- October 01, 2001, William E. Hawkins, Film Handling Solutions Ltd.
Low-strength films, such PE or cast PP, sometimes are coated with an adhesive and slit into customer-sized rolls. Low-strength films do not have sharply defined yield points. Thus, they are subject to permanent width loss when put under sufficient tension for good operability in a slitter.
And, there is a significant caliper increase at the web edges, to the point where trim must be removed prior to rewinding. Because the adhesive presents a problem with pneumatic conveying of the trim away from the slitter, the trim may be wound on waste roll cores on another mandrel or wound on separate, even-wind trim winders at the machine sides. But good rewind roll formation on large-diameter production rolls often is lacking because of air entrapment in the rewind rolls.
One of the contributors to air entrapment is the edge thickening that occurs as the web is being slit, especially on older machines that use razor blades for cutting. Razor blades can present sufficient drag on a web coated with adhesive to thicken the caliper of the web edges appreciably. Part of the drag of the blade is due to the adhesive buildup, and the other part is due to the dulling of the blade by the adhesive.
Cutting drag results in thicker edges during winding that make the production roll very hard on the edges. The hard edges prevent sufficient stack compression of the winding layers by the lay-on roll to exclude the proper amount of boundary air in the production rolls.
You may obtain some reduction in edge thickening on these products by using blades that are extremely hard-coated. Also, these blades should be oscillated, crossing through the plane of the web to increase the amount of blade exposed in the cutting process. Oscillating the blades can improve blade life by 3x. Blades that are hard-coated can add another 2x to blade life.
One method of eliminating edge thickening with razor cutting is to use a pair of opposed, one-side-beveled blades for each inside cut and take away the bleed trim by a suitable means. The outside trim cuts need only one blade. The beveled side of each blade must face the trim; therefore this method requires the operator to orient the blades properly during blade change-out.
Orientation is easy to remember, because the bevel side always faces the trim. Thicker blades are required for this method of cutting than when double-bevel single blades are used. The reason that thicker blades must be used is the side pressure applied to the blade by the trim. I recommend the single-bevel blade be at least 0.040 in. thick.
Double blades for bleed trim should be mounted on a blade-holding yoke in a single oscillating blade holder, so they can move together during blade oscillation.
Free-turning rotary knives can be used in the same fashion as described above, except there is no need to oscillate the knives in the plane of the web. One of the advantages of rotary knives is the entire cutting surface is exposed to the web. Rotary knives with double bevels work exceedingly well on weak films, but they still cause edge thickening due to the drag caused by the adhesive and the bevels on the blades.
Pairs of single-bevel rotary blades mounted on yoke holders with bevels opposed for mitering the product edges can eliminate this edge thickening. Bleed trim must be removed properly from the inside cuts as discussed above.
Custom-built, driven shear knives could be used in the same manner described above for rotary knives. Weak films tend to develop thickening on the product edge that is exposed to the bevel on the male knife. And slitting adhesive-coated films with driven shear knives could be improved. However, the slitter bars would probably have to be custom built, since it is not practical to use standard shear knife designs for cutting as described above.
William E. Hawkins has 30-plus 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; e-mail: firstname.lastname@example.org.