- September 01, 2006, By Timothy J. Walker TJWalker & Assoc. Inc.
Last month I introduced you to roll modulus ratio, the first of the terrible trifecta of difficult winding. This month let's move on to the next two of winding's troublesome trio: roll buildup ratio and coefficient of friction (COF).
A roll's buildup ratio is simply its final diameter divided by the core's outer diameter; the trouble starts when buildup ratios go beyond three or four. Customers and accountants love larger buildup ratios, since larger buildups mean longer time between splices, lower core costs, and denser packaging. Be cautious of these apparent savings opportunities. If larger buildup ratios create more winding waste, the potential savings quickly goes in the dumpster.
Why are large roll buildup ratios difficult? Larger buildup in center winding or unwinding means more torque transmission from the core to the outside layers, leading to more cinching and telescoping. More buildup ratio tends to create more pressure within a roll and more of the associated high stress defects of starring, blocking, and core crushing.
The third factor in winding difficulty is a product's front- to back-side COF. The standard and ideal product COF is between 0.2 to 0.5. Troublesome COFs fall into three categories: slippery, tacky, and pressure dependent. I consider a product slippery if the COF is lower than 0.1 and tacky if the COF is close to or above 1. If you measure a COF over 1, you've gone beyond friction into adhesion.
The most troublesome tribological condition is when the COF is a function of pressure. Why? When the web hits the top of the winding roll, there a small slip zone as the web lands and adjust to the roll's diameter variations. If this slip isn't uniform, the web will shear and buckle, creating a defect called slip knots commonly seen in uncoated, smooth films.
My advice on these three winding challenges (isotropic modulus, large buildups, and unusual frictions) is to take on one, maybe two, but if you're faced with all three, run.
I can figure a way to wind isotropic materials if either the buildup is reasonable or the friction is normal. I can wind large buildup ratios if the product is relatively soft in the radial direction or the friction is optimized. And I'm willing to try some challenging frictional materials if the roll buildup is small or relatively soft radially. Just don't be cruel and make me (or yourself) face all three.
With this knowledge of what makes winding difficult, what do you do? First, you can look at a product design and decide whether you should take it on or delegate it. Second, armed with this knowledge, you can start further back in the product design cycle and make material or process decisions with windability in mind.
What makes a product easier to wind? Many films and coated products are modified intentionally to create cooperative frictional properties. PET films have internal slip particles to avoid tackiness. Videotape has a special back-side coating largely to improve winding friction. Some expensive and difficult-to-wind products will use a sacrificial interleave layer in winding to change the radial modulus of the roll and improve windability.
Changing either friction or radial modulus requires major product or process changes, which often are unacceptable or overly expensive avenues to windability.
What does this leave? Roll buildup ratio. Almost all winding processes will be more forgiving by reducing buildup ratio. If you can't sacrifice roll length, this means increasing core or hub size.
It may be difficult to convince your customers to accept a larger core and the associated expenses, but there are many cases where the roll you wind is being shipped inside your company. Winding on larger cores always will improve quality as jumbo rolls are shipped to the next converting process.