High-Speed Rewinding of Thin Films
Modern inspection rewinders solve the process issues that are inherent when running film materials.
The use of film substrates for labeling applications continues to increase. Films are at the center of several popular product labeling and decoration techniques, such as clear (no-label-look) pressure-sensitive labels, shrink films and in-mold labels. As such, printers and converters in the narrow web segment are processing more and more products using a range of film substrates, from polyesters and polystyrenes to polyethylenes and polypropylenes.
Film materials pose a higher level of processing challenges for narrow web converters versus traditional paper substrates. Tension, static and web guiding are some of the more important parameters that need to be controlled. These challenges are made even more difficult by the fairly recent — and successful — trends to down gauge substrates and liners. As companies throughout the supply chain focus on sustainability, using thinner label materials reduces material usage and increases the number of labels on a given sized roll diameter, which in turn helps reduce transportation costs and related carbon emissions.
Film materials can be a special challenge in the finishing department when operating inspection rewinders. These machines typically provide three functions: slitting, inspection and rewinding. Since they represent the last value-added processing before product is shipped to customers, there’s a lot riding on rewinder performance.
Much has been done in recent years to make sure inspection rewinders are up to the challenge, especially when it comes to processing film materials. For instance, certain lines of inspection rewinders have been specially designed to process films and other tension-sensitive materials. Where just 10 years ago 800 fpm was considered fast, these machines can run at speeds up to 2,000 fpm. Market requirements are also driving the need for wider machines, now available at 28 inches. This allows narrow web converters to expand their capabilities by moving into the mid-market, which includes both labels and packaging products.
The key technology that has supported these performance gains in high-speed rewinding of thin films is the use of servo drives. Servos are required to maintain proper tension when running the wide range of thin film materials at these elevated speeds. And tension is just one component of what is really motion control. When third-party peripherals, such as vision or inkjet systems, are added into the rewinding process, the motion of the web must be controlled within the different tension zones. Servo technology provides the level of control needed.
As mentioned, most inspection rewind systems provide three primary functions: slitting, inspection and rewinding. Recent advancements in each of these areas have allowed the productivity gains in high-speed rewinding machines.
The growing range of film materials used in labels and packaging has driven improvements in slitting systems themselves. Understanding that different materials will interact differently with the available cutting systems (crush, shear or razor slitting), converters need the flexibility to easily select and engage the slitting option that will provide the best solution for their application. Modern rewind systems are built for easy setup and quick changeovers, so some systems use a common housing with quick-change cartridges providing the desired slitting method.
Tension control provides the most significant challenge for high-speed rewinding of film materials. Films are stretchable materials and have to be handled very carefully throughout the entire inspection rewind process. This is further aggravated by the frequent starting and stopping that is the nature of inspection rewind systems. Since tension changes throughout the acceleration and deceleration cycles, it requires the precise control provided by servo technology, especially when running unsupported mono films, such as shrink films.
Two factors that can have a detrimental effect on tension in high-speed rewinding are the buildup of both static and air pockets throughout the process. The challenge is to reduce the static and get air out and away from the film as fast as possible.
Static problems vary from country to country, season to season and plant to plant. In addition to being a safety concern, static buildup changes the tension characteristics on the surface of film materials. In some cases, it can be effectively controlled by grounding the rewinder at multiple points. To help resolve this issue, it has become more common in film applications to include active static elimination systems on the rewinder.
With very thin films moving at 2,000 fpm, air pockets can become a problem, causing the film to “float,” effectively bypassing the tension control and moving the film. This movement and loss of tension control can result in telescoped rolls, which are unacceptable to most customers. The side profiles of the finished rolls of labels must be perfectly flat as high-speed label applicators in customers’ operations are very sensitive to side-to-side movement.
The use of fluted idler rolls and layon rollers (also called ironing rollers) are used on high-speed rewinders to dissipate and remove air buildup before it can cause a problem. Fluted idler rollers have effectively replaced spiral idlers, which had provided a sufficient solution when rewinders were running thicker films at slower speeds. Layon rollers have been in use for many years, and when located at the rewind section they do an effective job of getting air away from the film.
Along with tension control, web guiding systems are typically placed prior to the rewind section. Although multiple web guiding systems can be incorporated on rewinders, one system provides sufficient control in the majority of applications.
Inspection: It’s All About Data
Modern inspection systems for printing and converting include impressive capabilities for 100 percent inspection of printed webs, full width at commercial running speeds. The trend has been — and will continue to be — to move the inspection onto the printing press. Defects and printing problems can be identified further upstream in the process to minimize and even eliminate the production of off-quality product. While this evolution might reduce the need for inspection on rewinders, it requires the ability to clearly communicate defect information to the finishing department for further processing.
Roll mapping is a means to communicate the location of defects within individual rolls coming off printing presses. Defect information for each roll is incorporated into a file, which is used to automatically stop the rewinder when it’s necessary to remove defective product from that specific roll. With the ability to link up a pressroom vision system with the finishing department, communication is significantly improved and the rewinder becomes a downstream extension of the printing press. Off-quality product is removed much more efficiently and effectively.
While there has been a lot of focus in the past 10 years on refining the pressroom to reduce waste, the finishing department is beginning to get more and more attention. There’s no use making gains in waste reduction in the pressroom only to lose it in finishing. Companies in all industries, including printing and converting, are putting a significant effort into data collection and analysis. It’s critical to future success. Data can be collected automatically concerning footage, waste and number and types of defects. This information is used to generate customizable reports to analyze data for ongoing waste reduction efforts. In addition, many rewinders are beginning to include Wi-Fi connectivity for easy, remote access of live, real-time data.
What the Future Holds
Tremendous progress has been made in recent years on the productivity and effectiveness of inspection rewind systems, especially used for high-speed processing of films. Data collection and analysis will be the next frontier that helps drive improvements on the bottom line. Many companies offer good presses and rewinders. The key will be what value-added features are available that give customers an advantage.