Web edge guiding – an indispensable necessity?
Substrate webs running through reel-to-reel printing or converting machines from unwind to rewind are guided by numerous rollers, whether they are servo driven or not. Such web paths usually are not straight-line through the machine but travel a winding path. Therefore, this article deals with the prerequisites necessary for the best web guiding technique.
by Dr Renke Wilken
To achieve accurate register in manufacturing steps, the exact position of the web on the respective rollers has to be defined and always kept constant. This is an absolute necessity to ensure that all printing and die-cutting units work in a register accurate way. Each change of the web position and each movement of the web is a potential threat to the precision of the respective process steps which would mean acceptable product quality levels cannot be achieved.
Exact and parallel
To achieve a constant web path, there are many preconditions which must be fulfilled in connection with the web itself and the condition of the processing machine. As far as the machine is concerned, all web guiding rollers must be aligned in the longitudinal machine direction in an exact parallel and rectangular way. In addition, the rollers must be correctly cylindrical, as any deviation may damage the web irreparably.
Moreover, the reel must be precisely wound and perfectly positioned in the processing machine and the paper web must pass over the rollers almost slip-free. With servo-driven rollers, this is important for the best torque transmission, whereas with non-servo-driven rollers this is important to overcome friction at the roller bearings. Any slip between paper and roller would result in friction which may result in marks on the paper surface. Drive torque in combination with brake torque at the unwinding area would generate the necessary web forces required to achieve a certain web tension. In turn, such forces generate expansion conditions, which should not exceed the material-specific elastic deformation range as this would result in irreversible elongation of the web. The elasticity limit of many substrates is quite low and the rough reference value for paper is 0.1%.
However, this does not mean that the objective has been achieved as there are also certain requirements on the substrate web. In general, all longitudinal and lateral expansion caused by the web forces must be the same everywhere.
It is apparent, that all the ideal conditions described above can be hard to find in everyday production environments. However, it can be confirmed that the mechanical prerequisites for accurate web guiding are given. It is much more difficult to ensure that following a reel change the new reel is positioned in exactly the same position as the old one. Of course, the paper web does not show the same characteristics throughout. Particularly critical are the hanging cross profiles of attributes effecting the substrate expansion. The consequences of non-ideal conditions occurring can result in deviations of the web path from the specified value. However, to achieve the required constant web path, corrective measures must be taken which can be summarised by the term “web guiding”. To implement these measures successfully a reference variable is required and the web edge is particularly suitable, as its position can be determined precisely optically or by the use of ultrasonic sound. Levelling devices for correcting the web path can cause more difficulties, as they have to intervene in an active way often causing new problems if they are constructed incorrectly.
The effects of tilted rollers
Figure 1 shows an arrangement of three non-servo-driven guiding rollers. Given the assumption that the second of the three rollers is tilted, this results in an arrangement as shown in figure 2. It can be seen that the roller gap on the left (L) is wider by the factor L1 in this case the upper roller is tilted relative to the others by the angle .
This means, the web must travel a longer path on the left side than on the right which is determined by the factor L+L1. This is only the case if the web is stretched more on the left side than on the right. Stronger stretching requires higher web tension. In general, a tilted roller generates a web tension profile suspending from left to right.
Given that a = 0.1° and B = 1000 mm (39.4”) this results in L1 = 1.7 mm (0.07”). Whether this length change is critical or not depends on L and the stretch factor e which is generated by the web tension and the cross profile is assumed to be absolutely even. Given L = 200 mm (7.9”) this results in an additional web stretching on the left edge of e1 = 0.85 %.
Close look at the details
If the web stretching factor e caused by the web tension is already near the elasticity limit of the paper substrate, it would be significantly exceeded by the additional stretching on the left side. This would result in irreversible elongation of the left side. As far as the web running out of true is concerned, the right edge is tighter than the left which may even flutter. During rewinding, the right side of the reel would be wound in a firm way and the left in a soft way respectively.
Due to the higher web tension on the left side higher forces are generated on the left side compared to the right side. When the web exits from the tilted roller this creates a tendency to give way to the right side by deviating at an angle of a. The wrong way to counteract this tendency and avoid fluttering is to tighten the web by increasing the web tension as this would overstretch the previously intact right web edge causing irreversible damage. This example indicates the critical situation caused by tilted rollers. Often, web path faults can be attributed to this phenomenon. Sometimes the rollers supplied are faulty as already tilted rollers generate a hanging stretch profile. This is indicated by the fact that one side of the substrate reel is wound ultra-tightly whereas the other one is and the other side roller is wound noticeably smoother.
Web edge guiding
For web guiding purposes roller arrangements as shown in figure 3 may be used. Rollers 1 and 4 are mounted firmly whereas rollers 2 and 3 are mounted commonly on a rotating table and a rotating axis is levelled to the incoming web between the rollers 1 and 2. The reference variable is provided by a web guiding sensor at the web which exits between rollers 3 and 4. If the rotating table is twisted by the small angle a the roller 2 acts analogous to the tilted roller in section 2. This generates web path differences and therefore stretching differences which causes the web to leave roller 2 with the rotation angle a perpendicular to the roller axis. As soon as the web has passed the parallel roller 3 and moves to roller 4 the same happens as between roller 1 and 2 but this time on reversed sides. Afterwards the web edge is again parallel to the machine direction however shifted by the value K. This value results from the rotation angle a and the distance Lü between the rollers 2 and 3:
The connection between rotation angle a, roller distances L1,2, web width B and generated rotation e is defined as follows
The stretching of the web or rather the web tensions causing the stretching generated an almost symmetrical but hanging cross profile following roller 4. The already generated stretching of the web passing roller 1 and the additional stretching caused by the relation expressed in the above formula are important factors if the elasticity limit of the web is exceeded and the web is irreversible damaged during the process of web edge guiding or not. As a rule of thumb, the distance L between the single rollers should correspond to the web width B. If very stretch sensitive substrates are used, it may be recommended to switch to bigger distances between the rollers.
Vertical folds and spreading
Vertical folds appear if the web tension F exceeds a certain critical value Fcr. The reasons for this are cross forces which affect the web through the impact of tension forces which result in lateral contraction. This is similar to the bending or buckling of long thin metal plates. The critical web tension value can be determined by using the following formula:
E stands for the elasticity module, D stands for the thickness of the substrates, L stands for the span length of the web (distance between the guiding rollers), µ stands for Poisson’s ratio (transversal contraction) and m stands for the number of folds. The web tension factor ß is defined as follows:
This formula indicates, that the web tension factor ß is not a constant but depends on the web width B and the free span length L. It also indicates, that the number of folds are caused by web tension and web width. Such longitudinal folds should be avoided, as they may cause a squeezed fold with Ω-shaped cross section when the web passes the guiding rollers. Such thick spots can be three times thicker than the original web which may cause trouble for subsequent processes.
There are several potential measures which can be taken to avoid the generation of folds:
- Avoiding exceeding the critical degree of web tension
- Avoiding the use of out-of-round guiding rollers
- Reducing the free span length through the mounting of more guiding rollers
- Avoiding of hanging web tension profiles thus avoiding the risk of local fold generation
- Mounting of spreader rollers or expanding devices
Spreader rollers or spreader bars can be found in many web guiding systems. These are curved rollers or bars, whose apex is at the centre of the web. They behave like tilted rollers generating a web tension profile declining from the centre of the web down to the edge. Such distribution of the transverse forces squeezes the web from the centre to the edge ironing out the folds. Figure 4 shows a schematic drawing of this process.
Spreader rollers or spreader bars can also be used to increase the distance between small substrate reels cut from the mother reel (figure 4).
Spreader rollers are also characterised by the fact that the circumferential speed of the roller is equal to the web speed. Technically this can be achieved as follows: An axis suitable for hydraulic curving is provided with a segmented outside shell with a smooth surface. The curve is shaped through the arrangement of the single segments. To avoid marks caused by the joints between the segments, the outside shell can be include a flexible covering. Through adjustments of the roller plane and/or the roller height the spreader roller can be adapted to the requirements of the substrate web.
The accurate arrangement of the spreader roller between the neighbouring guiding rollers is most important. The distances shown in figure 5 indicate as a rule of thumb that the distance A has to be roughly two times larger than the distance E, whereas E should be at least two to three times larger than the diameter D of the spreader roller. Another influencing quantity is the wrap angle of the spreader roller. For paper substrates it should be about 20°, whereas for polyethylene (PE) or polypropylene (PP) films and textile substrates it should be about 60° C. When using spreading bars the curved surface does not rotate so the web has to glide over the surface. To avoid damage of the web through friction, the wrap angle should be as small as possible.
Meeting high demands
To achieve satisfactory web guiding in printing and converting high demands have to be met. Tilted guiding rollers inevitably result in faults like hanging cross profiles in web tension. As long as the corresponding stretching remains within the limits of the elastic deformation range, the potential damage is rather low. On the contrary, exceeding this range results in irreparable damage to the web. Other disruptive factors include errors in parallelism, rollers with insufficient regular concentric running or eccentrically mounted rollers. However, the web edge guiding device may also generate faulty tension profiles. Also the web tension may cause undesired folds in the longitudinal direction if critical values are exceeded. Therefore it makes sense to take a close look at every single element of web guiding.