- April 01, 2001, William E. Hawkins, Contributing Editor
Buyer beware! Before investing in new slitting/rewinding equipment, carefully consider the web handling issues that are involved.
If you are thinking about upgrading your slitting/winding equipment in the near future, it is probably time to review some major web handling considerations. In this special supplement, we will review several basic issues and offer advice on making decisions for matching equipment to your product needs.
Successfully matching the behavioral characteristics of the products with slitting, winding, and control technologies that are available from the equipment vendor will determine the success of the upgrade.
Product Machine Issues
There are many related product/machine issues to review before spending money on new slitting and winding equipment. Listed below are a few of the more prominent ones.
- 1. Breaking Strength
The ultimate breaking strength of the web is a very important parameter. Normally, for good web control, web tension (T) through the slitting and winding machines should run at equal or less than 10% of the yield point of the product. Because the yield point is not well defined in thermoplastic webs, it is usually taken between 1% and 3% of the ultimate strength.
You can calculate the entire operating tension range for your proposed machine when you know the ultimate tensile strength of each product, thickness range, and width range. Most machines will be expected to work on a 2x or more web width range. The total tension range should be used to determine the size of tension transducers and motor drive systems.
Sometimes multiple range controls are necessary when the machine is expected to operate on many different product types and thicknesses. This is because the drive control systems tend to work more accurately when the range between minimum and maximum is limited.
- 2. Catastrophic Shutdown
In the event of catastrophic shutdown, your machine components must be strong enough to break the entire web without damaging the rolls or transducers. The side frames, roll shafts, and bearings on the machine must be well oversized to prevent any damage during a catastrophic machine stoppage.
Some transducer manufacturers make units that can take large overloads without losing their calibration. These are the units to specify when frequent catastrophic shutdowns are expected.
The factors of safety for the rolls, shafts, bearings, and side frames are best left to the machine vendor. Although vendors may have very different philosophies on how robust your machine needs to be, your personal preference on this point should prevail.
If you are planning to operate at speeds of more than 1,000 fpm, keep in mind that a more rugged machine has better potential for high quality finished rolls. Also, you can expect lower maintenance costs and less downtime with the more rugged machine.
The down side is that the initial cost of the basic machine goes up with robust design. Most converters must make a compromise on what they want and what their budget will allow. My preference is to err on the robust side.
- 3. The Modulus of Elasticity
The modulus of elasticity—the ratio of web tension divided by web elongation in the elastic region of the product(s)—is an important parameter for calculating tension increases in the web. You can calculate the tension increase in the web due to elongation if you know how much elongation (draw) there is between tension isolation points, the length of web path between the isolation points, the thickness of the web, and the modulus of elasticity [see Formula (a)]. Also, you can calculate how much neck-in there will be at the operating tension level if you also know Poisson’s ratio (see Formula (b)]. The two formulas are as follows:Formula (a)
T = (M x t) x (?L/L)
T is web tension force per unit width, M is the modulus of elasticity, t is web thickness, (?L is amount of elongation between the tension isolation points, and L is the web path length between the tension isolation points.Formula (b)
e = - (µ x T)/(t x M)
e is transverse neck-in in unit width/unit width, µ is Poisson’s ratio, T is machine direction tension in unit width, t is web thickness, and M is the modulus of elasticity.
- 4. Machine Speed
You also must consider machine speed when selecting how rugged your machine should be. Web speeds above 2,000 fpm require that the machine be more rugged in all basic structures. Vibration of critical parts at high web speed can be a limiting parameter of a light machine design. Some of these parts may vibrate at their natural frequency even though they are not moving or rotating. Serious vibration in the rewind section of a slitter at high speeds can lead to lost production through telescoped rewind rolls.
Machine speed must be considered when selecting the type of roll surfaces that will work best for your product(s). High speed requires that transfer roll surfaces have a texture sufficient to provide relief volume for storage of entrapped boundary air under smooth surface webs to keep them tracking properly through the machine. Tracking friction is broken when the boundary air that clings to both roll and web surface lifts the web above the interlocking surface roughness of the roll.
You can calculate the height of the entrapped boundary air between a smooth roll surface and a smooth web by the following T.L. Sweeny and K.L. Knox (1967) formula:Formula (c)
H = (0.65 x R) x (12 x µ x (V/T))2/3
H is the gap between the web and the roll, R is the radius of the transfer roll, µ is the viscosity of air at room temperature, V is velocity of the two surfaces, and T is the tension per unit width on the web.
- 5. Surface Roughness
Surface roughness of your products is another important parameter, because web tracking and roll winding are very much related to the height and density of the web surface asperity.
Surface asperity determines the slip characteristic of the web. The roughness limits the amount of web-to-web contact and keeps the webs from blocking (adhesion of layers) in the wound roll.
The roughness is also a significant variable in the amount of stack compression that can be obtained in the winding layers of the roll when using a contact roll. Stack compression helps improve rewind roll smoothness by permitting some reduction of diameter buildup at locations where web caliper thickness is greater than the base web.
Stack compression is also a function of the volume of entrapped boundary air. Normally, converters spend much effort trying to limit excess entrapped boundary air during the winding process. However, metered amounts of air between the wraps can be beneficial, especially on clear webs with little surface asperity.
While boundary air entrapment and asperity height and density are not related per se, they do work together during winding to give a smoother surface rewind roll through stack compression.
- 6. Surface Clarity
Surface clarity is an important variable in the winding process, because a very clear web is normally very smooth and can be marred by roll surfaces before and during the winding process.
For example, you must take special precautions when a contact roll is used on very clear, tacky surfaces to prevent defects known as slip pimples (sometimes referred to as slip dimples). Slip pimples are the result of very small areas of adjacent wraps blocking during stack compression. They may be generated under the contact roll during the winding process, or they can be generated after the roll has been wound.
Rolling a clear web roll on a table or floor is a very good way to make slip pimples. Clear film rolls should always be supported from the core because of the potential of generating slip pimples.
Therefore, before investing in new equipment, you should review critically the technique for the offloading and handling of finished rolls from the winder.
- 7. Quality of the Web
The quality of the web to be wound also is significant. Caliper differences or gauge bands from the casting or extrusion areas are the bane of web winders.
Sometimes it is possible to distribute the thicker lanes over more rewind roll area by oscillating the mill roll in the transverse direction (TD) as it is unwound on the converting machine. However, in many cases, the amount of required lateral oscillation for uniform mass distribution in the rewind rolls is prohibitive because of the amount of trim that must be discarded. The period length of windup oscillation required for optimum gauge band distribution has been found to be three-quarters of the distance between standing gauge bands. The optimum speed of oscillation has been found to be about 1½ in./min.
Standing gauge bands are sometimes so severe that they result in stretched web at the gauge band areas. Baggy lanes form in the web at these points when the web is unwound from the mill roll, because the web has been stretched beyond its elastic limit.
Incoming webs that exhibit baggy lanes or edges do not yield smooth rolls even on the best winding equipment, because the baggy lanes are longer than the rest of the web. Attempting to pull the baggy lanes smooth by increasing the web tension does not work, because the web neck-in forces will narrow the flat sections of the web, and the baggy lanes will turn into machine direction (MD) wrinkles.
Sometimes it is possible to remove baggy lanes on-line by special processes before they approach the windup. Stress-relaxing technologies such as the combination hot/cold roll technology may be used to relieve many types of baggy lanes.
- 8. Tension Isolation of the Web
Tension isolation of the web before and after any process is a must for ultimate control of that process.
Many machines run open to the atmosphere, and they are candidates for using a vacuum roll for tension isolation. However, vacuum rolls must be well designed in order to work well. A vacuum roll that is well designed may be used on very clear webs without marking the surface, and it can be used on wet or dry webs. Vacuum rolls tend to be very reliable when properly constructed, and they usually stay very clean in the area of the web path.
Most superior designs are easy to adjust for web width changes, and some can even be adjusted for wrap angle changes without excessive downtime. Vacuum rolls should be installed with wrap angles from 90–180 deg.
For best performance, a vacuum roll should have two or more fine-mesh (100–150) screens covering a very porous shell. The inner screen may be made of heavier wire and be more open than the outer screen. The screens must be either endless or joined by welding to form a tube.
The welded joints must be worked very smooth to prevent marking clear surface webs. Also, these screens must be fixed to the shell to prevent movement between the screens and between the screens and the shell.
Screens are necessary to distribute differential air pressure under the web and maximize the amount of tension isolation possible. The amount of tension that can be isolated on a properly designed vacuum roll can be calculated from Formula (d) below.Formula (d)
T2 – T1 = (T1 x e( µ x K x q )) + (µ x K x ?P x R x q) – T1
T1 is the low web tension, T2 is the high web tension, µ is the coefficient of friction (COF) between the web and the outside screen surface, K is the % surface area in contact with the web, ?P is the air pressure difference side to side of the web, q is the wrap angle in radians, and R is the vacuum roll radius.
Formula (d) gives the amount of tension isolation capability per unit width. A larger coefficient of friction, wrap angle, and roll radius will directly increase the amount of tension isolation capability. However, maximizing K reduces the effective ?P by decreasing the actual surface area that is acted on by ?P.
The above formula generally describes the tension isolation of all “S” wrapped rolls, whether open to atmosphere or in a vacuum chamber. K becomes 1 and ?P becomes 0 when operating on solid surface rolls in a vacuum chamber.
Nip rolls should never be used for tension isolation unless there is no other alternative, because they can damage the web, especially clear webs. Properly designed nip rolls work well in limited situations where the nip loading pressure does not have to change as different products are run on the machine. Effective tension isolation can usually be obtained at 10–15 PLI for most applications.
There is only one nip roll loading pressure that is correct for each set of nip roll shells. Nonuniform nipping pressure will result when nipping pressure is changed. Good design requires a torsion shaft between the pivot arms of the movable nip roll. This shaft should be split and coupled with a precision shaft alignment device so that the centerlines of the two rolls operate parallel in the nipped position. The torsion shaft must be flexible enough to let a large wrap occur without destroying the nipping alignment in running position.
- 9. Surface Profiles
Surface profiles of web handling rolls should be flat or slightly concave. And they should be textured to provide a relief reservoir for excess boundary air that clings to the web and roll surfaces during operation. The texturing should be machined into the roll surfaces in order to provide good control over the roll diameter throughout the working surface.
Textured concave surface rolls are excellent web transfer rolls, because they apply a positive spreading action to the web. Generally, up to one-half of the total number of rolls in the thread path of your machine may have a concave profile. But you have to be careful where textured concave surface rolls are located. The rolls should operate with a web wrap angle of 90–180 deg. They should never be used where the web temperature is high enough to lower the yield point to where the web will be elongated beyond its elastic limit. The preferred surface profiles for concave rolls are given by Formula (e).Formula (e)
Y2 = 180000 x X
The origin of Y and X is at the center of the surface where Y represents one-half of the surface length and X represents the radius extension of the working face from the minimum radius at the origin.
- 10. Web Spreading
Web spreading is a major process item for consideration. Whether the web is to be slit or wound, it needs to be spread flat before the next operation. One of the methods for spreading already has been discussed in item 9 above.
Another method of spreading is the raised edge surface or (under-cut) roll. This method probably has been used for web spreading since webs were first made to run over rolls through a machine. I do not know the origin of this technique, but in all probability an operator found that wrapping two or three layers of masking tape under the web edges on the roll where wrinkles were appearing would make the wrinkles disappear.
One thing that these operators probably learned quickly was that a small increase in radius works well, but too much increase fails. I have found that a buildup of 0.007–0.010-in. radius works well on most webs. Also, only 1–1½ in. of extended radius needs to be in contact with most webs.
The method is simple and follows sound scientific principle. Because the roll radius has been increased, the web edges are pulled at a higher velocity than the rest of the web. Since the web edges are running at a higher tension than the rest of the web, they tend to draw the adjacent web toward the higher tension zones, which spreads the web. Care must be taken to keep the web edges centered on the raised sections of the roll. Wrinkles will appear on the raised sections if the web edges are allowed to extend beyond the raised areas.
Under-cut rolls can be used on webs that are always the same width. The raised radius sections must have surface texture to provide high surface friction so that the edges can provide the TD stretching forces. Surface texture also is necessary on any tape that may be used to raise the roll radius under the web edges.
Bowed rolls are positive spreading devices with a long tradition in web handling, and they are found on a great many machines. The spreading action comes from the angle of tracking friction that each web lane encounters as it moves around the roll. All points on the roll surface run perpendicular to the roll axis at each cross-section plane. When the roll axis is properly bowed in the web path, the traction vectors of the surface diverge toward the downstream direction, and these forces affect spreading.
Maximum wrap angle on a bowed roll is 90 deg. One of the limitations in application is that bowed rolls require a significant web path span. The reason for this restriction is that the length of the thread path centerline is longer than at the outside web edges in the span where the bowed roll is used. The web edges often go slack after they pass over a bowed roll. The extra length at outside edges must be flattened and the web tension profile adjusted before further processing, especially if the web is going through a slitting section.
Sometimes bowed rolls are misused to correct wrinkle problems that should be corrected by other means. For example, bowed rolls should never be used as an attempt to correct the effects of nonaligned rolls. Bowed rolls often are run with excessive bow when machine nonalignment is severe, as operators attempt to correct serious web flatness deficiencies. Often this action (excessive bow) leads to abrasion of the bowed roll elastomeric covering and the unnecessary generation of debris. Also, running a bowed roll in this fashion will degrade the roll’s surface friction with the web by causing the elastomeric surface to glaze over.
The rule of thumb for operating a bowed roll is to maintain minimum bow. Although bowed rolls have much less turning friction today than in the past, they should still be driven when used on high-speed and thin-web processes, because the energy to turn the bowed roll plus the work that the bowed roll is doing add significant tension to the web. Increasing web tension tends to defeat the spreading function and cause significant web neck-in downstream.
Many concepts for web spreading have limited value. One method that you can use to test whether the spreading device is working is very simple: Insert a knife just before the roll and observe the web after the roll. The knife cut will widen if the roll is doing any spreading. This simple test may save you from buying one of these devices.
Slitting Process Guidelines
The converter must decide which method of cutting (razor or shear knives) is best for his/her product. Following are points to consider.
- Razor Slitting
One of the problems encountered when you are razor slitting is edge thickening at the cut edge. This occurs because the blade retards the flow of the web and slows some of the mass to create a thicker web edge during the cutting process. The additional thickness is dependent on how much resistance the knife presents to the web.
The thickening phenomenon is exacerbated by buildup on the slitter blades. One way to reduce buildup on the blades is by oscillating the blades in the plane of the insertion.
Thicker web edges significantly increase the winding roll radius and make rewinding good rolls very difficult as they tend to keep the contact roll from properly nipping most of the surface of the rewind roll. This allows excessive boundary air to be entrapped in the wound roll.
Excessive entrapped boundary air usually results in TD or MD wrinkles. Also, thicker edges tend to wear grooves in the contact roll covering. This is detrimental when the contact roll is to be used for winding production rolls of different widths.
One method of eliminating thicker edges is to use two blades with opposed edge bevels that are positioned to cut a very thin strip between the blades, which is then removed as waste trim. The flat side of the blades must face the production web edges, and the bevel side should face the trim. This technique will produce a mitered cut on the production web edges at the expense of a small amount of trim waste. When you use this technique, you should use rigidly designed knife holders and thicker blades.
Razor slitting generates a significant amount of debris during the cutting process. This debris often has static charge, which makes it cling tightly to the web surface near the cut edge. Slitter debris is not removed easily from the web and is a frequent source of slip pimple winding defects. Good razor slitting technique is required to minimize the debris.
Good razor slitting technique requires that the web be slit at the minimum possible tension required to keep the web flat in the slitting span. Low tension reduces the amount of elongation of the web lane being cut as it is pulled past the blade. This is because the web tension of cutting is added to the web tension overall in the slitting lanes. Operating the slitting machine with excessive web tension in the slitting zone will produce a cut with wavy edges. Thus, the best slitting technique is to isolate web tension on each side of the slitting zone.
Knife angle also is important for good slitting technique. Knife angle depends on the thickness and type of web material. Generally for any one type of material, the ideal blade angle approaches more to the vertical with web thickness.
- Shear Knife Cutting
Shear cutting is superior to razor cutting in most slitting applications. It also is significantly more expensive than razor cutting, both in initial installation and maintenance costs.
Advantages of shear cutting include the following: less debris generation; higher available cutting speed; less edge thickening (especially on one side of the cut); more accurate cut; and less machine downtime for slitter blade changes.
Shear knives must be set accurately for good performance. For proper cutting the web must be supported by the anvil roll where the male knife first touches the web. This is achieved by offsetting the male knife axis downstream of the web contact point on the anvil roll, so that the male knife can penetrate the anvil roll groove without depressing the web (wrap shear).
The web is sheared as the rotating male knife pushes one side of the cut into the groove of the anvil roll. It is important to note that only one side of the cut is mitered. Blade depth should be kept to a minimum to reduce abrasion on web cut edge. Smaller blade diameters may be set at a more shallow depth than larger ones. Male knife offset, cant, and rake angles also must be properly set. You should follow the recommendations from the shear knife vendor for proper setup.
The beveled edge side of the knife wheel sometimes thickens the edge enough to cause winding problems. The thickened edge will form one raised edge on the rewind roll and lift the contact roll away from proper nipping position. This problem may be overcome by taking a narrow trim between the reversed bevel blades as described in the razor slitting section above. This technique is sometimes called taking bleed trim between blades.
Removing bleed trim may present problems during the slitting operation when many trims are being removed. Probably the best solution for trim removal is to use vacuum pick-up nozzles near each bleed trim station. These trims are joined in a common header, fed to a chopper, and blown from there to an appropriate disposal device.
Other Winding Process Guidelines
Most new winding machines offer a variation of the thread path. This configuration provides good web control near the laydown point. It also facilitates automatic roll doffing and, in special cases, automatic threadup.
But you should be aware that winding roll eccentricity has the greatest effect on web tension variation in the span between the last tension isolation roll and the wind-up roll when this configuration is used.
For optimum winding, winder mandrel eccentricity should be kept to less than 0.010 in. TIR (total indicated runout). Precision mandrels should be used for winding speeds of more than 200 fpm and must be used at speeds of more than 1,000 fpm.
You can calculate the change in web tension in the incoming span by the following formulas:
?L = 2 e + (4 e x (r/r'))
?L is the change in span length due to eccentricity in the winding mandrel, e is an average TIR of the winding roll, r is the radius of the two lead-in rolls, and r' is the length of the contact roll pivot arm.
?T = (M x ?L x t)/L
?T is the tension change from winding roll eccentricity, M is modulus of elasticity of the web, ?L is the change of span length calculated above, t is the thickness of the web, and L is the true span length between the last tension isolation roll and the winding roll.
Choosing the correct contact roll hardness for your new winder is very important. Generally, it is better to use roll coverings that are less hard on smooth clear film products. The range of reliable roll covering hardness varies from 40–75 durometer.
However, roll covers must be of sufficient hardness to enable them to be ground into very precise cylindrical shapes. Normally, soft covers (hardness <40 durometer) do not grind well, and the roll shape is not true. Also, during the winding process, very soft covers are greatly deformed. This flattening (deformation) of the contact roll surface in the nip is referred to as the roll footprint. A larger footprint means there is less contact roll pressure (PSI) with the same actuator pressure on the contact roll cylinders. And a smaller footprint means there is higher contact pressure with the same actuator pressure.
The smaller footprint excludes boundary air more efficiently from the nipping surfaces than the larger footprint at the same actuator pressure. There is more stack compression of the winding layers with harder covers than with softer covers at the same actuator pressure, and this sometimes permits harder covers to make rewind rolls that have fewer MD wrinkles (tin canning) and other imperfections. In general, harder covers require less actuator pressure to exclude the same amount of boundary air than soft covers, and it is easier to regulate the amount of boundary air that is entrapped in the winding roll.
The converter should understand what takes place in the contact roll nip on the winder.
Normally the winding roll layers are compressed (stack compression) by the contact roll. There is a very low slip zone at the center of contact between the winding roll and the contact roll. This is the area of highest compression in the nip and, therefore, has the highest frictional force of web-to-web and web-to-contact roll surface. Thus, assuming little or no slippage, the velocities of the incoming web, the velocity of the outside winding roll wrap, and the surface of the contact roll all are nearly equal at this point. This means that there is slip between the incoming web on the contact roll and the outside web on the winding roll in a zone where the stack compression starts and the center low slip zone begins.
The greater the stack compression, the larger this slip becomes. On the downstream side of the contact roll nip, there is slip between the new outside wrap and the contact roll surface as the radius of the winding roll increases as the nipping pressure is relieved.
Smooth-surface webs are prone to develop a winding defect known as slip pimples (described earlier) when a contact roll is used because of the slip between the webs in the stack compression zone. Usually, softer roll coverings are required on these webs.
However, sometimes it is possible to prevent slip pimples by using a hard-surface contact roll that has a roughened surface. The rough surface meters boundary air between the web layers and prevents localized blocking around debris particles that are often the source of slip pimples. This technique requires special technology, and if you plan to implement it, study it carefully first.
Roll Starts on Automatic Equipment
You should be concerned with the quality of roll starts on automatic winding equipment. The most serious problem is that of leading-edge foldover on the incoming roll after the web is severed.
This is due to the sudden release of tension as the cut-off knife breaks the web. Snapback will occur unless the leading edge is secured to the incoming core.
Because of the geometry of roll placement during the cutoff, securing the leading edge to the new core with some kind of nipping arrangement almost is prohibitive. I only know of one vendor who claims to have solved this problem. I know of one other vendor who solved the problem by using a transverse strip of tape in conjunction with a stationary cut-off knife. I am informed that both systems seem to work well in test trails.
Smooth roll starts are necessary to build production rolls that maintain their quality close to the core, especially on clear webs.
Auxiliary contact rolls are required to exclude surface boundary air on the outgoing roll during auto roll doffing. Usually two auxiliary rolls are required on turret windups. These rolls should be designed carefully to operate without introducing wrinkles during turret rotation and web cutoff.
The main contact roll is the heart of any winding system. Optimum winding requires a near uniform contact pressure across the entire winding surface from core to full roll.
There are three key variables to consider when choosing the best contact roll for your products. Contact roll hardness already has been discussed. Contact roll stiffness is another key variable. In general, to minimize contact roll deflection, use the stiffest, lightest contact roll shell that meets the design criteria. There is no optimum contact roll diameter for all products. Larger diameters will add stiffness at the expense of adding weight and increasing the size of the roll footprint.
Surface roughness of the contact roll is the third key variable. Surface roughness should be <50 micro in. when winding webs of smooth, clear, thin film (web <0.15 mil) at speeds of more than 200 fpm.
Optimum surface roughness will vary significantly on other products, depending on the web’s surface roughness. Usually the optimum surface roughness for your contact roll on your products must be found by trial and error.
Finally, contact rolls should never be counterbalanced. Counterbalancing adds rotational inertia to the system. Additional inertia works to prevent good contact by adding resistance to the pivotal rotation of the contact roll arms.
We have covered the major issues you must consider when upgrading your slitting/winding equipment. It may seem like a lot to digest, but the results will be worthwhile. You will improve your operation genuinely rather than simply replacing one set of problems with another. The improvements will show up in a smoother operation and in your bottom line.
William E. Hawkins is the author of the monthly “Web Solutions” column. He has more than 30 years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/474-5840; fax: 740/474-3148.