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Cores | The Foundation of Winding

Where would you prefer to build your next house — on swamp land or solid bedrock? This should be an easy question since it's clear a house starts with the earth upon which it is built. If the earth moves, not much good happens. Doors don't line up anymore; walls separate from the ceiling; concrete cracks; and basements leak. It becomes quite clear the importance of a good foundation and that a home is more than just bricks and lumber.

A core is the foundation of winding. Instead of building upon the core with bricks and lumber, we build atop a core by wrapping hundreds or thousands of tensioned layers. Like a stack of bricks, each layer may add to the pressure exerted on a core.

Core pressures are commonly 10?100 psi but easily can reach 1,000 psi in winding stretch films. The core will respond to pressure by compressing and, in cases of extreme pressure, by collapsing.

Core compression is impossible to avoid but can be minimized. All materials respond to stress (pressure) by straining (dimensional change). A core will respond to the pressure of the winding roll by losing both outer and inner diameter. Excessive inner diameter loss can make a roll impossible to remove from a shaft.

Core compression is especially troublesome for stiff materials (foils, polyester, many papers) in which even a subtle outer diameter loss will loosen the layers near the core, possibly leading to a telescoped roll (see “Cinching: Belt Tightening Gone Bad, Part 1” and “Cinching: Belt Tightening Gone Bad, Part 2”). However, some core compression isn't necessarily a bad thing.

If the core doesn't give a little, core pressures will be many times the pressure elsewhere in the roll. In many film and foil products, high core pressure will create more waste near the core. Any imperfection of the roll start, even a single piece of tape used to attach the web to the core, will imprint and damage hundreds of layers of product. The nightmare defect of high core pressure is blocking, in which the layers of the roll fuse together and tear out rather than unwind.

Since a core is not a solid cylinder, how it responds to pressure is a combination of the geometry (inner diameter and wall thickness) and material (the modulus of elasticity in the hoop and radial directions). The effective modulus of a core always will be much lower than the material of which it is made, as low as 5% for thin-walled cores or as high as 30%?40% for thick-walled cores.

For a given wall thickness, smaller diameter cores will be stiffer than their larger brothers since core stiffness is proportional to the ratio of wall thickness to diameter. To have the same stiffness, a 6-in. core must have twice the wall thickness of a 3-in. core.

Calculating core stiffness for uniform materials, such as aluminum or plastic, is straightforward. The core stiffness of complex core structures, such as dual material cores or non-uniform materials, like traditional paper cores, may be found only from core compression testing.

To avoid defects associated with too hard or too soft cores, the core compressibility should be tuned to your product's radial modulus of elasticity (which is a function of material properties and how tight you wind the roll). Matching core to product will create the smoothest transition of stresses from the body of the roll to the layer near the core and the least high or low pressure defects near the core.

Core matching may include changing paper core geometry or hard coating but also can include switching materials. If paper cores prove too soft and metal cores too hard, consider intermediate modulus materials such as plastics or phenolics.

Core compressibility is only one of many considerations in choosing the best core for your product, but it may be the most important one. No matter how beautiful a home is, it will lose its luster if it slides off the cliff during the rainy season.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.


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