Edge slitters

Fig. 7. Extmsion of roll-cast film A, die inlet B, cast-film die C, air gap with molten web D, casting roll E, stripping roll F, idler roll G, edge-trim slitter ...

With pull-through slitters, the distance from the slitter to the recoiler is usually between 15 and 25 ft. This relatively long distance allows the strands to sway back and forth. Loose strands, in particular, will show edge position deviation upon arrival at the recoiler. Fan-out to the separator disks only adds to this condition. 

Therefore, in order to minimize interacting forces which negatively affect the quality of the slit edge, it is desirable to arrange slitter lines for tensionless slitting. ... 

With all due respect to the metallurgical characteristics, temper, thickness, or material shape, over which the slitter has no control, the quality of the slit strand is still a function of its edge condition, accuracy of width, flatness or crosscurvature, camber, and surface finish. 

Uneven side clearance on one side of the strip relative to the other side of the strip is the major cause of slitter-induced camber. Unbalanced clearance results in a different shear-to-fracture ratio on each side of the strip. The fracture tends to be stress-free, whereas the shear section tends to induce stress in the strip edge. Therefore, the edge with more shear will force curvature into the strip toward the edge with less shear. This condition is virtually undetectable in a wide strip with a width-to-thickness ratio of 100 1, but let that ratio reduce to perhaps 5 1 and you will easily see camber as a result of uneven side clearance. The setup must be equal on each side to eliminate camber from this condition. Even though an operator may use improper side clearance for the optimum slit edge, as long as they are equal, camber will not be a problem. Also, do not overlook equal knife sharpness. A setup that utilizes one dull knife and one sharp knife to a common strip can also cause an uneven shear-to-fracture ratio even though the clearance is balanced. 

The most important factor in achieving an ideal slit edge is the horizontal clearance between a pair of slitter knives (C in Fig. 2). The proper horizontal clearance depends primarily on the thickness and the tensile strength of the material. In general, as the gauge of the strip and/or its tensile strength increases, so should the horizontal clearance between the knife faces. 

Because more force than normal is required to separate the strip when the knives are too tight, the knives wear more quickly and unnecessary forces are imparted to the strip and to the slitter head itself (Fig. 3). This often causes knives to chip. When the knives are set extremely tight (Fig. 4), there are actually two fractures in the metal. The resulting edge may have a nick that extends through most of the thickness of the strip, little or no fracture zone, a double... 

When the knives are set too far apart, the strip tears. The resulting edge may have no noticeable nick, a heavy rollover on the top, and a heavy burr on the bottom of the strip. It is easy to see the rollover and the burr on the slit edge. In effect, the slitter knives are acting as bending dies. Excess clearance is rare (Fig. 5). Most operators tend to set knives tight because setting them with excess clearance can lead to immediate rejection of the slit strip. 

Slitter knives usually have two cutting edges. However, in some cases, knives are tapered on the outer diameter and thus have only one cutting edge. This is usually done when slitting aluminum and other soft nonferrous metals, in order to eliminate knife marks on the strip. 

Figure 8 Edge trimmers and slitters do not eliminate camber. At best, they duplicate the camber already present in the strip. W = width.

If the tape is to be slit into narrower strips, the carrier is almost always removed from the tape prior to the slitting operation. This is usually accomplished by having the carrier make a 180° bend around either a knife edge or a roller. The tape-cast product continues in a straight path once it is stripped from the carrier, and the bare carrier is rolled onto a take-up roller. The tape-cast product then passes into the slitter section. 

There is a choice of several winding systems. For lines with a line speed less than 200—400 m/min, a winder with inline slitting is normally used. Thus, a web of the required width and length is directly produced in the winder. The edge trim can be fed into a recycling extruder and from there into the main extruder. For lines with a higher line speed, a jumbo-reel winder with a slitter rewinder unit is used. The non woven is reeled by the jumbo-reel winder to a diameter of up to 3.5 m, and this reel is rewound and slit to the required lengths and cut sizes in the slitter rewinder. 

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