Slitting Shear

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.

Modes of
Crack Displacement

Shear Slitting

Forces are
perpendicular to the
crack

Crack is sheared
out-of-plane

Crack propagation
is concentrated in a
precise and focused
location.

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 Tension: as needed for
slitter tension zone roll profile

Tension: as needed for
unwinding

Slitter Tension Zone

Unwind Tension Zone Tension: <10% of Rewind Tension Zone

product yield strength*

*Ref: Wm Hawkins Plastic, Film and Foil Web Handling Guide

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 Web “bubble” ahead of slitter
fpm/mpm

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 tensile stress imposed
by narrow blade
Lateral crowding avoided
Blade acts as a wedge to
Single bevel blade with low “smear” against the cut edge.
grind angle minimizes edge
deformation Raised edge bead and
stretched edges likely
True shear geometry
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.

Recommendations

Tangent shear system

Permits slitter nip synchronization

Large lower slitters

>1.5:1.0 lower anvil diameter ratio

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