- August 31, 2004, Timothy J. Walker TJWalker & Assoc. Inc.
Delaminating — undesired separation within a multilayer product — is a major problem for many converters.
Laminate products are a marriage of two or more layers with combined properties not found in any individual layer. Delamination is the breakup of this marriage when outside forces exceed the ties that bind.
If coatings are counted as layers, almost all converted products could be considered laminates. As I refer to laminates in this column, I'm generally thinking about substrate layers and stiffer and thicker coated layers. For a laminate to work, we usually want the layers to stay together. But, look out! There are forces at work to undermine the marriage of your laminate layers.
Products with intentionally low laminate bonds will be more susceptible to unintended delamination. For example, label stock will need more care in processing to avoid an early breakup, since ease of delamination is a desired end-customer feature.
Many laminating processes create a product with weak initial bond strength, sometimes called “green strength.” Often a laminate bond involves curing, whether chemical or added by outside energy, developing a stronger bond over time. Therefore, the most important period to deter delamination is just after initial bonding.
In web handling, the excitement usually happens at the web-roller interface. Going around a roller should be an elastic experience. When your flat web is forced around the cylinder of a roller, the curved shape will impose tensile and compressive stresses on the outer and inner surfaces, respectively. These stresses are proportional to the product thickness divided by the roller radius.
Many web processes have specifications for minimum bending radius or roller diameter, aimed at avoiding curvature-induced stresses beyond a product's elastic limits. However, for laminates, you may need to set a larger minimum roller diameter, since the forces to cause delamination may be well short of elastic limits.
As a laminate wraps around a roller, the layers respond with a through-thickness stress and strain change. At the interface, the inside is attempting to elongate while the outside layer is trying to contract. This mechanical opposition creates a shear force at the bond point. If the shear load is greater than the laminate bond, the laminate will split apart.
When two webs are transported over the same roller, there is a natural tendency for the top web to have greater velocity. Assuming no slip, the average web speed going around a roller is determined by the roller revolutions per minute and the circumference of the center line of the web. When two webs wrap a roller together, the small difference in the centerline circumference will create a significant long-term length variation.
For example, two 1-mil webs will have a 1-mil radial centerline difference and a 0.006-in. feed length variation per roller revolution. This doesn't seem like much, but at 100 fpm on a 4-in.-dia roller, this adds up to 0.6 in./min of extra material in the bottom web.
When two webs have low or no bond, this extra material will pile up on the floor, wrap on the roller, or fold over in a crossweb wrinkle — all undesired outcomes. If a laminate has high bond strength, the two layers will act as one, the top web will stretch, the bottom will contract, and there will be no delamination or feed rate variations.
Here are four tips to avoid roller curvature and feed rate-induced delamination:
- Move to large-diameter rollers. The curvature-induced delaminating stresses go down inversely proportional to diameter.
- Avoid rollers until bond strength increases. If you postpone roller curvature until green strength goes up, you may avoid delamination.
- Increase tension. The pressure created by the outer web's tension will add to the laminate bond force, increasing the resistance to delaminate.
- Use lateral or low-angle spiral-ridged rollers. Roller ridges may not prevent delamination, but they can burp through small feed variations, preventing the catastrophe of web wrinkling or wrapping rollers.
FYI, in August my wife and I completed our move back “Up North” to St Paul, MN. Please note my new phone number and expect mood swings associated with Viking losses.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 651/686-5400; email@example.com; tjwa.com.