|One of the more common problems converters encounter
during razor knife slitting on thermoplastic films
is the thickening of the web material in the area
of the cut. This occurs because the blade blocks
the flow of the web, much like a protruding rock
blocks and divides the flow of water in a stream.
The stream thickness increases in front of and near
the sides of the rock due to blocked and retarded
During razor slitting of a plastic web, flow
is retarded directly in front of the angled cutting
surfaces of the blade. The retarded flow increases
the thickness of the web edges, and the amount
of thickness increase is dependent on how much
resistance the blade presents to the web flow.
Thicker edges significantly increase the winding
roll edge radius and are responsible for many
winding defects. For example, the increased roll
radius at the edges prevents the lay-on roll from
applying sufficient contact force on the rest
of the roll to properly exclude boundary air,
which, in the case of films less than 2 mils thick
and rolls wider than about 3 in., will result
in machine-direction (MD) wrinkles in the wound
Another problem caused by increased edge thickness
on very smooth films is the generation of slip
dimple distortion (resembling small bumps) on
the roll edges due to excessive pressure in these
locations. This distortion often results in the
roll - and possibly the adjacent rolls - being
scrapped because of single-sheet distortion in
the wound rolls.
Still another problem that raised edges cause
is the wearing of grooves in the lay-on roll surface.
Wear is accelerated by the high pressure applied
at these locations. While this wear may actually
help the lay-on roll maintain contact with the
rest of the roll, it will prevent using the lay-on
roll for wider winding rolls. MD grooves in the
lay-on roll will entrap boundary air, which distorts
the web in a narrow balloon band at each site
on the lay-on roll and results in permanent single-sheet
distortion in the winding roll.
Razor slitting also generates debris during the
slitting process. As the web is pulled through
the knife, strands of polymer are torn from the
web matrix by the blade-plowing action and are
then stretched before shearing or breaking at
their anchor points in the matrix. These free
pieces then tend to curl into tiny balls as they
are thrown free by elastic forces when the breaks
occur. This debris often has static charge on
it due to relative motion with the knife and adjacent
matrix material. And, it tends to stick where
it lands, near the slit edge. Rolls wound from
smooth webs often have slip dimples near the slit
edges due to slitter debris even when edge thickening
is minimal. Slitter debris is not easy to remove
from the web. Hence, prevention of debris by employing
good web handling techniques during razor slitting
is important for all converters who slit webs.
One very important parameter is web tension at
the slitter knife. Generally, the web should be
at less than 10% of yield stress in the slitter
zone. Low web tension helps reduce the amount
the web is elongated as it is pulled past the
blade. This is because the blade drag force tension
is added to the overall web tension in the slitter
zone. Excessive web tension in the slitter zone
will result in edges that are stretched beyond
their elastic range, and the slit rolls will exhibit
a wavy or waffle appearance. Thus, web tension
should be isolated on each side of the slitter
zone to provide optimum tension for razor slitting.
Razor slitting is best accomplished by slitting
in air between close transverse support rollers.
An ideal slitting zone is depicted in the figure
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 740/4745840; fax: 740/474-3148.