The gold standard of slitting design is spread > slit > spread > wind. In this ideal, we use two different spreaders. The first is called the pre-slitter spreader, and the second the post-slitter spreader. Each has a separate and vital function for optimum converting health. The venerable paper industry has known this for decades, but the message has been slow to reach converters. Let me try to explain why spreading should (almost always) be paired with slitting.

The pre-slitter spreader has two functions. The first is to make the web dead flat going through the slitter section. Even the slightest of puckers, barely visible to the eye, will be enough to destroy tight width tolerances of the slit product. The reason is simple—the width of the web with a pucker is slightly wider than the distance between the slitter blades. This is not a problem with a constant pucker size, but this is also not usually the case.

The second function of a pre-slitter spreader is to provide a small amount of CD tension. This can assist the other stresses at the slitter blade(s) to boost cut quality. Beware, however, that there is a near epidemic of oversizing of almost all spreaders, resulting in web instability, wrinkles and loss of spreading.

The post-slitter spreader has just one function—to provide a tiny separation between the slit lanes. This keeps the individual strands from tangling each other. Some of this separation is the result of a possible slight tension increase at the slitting span with respect to the one just upstream.

If we are merely edge-trimming, rather than cutting a wider web into narrower webs, we do not use a “spreader” in the conventional sense. Rather, the trim chute pulls the trim at a very tiny angle away from the main web. Converting operators who yank the web sideways in the belief that cutting will be improved find the result is actually the opposite. Cutting is greatly degraded when the trim is directed at even a small angle because it forces a shear pucker in front of the blade.

Just as good slitting requires a rock-steady web in that area, so too does good edge trimming require a rock-steady trim. This can be done several ways. One is to use a trim pan at the slitter instead of a trim opening some distance away. If the opening is away from the cut point, the trim can be stabilized by running it over the next roller. Some pull their trim at a ZD angle instead of a CD angle, which is more benign to cut quality, but the above principles apply there as well.

We now see the reasons for the two spreaders. We also see these reasons are different. Thus, if we equip our slitters with two spreaders, they would be sized differently. Using a bowed roller on paper, for example, the pre-slitter bow may be about 0.125% and the post-slitter bow about 0.5%. The after-slitter bow has more (separation) work to do and the individual lanes are easier to move than increasing the width of an unslit web; thus two reasons for the difference in size.

920/725-7671, drroisum@aol.com, www.roisum.com