Improving Edge Quality in Film Applications with Tangent Shear Slitting Presented by: Sean Craig General Manager, Maxcess-Tidland Date: October 17, 2017
What is Fracture Mechanics? The study of how things come apart Understanding why a crack propagates The study of the forces that separate things Most analysis concentrates on how to prevent fractures
but in slitting, we want to control and focus the fracture.
Plastic Flow Films react to obstacles in their path, depending on velocity, density, rigidity, temperature, crystalline structure, shear characteristics, etc
A less than desirable result For certain films, poor edge quality is disastrous to the process.
Why Razor Sharp isn t good enough Lateral (cross-machine Y mode) tensile stress forces the web laterally. Razor Slitting creates a "controlled crack" ahead of the blade edge. The physical properties of the material and the shape of the blade edge determines how and where this crack forms. The farther away from the tip the crack forms, the less stable the process. Edge flaws may develop, and uncontrolled tearing or splitting may occur.
Why Razor Sharp isn t good enough Fracture (crack propagation) occurs ahead of the razor Low Elongation or thick materials "crack" farther ahead of the blade edge. High Elongation or thin materials "crack" closer to the blade edge.
Why Razor Sharp isn t good enough The ratio of web tension to the material s yield stress must be considered. Since the blade is dragging against the web its resistance must be added to the tension force. This has the potential of exceeding the film s elastic limit as it encounters the blades shoulders immediately adjacent to the slit. Fragments, stretched, thickened, and deformed edges are the result. A general rule of thumb is that the web tension in the slitting zone should not exceed about 10% of the material s elastic limit*. * Ref: Wm Hawkins Plastic, Film, & Foil Web Handling Guide
Why Razor Sharp isn t good enough 50µ Standard shrink, oriented PVC film
A better solution: Tangent Shear Slitting Films Shear Shear stress in the vertical ( Z mode) avoids lateral cross-web conflict.
Shear Slitting Forces are perpendicular to the crack Crack is sheared out-of-plane Crack propagation is concentrated in a precise and focused location. Modes of Crack Displacement
Starting point for films: Tangent shear slitting Closely spaced infeed and outfeed idler rolls to suppress flutter 90mm/150mm upper blade and lower anvil (1.5:1.0 ratio) The Slitter Table The Slitter Table is a sacred zone The web must not be disturbed by any outside force as it passes over the Slitter Table. Maintain uniform tension, speed, flatness, guiding etc. across the entire length of the Slitter Table.
The Slitter Table
Manage tension in the slitter tension zone Tension: as needed for unwinding Tension: as needed for roll profile Unwind Tension Zone *Ref: Wm Hawkins Plastic, Film and Foil Web Handling Guide Slitter Tension Zone Tension: <10% of product yield strength* Rewind Tension Zone
Web properties critical to slitting Caliper: The measured thickness of the web Density: More than the basis weight or gsm. Elongation: How much does it stretch? Stiffness: What happens when it is bent? Tensile Strength: How resistant is it to slitting? Abrasiveness: Will it eat my blades? Compressibility: If it compresses, will it recover?
Critical factors to control in tangent shear slitting Slitter geometry Blade profile/sharpness Overlap Cant angle Overspeed Sideforce
Slitter Geometry 90mm/150mm top blade to bottom blade recommended. Influences Setback for tangent slitting Typically 1/8 to 3/8 Closing the nip Supporting the web
Overlap Upper blade overlap of lower anvil Influences: Web distortion Nip speed Nip geometry Blade wear and life
Overspeed Web speed fpm/mpm Web bubble ahead of slitter Influences Slit quality Blade wear and life
Blade geometry influences closing the nip and supporting the web. Overlap influences web distortion and nip speed, Overspeed influences slit quality and blade wear.
Blade profile/sharpness Influences: Web path through slitter Material distortion around blade Slit quality Blade life
Micro-finished, single bevel upper slitters with polished rms 2-4 finish
Wider blade profiles deflect the web down to avoid lateral web edge tensile stress typical with razor profile blades.
Slitter Ring Spacing Generous space between rings Lateral crowding avoided Single bevel blade with low grind angle minimizes edge deformation True shear geometry Lateral tensile stress imposed by narrow blade Blade acts as a wedge to smear against the cut edge. Raised edge bead and stretched edges likely Thin, acutely ground, highly polished blade tip is needed
Shear 40x Oriented PVC Supported Edge Unsupported Edge (50µ Std PVC)
Cant Angle Influences: Closing the nip Blade wear and life Sideforce Slit edge quality
Keep the nip closed Cant Angle Upper blade cant angle is a critical component to keeping the nip closed.
Keep the nip closed Open Nip No slitting will occur in an open nip. Tearing, ripping, shredding, bending, yes but no slitting. The fracture is no longer controlled.
Side Force 1 to 20 lbs typical force Influences Traction to drive upper blade Closing the nip Blade wear and life
Speed of upper blade is controlled by traction with the lower anvil. Side force directly affects traction. Not enough side force, not enough traction to drive the upper blade.
Tangent shear system Permits slitter nip synchronization Large lower slitters >1.5:1.0 lower anvil diameter ratio Recommendations Manage the tension at the nip point Do not exceed 10% of the material yield strength Isolate the tension at the slitter table when possible if material tension varies Permits varying tension to match the film's thickness and yield characteristics Microfinished, single bevel upper slitters Rm 2-4 finish Adequate slitter gap Tangent system, unsupported side Adequate overspeed Accomodates re-grind diameters variables