Not all films are created equal. This creates problems for both the winder and the operator. Here’s how to deal with them. #processing tips #best practices
On central surface winders, web tension is controlled by surface drives connected to stacker or pinch rollers to optimize web slitting and web distribution. Winding tension is independently controlled to optimize coil stiffness.
When winding the film on a purely central winder, web tension is created by the winding torque of the central drive. Web tension is first set to the desired roll stiffness and then gradually reduced as the film winds up.
When winding the film on a purely central winder, web tension is created by the winding torque of the central drive. Web tension is first set to the desired roll stiffness and then gradually reduced as the film winds up.
When winding film products on the center/surface winder, the pinch roller is actuated to control web tension. The winding moment does not depend on the web tension.
If all webs of film were perfect, producing perfect rolls would not be a big problem. Unfortunately, perfect films do not exist due to natural variations in resins and inhomogeneities in film formation, coating, and printed surfaces.
With this in mind, the task of the winding operations is to ensure that these defects are not visible visually and do not increase during the winding process. The winder operator then has to make sure that the winding process does not further affect product quality. The ultimate challenge is to wind the flexible packaging film so that it can work seamlessly in the customer’s production process and produce a high quality product for their customers.
The Importance of Film Rigidity Film density, or winding tension, is the most important factor in determining whether a film is good or bad. A roll wound too softly will be “out of round” when wound, handled, or stored. The roundness of the rolls is very important to the customer in order to be able to process these rolls at maximum production speed while maintaining minimal tension changes.
Tightly wound rolls can cause problems of their own. They can create defect blocking problems when the layers fuse or stick. When winding a stretch film on a thin-walled core, winding a rigid roll can cause the core to break. This can cause problems when removing the shaft or inserting the shaft or chuck during subsequent unwind operations.
A roll that is wound too tightly can also exacerbate web defects. Films typically have slightly high and low areas in the cross section of the machine where the web is thicker or thinner. When winding the dura mater, areas of great thickness overlap each other. When hundreds or even thousands of layers are wound, the high sections form ridges or projections on the roll. When the film is stretched across these projections, it deforms. These areas then create defects called “pockets” in the film as the roll unwinds. A hard windrow with a thick sliver next to a thinner sliver can lead to windrow defects called waviness or rope marks on the windrow.
Small changes in the thickness of the wound roll will not be noticeable if enough air is wound into the roll in the low sections and the web is not stretched in the high sections. However, the rolls must be wound tightly enough so that they are round and remain so during handling and storage.
Randomization of machine-to-machine variations Some flexible packaging films, whether during their extrusion process or during coating and lamination, have machine-to-machine thickness variations that are too great to be accurate without exaggerating these defects. To streamline machine-to-machine winder roll variations, the web or slitter rewinder and winder move back and forth relative to the web as the web is cut and wound. This lateral movement of the machine is called oscillation.
In order to oscillate successfully, the speed must be high enough to randomly vary the thickness, and low enough not to warp or wrinkle the film. The rule of thumb for maximum shaking speed is 25 mm (1 inch) per minute for every 150 m/min (500 ft/min) winding speed. Ideally, the oscillation speed changes in proportion to the winding speed.
Web Stiffness Analysis When a roll of flexible packaging film material is wound up inside the roll, there is tension in the roll or residual stress. If this stress becomes large during winding, the inner winding towards the core will be subjected to high compressive loads. This is what causes “bulge” defects in localized areas of the coil. When winding non-elastic and highly slippery films, the inner layer may loosen, which may cause the roll to curl when wound up or stretch when unwound. To prevent this, the bobbin must be wound tightly around the core, and then less tightly as the bobbin diameter increases.
This is commonly referred to as the rolling hardness taper. The larger the diameter of the finished wound bale, the more important the taper profile of the bale. The secret to making good stranded steel stiffness construction is to start with a good strong base and then wind it up with progressively less tension on the coils.
The larger the diameter of the finished wound bale, the more important the taper profile of the bale.
A good solid foundation requires that the winding start with a high quality, well stored core. Most film materials are wound on a paper core. The core must be strong enough to withstand the compressive winding stress created by the film tightly wound around the core. Typically, the paper core is dried in an oven to a moisture content of 6-8%. If these cores are stored in a high humidity environment, they will absorb that moisture and expand to a larger diameter. Then, after the winding operation, these cores can be dried to a lower moisture content and reduced in size. When this happens, the foundation of a solid injury throw will be gone! This can lead to defects such as warping, bulging and/or protrusion of the rolls when they are handled or unrolled.
The next step in getting the necessary good coil base is to start winding with the highest possible stiffness of the coil. Then, as the roll of film material is wound, the rigidity of the roll should decrease evenly. The recommended reduction in roll hardness at the final diameter is typically 25% to 50% of the original hardness measured at the core.
The value of the stiffness of the initial roll and the value of the taper of the winding tension usually depend on the build-up ratio of the wound roll. The rise factor is the ratio of the outer diameter (OD) of the core to the final diameter of the wound roll. The larger the final winding diameter of the bale (the higher the structure), the more important it becomes to start with a good strong base and gradually wind softer bales. Table 1 gives a rule of thumb for the recommended degree of hardness reduction based on a cumulative factor.
The winding tools used to stiffen the web are web force, down pressure (press or stacker rollers or winder reels), and winding torque from the center drive when winding film webs on the center/surface. These so-called TNT winding principles are discussed in an article in the January 2013 issue of Plastics Technology. The following describes how to use each of these tools to design hardness testers and provides a rule of thumb for initial values to obtain the required roll hardness testers for various flexible packaging materials.
The principle of web winding force. When winding elastic films, web tension is the main winding principle used to control the stiffness of the roll. The tighter the film is stretched before winding, the stiffer the wound roll will be. The challenge is to make sure that the amount of web tension does not cause significant permanent stresses in the film.
As shown in fig. 1, when winding film on a pure center winder, web tension is created by the winding torque of the center drive. Web tension is first set to the desired roll stiffness and then gradually reduced as the film winds up. The web force generated by the center drive is usually controlled in a closed loop with feedback from a tension sensor.
The value of the initial and final blade force for a particular material is usually determined empirically. A good rule of thumb for a web strength range is 10% to 25% of the film’s tensile strength. Many published articles recommend a certain amount of web strength for certain web material. Table 2 lists suggested tensions for many web materials used in flexible packaging.
For winding on a clean center winder, the initial tension should be close to the upper end of the recommended tension range. Then gradually reduce the winding tension to the lower recommended range indicated in this table.
The value of the initial and final blade force for a particular material is usually determined empirically.
When winding a laminated web composed of several different materials, to obtain the recommended maximum web tension for the laminated structure, simply add the maximum web tension for each material that has been laminated together (usually regardless of the coating or adhesive layer) and apply the next sum of these tensions. as the maximum tension of the laminate web.
An important factor in tension when laminating flexible film composites is that the individual webs must be tensioned prior to lamination so that the deformation (elongation of the web due to web tension) is approximately the same for each web. If one web is pulled significantly more than the other webs, curling or delamination problems, known as “tunneling”, can occur in laminated webs. The amount of tension should be the ratio of modulus to web thickness to prevent curling and/or tunneling after the lamination process.
The principle of spiral bite. When winding non-elastic films, clamping and torque are the main winding principles used to control roll stiffness. The clamp adjusts the stiffness of the roll by removing the boundary layer of air that follows the web into the take-up roller. The clamp also creates tension on the roll. The stiffer the clamp, the stiffer the winding roller. The problem with winding flexible packaging film is to provide enough down pressure to remove air and wind up a rigid, straight roll without creating excessive wind tension during winding to prevent the roll from binding or winding in thick areas that deform the web.
Clamp loading is less dependent on material than web tension and can vary widely depending on material and required roller stiffness. To prevent wrinkling of the wound film caused by the nip, the load in the nip is the minimum necessary to prevent air from being trapped in the roll. This nip load is usually kept constant on center winders because nature provides a constant nip load force for the pressure cone in the nip. As the roll diameter becomes larger, the contact area (area) of the gap between the winding roller and the pressure roller becomes larger. If the width of this track changes from 6 mm (0.25 inch) at the core to 12 mm (0.5 inch) at the full roll, the wind pressure is automatically reduced by 50%. In addition, as the diameter of the winding roller increases, the amount of air following the surface of the roller also increases. This boundary layer of air increases hydraulic pressure in an attempt to open the gap. This increased pressure increases the taper of the clamping load as the diameter increases.
On wide and fast winders used to wind large diameter rolls, it may be necessary to increase the load on the winding clamp to prevent air from entering the roll. On fig. 2 shows a central film winder with an air-loaded pressure roll that uses tension and clamping tools to control the stiffness of the winding roll.
Sometimes the air is our friend. Some films, especially “sticky” high-friction films that have problems with uniformity, require gap winding. Gap winding allows a small amount of air to be drawn into the bale to prevent web stuck problems within the bale and helps prevent web warping when thicker strips are used. To successfully wind these gap films, the winding operation must maintain a small, constant gap between the pressure roller and the wrapping material. This small, controlled gap helps meter the air wound onto the roll and guides the web straight into the winder to prevent wrinkling.
Torque winding principle. The torque tool for obtaining roll stiffness is the force developed through the center of the winding roll. This force is transmitted through the mesh layer where it pulls or pulls on the inner wrap of the film. As mentioned earlier, this torque is used to create web force on the center winding. For these types of winders, web tension and torque have the same winding principle.
When winding film products on the center/surface winder, the pinch rollers are actuated to control web tension as shown in Figure 3. The web tension entering the winder is independent of the winding tension generated by this torque. With a constant tension of the web entering the winder, the tension of the incoming web is usually kept constant.
When cutting and rewinding film or other materials with a high Poisson’s ratio, center/surface winding should be used, the width will vary depending on the strength of the web.
When winding film products on a central/surface winding machine, the winding tension is controlled in an open loop. Typically, the initial winding tension is 25-50% greater than the tension of the incoming web. Then, as the web diameter increases, the winding tension is gradually reduced, reaching or even less than the tension of the incoming web. When the winding tension is greater than the incoming web tension, the pressure roller surface drive regenerates or generates a negative (braking) torque. As the diameter of the winding roller increases, the travel drive will provide less and less braking until zero torque is reached; then the winding tension will be equal to the web tension. If the wind tension is programmed below the web force, the ground drive will pull positive torque to compensate for the difference between the lower wind tension and the higher web force.
When cutting and winding film or other materials with a high Poisson’s ratio, center/surface winding should be used, and the width will change with web strength. Center surface winders maintain a constant slotted roll width because a constant web tension is applied to the winder. The hardness of the roll will be analyzed based on the torque at the center without problems with the taper width.
Effect of film friction factor on winding The film’s interlaminar coefficient of friction (COF) properties have a large impact on the ability to apply the TNT principle to obtain the desired roll stiffness without roll defects. Generally speaking, films with an interlaminar friction coefficient of 0.2–0.7 roll well. However, winding defect-free film rolls with high or low slip (low or high coefficient of friction) often presents significant winding problems.
High slip films have a low coefficient of interlaminar friction (typically below 0.2). These films often suffer from internal web slippage or winding problems during winding and/or subsequent unwinding operations, or web handling problems between these operations. This internal slippage of the blade can cause defects such as blade scratches, dents, telescoping and/or star roller defects. Low friction films need to be wound as tightly as possible on a high torque core. Then the winding tension generated by this torque is gradually reduced to a minimum value of three to four times the outer diameter of the core, and the required roll rigidity is achieved using the clamp winding principle. Air will never be our friend when it comes to winding high slip film. These films must always be wound with sufficient clamping force to prevent air from entering the roll during winding.
A low slip film has a higher coefficient of interlaminar friction (typically above 0.7). These films often suffer from blocking and/or wrinkling issues. When winding films with a high coefficient of friction, roll ovality at low winding speeds and bouncing problems at high winding speeds can occur. These rolls may have raised or wavy defects commonly known as slip knots or slip wrinkles. High friction films are best wound with a gap that minimizes the gap between the follow and take-up rolls. Spreading must be ensured as close as possible to the wrapping point. FlexSpreader coats well-wound idler rolls prior to winding and helps to minimize slip creasing defects when winding with high friction.
Learn more This article describes some of the roll defects that can be caused by incorrect roll hardness. The new The Ultimate Roll and Web Defect Troubleshooting Guide makes it even easier to identify and fix these and other roll and web defects. This book is an updated and expanded version of the bestselling Roll and Web Defect Glossary by TAPPI Press.
The Enhanced Edition was written and edited by 22 industry experts with over 500 years of experience in reel and winding. It is available through TAPPI, click here.
R. Duane Smith is the Specialty Winding Manager for Davis-Standard, LLC in Fulton, New York. With over 43 years of experience in the industry, he is known for his expertise in coil handling and winding. He received two winding patents. Smith has given over 85 technical presentations and published over 30 articles in major international trade journals. Contacts: (315) 593-0312; dsmith@davis-standard.com; davis-standard.com.
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Post time: Mar-24-2023
