Web Spreading Systems
Table of Contents
Spreaders are found on most converting machines. Their most common role is to flatten a web prior to a critical process such as coating, embossing, laminating, printing, slitting, winding and so on. Sometimes spreaders are needed to reduce wrinkling so that a web might merely cross a roller without creases and foldovers. Spreaders are also used to separate slit webs prior to winding and to permanently widen some nonwoven products.
A spreader is any device which makes the web wider. As seen in Figure 1, the web is wider at the spreader than it is upstream. Much or all of this extra width, which results from CD (cross direction) tension is temporary. This is why one should place the spreader immediately upstream of the critical process. On some materials, such as tissue, stretch film and some nonwovens, one can easily measure the width increase with a tape. On less extensible materials, such as paper, the width increases are there, but are imperceptibly small.
Thus, one way to determine if a spreader is functioning properly is to check for a width increase. Similarly, we should expect to see a gap width increase as slit webs proceed from the slitter, over the spreader and to the windup. Finally, we could check for a visible flattening of the web at the spreader. If spreading is not detected by these means, the spreader may not be functioning, effective and/or needed.
Indeed, many spreaders are actually causing wrinkling due to setup or application problems. A common example here is a loss of traction on a bowed roller. Sometimes the so called ‘spreader’ is not based on sound physical principles. A common example here is the spiral grooving which is erroneously thought to provide a spreading action. Table 1 lists known spreading systems.
Table 1 – Spreaders
- Concave Roller
- Bowed Roller
- Bent Pipe & D-Bar
- Dual Bowed Roller
- Pos-Z Spreader
- Expander Rollers
- Edge Pull
- Nipped Stretcher Rollers
Concave Spreader Roller
The simplest and least expensive spreader is the concave roller (sometimes called a bowtie roller). As seen in Figure 2, this spreader is a conventional roller whose diameter at the ends is slightly greater than at the center. In its ideal configuration, the diameter profile of the roller is cut as an arc of a circle. However, a simpler version can be made for unslit webs by cutting a roller with conical ends and a cylindrical center. A rough starting point for a diameter reduction at the center would be 10-25% of the MD (machine direction) strain induced by web line tension or draw control.
A useful tip is that one can crudely simulate the concave spreader by banding a roll with a couple of wraps of tape at the edges of the web. However, complete traction is required for this spreader so that the roller must be well wrapped and the surface grippy. Though this spreader is an inexpensive fix, it is not particularly powerful.
The bowed roller, as seen in Figure 3, has a curved stationary axle upon which a rotating sleeve(s) is mounted over numerous bearing sets. The axle may have either a fixed or variable bow. The sleeve is typically a one-piece flexible tube of a soft synthetic composite, or may consist of numerous narrow metal shells.
The amount of bow is very important because too little will reduce the effectiveness of the spreader, while too much will also reduce spreading and may even generate wrinkles. Unfortunately, the historical tendency has been to overbow. As a rule of thumb, most bowed rollers should have a 1/8% bow, unless the material is slit, very flexible or very troughed where the bow might be increased to as much as 1/2-1% of roller width. Since traction is vital for the spreading function, the bowed roller should be wrapped about 15-45 degrees and the cover must be grippy The feel of an acceptable cover is like the surface of a tire, while one that is worn and in need of replacement feels slippery like plastic pipe. Finally, the cover may need grooving for high speed applications and the roller may need to be driven at web speed when used on light or weak webs.
The setup of a bowed roller is straightforward. First, the bow should be pointed downstream in a direction perpendicular to the bisector of the wrap as seen in Figure 4. However, the bow can be rotated into the sheet slightly to accommodate a web with a baggy center and turned away from the sheet slightly to accommodate baggy edges. Second, the entering/exiting span length ratio is desirably about 2: 1, but will work at different ratios.
Bent Pipe and D-Bar Spreader
The popularity of the bent pipe spreader lies with its simple construction. Unfortunately, this simplicity often leads to crude manufacturing practices and dubious spreading qualities. A D-bar spreader takes its name from the characteristic ‘D’ cross-section of the bar as seen in Figure 5. The only functional difference between a D-bar and bent pipe spreader is that the D-bar’s shape can be adjusted by intermediate jacks. With this adjustment, the operator can spread a local baggy or wrinkled spot, or open up a particular slit position. The bow of the bent pipe or D-bar is pointed directly into the web.
One application limitation of pipe, D-bar and other sliding spreaders is that web scratching may be unacceptable for many grades, particularly those that are coated. Indeed, even a hardened steel bar may wear out prematurely on paper grades which can be so abrasive that they generate sparks on the bar. Another issue with the bent pipe on unslit webs is that the path of the center is longer than the edges, thus inducing a degree of temporary slack edges.
There are several types of dual element spreaders that are occasionally used for the spreading of narrow slit webs. The dual bowed roller, as seen in Figure 6, consists of two bowed rollers pointing perpendicular to the ingoing and outgoing web runs. The amount of spread may be adjusted by pivoting the spreaders or by changing bow magnitudes.
The Pos-Z, as seen in Figure 7, consists of two air float bars pointed upstream. Finally, another version is a bowed spreader followed by a D-bar. The dual spreaders operate by entirely different principles than their single element counterparts.
Expander rollers, as seen in Figure 8, have shells that stretch outward from the centerline as the web transits from the ingoing to outgoing tangents. In one version, cam operated half-width slats slide at their junction in the middle. Another version has numerous elastomeric bands connecting across the width to adjustable cams on each end. Finally, another variant has a very flexible cover attached to the end cams and supported elsewhere by bristles mounted on a central shaft. One very unusual characteristic of these spreaders is that they operate successfully in traction, sliding or even transitions between traction and sliding. However, these spreaders can be speed limited and geometrically crude.
Edge Pull Web Stretchers
Edge pull stretchers are the most powerful spreaders, and are sometimes capable of increasing web width by hundreds of percent. Obviously, strains this large are limited to very stretchy materials such as films, nonwovens and textiles. Indeed, the intent may be to draw the web permanently wider.
Edge pull stretcher rollers are a pair of narrow, soft covered nipping rollers on each edge of the web which are canted outward, as seen in Figure 9. Unfortunately, these spreaders are often finicky to set up. Another edge pull spreader is the tenter used in the manufacture of some films and textiles. This spreader has an endless track which guides a chain with numerous clips which pull the web outward following the shape of the tenter track, and release it on the downstream side. A disadvantage of edge pull stretchers are severe stress gradients. Sometimes the edges are damaged so that they must be trimmed off.
As we’ve seen, web spreading is an active process which temporarily forces a web to be wider than it would like to be. However, spreaders are expensive and not always required if web flattening would suffice. Web flattening is a passive process which allows the web to be as wide as it would prefer to be.
Flattening works on the principle that the web would avoid compound curvature because it is a high energy state. The web which must first follow the arc of the roller or bar, would not like to hold a second arc as a trough or wrinkle crossing the roller or bar. As seen in Figure 10, the wrinkle crossing the roller induces compressive CD forces that would tend to push the edges outward. These forces are very weak and thus can seldom overcome the friction or traction between the web and the roller. If the friction can be reduced sufficiently, however, the trough will push itself outward until the web is flat.
Traction can be reduced by increasing the roller diameter, wrapping it lightly and decreasing the coefficient of web/roller friction. Ideally this device would be a bar, however, a roller could also be used if traction could be reduced enough to allow some but not wholesale slippage. While this technique is very effective for thicker webs, webs less than a few mils may not have enough push at a wrinkle to precipitate flattening.
In a perfect world we might not ever need spreaders. However, most machinery, products and process could use the benefits provided by spreaders which include wrinkle reduction, web flattening or opening up die gaps between slit webs. The keys to achieving these benefits are to understand why spreading is needed, where it is needed, and how to best accomplish it.
For Further Information
Roisum, David R. The Mechanics of Web Spreading Parts I & II. Tappi J., vol 76, no 10, pp 63-70, October 1993 and vol 76, no 12, pp 75-86, December 1993.
Roisum, David R. The Mechanics of Wrinkling. Tappi J . , vol 79, no 7, July 1996.
Written for Faustel, Inc. by David R. Roisum, Ph.D.