper the roll’s hardness. The greater the build-up ratio (core OD to finished roll diameter), the more important the hardness of start and the hardness taper becomes.

Rules of Thumb for Starting Values of Web Tension, Nip and Winding Tension (Torque)
The web conveyance Tension will depend on the type of material being wound and the thickness of this material. The following chart provides good starting points for web conveyance tensions for typical materials used for flexible packaging.

The Nip loading is less material dependant than web tension and will vary greatly on the material and amount of roll hardness required. The nip removes the air following the web and winding roll and the loading produces increased roll hardness.  The rule of thumb is to use as little nip as possible to achieve a firm roll with the desired roll hardness.

The Torque transmitted through the center of the roll is determined by programming the winding tension needed at the start and finish of the wind. The rule of thumb is to start winding tension at 25 and 50 percent of the web conveyance tension from the chart and then taper the winding tension for the desired roll hardness at the final roll diameter.

Laminated Structures
For laminating flexible film composites, tension individual webs before they are laminated so that the strain (elongation of the web due to web tension) is approximately equal for each web. If one web is strained signficantly more than the other web, curl or tunneling can occur. The amount of tension should be a ratio of the modulus and the web thickness to prevent curl and/or tunneling after the lamination process.

In conclusion, it’s important to remember that the process of finding the right combination of TNT is more of an art rather than a science. Understanding the winding principles and how they are applied on different winders is the first step to develop the best combination of the TNT roll structure tools.   I hope that you found this helpful to assist you in perfecting this “Art” for your specific products and winding conditions.

 

Films Tension Range (Metric) Tension Range (English)
Polyester 35 to 105 N/cm/mm 0.5 to 1.5 lbs/inch/mil
Polypropylene 14 to 35 N/cm/mm 0.2 to 0.5 lbs/inch/mil
BOPP 21 to 70 N/cm/mm 0.3 to 1.0 lbs/inch/mil
Polyethylene 7 to 21 N/cm/mm 0.5 to 1.0 lbs/inch/mil
Polystyrene 35 to 70 N/cm/mm 0.5 to 1.0 lbs/inch/mil
Vinyl (uncalendared) 3.5 to 14 N/cm/mm 0.05 to 0.2 lbs/inch/mil
Aluminum Foils 35 to 105 N/cm/mm 0.5 to 1.5 lbs/inch/mil
Cellophane 35 to 70 N/cm/mm 0.5 to 1.0 lbs/inch/mil
Nylon 7 to 21 N/cm/mm 0.1 to 0.3 lbs/inch/mil

 

 
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