Note: Descriptions are shown in the official language in which they were submitted.
CA 02219937 1997-10-31
CUTTING AND CREAS=NG APhARATUS
This invention relates to an apparatus for the cutting and
creasing of plastics sheet materials, in particular such..
materials having voids therein sometimes referred to as
corrugated sheets.
Cutting and creasing of such sheet materials is carried out
mostly to produce blanks , a . g . for the production of cartons or
boxes. Traditionally, box blanks of paper board and many
plastics sheets have been cut and creased cold using pressure
only. While this is satisfactory for some materials,
plastic sheet material and blanks often retain a "memory" and
tend to revert to their original configuration after creasing.
This memory is particularly pronounced in polypropylene and
other polyolefin-based materials and increases with the
buckling resistance of the sheet. Furthermore, as the weight
of corrugated sheet per unit area decreases in relation to
thickness, thus giving rise to thinner sections, it becomes
more .difficult to convert into a useful blank free from
cracking and able satisfactorily to resist shearing and other
forces to which it is likely to be subject in use.
Another approach is to use a heated cutting and creasing tool
which, provided sufficient heat is introduced into the sheet,
overcomes both the general 'memory' problems of plastics sheet
and the cracking, as well as the problems due to unsatisfactory
resistance to shearing and other forces, but introduces
problems of its own. In order that the strength and
appearance of the sheet material is not impaired to too great
an extent, it is necessary to protect the greater area of the
sheet from the hot tooling during the cutting and creasing
process so that the heat is applied principally in the areas to
be cut and creased. This militates against the use of a flat
metal bottom plate or platen. The use of a bottom plate or
platen having raised "anvils" is successful but is relatively
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expensive and inflexible. That is, should the outline of the
blank be changed in any degree it is necessary to produce a new
bottom plate, often milled out of a solid block, and this is
expensive. Moreover, it has been found that the repeated
action, particularly of the cutting blades, can score the metal
surface. In view of the heated environment, plastic
material from the sheet can get trapped in these score lines
thus rendering the tooling ineffective in that the products are
not properly formed or are unsightly. Furthermore, the action
of the cutting blades being arrested by a metal bottom tool
blunts them and leads to the requirement for frequent
refurbishment of them.
The invention seeks to provide a tooling apparatus improved in
the above respects.
According to the present invention there is provided an
apparatus for the cutting and creasing of plastics material
sheet which comprises a cutting and creasing tool and a
corresponding plate arranged for mutual contact characterised
in that the plate is free of raised surfaces and is covered
with a resilient material.
We have surprisingly found that excellent results can be
obtained using a flat platen or bottom plate provided that its
surface is, or is covered by, a layer of resilient material,
especially a plastics material. The plastics material must
be sufficiently tough and heat resistant to cope with the heat
and-the-iue~hz3i'ii~cr3.-zzeti~Tf'W3f ~~'ic'~tinf~ and-i.re&5tng-prowess
and, while other materials may be used, we have found that a
sheet of nylon 66 provides excellent results. In a preferred
embodiment, two such sheets are employed, one thicker than the
other, with the thinner sheet outermost. It has been found
that when wear finally takes place on the outer sheet it can
firstly be turned over and then be replaced without needing to
replace the inner sheet for a further period of time.
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i
It has been further found that the use of a continuously or
intermittently indexed thin sheet of a cheap material, far less
robust than nylon, for example brawn paper, fed across the
surface of the resilient material sheet to present a fresh
portion, wholly or partly, each cycle further increase
flexibility, reduces changeover time, reduces tool wear, and '
increases the life of the plastics material forming the surface
of the platen while enhancing product quality.
The tool will normally be heated and the plate may or may not
be heated. The plate may be a flat 'bottom' plate, in which
case the tool and plate will be mounted for mutual
reciprocation, or may be mounted around a drum which will be
rolled in contact with the tool to effect creasing.
Alternatively, the cutting and creasing tool can be on a drum
and rolled onto the flat plate. Alternatively, both the
cutting and creasing tool and the plate may be mounted on
drums.
It has also been found that, when the platen is heated, it is
not necessary to hold the temperature of the platen or bottom
plate at as high a level as had been employed. Commonly, and
depending on cycle times together with sheet weight/thickness,
temperatures of between approximately 70 and 150°C had been
employed. We have now found that temperatures as low as room
temperature can nevertheless give satisfactory results.
However, for reproducibility, it is preferred to maintain the
platen or bottom plate above ambient temperature, e.g. at not
less than approximately 40°C. The temperature of the cutting
and creasing tool will be maintained at a similar temperature
to formerly, namely in the region of 150°C (+/- 30°C).
The apparatus of the invention is designed to work with
corrugated plastics material sheet, and in particular with
corrugated or "fluted" material sheet, in particular polyolefin
material. This is a non-polar material and has in the past
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been difficult or impossible to cut, weld and crease with
apparatus of the type described, with the difficulty increasing
as the sheets' weight is decreased in relation to its thickness
resulting in thinner sections. An additional problem with
such corrugated sheet during the creasing process, is that the
vertical corrugations or flutes tend to deform in different'-
directions and at the bottom of the stroke of the cutting and
creasing tool there may be anything from two to six or more
thicknesses of the material at the creasing zone. It has been
found that the use of a plastics material sheet on the platen
gives sufficient flexibility to accommodate this variability
and still give a satisfactory cut or crease. It will be
appreciated that creasing is often produced by a welding action
in which the top and bottom "skins" of the sheet material are
welded together (with any intervening corrugations or flutes
being sandwiched therebetween).
Particularly when operating the bottom platen at the lower
temperatures which the apparatus of the present invention
allows, it is preferred to preheat the corrugated sheet
material before cutting and creasing. While the material can
be preheated by exposing the sheet to a heat source, this is
not preferred as it tends to heat the outer skins more than the
interior when the opposite heat profile is preferred. The
heat profile of just-extruded sheet material is ideal but for
many reasons it is often not practical to have the cutting and
creasing apparatus immediately down stream from an extruder.
According to another aspect of the present invention there is
provided an apparatus for preheating corrugated plastics sheet
material which comprises a flat bed portion adapted to receive
a stack of precut corrugated sheet material blanks with their
direction of corrugation aligned, heating means at at least one
end of the stack, and fan means at at least one end of the
stack, the heating and fan means being arranged such as to
direct heated air along the corrugations of at least some of
the sheets in the stack and back via the remainder of the
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sheets in the stack in an endless loop.
Preferably, a fan and heating means is provided at each end of
the stack in communication with the open ends of the
corrugations, and the stack may be divided approximately in
half vertically so that hot air is directed from one fan and''
heating means through half the stack of sheets to the other
side, and is then directed back via the other fan and heating
means through the other half of the sheets back to the first
fan and heating means. The hot air is then both 'pushed' into
the flutes from one side and 'pulled' from them on the other
giving a very effecient circulation. In this way the sheet
blanks are heated from the inside out in contrast to being
exposed to radiant heaters from the outside. Ideally, heated
sheets are removed from the bottom of the stack and fresh
unheated sheets added to the top of the stack, so that the
sheets acheive optimum temperature as they pass down the stack
in a continuous manner.
The invention will be described further, by way of example,
with reference to the accompanying drawings, in which:
Figure 1 is a bottom plan view of a cutting and creasing tool;
Figure 2 is a sectional view corresponding to figure 1;
Figure 3 is a diagrammatic sectional view of part of a
corrugated plastics sheet material;
Figure 4 is a side elevational view of a bottom plate;
Figure 5 is a sectional view of the heating apparatus; and
Figure 6 is diagrammatic perspective view of another
embodiment.
Referring to the drawings, and in particular figures 1 to 4, a
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cutting and creasing apparatus comprises a bottom plate or
platen 10 which will normally be statically mounted. Located
above the platen 10, and in register with it, is a cutting and
creasing tool or top plate generally designated l2.mounted for
reciprocation towards and away from the platen 10. The tool
12 as illustrated has a cutting rule 14, a welding rule 16 and'
creasing rules 18.
The platen 10 is provided with heaters 20 and, in accordance
with the invention, with one or more covering layers of
plastics sheet material 22. In a preferred embodiment a 1 mm
thick sheet of nylon 66 is placed over the platen 10 and a
second, 0.5 mm thick, sheet of nylon 66 is placed over the
first sheet. The sheets may be secured at the edge of the
platen 10 by any suitable means.
While such an apparatus can be used to cut and crease a variety
of sheet material, it is particularly designed for use with
corrugated plastics sheet material such as that illustrated in
figure 3. It will be seen that such a sheet 23 has upper and
lower. skins 24 separated by generally vertical "flutes" 26 such
material has a very high strength to weight ratio.
In use a length of sheet material is cut to size and placed on
- the bottom platen 10 over the plastic sheet layer 22. The
top plate 12 is reciprocated towards the platen 10 and the
cutting rule 14 cuts the sheet. The welding and creasing
rules 16, 18 then come into contact with the sheet as the top
plate continues in its cycle towards the platen lo. The
limit of movement will normally be set by the contact desired
by the cutting rule 14, the welding rule 16 and the creasing
rules 18 with the sheet 22 . The rules 16 ,18 are normally
made of brass and are heated, e.g. to a temperature of around
150°C, at which they will melt or at least soften the
polyolefin material of the sheet 23. The rules 16 and 18
deform the sheet 23 and crush any flutes 26 in the area of the
rules. They weld the top and bottom skins 24 together with
CA 02219937 1997-10-31
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any intervening corrugations or flutes being sandwiched
therebetween. This then produces a permanent creased area
which will not return to it original configuration through
plastic "memory".
The brass welding rule 16 welds the edge of the sheet 23'
adjacent to the cut made by the cutting rule 14 and in so doing
forms a "bead" in the channel between the cutting rule 14 and
the welding rule 16. This gives a neat sealed and reinforced
finish to the cut blank which needs to be comfortable to the
touch.
Turning now to figure 5, a stack of sheets 23 is laid on a bed
28 with all the flutes 26 aligned in the same direction (left-
right as shown in figure 5). At one end of the bed there is
provided a chamber 30 of approximately the same width as the
stack of sheets 23 and containing one or more axial fans 32 and
one or more electrical heating elements 34. The chamber 30 is
divided approximately half way up down the length of the stack
by a divider 36. At the other side of the stack of sheets 23
is a corresponding chamber 38 in which the disposition of the
fan and heaters are reversed with respect to the chamber 30.
' Air is forced by the fan 32 through the heating element 34 down
the internal corrugations defined by the flutes 26 of the
topmost sheets 23 in the stack. The air then enters the
chamber 38 where it is pushed through the further heating
element and back through the corrugations in the bottommost
sheets of the stack back to the chamber 30. In this manner
hot air is continuously circulated through all the sheets in
the stack until they have reached the desired temperature.
They may then be fed to the cutting and creasing apparatus and
a fresh stack of sheets heated up. The heating action, by
concentrating the hot air down the interior corrugations of the
sheets, produces the desired kind of heating effect from the
inside out rather than the less satisfactory heating, from the
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outside in, produced by exposing individual sheets to external
radiant or convected heating. In a preferred method of
operation, fresh sheets are fed to the top of the stack and
move continuously downward from the top of the stack to the
bottom where they are removed. While the heating apparatus of
figure 5 is ideally suited to preheat sheets of plastics
materials for the cutting and creasing apparatus of the
invention, it may be used to heat such sheets for other
purposes.
Turning now to Figure 6, and using like numerals for like
parts, an embodiment is illustrated in which a continuous sheet
22' is indexed across the platen 10 at an angle thereto. The
platen 10 is covered with a static sheet 22, e.g. of nylon,
which corresponds to the 'thicker' sheet referred to above ( and
the thinner sheet of nylon may also be present). The sheet 22'
is led from feed roller 40 to take-up roller 42 via guide bars
44 and is indexed forward a small amount in between cutting
cycles thereby presenting a fresh portion each time to the
cutting rules 14. In view of this, a far less robust material
than nylon can be used for the sheet 22', and brown paper has
been found to be satisfactory. The sheet 22' nevertheless
protects the sheet 22 and ensures long life for it. The guide
bars 44 ensure even tensioning of the continuous sheet 22'
The cutting and creasing apparatus of the invention has many
advantages: tool wear is reduced since metal to metal contact
is eliminated; the difficulty and thus cost of getting the
necessary accuracy of fit between the mating surfaces of the
cutting tool with an anvil type bottom plate is eliminated; the
impairment to the strength of the sheet's load bearing sections
through use of high temperatures is reduced; the appearance of
the sheet is improved as less heat may be used; splitting of
the sheet which could take place, particularly around sharp
corners or when creasing in the direction of the flutes 26, may
be eliminated; and design changes in the cutting and creasing
tool do not require any changes in the platen.
