- August 01, 2006, By Timothy J. Walker TJWalker & Assoc. Inc.
What is it that allows one product to run all year with 2% waste and another can't get on the core without problems? Consider this a lesson in product design for manufacturing or design for winding — or in some cases, design for difficult winding.
In winding challenges, there are many things to consider, but I look at the terrible trifecta of winding: roll modulus ratio, roll buildup ratio, and product front-to-back COF.
This month, let's take on the number one difficulty factor — roll modulus ratio.
I first learned this concept in the early 1980s from Dr. Lien Struik of TNO, the famous Netherlands research institute. Dr. Struik described the two extremes of winding as “spongy” and “fully compressed” rolls, where the difference between the two was the roll modulus ratio. This ratio is found by comparing the stiffness or modulus of a material in the machine direction (Et, also called the Young's modulus or tensile modulus) and the stiffness or modulus of a stack of material (a.k.a. Er or radial modulus).
If Et/Er is near one, a wound roll is “fully compressed.” Rolls approaching the fully compressed state are trouble. However, if Et/Er is much greater than one, what Dr. Struik called a “spongy” roll, you have a good chance for trouble-free winding.
The Young's modulus of a material is a straightforward concept. The modulus of aluminum is approximately 10 million psi; polyester films or many papers are about 500,000 psi; an elastic rubber band will be much lower, say 5,000 psi. Many QC labs have this value readily available or easily found from tension-elongation testing.
The stack or radial modulus is tougher to characterize. It's a difficult measurement to take. Loads are quite high, and strains are quite small. In measuring stack modulus, the real kicker is that there is no one value. Stack or radial modulus is a nonlinear variable and can be described only with an exponential or polynomial function. Fun, eh?
In the May column, I described what determines how internal roll pressure builds up (or doesn't). The key to whether roll pressure continues to grow with increasing roll size, what I call the tourniquet effect, is dependent on how much the core and inner layers compress and how much that drops the circumferential tension in outer layers.
The Et/Er ratio is the key to this. When these material properties are similar, roll compression has little effect on roll pressure buildup. Thus, similar to wrapping additional layers of a tourniquet, larger rolls mean more pressure, more stress, and more diameter differential from crossweb thickness variations.
On the good side, when Et/Er is high, roll compression reduces diameter variations and relieves pressure buildup. You can build your rolls as high as the ceiling.
What are examples of the modulus ratio working for or against you? Paper products are relatively easy to wind; paper usually is high modulus in the sheet direction but fluffy in a stack, at least at typical wound roll pressures. Paper commonly winds on a 3-in. core and ends at people-sized rolls. Don't try that with most films.
Uncoated films and foils usually are high on the difficulty scale. A film or foil will have similar modulus properties in sheet or stack. If you have crossweb thickness variations in your film or foil, look out! Even minor thickness variations quickly will create roll diameter variations, pulling in the web nonuniformly and creating wrinkles, hardbands, and baggy webs.
How do you make film or foil winding easier? Anything that lowers the stack modulus will help. Lower the tension and pressure in the roll, wind on a compliant core, increase the surface roughness, add a soft coated layer, or wind with a soft interleave material.