Method of die-cutting packaging material, punch form and die-cutting machine including stoppers

The present invention concerns a method of die-cutting packaging material wherein the packaging material is brought in between an anvil and at least one cutting knife, whereby the anvil and the cutting knife are moved relative each other so that the cutting knife cuts through the packaging material and hit the anvil; a punch form for use in a method for die-cutting packaging material comprising at least one cutting knife; and a die-cutting machine for use in a method for die-cutting packaging material comprising a support with a die-cutting part, in which an exchangeable punch form comprising at least one cutting knife is mountable, and an anvil.

The invention is within the technical field of die-cutting and especially die-cutting packaging material, such as paper board, corrugated card board and other material based on wood fibres. The expression "packaging material" shall here be considered to have a broad meaning and also comprise, for example, display material.

Die-cutting of packaging material may be executed in different ways. Thus, a punch form may be stationary or movable. In the first case the packaging material is fed in between the punch form and an anvil and in the second case the packaging material is fed together with the punch form. In the punching nip, i.e. where the punching takes place, the packaging material is clamped between the cutting knives and/or scoring rules of the punch form and an anvil, and die- cutting occur rapidly.

Cutting means, i.e. cutting knives, balance knives, scoring rules etc, are generally mounted to a flat or cylindrical bed, a so called flat punch form and rotational punch form, respectively. For each package to be produced a specially designed punch form

is required, which is mounted in the present die- cutting apparatus at the time for production.

The punch anvils may have the form of a flat plate or a rotating cylinder. In the punch nip the cutting knives cut fully through the packaging material while the scoring rules, just as some other means, only partly cut through or only deform the packaging material. Generally, score or fold lines are attained, where the die-cutting reduce the bending rigidity, so that die- cut package with great precision of measures and high speed will be able to be folded and closed around the article in question.

During die-cutting, cracks easily arise in the die-cut packaging material, especially in the vicinity of the cutting knives and the scoring rules, due to severe stress in the packaging material.

The die-cut packaging material is pushed out of the punch form by means of resilient elements, generally so called rubber ejectors. Such rubber ejectors are generally in direct contact with the packaging material.

After the ejection from the punch form, the die-cut packaging material is passed on and overflow material, so called die-cutting waste, is removed from the packaging blanks, either in connection with the ejection of the packaging blanks from the punch form and leaves the punching nip in question or at a separate operation directly afterwards. One problem is damages in the surface of the packaging material, local crushing and crack propagation initiated by the pressure/shear forces of the rubber ejectors. This problem is considerable when die-cutting of corrugated card board and other pressure sensitive packaging material and at high production speeds.

The last mentioned problem may be alleviated by providing a sheet material between the punch form

and the packaging material, as described in EP 1432558, the content of this is hereby incorporated into this description. The sheet material in question may be dressed on portions of or over the whole punch form, depending on technical requirements and economical considerations.

Background art

The present invention is mainly concerned with flat die-cutting with a flat punch form against a flat anvil, where the anvil is hard, for example made of steel, but may of course be used in other types of die-cutting. The flat punch form comprises at least cutting knives and rubber ejectors. Often at least scoring knives are mounted and the punch form may be provided with crack reducing material if so desired.

A considerable problem in the art is the cutting dust that occurs along the cutting lines, a dust comprising microscopically thin and thick strips of the packaging material, which occur between the cutting knives and the anvil. Whereupon the knives wear said dust particles get broader and thicker. The dust particles are often called within the art angel's hair and when they have become thicker witch's hair. Another great problem is the cost for the wear of the knives and anvils, the interruption of service and so on for maintenance counted up. The knife must cut so far that full cut through of the packaging material is achieved. Otherwise it is not possible to sell the cut out product and great problems arise when die-cutting waste is removed from the blanks. Thus, the goal is at first hand full through-cutting and at second hand small knife wear.

The knife movements just adjacent the anvil is not always under full machine control due to instable machine, despite accurate setting before the cutting operation. Sometimes the return position for the knives are incorrectly set so that the knives cut

further down into the anvil than intended, whereby both knives and anvil wear.

With dry and brittle packaging material a brittle fracture will occur before the knives passed through the whole thickness of the packaging material. In this ideal condition the knives thus do no need to touch the anvil, which will give minimal knife wear.

However, packaging material such as paper, paper board and corrugated card board based on wood fibres are rarely homogenously brittle or homogenously dry, which inter alia is dependent on the structure of the material, fibrous structure and moisture content, how it has been made, stored and handled before the die-cutting operation starts. In practice the moisture and brittleness vary a lot.

Due to this variation, the knives do not always cut through the packaging material at all positions. The machine operator will then set the machine again so that the knives come closer to the surface of the anvil or into this. In order to secure that full through cutting always is achieved and that one thus avoids that more disruptions occur due to the variations in the material, the machine is reset as a rule so that the knives go into the anvil and with such a force that an accelerated wear of the knives or the anvil become unavoidable.

Of course it easily happens that the edge of the knives become bent at such a resetting, whereby a further reset will be done, some knives must be changed or be adjusted in height, and so on in a spiral of new costs. With the machines of today it is additionally not possible to set as exact as the cutting in the ideal case only cuts down to exact the position above the surface of the anvil, where brittle fracture occur, since said position lies very close the surface of the anvil. Naturally, the lack of technology for fine adjustments of the movement of the

knife in the vicinity of the anvil strongly contribute to the above mentioned wear costs.

Summary of the invention The object of the present invention is to in an easy and cost effective way improve the known die cutting technique in order to minimize unwanted deficiencies in the die-cut packaging blanks, minimize the wear of the knives and the anvil, increase the life of the punch form and at the same time permit increased production speed.

In accordance with the invention, this is achieved by means of a method, a punch form and a die- cutting machine, according to the preamble, which show the features of the enclosed claims.

The self-evident when using stoppers may seem to be stopping the contact of the knives with the anvil, i.e. the use of so called positive stoppers. Systems with such positive stoppers are known within related fields of technology. For example, EP 35 34 79 Bl shows how positive stoppers in combination with electronic pressure sensors may be used for automatic controlling and adjustment of the punch pressure punching machines are exposed to. But in the field of technology in question for die-cutting half-ductile or ductile packaging materials positive stoppers would not work since the movement of the knives through the packaging material would be stopped before they reached the surface of the anvil giving at least partly incomplete cutting through as a result, which is not acceptable.

Surprisingly, it appeared that negative stoppers in a very cost effective way create a stability between the knives and the anvil, so that a

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full cutting through is secured at the same time as accelerated wear of the cutting knives and anvil is prevented. By negative stopper is meant that the height of the cutting knife is greater than the stopper, such that knife by force is brought in immediate contact with the anvil, since the negative stopper hit the anvil not until the cutting knife has hit the anvil.

By option and adaptation of different negative stoppers relatively the thickness and stability of each cutting knife against crush-buckling, the properties of the knife edge included, one may achieve a fine adjustment of the motions of the knife in the area just adjacent to the surface of the anvil, such that the cutting knives during production only in principal are subjected to elastic forces, i.e. that the cutting knives are prevented from being subjected to plastic forces and accelerated deformations by the stoppers . As a result, the wear of the knives and the cutter dust are reduced, the conflict between full through cut and wear of knife is solved, the sensitivity for incorrect setting and unnecessary high punching pressure during production is radically decreased, and the technical life of the punch form is multiplied.

A punch form according to the invention for use in a method for die-cutting of packaging material comprises at least a cutting knife and is provided with at least a stopper that is arranged such that the cutting knife projects further than the stopper in the direction towards the anvil and reaches the anvil before the stopper during the die-cutting operation. The stopper is arranged to stop a relative motion

between the punch form and the anvil when the cutting knife has reached the anvil during the die-cutting operation, such that the cutting knife only is subjected to substantially elastic forces during the die-cutting operation.

A die-cutting machine according to the invention for use in a method for die-cutting of packaging material comprises a support with a die- cutting part, in which an exchangeable punch form comprising at least one cutting knife is mountable, whereby the die-cutting part, the punch form and the cutting knife forms a punching assembly, and an anvil, on which the packaging material to be die-cut is placeable. The anvil and the punching assembly are movable with respect to each other such that the cutting knife cuts the packaging material and hit the anvil. The die-cutting part is provided with at least one stopper that is arranged such that the cutting knife projects further than the stopper in direction towards the anvil and reaches the anvil before the stopper during the die-cutting operation. The stopper is arranged to restrain a relative motion between the punching assembly and the anvil when the cutting knife has reached the anvil during the die-cutting operation, such that the cutting knife only is subjected to substantially elastic forces during the die-cutting operation.

According to the invention, the cutting knife projects so much further than the stopper, such that a full through-cut is achieved at the same time as the stopper prevents accelerated wear of the cutting knife .

Preferably the cutting knife extends at least 0.02 mm further than the stopper in direction against

the anvil and at most 0.08 mm further than the stopper.

In order not to obtain different result on knives having different position in the punch form, due to uneven or fluctuating pressure distribution on the anvil, angular adjustment and tilting motions, at least four negative stoppers are arranged. The last mentioned stabilization function that is provided by the stoppers, is normally complied with by so called balance knives, that according to prior art are put together with other knives in the punch form, in the case the punch form is small and not covers the whole pressure surface of the anvil. However, punch forms according to the present invention that uses negative stoppers, do not need any balance knives, which increases the cost efficiency.

Brief description of the drawings

The present invention will now be described by exemplifying embodiments with reference to the attached drawings, in which

Fig. 1 shows a basic die-cutting technique. Fig. 2 shows a die-cutting operation at ideal conditions for brittle material. Fig. 3 shows the stress curve for die-cutting at ideal conditions for brittle material.

Fig. 4 shows the stress curve for die-cutting at ideal conditions for ductile or inhomogeneous brittle material . Fig. 5 shows the stress curve for die-cutting at normal conditions for ductile or inhomogeneous brittle material .

Fig. 6a and 6b shows a punch form according to the invention in a cross-section and from below, respectively.

Fig. 7a and 7b shows a die-cutting machine according to the invention in a cross-section and from below, respectively.

Fig. 8 shows prior art with respect to applying of tape.

Detailed description of preferred embodiments

In fig. 1, the basic die-cutting technique is shown. At least one cutting knife is mounted in a plate 2, usually made of plywood, such that the back of the knife 1 abuts against a plate-shaped attachment 3, i.e. the attachment of the punch form in the die- cutting machine. The knife 1 is subjected to a pressure per length of the knife P by the die-cutting machine. The height of the knife 1 is shown by the reference number 4. This height of the knife 4 always includes any additions or amendments along the back of the knife 1, see fig. 8, such as tape 27.

Packaging material 5 that shall be die-cut is positioned on an anvil 6, usually of steel. This anvil 6 may in some cases be movable in vertical direction 8, and in other cases it is the part where the punch form is movably mounted in vertical direction. During the die-cutting operation in itself, the knife 1 is moving in a motion range 7 within two turning positions . In fig. 2, die-cutting of brittle packaging material 5 during ideal conditions is shown. For a certain knife pressure, P-brittle, and at a position 12 before the knife 1 reaches the surface of the anvil 6, the through-cutting propagates through the whole

thickness 14 of the whole packaging material 5 like a brittle fracture. The surface of the anvil 6 has an X- coordinate that is equal to zero. The brittle fracture occurs at a coordinate X that is positioned above the surface of the anvil, in the motion range 7, on the distance X-brittle above the surface. The knife 1 has a thickness 11.

In fig. 3, the stress curve for the knife 1 for ideal die-cutting in a brittle packaging material 5 is shown. The Y-coordinate shows the pressure per knife length P and the X-coordinate shows the knive 1 position relatively the surface of the anvil 6, i.e. within the motion range 7, where the surface of the anvil 6 has the X-coordinate zero. The compression of the die-cutting machine occurs along the X-axle relatively the anvil 6.

The stress in the knife increases until then the pressure P-brittle is reached, whereby a homogeneous brittle fracture arises, i.e. the material is totally divided, all over the thickness and along the length of the knife, and the stress disappears. This occurs on the distance X-brittle above the surface of the anvil, i.e. before the knife 1 reaches the surface X=O of the anvil 6. In fig. 4, stress curve for the knife 1 for ductile packaging material 5 during ideal conditions is shown. In this case too, the stress of the knife 1 is rising up to P-brittle, but instead of resulting in a homogeneous brittle fracture, a successive through- cut 15 of the ductile or inhomogeneous brittle material occurs where the stress in the knife 1 successively decreases up to the knife 1 comes in contact with the surface of the anvil 6 at X=O . However, in order to reach a total through-cut,

contact between the knife 1 and the surface of the anvil 6 is not only required, but also a certain contact pressure P-ductile , that can be higher as well as lower than P-brittle depending on the present packaging material, knife and anvil. In that position, an additional compression has occurred to X-ductile , i.e. the knife has been compressed and has a knife stress 17.

In fig. 5, the stress curve for the knife 1 for the packaging material 5 during normal conditions is shown. With certainty during such conditions, in order to attain total through-cut as well as simultaneous not start an accelerated wear of the knife, the knife compression must exceed a certain lowest knife stress 18 corresponding to the compression X-min in fig. 5, and go below a certain knife stress 19, corresponding to the compression X-max, according to fig. 5. Thus, the knife compression must end up within a certain functional area limited by X-min and X-max, where X- max of course is dependent on the condition of the knife, such as material, its thickness 11, free length above the plate 2, material and buckling/breaking tendency. The position 20 in fig. 5 relates to a knife of a kind that withstands relatively more compression or stress. With respect to knives and anvils of steel, it applies that X-min preferably is at least 0.02 mm and X-max as a rule always is 0.08 mm and in most cases is in the range of 0.04 - 0.08 mm.

In order to prevent that the stress in the cutting knife 1 exceeds P-max (at 19) , where accelerating deformation appears, at least one stopper 23, see fig. 6 and 7, is arranged with a difference in height between the projection of the knife 1 and the projection of the stopper 23, such that the knife 1

protrudes further than the stopper 23 in the direction towards the anvil. The difference in height is adapted such that the compression of the knife 1 ends up within the functional range 21, i.e. where through- cutting as well as a minimum of wear of the knife is obtained for knives in question.

In fig. 6a, a preferred embodiment of a punch form according to the invention is shown in cross- section and in fig. 6b is the punch form shown from below. The punch form is preferably provided with four stoppers 23, two at the front edge spaced apart and two at the back edge spaced apart, such that the effect definitely becomes equally independent of the position of the knife in the punch form, e.g. due to that the anvil or the punch form for some reason has a tendency to tilt itself or swing up and down during the die-cutting production. The propagation 22 of the packaging material 5 is shown with broken lines. The propagation 24 of the anvil 6 is shown in fig. 6b. In fig. 7a, a preferred embodiment of a die- cutting machine with a mounted punch form in cross- section is shown, and in fig. 7b the die-cutting machine and the punch form is shown from below. The die-cutting machine is preferably provided with four negative stoppers 23 outside the punch form but preferably in the immediate vicinity of the punch form, in order to completely cooperate with the punch form in question or other punch forms that are used in the machine in question. The negative stoppers 23 may preferably be made of steel or of non-metallic material, such as kevlar.