The Role of Blade Sharpness in Cutting Instabilities of Polyethylene Terephthalate

Industrial polyester ®lms are produced in large rolls that need to be cut to speci®c dimensions to make ®nal products. The deformation and secondary fractures during the cutting or slitting processes reduce the amount of usable material. An ideal cut should be perpendicular to the web, leave minimal debris, be performed at high speeds and have no deformation at the edges. The dominant mechanical factors in cutting polyester ®lms are tension in the web, blade sharpness, cutting speed and material properties [1]. Arcona and Dow [2] examined the role of blade sharpness in the slitting of polyethylene terephthalate (PET) using commercially available scissors. One blade was ®xed and the other driven by a variable speed motor. The cutting force was measured with a load cell attached to the scissors. Impressions of the blade, made with vinyl polysiloxane, were sectioned and viewed under a light microscope to determine the blade radius. The cutting speed and the angle of the blade entering the web varied over the length of the cut. Their results showed the cutting force strongly decreasing with cutting speed and increasing with blade radius. Our experiments on cutting a PET web, with rate and knife angle independently controlled, showed that for a ®xed blade angle, the cutting force varied by less than 5% when the cutting speed changed from 10 mm sy1 to 10y2 mm sy1 [3]. We concluded that the blade cutting angle can cause the web to buckle locally but that the cutting force is nearly speed-independent for sharp knives. In this letter, we have varied the knife blade radius and cut a PET web again with a wide selection of cutting speeds. At very large cutting radii, we observed a negative strain rate±sensitivity [4] type of effect. This rate instability in the cutting force shows a decrease in cutting force for increasing cutting rate. The effects of blade sharpness and cutting speed were examined on 178 2 im PET. The test ®xture measured the cutting force and allowed independent control of the cutting speeds and blade angle [3]. Stanley #11-921 knife blades were dulled using a special tool designed to round the edge at the blade tip. The blades were dulled by abrasion on 600 grit silicon carbide paper. The blade direction was reversed at the midpoint of dulling to ensure a symmetric edge. It was found that 100 passes over silicon carbide paper increased the radius of curvature of the blade by approximately 1.4 im. The blade tip radius was measured using 3M Express 7322H vinyl polysiloxane impression material. The impressions were made with the blade vertical and held still during the hardening process so the radius was not skewed. The mold was sectioned into pieces approximately 1.5 mm thick. These sections were viewed under a light microscope, and the radius was determined by ®tting the blade tip to a circle template and recording the microscope's magni®cation. This method was veri®ed by mounting a section of the blade in a molding material and metallurgically polishing it for viewing under the light microscope. The radii for the two methods differ by less than 1 im, which con®rms the validity of the radius found using the impression material. Sections of the blade were also viewed in a scanning electron microscope (SEM), and it was found that the radii matched those measured by the impression material. All testing was conducted using the slitting apparatus, seen in Fig. 1, mounted on a screwdriven test machine (Instron Model 1115). The slitting apparatus was designed to deliver a long, straight slit to the PET and accurately record the force needed to make the cut [3]. The web was