- January 13, 2012, Timothy J. Walker
Hockey pucks. That is one name for the worst case of blocked rolls. You can’t unwind them, so what are they good for? Answer: hockey pucks.
Unwinding a roll should be a simple delamination process. Pull on the top layer of a roll of paper, film, foil, or even sticky tape, and it should not require much force to dispense a desired roll length. Blocking is the failure to unwind and delaminate as expected.
Blocking occurs when the bond between layers increases. If the bond between layers is high enough, a blocking roll could stall the unwinding process, but more likely it will concentrate all the available tension to one spot or the web’s edge, likely yielding or breaking the web.
I’m not an expert is the chemistry of bonding, but I know from experience that there are certain products that are more prone to blocking and certain characteristics of winding and wound rolls that will promote blocking. Two of the biggest contributors to blocking, outside of chemistry, are pressure and contact area.
Blocking tends to occur in the highest-pressure regions of a wound roll. Some products tend to hold their tension in winding and will create higher internal roll layer-to-layer pressure. For a given winder design, winding torques, and winding nip loads, films, especially stretchier films, will wind to higher pressures than most all papers and foils (or laminates with paper or foil in them).
For nearly any wound roll, the highest average pressure will be at the core (especially when winding on hard plastic or metal cores) or in the layers immediately off the core. Across the roll’s width, the highest pressures will be aligned with thick lanes of material that form hard bands, especially when gauge oscillation is off or low.
Regarding contact area, blocking is most common in products that are smooth. High surface roughness on either a micro or macro level will reduce blocking. In polyester films, micro-roughness is intentionally engineered into the film, typically by adding finely milled calcium carbonate or silicon dioxide slip agents in low percentages to the polymer resin. In extrusion or biaxial orientation, the small mineral particles create an engineered roughness to help windability. The small micro-mountains prevent the high bonds that would develop between mirror-smooth surfaces. Slip additives are acceptable for many polyester applications, but for optical films where clarity is a virtue, there always will be a conflict between an slip additive’s benefits to winding vs. end-use problem with higher haze values. Of course, an optical film isn’t useful if you can’t unwind it.
The macro scale of roughness to reduce real contact area includes embossing, pattern printing, roughness coating (like micro or macro abrasives), and the natural surface roughness features of many paper webs. For film manufacturers, I am commonly asking if they measure their surface roughness and whether they would consider embossing one or both sides of the product. Super-smooth films may look nice, but they tend to wind into blocks. If your customer is just going to coat over one or both sides of your product, adding some micro-roughness can improve your winding yields, stop their blocking headaches, and have no impact on final performance.
Recommendations to reduce blocking:
- Wind Looser | Pressure within wound rolls of film is controlled by many variables, including the core, the final roll diameter, the winding speed, the cross-web thickness variations, but primarily by control of winding tension and winding nip roller load and their profile set points vs. roll diameter. Wound rolls will have lower internal pressure when they are wound with lower starting tension, lower nip loads, higher speeds, and more taper of tension and nip load vs. diameter.
- Big Cores, Short Rolls | Wound rolls will have lower pressure when winding on larger cores winding to shorter lengths. The ideal wound roll is a giant core with one wrap on it. Yes, this is ridiculous, but any move toward that ridiculous ideal will reduce many, many winding defects. No one ever likes to hear this solution. They immediately assume that larger cores and shorter-length rolls will be a cost-prohibitive option, but don’t throw out this option without truly considering it. The most likely application of big cores is when you have an in-house customer. Most paper, film, and foil makers send master rolls to a slitter before they go out the door. These in-house master rolls should be wound on large diameter, reusable cores.
- Oscillate To Reduce High Pressure in Gauge Bands | Most flat film and papermaking processes use lateral oscillation ahead of their first winding process to shift the lateral position of the thick lanes and reduce the wound roll hard bands and associated high pressure regions. Proper oscillation at winding reduces the high pressures that occur within a roll. Dropping average pressure within a roll usually will have less effect than dropping the local increases of pressure from gauge bands.
- Increase Roughness | Reducing contact area reduces the total force that can form in layer-to-layer bonding. Surface roughness also will tend to lower coefficient of friction and decrease the stiffness of the winding roll in the radial direction, both effects that will reduce average pressures within a roll.
These are the things that can be done from a mechanical point of view. If these don’t work, then you likely need to leave mechanical engineering and move over to the materials and chemical engineering departments for your solution (unless you are in the hockey puck business).
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; email@example.com; www.webhandling.com.