Method for producing, by means of ultrasound, a plastic film provided with a local material weakening arrangement

Abstract

A method for producing a plastic film (48) provided with a local material weakening arrangement (12) comprises the following steps: i. providing an ultrasonic tool arrangement (25) comprising at least one sonotrode (26(i)) and at least one anvil (30) as ultrasonic tools (28, 32) with cooperating active formations (33, 35) that can be moved toward and away from one another, wherein at least one ultrasonic tool (28, 32) has a structural active formation (33) which has a shape complementary to the shape of at least one portion of the local material weakening arrangement (12), ii. introducing a plastic film blank (22) into a working gap (24) between the ultrasonic tools (28, 32), iii. reducing a working gap width (W) to a first working gap final width (w) that is smaller than the film thickness (D) of the plastic film blank (22), iv. activating the at least one sonotrode (26(i)) at least during step iii., in which the working gap width is reduced, and v. increasing the working gap width (W) to a second working gap final width (Wmax), which is larger than the film thickness (D) of the plastic film blank (22).

Description

[0001] 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 1 - Method for producing a plastic film with a local material weakening arrangement using ultrasound

[0002] Description

[0003] The present invention relates to a method for producing a plastic film, in particular a plastic lid film for push-through packaging, provided with a local material weakening arrangement. The material weakening arrangement has a defined shape.

[0004] The material weakening arrangement is typically a linear arrangement whose line extends across the surface of the plastic film, particularly the lid film, and penetrates the film to a certain depth in the thickness direction. The material weakening arrangement generally has a relatively long linear extension, usually to ensure that the plastic film can be punctured at any given location. "Puncture" in this context refers to a local tearing or bursting of the plastic film, which occurs, for example, during the use of a push-through package formed with the plastic film as a lid film. This happens when a packaged rigid product is pressed against the surface of the plastic film or lid film facing it, thereby locally exceeding the film's strength limit.To reduce the strength limit locally to a manageable level, the material weakening arrangement is integrated into the plastic film. Due to the large overall length of the material weakening arrangement and its distribution over at least a large portion of the surface of the plastic film, particularly the lid film, a local reduction in the strength limit is so close to any possible point of contact with the packaged rigid product that the applied force, regardless of the specific point of contact, is sufficient to open the film at the location of the material weakening arrangement. Push-through packaging is also known as "blister packaging" in the relevant industry. 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 2 -.

[0005] The material weakening arrangement can be a continuous, long weakening that changes direction multiple times, or it can comprise a multitude of separate, shorter weakenings that interact to form the arrangement. The individual weakenings of the arrangement can intersect to create particularly weakened areas of the plastic film at their intersection points.

[0006] Such a plastic film used as a lidding film is disclosed in US 2008 / 0230432 A1 with a thickness of 6 to 50 µm. The known lidding film features a material weakening arrangement with a plurality of material weakenings which, in order to protect the product packaged in the push-through packaging formed with the lidding film, do not completely penetrate the lidding film in its thickness direction.

[0007] US patent 2008 / 0230432 A1 discloses two possible methods for forming such a material weakening arrangement: laser cutting and a so-called "kiss-cut" method, i.e., a punch cutting process with a mechanical blade in which the blade entering the film material to be weakened does not completely penetrate the film material.

[0008] The aforementioned methods currently represent the standard procedures for producing locally weakened plastic or lidding films. While laser cutting allows for highly customized design of the material weakening arrangement, it is relatively expensive. Cutting with a mechanical blade is considerably more cost-effective for larger quantities of lidding film relative to the surface area produced, but leads to less consistent results in the weakened product. Due to material elasticity and unpredictably fluctuating adhesion phenomena between the blade penetrating the lidding film blank and the blade's surface of the plastic material, the dimensions of mechanically introduced material weakenings in the lidding film blank can vary considerably, particularly in the thickness direction.An undesirable further propagation of the incised material weakening, manifesting as a crack when the blade is pulled out of the plastic material adhering to it, sometimes occurs. 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 3 -.

[0009] From DE 10 2013 225 042 A1, an ultrasonic processing device for welding and cutting material is known. The sonotrode of the device known from DE 10 2013 225 042 A1 can be designed as a rotating roller. A rotatable anvil as a counter-tool for the sonotrode is also known from this publication.

[0010] The object of the present invention is to provide an improved method for producing a locally weakened plastic film, in particular as a lidding film for push-through packaging.

[0011] The present invention solves this problem by means of a method having all the features of claim 1. Such a method comprises the following steps:

[0012] i. Providing an ultrasonic tool arrangement comprising at least one sonotrode and at least one anvil as ultrasonic tools with mutually approachable and mutually retractable cooperating action formations, wherein at least one ultrasonic tool has a structure action formation which has a complementary shape with respect to the shape of at least one section of the local material weakening arrangement,

[0013] ii. Inserting a plastic film blank into a working gap between the ultrasonic tools,

[0014] iii. Reducing the working gap width to a first final working gap width which is less than the film thickness of the plastic film blank, iv. Activating the at least one sonotrode or maintaining an activation state of the at least one sonotrode at least during step iii. of working gap reduction, and

[0015] v. Increasing the working gap width to a second working gap end width, which is larger than the film thickness of the plastic film blank.

[0016] The basic concept of the present invention is the formation of the material weakening arrangement by ultrasound. In step ii, the working gap has an initial working gap width that is larger than the film thickness of the plastic film blank. This significantly facilitates the insertion of the plastic film blank into the working gap. The working gap width is the distance between the cooperating working formations of the two ultrasonic tools across a plastic film blank arranged between them.

[0017] Step iii ensures that the structure-action formation of at least one of the ultrasonic tools penetrates the film material of the plastic film blank, thus locally disrupting its material cohesion. At points unaffected by the at least one structure-action formation, the original material cohesion of the plastic film blank is maintained.

[0018] By activating at least one sonotrode or by maintaining an activation state of at least one sonotrode, it is ensured that it performs its necessary micro-movement for the separating processing of the film material at least when the structure-action formation locally separates into the originally contiguous plastic material.

[0019] Locally disrupting the material bond through the structure-action formation of an ultrasonic tool offers the advantage that the sonotrode, during penetration into the plastic film blank, can generate micro-relative movement between the structure-action formation and the plastic material. This allows for very precise local disruption of the material bond, significantly reducing or even eliminating adhesion of the plastic material to surfaces of the penetrating structure-action formation. Consequently, local dimensional variations in the material weakening arrangement created by ultrasonic cutting are minimal.In particular, the depth dimension of the material weakening arrangement, which is oriented parallel to the thickness dimension of the plastic film blank, is significantly more uniform and constant than with a purely mechanical cut to form the material weakening arrangement.

[0020] In principle, it is conceivable to use the ultrasonic tool carrying the structure-action formation to cut the plastic film blank into its thickness- 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 5 -

[0021] to form a fully penetrating material weakening arrangement. Following the formation of the material weakening arrangement, the plastic film blank can, if necessary, receive a further layer by lamination, which is closed and without material weakening. However, in this case, it is likely to be more economically advantageous to form the fully penetrating material weakening arrangement purely mechanically with blades. Completely penetrating a plastic film with mechanical blades is technically considerably less demanding than the targeted, uniform introduction of material weakening over only a portion of the thickness dimension of a plastic film.The tooling costs for a purely mechanical blade tool are lower compared to the manufacturing costs of an ultrasonic tool, as are the operating costs incurred during operation, since the purely mechanical cutting blades are passive components that certainly do not need to be stimulated to perform micro-movements.

[0022] However, it is advantageous to use a plastic film blank whose layer structure fully corresponds to that of the finished plastic film, particularly the lid film, and which is then refined into the weakened plastic film, particularly the lid film, by incorporating the material weakening arrangement. Since the material weakening arrangement, intended to protect a product packaged with the weakened plastic film, preferably does not completely penetrate the plastic film in the thickness direction, it is preferred that the first working gap width is greater than zero.Thus, between a structural-active formation of one ultrasonic tool consisting of sonotrode and anvil penetrating the material of the plastic film blank and the opposite, preferably unstructured, particularly smooth, active formation of the other ultrasonic tool, a residual working gap width remains, in which material of the plastic film blank introduced into the working gap can remain unchanged and interconnected.

[0023] Preferably, the plastic film blank is a lid film blank, particularly preferred for push-fit or blister packaging. 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 6 - When cooperating working formations of the ultrasonic tools approach each other, only the relative movement of the working formations is decisive. A working formation of the anvil can be moved towards a working formation of the sonotrode, or a working formation of the sonotrode can be moved towards a working formation of the anvil, or the working formations of both ultrasonic tools can be moved towards each other. More economically viable, because technically simpler and therefore more cost-effective to implement, is the movement of at least one working formation of only one ultrasonic tool towards and away from the working formation of the other ultrasonic tool in order to change the working gap width.For the same reason of simpler design and operation, the working form of the anvil is preferably movable towards and away from the sonotrode. The reference form for determining movement is a stationary frame of the ultrasonic tool assembly.

[0024] In principle, both ultrasonic tools can each have a structural element designed to penetrate the film material of the plastic film blank. This applies particularly when the plastic film blank is to be treated with a material weakening arrangement on each of its two opposing exposed surfaces.

[0025] In most cases, however, it is sufficient to apply a material weakening arrangement to only one of the exposed surfaces of the plastic film blank. In this case, it is also sufficient to provide only one of the two ultrasonic tools—the sonotrode or the anvil—with a structure-active formation for creating the material weakening arrangement. Preferably, this is the anvil, which is not directly excited to micro-movements.

[0026] The structure-action formation advantageously has a shape complementary to the section of the material weakening arrangement it produces. To generate the most frequently desired linear material weakening arrangement, the structure-action formation is preferably blade-shaped and tapers towards the end of the ultrasonic tool carrying the structure-action formation that is closest to the working gap.

[0027] The working element of the other ultrasonic tool, consisting of a sonotrode and anvil, which cooperates with the structural working element of the first ultrasonic tool, and which is complementary with respect to the material weakening arrangement section or the aforementioned material weakening as a material weakening arrangement section, can have or be a simple flat or curved counter-pressure surface on which the plastic film blank rests during the generation of the material weakening arrangement section, optionally in a sliding manner. Preferably, this simple flat or curved counter-pressure surface is unstructured, i.e., smooth. Alternatively, the cooperating working element of the other ultrasonic tool can also be designed as a structural working element, for example, blade-like with a linear apex, in particular adapted to the structural working element of the first ultrasonic tool.The cooperating structure-action formations can be mirror images of each other, with the plastic film blank arranged between them serving as a virtual mirror symmetry reference formation.

[0028] In principle, it can be conceived that the entire material weakening arrangement is generated by an approximation movement of the respective working formations of the ultrasonic tools relative to each other. This is possible, for example, when a single sheet of plastic film blank is processed by a plate-shaped sonotrode and a plate-shaped anvil cooperating with it.

[0029] Typically, the plastic film blank is a web of film that is unwound from a supply at the beginning of a processing line, moved along one machine direction through the processing line, and rewound at the end of the processing line for simplified transport or storage. Such a quasi-endless plastic film blank, or generally a large plastic film blank, cannot usually be completely provided with the required material weakening arrangement in a single processing step. Therefore, according to a preferred embodiment, the method comprises the following additional steps:

[0030] vi. Advance the plastic film blank and

[0031] vii. Repeat steps iii. to v.

[0032] Preferably, the material weakening arrangement exhibits a patterned weakening. Due to the operating procedure described above, with a repetition of steps iii. to v., the patterned weakening pattern is preferably a periodically recurring arrangement of weakening formations along a feed direction of the plastic film blank. This includes a continuous line running along the feed direction of the plastic film blank, the dimension of which is larger along the feed direction than the dimension of the structure-effect formation that generates it.

[0033] In principle, the structural-active formation penetrating the plastic material can only disrupt the material cohesion of the plastic. Preferably, the sonotrode generates a relative movement between the structural-active formation and the plastic material, either directly if the sonotrode carries the structural-active formation, or indirectly if the anvil carries the structural-active formation. This movement allows the interface between the penetrating structural-active formation and the plastic material to be locally melted. The melting and the subsequent inevitable cooling and solidification of the interface of the weakening formation ensures its shape retention. The ultrasound-induced micro-relative movement between the structural-active formation and the plastic material can cause the molten plastic material to be ejected in droplet form or even vaporized.Then at least some of the plastic material is removed from the plastic film blank during the creation of the weakening formation.

[0034] The feed direction in step vi. is, in the case of a material web as a plastic film blank, the machine direction mentioned above. Step vi. or steps vi. and vii. can also be repeated to provide as large an area as possible of a plastic film blank with a material weakening arrangement.

[0035] In a particularly advantageous embodiment, the at least one ultrasonic tool can be a rotatable ultrasonic tool, such as a roller. The method can then include a step of rotating the at least one rotatable ultrasonic tool. It is particularly preferred that the ultrasonic tool is rotatable and has at least one structural element designed to penetrate the film material of the plastic film blank. When using a roller as the ultrasonic tool, a plurality of radially outwardly exposed structural elements are preferably arranged around the circumference and / or along the axis of rotation of the roller-shaped ultrasonic tool.

[0036] To prevent slippage, both the at least one sonotrode and the anvil opposite it can be designed as a rotatable roller. Preferably, the two ultrasonic tools then have parallel axes of rotation. The axis of rotation of a rotatable ultrasonic tool preferably lies in a plane that is oriented orthogonally to the surface of the plastic film blank to be processed and orthogonally to the feed direction of the plastic film blank. A rotatable, cylindrical ultrasonic tool often has a constant cutting circle, i.e., an enclosing cutting cylinder, along its axis of rotation. The axis of rotation of the ultrasonic tool is then preferably parallel to the surface of the plastic film blank to be processed in the working gap.

[0037] A rotating, cylindrical ultrasonic tool, in particular two rotating, cylindrical ultrasonic tools interacting to form a tool gap, ensures an engagement zone parallel to at least one axis of rotation for the majority of structural activity formations with the plastic film blank. This engagement zone is short, approximately 1 mm to 5 mm, in the feed direction of the plastic film blank, i.e., orthogonal to at least one direction of rotation. Within such a narrow engagement zone, the activity formations on the 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 10-

[0038] The force acting on the plastic film blank for forming the material weakening arrangement can be set and dosed more precisely than in an intervention zone with a larger contact area.

[0039] At a feed rate of 100 m / min of a plastic film blank unwound from a roll as a plastic film web, the engagement time of an engagement of a structural action formation in the plastic film blank is approximately 6 10'. 4 up to 3 10' 3 See. When using an ultrasonic tool with an operating frequency of 30 kHz, the active formation of the sonotrode performs approximately 18 to 90 oscillations during the engagement of the structural active formation in the plastic film blank. This enables a significantly more precise and finer formation of a material weakening arrangement than the same engagement with the same engagement duration without vibration support, during which the active formation only dips into and retracts from the plastic film blank.

[0040] The size of the penetration zone can be influenced by selecting the diameter(s) of the at least one rotating ultrasonic tool. The penetration time of a rotating ultrasonic tool can be adjusted by the size of the penetration zone and the feed rate of the plastic film blank.

[0041] For uniform processing and thus to achieve consistent product quality along the length of the plastic film blank, the rotational movement of the at least one rotatable ultrasonic tool is preferably continuous. This means, in particular, that the rotational speed of the rotatable ultrasonic tool, apart from transient acceleration and deceleration phases at the beginning and end of processing the plastic film blank, is essentially constant and in the same direction throughout the processing time. The feed movement of the plastic film blank can then also preferably be continuous, i.e., in particular, movement in a constant direction and at a constant speed. 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co.KG - 11 - In principle, the cooperating working elements of the two ultrasonic tools must be movable towards and away from each other in order to reduce and increase the working gap width. In the case of the previously discussed rotatable ultrasonic tool, one working element of the rotatable ultrasonic tool moves on a circular path, such that, in addition to the approach movement component towards and away from the working element of the opposite ultrasonic tool, the path of motion of the working element has a movement component orthogonal to this approach movement component.

[0042] In contrast, the relative movement of the cooperating working formations of the two ultrasonic tools in step iii can be exclusively a translational relative movement towards each other, for example, if one ultrasonic tool is moved towards and away from the other ultrasonic tool in the manner of a punch. In this case, the relative movement of the ultrasonic tools in step iii is not a continuous movement, but a pulsed movement. A working pulse in which the ultrasonic tools are brought closer together while reducing the working gap width is followed by another working pulse in which the ultrasonic tools are moved away from each other to realize step v.Again, the approach and the removal are to be understood as relative movement, whereby it is irrelevant which ultrasonic tool has the at least one working formation that is at rest relative to a stationary frame of the ultrasonic tool arrangement, and which ultrasonic tool has the at least one working formation that is moving relative to the stationary frame.

[0043] In this case of clocked processing, the plastic film blank preferably remains stationary while an ultrasonic tool's active element penetrates the plastic film blank, since the movement of the ultrasonic tool's active element is transverse or even orthogonal to the surface of the plastic film blank into which the material weakening arrangement is to be introduced. The movement of the ultrasonic tool's active element is also conceivable as being transverse or even orthogonal to the feed direction of the plastic film blank. Therefore, a 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 12 - feed movement of the plastic film blank is also preferably a clocked feed movement.

[0044] Regardless of whether one or both ultrasonic tools rotate or are moved predominantly or completely translationally during the execution of the process, the amplitude of an ultrasound-induced micromovement of one active structure towards the other active structure and its origin are preferably selected such that the structure-active structure is located outside the plastic material of the plastic film blank at one dead center of its ultrasonic vibration and has penetrated the plastic material at the other dead center. The feed motion can then be a non-timed, continuous feed motion.

[0045] The configuration of an advantageous ultrasonic vibration of the sonotrode described above, such that the structure-active formation is located outside the plastic material of the plastic film blank at one dead center of its ultrasonic vibration and has penetrated the plastic material at the other dead center, enables a particularly advantageous embodiment of the method discussed here, such that the ultrasound-induced vibration at least partially effects steps iii. and v. described above. Preferably, the ultrasound-induced vibration fully effects steps iii. and v. described above. An additional feed movement superimposed on the ultrasound-induced vibration of the ultrasonic tools relative to each other, moving them towards and away from each other, can then be unnecessary.

[0046] The plastic film or plastic film blank can be single-layered or multi-layered. To facilitate its recycling after its intended use, the plastic film or plastic film blank is preferably free of metal foils and most preferably consists of polymers. In a multi-layered configuration, the plastic film or plastic film blank can have a structure- and stability-determining layer and can additionally have at least one sealable layer with an exposed outer surface and / or at least a barrier layer made of a barrier polymer recognized in the field, such as a vinyl alcohol-based polymer and the like, and / or a printed layer, particularly in reverse printing.In the case of a multi-layered plastic film blank, the material weakening arrangement is preferably introduced into the layer that determines the structure and stability. To facilitate recycling, the plastic film is preferably composed of at least 90 wt.% polymers based on the same monomer.

[0047] For example, the plastic film blank can comprise a mono-film made of polyethylene terephthalate (PET) or polypropylene (PP), in particular polypropylene homopolymer (PPH), or polyethylene (PE), in particular HDPE, wherein the plastic film blank preferably comprises a sealing layer made of a mono-, co-, or terpolymer. The sealing layer preferably comprises 10% to 15% of the thickness of such a plastic film blank.

[0048] The thickness of a PET-based plastic film blank is preferably in the range of 35 pm to 50 pm. The thickness of a PP or PE-based plastic film blank is preferably in the range of 50 pm to 150 pm.

[0049] For incorporating material weaknesses into a plastic layer of the plastic film blank, a plastic with a melting point temperature in the range of 130 °C to 160 °C and a melt flow rate (MFR) in the range of 3 g / min to 12 g / min and a modulus of elasticity in the range of 900 N / mm is preferably used. 2 up to 2500 N / mm 2 Suitable. The melting flow rate is to be determined according to DIN ISO 1133, if in doubt at a test temperature of 216 °C.

[0050] To minimize or prevent the impact and deformation of the sealing layer during ultrasonic processing, a plastic with a melting point temperature below 135 °C, a melt flow rate (MFR) between 1 g / min and 4 g / min, and a modulus of elasticity below 700 N / mm² is preferably used for the sealing layer. 2 used. 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 14- The plastic film blank can have a polymer top layer on the side facing away from the sealing layer. An advantageous weakening, especially or exclusively of the stability-determining layer arranged sandwich-like between the top layer and the sealing layer, can be achieved by making the top layer from a plastic with a melting point temperature above 160 °C, a melt flow rate (MFR) below 1.5 g / min, and a modulus of elasticity around 700 N / mm². 2 up to 1200 N / mm 2is educated.

[0051] In principle, the desired material weakening can be achieved regardless of whether the sonotrode is in contact with the sealing layer and the anvil with the capping layer, or vice versa. However, the results are best when a smooth working element of the sonotrode acts directly on the sealing layer and when a structured working element of the anvil acts directly on the capping layer or, if no capping layer is present, on the stability-determining layer.

[0052] A preferred material weakening arrangement shows a zigzag pattern that oscillates around a virtual axis orthogonal to the feed direction of the plastic film blank.

[0053] The present invention is explained in more detail below with reference to the accompanying drawings. It illustrates:

[0054] Fig. 1 shows a rough schematic side view of a system with an ultrasonic tool arrangement during a process of continuously introducing a material weakening arrangement into one of two exposed surfaces of a plastic film blank in the exemplary form of a lid blank, and

[0055] Fig. 2 shows a rough schematic top view of the process or setup shown in Fig. 1.

[0056] The drawings are schematic and not to scale. 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 15-In Figures 1 and 2, only partially shown there, a system for forming a material weakening arrangement 12 (see Fig. 2) with individual separate material weakenings 13 is generally designated by 10.

[0057] The machine 10 processes a material web 14, which is moved through the machine 10 along a machine direction MD parallel to the respective drawing plane of Figures 1 and 2. The machine direction MD is therefore also a feed direction FD. The material web 14, formed from a multilayer plastic film 16 with an exemplary core layer 18 and a sealing layer 20, extends across its width in a transverse direction CD orthogonal to the machine direction MD. With the orientation of the material web 14 shown in Figures 1 and 2, the third spatial direction TD, which is orthogonal to both the machine direction MD and the transverse direction CD, is a thickness direction of the material web 14.

[0058] The material web 14 is unwound in machine direction MD upstream of the system 10 from a supply roll (not shown) and wound up downstream of the system 10 to a roll (also not shown).

[0059] The material web 14 is a plastic film blank in the exemplary form of a lid film blank 22, which is moved in the feed direction FD through a working gap 24 of an ultrasonic tool arrangement 25 of the system 10.

[0060] The working gap 24 is formed in the ultrasonic tool arrangement 25 between a sonotrode arrangement 26 as an active ultrasonic tool 28 and an anvil 30 cooperating with the sonotrode arrangement 26 as a passive ultrasonic tool 32. The sonotrode arrangement 26 has at least one sonotrode 26(i), wherein: (i) G a, b, c, d, ... The sonotrodes 26(i), in the present example 26a to 26h, are arranged successively in the axial direction with respect to a rotational axis R26 of the sonotrode arrangement 26 in order to generate a substantially uniform ultrasonic effect over the width or axial length of the sonotrode arrangement 26 with negligible damping along the rotational axis R26. The number of axially consecutive sonotrodes 26(i) of a sonotrod arrangement 26 is therefore, for a given power output, 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co.KG - 16 capability of the sonotrodes 26(i) and given damping of the construction of the active ultrasonic tool 28 is essentially dependent on the width of the active ultrasonic tool 28.

[0061] The sonotrode assembly 26 has radially outwardly projecting structural features on its outer circumference. For clarity, only one of these features, 33, is symbolically represented in Figures 1 and 2, although its cutting circle or cylinder 34 is indicated by a dashed line. The structural features 33 are radially outwardly projecting blades that cut into the material weakenings 13 shown in the top view of Figure 2 in the machine direction MD downstream of the sonotrode assembly 26. These weakenings begin at the surface 18a of the cover foil blank 22 facing the sonotrode assembly 26 and proceed parallel to its thickness direction TD.

[0062] In the illustrated embodiment, the sonotrode arrangement 26 causes an ultrasonic vibration movement of the active ultrasonic tool 28 towards and away from the passive ultrasonic tool 32.

[0063] The sonotrode assembly 26 rotates about the axis of rotation R26, which is orthogonal to the plane of Fig. 1 and parallel to the plane of Fig. 2, and is driven by an electric motor 36 to rotate counterclockwise at a constant speed as shown in Fig. 1. Identical structural elements 33 are arranged side by side on the sonotrode assembly 26 circumferentially around the axis of rotation R26 and along the axis of rotation R26 to produce the cross-sectional view shown in Fig. 2. The section circle 34 is thus a cylinder with the axis of rotation R26 as its axis. This cylinder forms an envelope of the radially outermost ends of the identical, blade-like structural elements 33.The material weakening arrangement 12 consists of identical material weakenings 13, which are arranged in parallel rows extending in the machine direction MD. The cross-sectional view in Fig. 2 is merely an example. It corresponds to a negative image of the structural features 33 of the 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 17- sonotrode arrangement 26. The actual design of the material weakenings 13 and the material weakening arrangement 12 may differ from the only roughly representative depiction in Fig. 2.

[0064] The anvil 30, which in the exemplary embodiment is also designed as a roller, is rotatable about a rotational axis R30. The anvil 30 can also be driven by an electric motor 38 at a constant rotational speed to enable slip-free movement of the lid foil blank 22 through the working gap 24.

[0065] The outer surface 30a of the active formation 35 of the anvil 30 is cylindrical, with the rotation axis R30 of the anvil being the cylinder axis of the outer surface 30a. The outer surface 30a of the active formation 35 of the anvil 30 is smooth. The free surface 20b of the lid foil blank 22 rests on the smooth surface 30a of the active formation 35 of the anvil 30 and supports the lid foil blank 22, while the structural active formations 33 of the sonotrode 26 penetrate into the lid foil blank 22 from surface 18a.

[0066] The working gap width W of the working gap 24 is measured as the shortest distance between a structural feature 33 and the point on the outer surface 30a of the feature 35 of the anvil 30 opposite the structural feature 33, parallel to the direction of the distance between the axes of rotation R26 and R30. Thus, the rotation of the sonotrode assembly 26 reduces the working gap width W between a feature 33 and the outer surface 30a of the feature 35 of the anvil 30 to a minimum working gap width w at the point of closest approach of the cutting circle or cylinder 24 of the sonotrode assembly 26 and the outer surface 30a of the feature 35 of the anvil 30. The minimum working gap width w in the present embodiment corresponds to the first working gap end width mentioned in the introduction.After reaching the minimum working gap w, the working gap W between a structural action formation 33 and the opposite location of the outer surface 30a of the action formation 35 of the anvil 30 increases again to a maximum working gap Wmax, which is shown in 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 18-.

[0067] The present embodiment corresponds to the second working gap end width mentioned in the introductory description. The working gap width Wmax is also the largest working gap width occurring on the entry side E into the working gap 24.

[0068] In the illustrated embodiment, at least the minimum working gap width w fluctuates around the amplitude of the ultrasonic vibration movement, which at the location of the minimum working gap width w runs in the same direction as the course of the minimum working gap width w.

[0069] The working gap width W, and with it the working gap widths w and Wmax, of the working gap 24 is / are variable. The sonotrode assembly 26 is movably guided by a boom 40 on a fixed guide rail 42 and can be moved along the fixed guide rail 42 away from and towards the anvil 30 by an actuator 44.

[0070] Before or during the insertion of the lid foil blank 22 into the working gap 24, the working gap width is preferably larger than the thickness D of the lid foil blank 22. During the formation of the material weakening arrangement 12, the minimum working gap width w is greater than zero, but smaller than the thickness D of the lid foil blank 22. As a result, a thickness region 46 of the lid foil 48 exiting the working gap 24 remains closer to the anvil 30 and is free of material weakenings 13. In contrast, a thickness region 50 of the lid foil 48 extending from the exposed surface 18a is provided with material weakenings 13. The thickness regions 46 and 50 complement each other to form the complete lid foil 48.

[0071] One advantage of forming the material weakening arrangement 12 by ultrasonic tools 28 and 32 is the uniformity of the thickness of the two thickness ranges 46 and 50 along the machine direction MD and along the transverse direction CD.

[0072] The system 10 comprises a control device 52, such as an electronic data processing device with at least one integrated circuit and at least one data storage device in which a control program can be stored 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 19- and which can serve to store data during an execution of the control program.

[0073] The control device 52 controls the electric motors 36, 38 and 44 according to its control program and, if necessary, based on input from a plant operator. The control device 52 also controls an ultrasonic generator 54, which is coupled via a converter 56 and a booster 58 in a manner known per se to the active formations 33 of the sonotrode arrangement 26 in order to set them into micro-motion.

[0074] A further advantage of cutting material weakenings 13 into the lid film blank 22, preferably only in its material layer 18, lies in the absence of adhesion phenomena between the resulting cut surfaces and the active formations that created them. Unlike mechanical blades, which do not perform any micro-movements during cutting, plastic material, especially thermoplastic plastic material, does not adhere to the structural active formations 33 that penetrate the plastic material. This significantly facilitates the emergence of the structural active formations 33 from the material.

[0075] The introduction of the material weakenings 13 can be carried out in such a way that the structural active formations 33 heat the flanks of the material weakenings 13 they create so intensely that they melt on their surface and cool down again after the active formations are withdrawn. If necessary, the flanks can locally fuse together again, which stabilizes the resulting cover film 48 despite its weakening, without eliminating the effect of the material weakening arrangements created.

[0076] In contrast to the illustrated embodiment, the sonotrode assembly can have a smooth surface and the anvil can bear the active features 33. Likewise, the sonotrode assembly can be designed with a smooth, flat surface. In this case, only the anvil can be designed as an ultrasonic tool, optionally rotating, with one or more structural active features.

Claims

68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 20 - Claims 1. Method for producing a plastic film (48) provided with a local material weakening arrangement (12), wherein the material weakening arrangement (12) has a defined shape, the method comprising the following steps: i. Providing an ultrasonic tool arrangement (25) comprising at least one sonotrode (26(i)) and at least one anvil (30) as ultrasonic tools (28, 32) with mutually approachable and mutually removable cooperating action formations (33, 35), wherein at least one ultrasonic tool (28, 32) has a structure action formation (33) which has a complementary shape with respect to the shape of at least one section of the local material weakening arrangement (12), ii. Inserting a plastic film blank (22) into a working gap (24) between the ultrasonic tools (28, 32), iii. Reducing a working gap width (W) to a first working gap final width (w) which is less than the film thickness (D) of the plastic film blank (22), iv. Activating the at least one sonotrode (26(i)) or maintaining an activation state of the at least one sonotrode (26(i)) during step iii. of reducing the working gap width, v. increasing the working gap width (W) to a second working gap final width (Wmax) which is greater than the film thickness (D) of the plastic film blank (22).

2. Method according to claim 1 , characterized in that the first working gap end width (w) is greater than zero, so that a material weakening (13) introduced into the plastic film blank (30) by approaching the cooperating active formations (33, 35) does not completely penetrate the plastic film blank (22) in the thickness direction (TD) of the plastic film blank (22). 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 21 - 3. Method according to claim 1 or 2, characterized by the fact that the procedure includes the following further steps: vi. Advance the plastic film blank (22) and vii. Repeat steps iii. to v.

4. Method according to claim 3, characterized in that at least one ultrasonic tool (28, 32) is a rotatable ultrasonic tool (28, 32) and the method comprises a step of rotating the at least one rotatable ultrasonic tool (28, 32).

5. Method according to claim 4, characterized in that a rotational movement of the at least one rotatable ultrasonic tool (28, 32) is a continuous rotational movement.

6. Method according to any one of claims 3 to 5, characterized in that a feed movement of the plastic film blank (22) is a continuous feed movement.

7. Method according to claim 3, characterized in that the relative movement to reduce the working gap width (W) in step iii. is exclusively a translational relative movement towards each other.

8. Method according to claim 4 or according to claim 7, characterized in that a feed movement of the plastic film blank (22) is a clocked feed movement.

9. Method according to any one of the preceding claims, characterized in that the amplitude of an ultrasound-induced micromovement of an active formation (33, 35) and its origin are directed towards the 68050P WO Huhtamaki Flexible Packaging Germany GmbH & Co. KG - 22 - each other active formation (33, 35) is chosen such that the structure-active formation (33) is located outside the plastic material of the plastic film blank (22) at one dead point of its ultrasonic vibration movement and has penetrated the plastic material at the other dead point.

10. Method according to claim 9, characterized in that the ultrasonic vibration movement of the ultrasonic tools (28, 32) at least partially, preferably completely, causes steps iii. and v.

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