Note: Descriptions are shown in the official language in which they were submitted.
1~77383
)
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a process for
cutting or perforating sheets of plastic film material,
especially as part of a process for making flexible plastic
film packaginq materials. More partlcularly, the invention
relates to a process for making plastic film packaging bags
and in a preferred embodiment, to a process for making con-
0 tinuous strips of interconnected, but separable, plastic film
packaging bags including the step of perforating the plastic
film material with a perforating blade having improved service
durability. The present invention is especially useful in
making packaging bags composed of low pressure-low density
polyethylene film.
Description of the Prior Art
Processes and equipment for making plastic film
wrapping sheets and bags in continuous strips, and providing
~0 tear-off lines for one at a time removal of the individual
wrapping units, have been well-known in the prior art and
commercially available. A known process for forming con-
tinuous and interconnected but separable plastic film pack-
aging bags includes the steps of extruding a tube of plastic
film material, such as polyethylene, by a tubular blown film
extrusion process and advancing the tube through a bag-making
machine wherein the advancing tubular film material is heat-
sealed across its width at spaced longitudinal intervals and
perforated across its width at the same intervals to allow
-- 2 --
773~3
later separation into individual bags. The resulting plastic
; bags have a variety of packaging uses, such as for garments,
trash, produce, meat, and the like. In most cases, the bag-
making machine (such as those available from Gloucester
Engineering Co.) is provided with shuttling means to momentarily
stop the tube advance for the sealing and perforating opera-
tions; however, apparatus is also known which travels along
with the advancing tube to seal and perforate same without
the necessity of momentarily stopping the tube advance. The
resulting continuous strip of interconnected and separable
plastic bags may be rolled for dispensing later or the bags
may be separated and stacked by methods and apparatus known in
the art. Single-ply plastic film wrapping sheets, in continuous
strips of interconnected and separable sheets or separated
sheets, may be made by a similar process by starting with a
single layer of plastic film or sheet material.
The perforating is usually performed by forcing a
serrated-type blade through the plastic film tube. 5uch
perforating blades are commercially available in various con-
figurations and are typically composed of a flat steel bodyhaving teeth along one edge thereof. The configuration of
the teeth depends, in part, on the type and size of bag being
made and the plastic film material of which it is formed.
Those skilled in the art are well aware of the considerations
governing the selection of a perforating blade for such
applications. As an example, in a process for making low
density polyethylene produce bags in a Gloucester Engineering
Model 418 bag machine, a 10-inch long perforating blade com-
posed of No. la95 carbon steel and having 40 teeth along one
edge thereof, may be employed. Each tooth is chamfered on one
side as a result of a sharpening operation to thereby provide
a cutting edge.
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Such plastic film packaging bags and sheets have
been made of various types of thermoplastic film materials,
including low- and high-density polyethylene, polypropylene
and the like. However, low density polyethylene i8 perhaps
the most important of the thermoplastic packaging films,
accounting for a significant portion of the total usage of
such films in packaging. Low density polyethylene possesses
a unique combination of properties essential for broad end
use utility and wide commercial acceptance in the packaging
field. These properties include film optical quality,
mechanical strength properties (such as puncture resistance,
tensile strength, impact strength, stiffness and tear
resistance), vapor transmission and gas permeability
characteristics and performance in film converting and
packaging equipment.
In processes for preparing, for example, low
density polyethylene bags, employing polyethylene made by
the conventional high pressure process, the expected service
life of a perforating blade may be on the order of 2 to 4
weeks. However, it has been found that when preparing low
density polyethylene bags by the same type of process, where
the polyethylene is made by the so-called low pressure
process, the service life of the perforating blade was
reduced to a matter of hours. In such a case it may be
necessary to close down an entire commercial line to change
perforating blades at frequent intervals or the resulting
- product may become unsuitable due to poor quality perfora-
tions. Either situation provides a totally unacceptable
commerical process.
Since low pressure-low density polyethylene film
is tougher than the corresponding high pressure material,
it was postulated that the cutting edges of the reduced-
life perforating blade were wearing. However, in one
~1'î'7383
instance, afteL a blade was no longer acceptable for per-
forating low pressure-low density polyethylene film, it was
found that it was nevertheless still useful in perforating
conventional high pressure-low density polyethylene for an
additional period of time of about 3 weeks. It was also
found that cutting edges coated on both sides with a hard
material did not significantly extend the useful service life
of a perforating blade in a process for making low pressure-
low density polyethylene film bags.
The prior art teaches various types and shapes of
cutting devices in various applications. For example, U.S.
Patent No. 4,064,776 discloses a method and apparatus for
making tear-resistant, separable end-connected plastic film
bags utilizing a perforating blade which is serrated in shape
and which is provided along its length with deep recesses
beyond the cutting edge to define connecting tabs between the
perforations across the width of the advancing film material.
The blade is also heated to effect annealing of the per-
foration edges.
U.S . Patent No. 4,161,382 discloses an apparatus
for making containers from thermoplastic sheet material,
including a cutting blade having at least one cutting edge
extending vertically at an oblique angle to the surfaces to
be cut, wherein slots in the sheet are formed when the blade
is moved vertically into the sheet material.
It is also known in the prior art that the dura-
bility of various types of cutting blades can be increased
by providing a coating of an extremely hard material such as
tungsten carbide on one side of the cutting edge. The prior
art discloses that ~his may be due in part to the fact that
since the tungsten carbide coating is significantly harder
1177383
than the substrate onto which it is coated, more and more of
the tungsten carbide coating is exposed as the softer sub-
strate is worn away during use, thereby presenting a sharper
cutting edae for longer periods of time. This self-sharpening
effect of one-sided coating has been recognized for use in
cutting devices such as household knives, paper cutting and
trimming knives and other types of beveled disk knives.
U.S. Patent No. 3,975,891 disclosec a rotary mower
blade made of outer layers of metal having an inner layer of
extremely hard material and shaped such that attrition in
use of the outer metal layers exposes the inner extremely
hard material to maintain a sharp cutting edge. It is dis-
closed that a fine cutting edge is formed and maintained as
a result of the wearing of the blade, instead of the blade
becoming more dull by such wear.
Other one-sided coated cutting instruments are
disclosed in the prior art. For example, U.S. Patent No.
3,618,654 discloses a blade for cutting plastic material
such as tire stock, having a body of steel with a flat back-
ing surface and a channel in one edge thereof which containsa ring of tungsten carbide to be ground flush with the steel
body on one side and projecting outwardly so that it is ex-
posed on both sides and terminates in a cutting edge. U.S.
Patent No. 2,634,499 discloses a similar cutting edge, com-
prised of a piece of tungsten carbide bonded to a substrate
and designed for cutting materials such as asphalt roofing
- and other abrasive compositions.
U.S. Patent No. 3,988,955 discloses a band saw
blade comprising a steel body having a plurality of teeth
spaced along one edge thereof. The tip of each tooth is
coated with a hard carbide material and then impulse hardened.
The coating overlaps both sides of the cutting edge.
-- 6 --
117~7383
SUMMA~ OF THF INVENTION
In its broadest aspects, the present invention
comprises an improved process for cutting or perforating
a plastic film sheet material employing a cutting or per-
forating blade which exhibits improved service life
durability, the blade being composed of a metal substrate
having a coating of a hard material on only one side thereof.
More specifically, and in a preferred embodiment, a process
for making a continuous strip of interconnected and separable
~i0 plastic film bags is provided which includes the steps of
providing, such as by tubular blown film extrusion, a tube
of plastic film material and sealing and perforating the
tube across its width at spaced longitudinal intervals,
wherein the perforating is accomplished with a blade coated
on one side only with a hard material such as tungsten
carbide. The process is especially useful in the manufacture
of continuous strips of interconnected but separable bags of
low pressure-low density polyethylene film, employing a
serrated metal perforating blade which is beveled or chamfered
on one side only and whose flat side only is coated with a
hard substance such as tungsten carbide. In such a process,
these blades exhibit a significantly improved service life as
compared to uncoated similar blades or similar blades coated
on both sides.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of a vertical
stroke heat-sealing and perforating apparatus which may be
used in a process for making continuous strips of intercon-
nected but separable plastic film bags.
773i~3
Figur~ 2 is an enlarged plan view of the teeth area
of a serrated and slotted metal perforating blade.
Figure 3 is an enlarged plan view of the teeth
area of a serrated, unslotted metal perforating blade.
Figure 4 is an enlarged side view of one of the
teeth of the perforating blade of Figure 3.
Fiqure 5 shows the blade tooth of Figure 4 with a
coating on the flat side thereof in accordance with the
invention.
o Figure 6 is an enlarged view of the tip area of
the blade of Figure 5.
Figure 7 is a view illustrating the tip of Figure
6 after some wear has occurred in use.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although the process of the present invention has
particular utility in a process for cutting or perforating
film sheets of low pressure-low density polyethylene, it is
expected to be useful in cutting or perforating many different
>o types of plastic materials, including conventional high
pressure-low density polyethylene, high density polyethylene,
polypropylene, polyvinyl chloride and the like. In addition,
the invention is expected to be useful in cutting or perforating
paper or paper-like sheets.
As is known by those skilled in the art, low
density polyethylene typically has a density of about 0.94
q/cc or lower while high density polyethylene has a density
of above about 0.94 g/cc. Conventional low density poly-
ethylene has in the past been made commercially by the high
pressure (i.e., at pressures of 15,000 psi and higher)
homopolymerization of ethylene in stirred and elongated
117'~383
tubular reactors in the absence of sol~ents using free
radical initiators. Recently, a low pressure process for
preparing low density polyethylene has been developed which
has significant advantages as compared to the conventional
high pressure process. One such low pressure process is
disclosed in commonly-assigned, copending U.S. Applications
Serial No. 12,720, filed February 16, 1979 and Serial No.
~` 892,322, filed March 31, 1978 (a foreign-filed application
corresponding thereto has been published as European Patent
, lO Publication No. 4647).
, The above-identified copending application discloses
a low pressure gas phase process for producing low density
~r, ethylene copolymers having a wide density range of about
0.91 to about 0.94 g/cc and a melt flow ratio of from about
22 to about 36 and which have a relatively lo~ residual
catalyst content and a relatively high bulk density. The
~ process comprises copolymerizing ethylene with one or more
f~ C3 to C8 alpha-olefins in the presence of a high activity
magnesium-titanium complex catalyst prepared under specific
activation conditions with an organo aluminum compound and
impregnated in a porous inert carrier material. The copolymers
thus prepared are copolymers of predominantly (at least
about 90 mole percent) ethylene and a minor portion (not
more than 10 mole percent) of one or more C3 to C8 alpha
; olefins which should not contain any branching on any of
' their carbon atoms which is closer than the fourth carbon
r~ ~ atom. Examples of such alpha-olefins are propylene, butene-
1, hexene-1,-4-methyl pentene-l and octene-l.
, The catalyst may be prepared by first preparing
, 30 a precursor composition from a titanium compound (e.g., TiC14),
a magnesium compound (e.g., MgC12) and an electron donor com-
, pound (e.g., tetrahydrofuran) by, for examplet dissolving the
titanium and magnesium compounds in the electron donor compound
_ g _
i~7 7383
,,
and isolating the precursor by crystallization. A porous
inert carrier (such as silica) is then impregnated with the
precursor such as by dissolving the precursor in the electron
donor compound, admixing the support with the dissolved
precursor followed by drying to remove the solvent. The
resulting impregnated support may be activated by treatment
with an activator compound (e.g., triethyl aluminum).
The polymerization process may be conducted by
contacting the monomers, in the gas phase, such as in a
fluidized bed, with the activated catalyst at a temperature
of about 30 to 105C and a low pressure of up to about 1000
psi (e.g., from about 150 to 350 psi). The resulting low
density ethylene copolymers may be formed into thin film
having improved puncture resistance, high ultimate elongation,
low thermal shrinkage and outstanding tensile impact strength,
by extrusion through an extrusion die having a gap of greater
than about 50 mils. One such process is disclosed in commonly-
assigned, copending U.S. Applications Serial No. 12,795,
filed February 16, 1979 and Serial No. 892,324, filed March 31,
1978 (a foreign-filed application corresponding thereto has
been published as European Patent Publication No. 6110).
The film thus prepared may contain conventional additives
and may have a thickness of about 0.1 mil to about 20 mils
and may be formed into a tube by the tubular blown film
extrusion process.
Although the present invention is applicable to
any process for cutting or perforating plastic film sheet
material, for purposes of convenience the invention will be
described herein by reference to a process for making con-
tinuous strips of interconnected but separable plastic film
packaging bags. It should be understood that it is not in-
,:
v -- 10 --
.
.,.
1~773~3
tended to limit the invention thereby; rather, it should beexpressly understood that the invention is limited only by
the scope of the claims appended hereto.
In addition, for ease of description only, the
` present invention will be described by reference to a process
for making such bags composed of the presently-preferred
material, low pressure-low density polyethylene, although it
is to be understood that the invention is or is expected to
be applicable to other materials such as conventional high
pressure-low density homopolymers and copolymers of ethylene,
high density homopolymers and copolymers of ethylene, homo-
polymers and copolymers of propylene, homopolymers and co-
polymers of vinyl chloride, paper and the like. For purposes
of definition, by "low pressure-low density polyethylene" is
meant ethylene polymers having a density of about 0.91 to
about 0.94 such as the ethylene-C3 to C8 alpha olefin co-
polymers described above.
As indicated above, plastic film bag-making machinery
and processes are commercially known and available. A
typical process includes the steps of forming a plastic film
tube by the tubular blown film extrusion process and advancing
the plastic film tube in flattened form through a bag-making
machine where the tube is heat-sealed across its width at
spaced longitudinal intervals and perforated at similar
intervals with a perforating knife to provide means for
later separating the individual plastic film bags from the
resulting continuous strip. Depending upon the type and size
of the plastic film bags being manufactured, any known and
commercially-available equipment can be employed. For example,
to make garment-type bags which have a width of approximately
2~ inches, Gloucester Engineering Company bag-making machines
identified by Model Numbers 417 or 419 may be employed. For
-- 11 --
~ '7383
.
i smaller produce-type plastic film bags, Gloucester Model Numbers
ç~'/~ 4~
~g or 425 may be used. All of these machines utilize a pair
- of rolls which shuttle back and forth to momentarily halt the
advance of the plastic film tube through the bag-making
machine and enable the heat-sealing and perforating operations
to occur.
Most commercially-available bag-making machines
employ perforating blades having a length of 10 inches, and
, 3 or more blades may be required across the width of the
bag-making line. In those machines such as the garment bag-
making type, which have a width of about 30 inches, only a
single flattened tube of about the same width is fed through
the machine. However, it is possible to feed one large tube
into the bag-making machine and produce more than one line of
bags simultaneously b~ conventional techniques. For example,
~ a single large tube may be fed into a single machine to
; produce separate rolls of interconnected but separable bags,
7 wherein the large tube is subjected to perforating and heat-
sealing operations and then is slit-sealed longitudinally to
provide several distinct rolls of bags. When it is desired
to separate and stack such bags, starting from a single
large plastic film tube, the operations are reversed; i.e.
7 the large tube is first longitudinally slit-sealed followed
s' by perforating and heat-sealing each of the resulting tubes.
Conventional means, such as nip rolls driven at a speed
greater than the tube advance speed in the machine, may be
- provided adjacent the exit of the bag-making machine to
effect separation of the interconnected strips into in-
dividual bags.
Regardless of the type of bag-making machine em-
ployed, the process of the present invention includes the
step of perforating a plastic film with a perforating
- 12 -
, ~
1~773t33
knife of improved durability. The perforating operation is
normally conducted by vertically stroking the perforating blade
into and through the plastic film tube. Figure 1 of the
attached drawings schematically illustrates a vertical
stroke heat-sealing and perforating apparatus of a co~mer-
cially available bag-making machine. Referring to Figure 1,
a tubular film 10 is shown advancing right to left in the
drawing over supporting means 11. Conventional means (not
shown) may be provided for momentarily stopping the advance
of the plastic film tube, at which time the heat-sealing and
perforating operations can occur. These operations are
normally performed simultaneously by vertically stroking a
heat-sealing means 12 and perforating blade 13 downwardly as
shown in Figure 1. A heat-seal is formed across the width
of the flattened tube to provide a sealed bottom portion of
a plastic bag. Simultaneously, the perforating blade 13
punctures the tubular film 10 by being forced through the
film into slot 14. Stripper bars 15 are also stroked down-
wardly at the same time to hold film 10 in place during the
pe.rforation and to facilitate removal of the perforating
blade from the tubular film during its upward stroke, as is
conventional.
These sealing and perforating operations, in com-
mercial processes, may be repeated at a rate of from about
10 to 170 times per minute. The cycle speed required of the
perforating blade, of course, depends upon how rapidly the
~ plastic film tube is advancing through the bag-making machine.
This, in turn, depends upon the size of the bags being made.
It is apparent that for a given speed of advance, the rate of
perforation is substantially less for long bags such as
garment bags in comparison to produce bags where the per-
forations are much closer. Generally, in processes for making
117~383
plastic film packaging bags, the plastic film tube may be
advanced through the bag-making machine at a rate of about
10 to about 400 feet per minute or higher. Normally, the
higher rates are for the longer bags such as garment bags.
The typical rate of advance in a process for producing
produce-type bags having a length of about 16 to 20 inches
is about 150 to about 160 feet per minute, at which speed
and for 16 inch-long produce bags, a perforating blade cycle
of about 147 times per minute would be expected. Of course,
the cycling time of the perforating blade may vary greatly
depending upon how rapidly one wishes to advance the plastic
film tube through the machine and upon the length of the bag
being manufactured. Those skilled in the art are well aware
of the controlling factors. Despite the particular speed of
advance, as the cycling speed of the perforating blade
increases, the wearing of the blade becomes more rapid and
hence this additional factor must be balanced in determining
the desired or optimum operating conditions. Those skilled
in the art may determine the various conditions of operation
given the desired results.
As the perforating blade exhibits wear and becomes
dull, the plastic film may not perforate cleanly, which is
undesirable since uneven elongated film areas surrounding
the perforation lines can lead to bag failure by premature
tearing. The severity of this problem is influenced by the
film gauge with the thinner gauge films presenting more
problems than thicker films. The thickness of the plastic film
bags may vary considerably depending upon the desired end use
and particular plastic film employed. As an example, for low
pressure-low density polyethylene, produce-type bags generally
may have a film thickness of 0.0003 to 0.001 inch and garment-
type bags may have a thickness of 0.0003 to 0.001 inch. It
is obviously desirable to use a perforating blade having as
- 14 -
1177383
.
long a life as possible since it is commercially un-
acceptable to shut down an entire bag-making line to replace
blades at frequent intervals. It is also commercially dis-
` advantageous to resharpen bag perforating blades if resharpen-
ing does not significantly extend their useful service life.
The problem of the short life of conventional perforating
blades, even those which have been resharpened, experienced
in processes for making low pressure-low density polyethylene
bags, is alleviated by using the perforating blade of the
,- present invention.
A portion of a serrated perforating blade of the
type which may be employed in the practice of the present
invention is shown in Figure 2. As shown in Figure 2 a
serrated blade 16 is composed of a main body portion 17 and
a plurality of teeth 18 projecting outwardly therefrom along
one edge thereof. Recesses or slots 19 are provided between
adjacent teeth. Although the slots are shown as being
~ rectangular in shape in Figure 2, they may take any form.
The edges of each tooth 18 are beveled such as by sharpening
to provide chamfered faces 23 and 24 and cutting edges 20
and 21 (formed by the intersection of faces 23 and 24 and
the flat side of each tooth 18).
Another type of perforating blade is shown in
Figure 3. Specifically, Figure 3 illustrates a portion of
the teeth area of an unslotted blade 25 which comprises a main
body portion 26 provided along one edge thereof with a
_ plurality of teeth 27. Each tooth 27 may be sharpened to
, provide chamfered faces 28 and 29 and cut~ing edges 30 and 31
formed by the intersection of the chamfered faces and the flat
side of each tooth. This is shown in greater detail in Figure
4 which illustrates, in exaggerated form, the tip area of
each tooth of the blade of Figure 3.
- 15 -
1~77383
The particular shape of the perforating blade em-
ployed in the practice of the present invention is not
critical. It may be slotted or unslotted and the number of
teeth or slots in the blade may vary depending upon the
particular material being treated, the sizes of the ~ag and
perforations, the speed at which the plastic film material
is moving through the apparatus, etc. Those skilled in the
art are aware of the various blade configurations that can
be employed for these purposes. Generally, any conventional
~;10 type and configuration of perforating blade can be used in
the practice of the present invention. It is preferred,
however, for perforating low pressure-low density polyethylene,
that the blade be unslotted of the type illustrated in
Figure 3. As stated above, many commercial bag-making
machines are designed to accept one or more blades each ten
inches in length. For unslotted blades of such length, best
results for perforating low pressure-low density polyethylene
have been obtained with blades having 40 teeth, although good
results have also been obtained with blades having 27 or 50
teeth.
Slotted blades may also be employed in the present
invention. Generally, any slotted blade of conventional
configuration may be employed, such as those represented by
Figure 2. It is not necessary that each tooth be separated
by a slot from each adjacent tooth which is the conventional
configuration. The dimensions of the slotted blades may
vary depending on the nature of the material being perforated,
the size of the bag being formed, the size of perforation
desired, etc. As a general rule, when perforating low pres-
sure-low density polyethylene, which is tough and tear-
resistant, the width of each slot should not exceed
about 0.030 inch. For conventional high pressure-low
- 16 -
~a.1'773E~3
density polyethylene, the slot width may be on the order of
0.060 to 0.070 inch although when making garment bags of
this material for example, slot widths up to about 0.125
inch may be tolerated. With either the slotted or unslotted
blade, the thickness thereof may be determined based on the
strength of the metal constituting the blade, its expected
level of use, the size of perforation required, etc. A
conventional perforating blade may be formed from a blank
by machining and the teeth area is typically of a lesser
thickness than the main body portion. ~enerally, for blades
formed of No. 1095 carbon steel, the thickness of the main
body portion may be on the order of about 0.050 inch while
the teeth may have a thickness on the order of about 0.010
to 0.015 inch. As is apparent to those skilled in the art,
the dimensions of a perforating blade are not particularly
critical and all of the foregoing dimensions may be varied
depending on the circumstances.
The choice between a slotted and unslotted blade
may depend on the degree of control desired or necessary in
the bag-making machine. When using a slotted blade, such
as in apparatus of the type shown in Figure 1, perforations
are obtained as a result of forcing the blade through the
plastic film at least as far as into the slot whose width
predetermines the distance between perforations. When using
an unslotted blade however, the depth of the perforation
Z stroke must be carefully controlled to provide the desired
- or necessary distance between perforations. Despite the
fact that finer control is therefore necessary with the
unslotted blades, they offer an advantage over slotted
blades in that there is more cutting edge along the length
of the blade since there are no slots and therefore one may
"
~ - 17 -
"
11'~7383
extend the useful life of an unslotted blade by suitable
adjustments in the perforating stroke as the cutting edge
dulls from wear ti-e., the stroke is controlled to force
more of the teeth through the plastic film). Depending upon
the need, the foregoing countervailing considerations may be
weighed by those skilled in the art in the selection of a
~ perforating blade configuration.
- ~ The materials of which the perforating blade may
be made are not particularly critical. An advantage of the
Lo present invention is that a relatively softer blade material
may be employed since it is the hard metal coating which
forms the cutting edge. In fact, as the cutting edges of
the blade undergo wear, the softer blade material is worn
away preferentially, thereby exposing the harder metal coating
on one side of the blade. In effect, the blade becomes
~; self-sharpening in use.
The blade substrate material is typically metal
and any conventional cutting blade metal having a hardness
of at least about 40 Rockwell may be employed, as is apparent
to those skilled in the art. An example of a suitable
material is the carbon steel known as No. 1095. Spring
steel is also expected to be useful. The preferred material
is No. 1095 carbon steel.
The coating which is on one side only of
; the perforating blade of the invention is a hard metal
material. Generally, this hard metallic coating should have
- a Rockwell hardness of at least about 50, preferably at
least about 70. Examples of hard metallic materials which
are, or which are expected to be, suitable for use as the
blade coating in the present invention include the metal
- 18 -
11773~33
carbides such as tungsten carbide; and nickel alloys, such
as the nickel alloy commercially available from Electro-
Coatings, Inc. under the tradename Ny-Carb (which comprises
a nickel-phosphorus matrix containing about 30 weight %
of silicon carbide particles- 1 to 3 microns in size-.
embedded therein). Tungsten carbide is the preferred hard
metallic coating material since it generally ~has a hardness
of over 70 Rockwell.
The term "tungsten carbide" as used herein is
meant to include both tungsten carbide per se as well as
tungsten carbide containing small amounts of other hard
metals such as cobalt. As an example, tungsten carbide
coatings are commercially available from Vnion Carbide
Corporation which comprise cobalt in a mixture of tungsten
carbides; for example, under the tradenames LW-30 (13 weight
% Co - balance tungsten carbides); LW-lN30 (13 weight % Co -
balance tungsten carbides); LW-lN40 (15 weight % Co - balance
tungsten carbides); and LW-lN20 (11 weight ~ Co - balance
tungsten carbides). The LW-30, LW-lN30 and LW-lN20 coatings
have hardness values of 74-75, 72-73 and 71-72 Rockwell,
respectively. The LW-lN30 is preferred.
The hard metal coating may be formed on the per-
forating blade by any convenient method. In a preferred
embodiment (i.e., a chamfered, unslotted blade), the coating
is formed on the flat side of the blade after sharpening as
shown in Figures 5-7 of the drawings. Figure 4 is a side
view of one of the teeth of the blade of ~igure 3, having a
hard metal coating on its flat side. Specifically, referring
to Figure 5, a blade tooth 27 is shown which has a chamfered
face 28 formed by edges 32 and 33 and cutting edge 30. A
-- 19 --
11'7738~
coating 35 (shown in exaggerated form) is provided on the
flat side of the tooth 27, thereby forming a new, hard cut-
ting edge 36.
The tip area of the blade tooth of Figure 5, be-
fore and after some wear, is schematically illustrated in
Figures 6 and 7, which are exaggerated for detail, and where
the same reference numerals indicate the same parts as in
the other drawings. As best shown in Figure 7, as the per-
forating blade is used, due to the more rapid wear of the
softer blade main body at 38 as compared to that of the
relatively harder metal coating at 37, the blade in effect
is self-sharpening.
The hard metal coating may be formed on the per-
forating blade by any suitable process. As an example, and
in a preferred embodiment, a tungsten carbide coating may
be formed by a commercially-available process of Union
Carbide Corporation known as flame-plating. In this process,
the coating material (tungsten carbide, with or without ad-
ditives) is fired from a detonation g~n at the part being
coating at supersonic speeds and at very high temperatures.
The process is more fullydescribed in U.S. Patent No.
2,714,563. Since the coating particles strike the part being
coated by this process with such high speed, it may be neces-
sary, and it is therefore preferred, to support the per-
forating blade teeth from the side opposite the side being
coated, in order to prevent deformation of the teeth. The
necessity for providing such support depends on the thickness
and rigidity of the blade and the specific conditions of the
flame plating operation.
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il773~33
The hard metal i,-~terial may be coated onto the
blade substrate to a thickness of from about 0.0005 to
0.0015 inch, preferably from about 0.0005 to 0.0007 inch.
In the case of tungsten carbide and a blade used to cut or
perforate low pressure-low density polyethylene, the tung-
sten carbide coating has a thickness on the order of about
0.0005 to 0.001 inch.
In addition to perforating plastic film packaging
materials as discussed above, the present invention also
` 10 contemplates other plastic film cutting operations, such as
punching and slitting wherein the cutting instrument is
coated on one side only with a hard metal coating of the
' types described above. In the case of punching, a circular
blade is generally used which may be formed of any con-
ventional blade-type material, such as No. 1095 carbon steel.
Such blades are generally unslotted and are provided with
teeth around the entire circumference of the blade. The
teeth may be sharpened by beveling the inside edges thereof.
When punching holes in plastic film material, such as low
pressure-low density polyethylene, the useful service life of
such punching blades may be extended by coating the outside,
flat surface thereof with a hard metal material such as
tungston carbide in the same manner as discussed above.
In addition to punching blades, slitting-type blades
may be coated to extend their useful service life when
slitting materials such as low pressure-low density poly-
ethylene film. Slitting blades are used in many different
applications such as forming flat films from a film tube
formed by a tubular blown film extrusion process. The tube
may be slit on one or both sides to provide the flat film.
,
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1~7~73t33
In one instance, it was the pr~ctice to sharpen the slitting
blades once a week when slitting conventional high pressure-
low density polyethylene. When low pressure-low density
polyethylene was employed in the process, it was found
necessary to sharpen the slitting blades twice a day. These
blades were curved and were sharpened on one edge only to
provide a chamfered surface on that side. By coating the
flat side of these curved blades with a hard metallic
material such as tungsten carbide, using the flame-plating
process described above, it was found necessary to sharpen
these blades only once a week even when slitting low pressure-
low density polyethylene film.
EXAMPLE 1
New 10-inch long 40-tooth unslotted blades of No.
1095 carbon steel were sharpened to provide a chamfered face
on one side of each tooth. The flat side of the teeth of
these blades was then coated with a LW-30 tungsten carbide
to a thickness of 3 mils and a set of 5 blades was installed
in a Gloucester Model 418 bag machine. The machine was
operated at a line speed of 170 feet per minute and a
perforating speed of 122 cycles per minute to produce produce-
type bags of low pressure-low density polyethylene having a
thickness of 0.5 mil. These blades produced good perfora-
tions even after 13 days of operation at which point 2 of
the blades were removed and installed in a so-called wet bag
- line employing a Gloucester Model 418 machine and a down-
stream separator designed to separate and stack the bags.
This line was operated at a speed of 170 feet per minute and
a perforating speed of 122 cycles per minute to produce
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117'7383
low pressure-low density polyethylene bags having a thickness
of 0.5 mil. These 2 blades produced good perforations for
an additional 14 days.
EXAMPLE 2
New 40-tooth unslotted perforating blades, 10 inches
long and composed of No. 1095 carbon steel and having a
hardness of 46-48 Rockwell in the teeth area, were installed
in a bag-making line using a Gloucester Engineering Model
No. 418 bag-making machine. The line was operated to make
low pressure-low density polyethylene produce-type bags
having a thickness of 0.5 mil. The line speed was 170
feed per minute and the perforating cycle was 122 cycles
per minute. These blades became too dull to provide good
perforations after only 24 hours of use. The blades were
removed from the machine and the tips of some of the teeth
were observed to have rolled back thereby dulling those
teeth.
EXAMPLE 3
A Ny-Carb coating (30 weight ~ of 1-3 micron particles
of silicon carbide embedded in a nickel-phosphorus ~atrix)
was coated onto both sides of new 10-inch long, 40-tooth
slotted perforating blades formed of No. 1095 carbon steel
and having slots 0.030 inch wide between each tooth, by a
con~entional plating process. The Ny-Carb coating had a
hardness of 60-68 Rockwell and a thickness of 3 mils.
These blades were then installed in the same type of Gloucester
bag-making line as used in Example 1. The line was operated
at 253 feet per minute and at a perforating speed of 184
cycles per minute to produce the same type of low pressure-
low density polyethylene produce-type bags as in Example 1
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11~73~3
having a thickness of 0.5 mil. Right at the start of opera-
tion, it was noticed that the perforations were not of good
- quality and the blades after 21 hours became too dull and
were removed.
A second set of the same type of perforating
blades was coated on both sides with the same Ny-Carb
coating by the same plating technique to a thickness of 3
; mils, and a hardness of 60-68 Rockwell and placed in the
same bag-making line and then used to make the same type of
low pressure-low density polyethylene bags. This set of
blades lasted only 20 hours before becoming dull enough to
warrant removal from the machine.
EXAMPLE 4
Two sets of new 10-inch long, 50 -tooth perforating
blades formed of No. 1095 carbon steel were spray-coated on
both sides with a conventional aerosol-type Teflon spray
and left to dry for six hours. The blades were coated again
on both sides with the same Teflon spray and left to dry
overnight. The thickness of the Teflon coating was 0.5 mil.
One set of blades was installed in a bag-making line using a
Gloucester Model No. 418 machine which was then operated at
a line speed of 170 feet per minute and at a perforating
speed of 122 cycles per minute to make low pressure-low
density polyethylene bags having a thickness of 0.5 mil.
The blades were removed after 26 hours when the perforations
became poor. The second set of teflon-coated blades, coated
in the same manner, lasted an additional 4 hours in the same
line when operated under substantially the same conditions.
:
1~77;~83
EXAMPLE 5
A set of new, 10-inch long 50-tooth slotted (each
slot being 0.035-0.040 inch wide) perforating blades formed
of No. 1095 carbon steel were dip-coated with Teflon 8-403
to a thickness of 0.2-0.3 mil followed by baking. The
resulting blades were installed in a bag-making line using a
, Gloucester Model No. 418 machine and the line was operated
at a speed of 170 feet per minute and a perforating speed
of 122 cycles per minute to produce low pressure-low density
polyethylene bags. The blades were removed after about 4
days due to poor perforations.
EXAMPL~ 6
The set of blades of Example 2 were resharpened
after removal and re-installed in the bag-making line which
was again operated under substantially the same conditions
as in Example 2. The resharpened blades failed to give good
perforations after only 8 to 24 hours.
It is apparent to those skilled in the art that
various other changes and modifications may be made in the
present invention without departing from the spirit and scope
thereof. It is the intention not to be limited by the fore-
going description, but rather only by the scope of the claims
appended hereto.
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