A method for producing a cellulose screw cap and a cellulose screw cap

Abstract

A cellulose High Density Dry Moulded Fibre (HD-DMF) screw cap (1) and a method for producing a cellulose High Density Dry Moulded Fibre (HD-DMF) screw cap (1) from a cellulose material (2; 9; 10), wherein the method comprises the steps of; heating a forming mould (3) to a forming temperature in the range of 100°C to 300°C; arranging the cellulose material in the forming mould (3), and forming the cellulose screw cap (1) from the cellulose material in the heated forming mould (3), by moulding the cellulose material (2; 9; 10) with a forming pressure to obtain a density of the cellulose screw cap (1) greater than 1 ,30 g / cm3.

Description

The present disclosure relates to a method for producing a high density dry moulded fibre (HD-DMF) cellulose screw cap from a cellulose material.BACKGROUNDCellulose fibres are often used as raw material for producing or manufacturing various products. Products formed of cellulose fibres can be used in many different situations where there is a need for having sustainable products of essentially non-flat shapes. An essentially non-flat shapes may refer to any suitable three-dimensional object shape. There is a wide range of products that can be produced from cellulose fibres and a few examples are disposable plates and cups, blank structures and packaging materials. Packages produced from cellulose fibres may for example be used for packaging of liquids, dry materials and other types of goods, where the packaging may be made in a three-dimensional shape or formed into a three-dimensional shape from a two-dimensional sheet material. Such products are often laminated with different films in order for the product to withstand liquids, grease, heat etc.Cellulose fibres are obtained by separating the cellulose fibres from a pulp derived from e.g. wood or other plants. Pulp is a lignocellulosic fibrous material that can be prepared either mechanically or chemically by separating cellulose fibres from wood or other plants. Wood pulp is e.g. obtained by grinding timber or trees in some kind of mill, e.g. a disc refiner, where the wood is ground to wood pulp. The pulp contains water, cellulose fibres, lignin and hemicelluloses. For some products, e.g. where the strength is not a key factor and / or when a low price is important, a lignocellulosic material, i.e. fibres where the lignin is not removed, can be used.There are different processes that can separate wood fibres. When preparing mechanical pulp, thermomechanical pulp or chemo-thermomechanical pulp, the fibres are separated but the lignin is not removed from the cellulose fibres. In a chemical pulp process, the lignin and the hemicelluloses are removed more or less completely from the pulp, leaving substantially pure cellulose fibres.One material commonly used for cellulose fibre products is wet moulded pulp. The pulp used for wet forming is often obtained from recycled paper boards and newspaper, where the cellulose fibres comprise lignin. This lowers the cost. Wet moulded pulp has the advantage of being considered as a sustainable packaging material, since it is produced from biomaterials and can often be recycled or composted after use. Consequently, wet moulded pulp has been quickly increasing in popularity for different applications. Wet moulded pulp articles are generally formed by immersing a suction mould into a liquid or semi liquid pulp suspension or slurry, while suction is applied, whereby a body of pulp is formed with the shape of the desired product by fibre deposition. The suction mould is then withdrawn from the suspension and the suction is generally continued to compact the deposited fibres while exhausting residual liquid. With all wet-forming techniques there is a need for drying the wet moulded product, where the drying is a very time and energy consuming part of the production, which is costly. Further, this method requires a large quantity of water. The demands on aesthetical, chemical and mechanical properties of products are increasing, and due to the properties of wet-formed cellulose products, the mechanical strength, flexibility, and chemical properties are limited. It is also difficult in the wet-forming process to control the mechanical properties of the products with high precision.Another known method for producing products from cellulose material is by pressing loose cellulose fibres in a dry state, known as Dry Moulded Fibres (DMF). These products can be made in a cost-efficient way without using water as a cellulose fibre bearer and with a reduced energy need. Such products can be used to replace disposable plastic products, but are somewhat limited when it comes to strength and the possibility to vary the thickness of a product to a great extend.In a DMF process, cellulose fibres are formed with a forming pressure between 10-20 MPa in a regular compression mould. In such forming, the cellulose fibres arranged in an air-laid cellulose fluff blank are drawn apart somewhat when a non-flat shape is created. If the shape or height difference is too large, the cellulose blank may be torn, which is one reason why deep drawn dry moulded fibre products are difficult to produce. Since the cellulose blank does not float or stretch, it is also difficult to produce dry moulded cellulose products where the difference in thickness varies over the cellulose product. DMF products can be produced at the same cost as disposable plastic products.One example of DMF product is a screw cap for a container, e.g. a bottle or the like. In order to provide a thread on the inside of the cap, indentations are arranged in the outer side of the cap corresponding to the internal threads. In this way, the wall thickness of the cap is substantially equal over the complete cap. Without the external indentations, the thread would be less compressed and thus weaker than the rest of the cap. WO2023144718 shows an example of such a screw cap.There is thus a need for improved sustainable cellulose products, where the cellulose products are having improved mechanical and chemical properties, can be manufactured with high precision, and where the production is costefficient and rational.SUMMARYAn object of the present disclosure is to provide a method for producing a cellulose screw cap where the previously mentioned problems are avoided. This object is at least partly achieved by the features of the independent claim. The dependent claims contain further developments of the method for producing a cellulose screw cap. Another object of the present disclosure is to provide a cellulose screw cap.The disclosure concerns a method for producing a cellulose high density dry moulded fibre (HD-DMF) screw cap from a cellulose material, wherein the method comprises the steps of; heating a forming mould to a forming temperature in the range of 100°C to 300°C; arranging the cellulose material in the forming mould; and forming the cellulose screw cap from the cellulose material in the heated forming mould, by pressing the cellulose material with a forming pressure to obtain a density of the cellulose screw cap greater than 1,30 g / cm3.Advantages with these features are that the method provides an efficient manufacturing process for cellulose screw caps with improved mechanical and chemical properties, where the cellulose screw cap is a high density dry moulded fibre (HD-DMF) product. The advantage with this method is that high density dry moulded fibre screw caps are provided, having a higher strength than regular dry moulded fibre (DMF) screw caps that are moulded with a forming pressure of 10-50 MPa. The forming pressure is greater than 100 MPa, preferably greater than 150 MPa, and preferably greater than 200 MPa or more.With a sufficiently high forming pressure, which may be combined with vibrations acting on the fibres, the cellulose material will become pseudoplastic during the pressing action, which means that the cellulose material comprising cellulose fibres will assume liquid-like properties during the moulding action. The induced vibrations may be axial, transvers or rotational, or may be a combination of these. The moulding of a HD-DMF product is in one example performed in a closed mould, where the cellulose material is completely enclosed by the mould. During a moulding action with a high forming pressure, where the forming pressure exceeds 100 MPa, the forces acting on the cellulose fibres will not only provide a compressing force but also a shearing force on the cellulose fibres when a three-dimensional product is moulded, since the cellulose fibres will be displaced somewhat relative each other. The shearing forces acting on the cellulose fibres will to some extent transform some of the cellulose fibres to micro fibrils and nano-cellulose.In one example, the forming pressure is higher than 150 MPa and may be higher than 200 MPa or higher. The forming pressure may be up to 500 MPa or even up to 1000 MPa or more, depending on the intended use and the actual type of the cellulose screw cap. If various additives are used in the cellulose material, this may also impact the most suitable forming pressure. The density of the moulded cellulose screw cap is greater than 1,30 g / cm3 and may be up to 1,40 g / cm3 or even higher.Even though a higher forming pressure will give a cellulose product with a higher strength and a higher density, the preferred forming pressure is a forming pressure where the desired parameters for the cellulose product are met, without exceeding these parameters. A higher forming pressure adds a cost to the cellulose product. This means that in the same press with the same rated pressure, fewer and / or smaller cellulose products can be made with the same forming pressure. There is thus a need to optimize the used forming pressure to the desired properties of the cellulose product. It has been shown that a forming pressure exceeding approximately 100 MPa will start to give the cellulose material pseudo-plastic properties, which allows for a HD-DMF screw cap having internal threads and a varying thickness.One advantage with a higher forming pressure where the cellulose material assume pseudo-plastic properties is that complicated shapes can be obtained, which are difficult to obtain with regular moulding of DMF products. With the inventive method, a complicated product such as a screw cap having internal threads and a smooth outer surface can be produced, where the thickness of the screw cap varies with up to 300-400%.The cellulose screw cap is formed in a forming mould which in one example comprises a first positive mould part and a second negative mould part. The forming mould parts are non-flexible, preferably made from steel, and may be heated to the desired forming temperature. The forming mould is in one example heated with integrated heating elements, preferably electrical heating elements, but also liquid heating is possible. The forming mould is preferably closed, such that the cellulose material is completely enclosed in the mould during moulding of the HD-DMF cellulose screw cap.In one example, the starting material is an air-laid cellulose blank structure used for regular dry moulded fibre products. Here, the cellulose material may be pre-pressed in a pre-forming mould with a low pre-forming pressure in the range between 1 - 10 MPa. The purpose of the pre-forming is to compress the cellulose material to a smaller volume such that it will be easier to insert the pre-formed cellulose material into the forming mould. If e.g. an air-laid cellulose blank material is used as starting material, a weight of between 1000 - 3000 GSM may be required, as compared to 400 - 600 GSM for a regular DMF product. Such a starting material may be difficult to insert in a forming mould without pre-forming it.In another example, the starting material is a cellulose cardboard paper or pulp sheet containing cellulose fibres. The cardboard paper or pulp sheet may be stacked in several layers in order to obtain a desired starting material. With such a material consisting of stacked paper or pulp sheets, a pre-forming may not be necessary, depending on how it is inserted into the forming mould. The starting material may also be cellulose particles or cellulose granules containing cellulose fibres. The granules or particles may be inserted directly into the forming mould.During the moulding of a three-dimensional HD-DMF cellulose product, different forces will act on the cellulose material. When a flat two-dimensional cellulose product is moulded, all forming pressure forces acting on the cellulose material will be in the same direction as the forming pressure direction, i.e. perpendicular to the mould surfaces. When a three-dimensional cellulose product is moulded, some of the forces will not be parallel to the direction of the forming pressure, but will be perpendicular to the mould surface. Since the cellulose material does not float at lower pressures, the cellulose material will be pulled apart somewhat in order to correspond to the three-dimensional shape of the mould, and this small displacement of the cellulose material will, together with the high forming pressure, induce some shear forces on the cellulose material, at least at some regions of the cellulose material. Together with the high forming pressure, these shear forces will not be neglectable and will help to create some local pseudo-plastic regions where the cellulose material will assume liquid-like properties. Depending on the design of the mould, more or less shear forces may acton the fibres. The shear forces may be increased by inducing vibrations to the fibres.In one example, the cellulose material is an air-laid cellulose pre-form blank comprising loose cellulose fibres. When an air-laid cellulose blank is used, the weight of the cellulose blank is preferably higher than the cellulose blank used for a regular DMF product, and may be in the region between 1000 - 3000 GSM. With such a material, a strong cellulose HD-DMF screw cap with a density exceeding 1,30 g / cm3 can be obtained when moulded with a sufficiently high forming pressure.In another example, the cellulose material is a paper sheet, either a cardboard paper or pulp sheet, comprising compacted cellulose fibres. When cardboard paper or pulp sheet is used, preferably arranged as a roll of paper sheet, the weight of the paper sheet may be in the region between 300 - 800 GSM, and the thickness of a paper sheet may be between 2 - 4 mm thick, having a density of e.g. around 0,50 g / cm3. With such a standardized paper sheet, several layers of paper sheets may be stacked on each other one by one or rolled together in order to obtain a desired starting material. By stacking several layers of paper sheets, a strong cellulose HD-DMF screw cap with a density exceeding 1,30 g / cm3 can be obtained when moulded with a sufficiently high forming pressure.In another example, the cellulose material is a granular cellulose material containing cellulose fibres particles, where the cellulose fibres particles are more or less compacted. When a granular cellulose material is used, the density of the granules may be lower than the density of the formed cellulose screw cap, and may be in the region between 0,4-0, 8 g / cm3. With such a material, a strong cellulose HD-DMF screw cap with a density exceeding 1,30 g / cm3 can be obtained when moulded with a sufficiently high forming pressure.The cellulose material may be made from mechanical pulp, thermochemical pulp or chemical pulp comprising at least some lignin and / or hemicellulose, also referred to as a lignocellulosic raw material. The cellulose material may in one example comprise more than 0,5% lignin. The cellulose material may also include additives, where the additives are used to decrease the liquid and / or gas permeability of the cellulose screw cap and to increase the resistance to e.g. hot and cold liquids, grease, oil etc. Such additives may also be applied to the surface of the cellulose screw cap after the cellulose screw cap is formed. In one example, the cellulose material comprises at least 90% cellulose fibres by dry weight. The additives used are additives adapted to alter the permeability of the cellulose material, and should not function as a binder material to bind the cellulose material together. By using untreated cellulose fibres, the cellulose fibres are bound together by hydrogen bonds and Van der Vaals bonds. Additives may decrease the possibility for hydrogen bonds, and binder material will definitely reduce the possible hydrogen bonds.One suitable product made from cellulose HD-DMF is a screw cap for a container, e.g. a bottle. The screw cap is provided with a top section and a concentric side wall having an inner surface and an outer surface, where the inner surface is provided with at least one internal thread section and where the circumferential outer surface is substantially even. Such a cellulose HD-DMF screw cap will resemble a regular plastic screw cap used for e.g. PET plastic bottles. The internal thread section may be a single thread or may comprise several thread sections that constitutes a screw thread. With the inventive method, a cellulose HD-DMF screw cap where the thickness of the screw cap varies with at least 200% can be obtained. A thickness variation up to 300-400% is possible if desired. In this way, it is possible to provide an internal thread on the inner surface of the screw cap, while the outer surface can be substantially smooth and even. It is of course also possible to provide the outer surface of the screw cap with some kind of gripping surface, a gripping rim and / or a tamper proof fixation rim.The cellulose HD-DMF screw cap is formed in the forming mould during a cycle time period in the range of 0,1 to 10 seconds, and preferably less than 5,0 seconds. A suitable holding time for the screw cap in the forming mould is less than a second, and may be e.g. 0,3-0, 7 seconds. The holding time together with the forming temperature and the forming pressure are important parameters in the forming of the cellulose screw cap.BRIEF DESCRIPTION OF DRAWINGSThe disclosure will be described in greater detail in the following, with reference to the attached drawings, in whichFigs. 1a-e show schematically a method for producing a cellulose HD-DMF screw cap from a cellulose pre-form blank according to the disclosure,Figs. 2a-d show schematically a method for producing a cellulose HD-DMF screw cap from a cellulose pre-form blank according to the disclosure,Figs. 3a-e show schematically a method for producing a cellulose HD-DMF screw cap from a paper pulp sheet according to the disclosure,Figs. 4a-d show schematically a method for producing a cellulose HD-DMF screw cap from cellulose granules according to the disclosure, andFig. 5 shows schematically an example of a cellulose HD-DMF screw cap according to the disclosure.DESCRIPTION OF EXAMPLE EMBODIMENTSVarious aspects of the disclosure will hereinafter be described in conjunction with the appended drawings to illustrate and not to limit the disclosure, wherein like designations denote like elements, and variations of the described aspects are not restricted to the specifically shown embodiments, but are applicable on other variations of the disclosure.In the present detailed description, a method for producing a cellulose HD-DMF screw cap from a cellulose material will be described. The method is suitable for different types of screw caps and other closures that should exhibit a higher strength and a higher density than regular DMF products, and that may have a more complicated shape with varying side wall thickness. Such products may be relatively small with a volume of e.g. a few cm3 due to the required high forming pressure, which is costly. It would of course also be possible to produce larger cellulose HD-DMF closures if desired. The cellulose HD-DMF products are disposable, but may be used several times, depending on the actual product and actual post treatment of the product. The cellulose HD-DMF products may be recyclable and / or compostable.The cellulose material used to form the cellulose HD-DMF screw cap is a cellulose material containing cellulose fibres and may also comprise at least some lignin and hemicellulose. Such a material is produced from mechanical pulp, thermochemical pulp or chemical pulp where some of the lignin and the hemicelluloses can be removed. Additives may also be added to the cellulose material, where the additives are used to decrease the liquid and / or gas permeability of the cellulose screw cap and to increase the resistance to e.g. hot and cold liquids, grease, oil etc. In one example, the cellulose material comprises at least 90% cellulose fibres by dry weight and at the most 10% lignin or additives by weight. The cellulose material will also comprise some water, e.g. between 6% to 20% by weight. Water is not seen as an additive, it is necessary to create hydrogen bonds between the cellulose fibres but will evaporate when the cellulose screw cap is heated in an oven.Figs. 1a-e show schematically a method for producing a cellulose HD-DMF screw cap from an air-laid cellulose blank, where the air-laid cellulose blank has been pre-formed to a pre-form blank 2. The basis weight of the cellulose pre-form blank 2 may e.g. be 1000 GSM (gram per square meter). When forming the cellulose blank in the air-laid process, the cellulose fibres are carried and formed to the cellulose blank structure by air as carrying medium. In the air-laid process, small amounts of water or other substances may if desired be added to the cellulose material in order to change the properties of the cellulose screw cap, but air is still used as carrying medium in the forming process. The layer of the dry-formed cellulose blank may have a dryness that is mainly corresponding to the ambient humidity in the atmosphere surrounding the cellulose blank. Additional water may be added to the cellulose blank, such that a water content of between 6 to 20% by weight is reached. A lower water content is possible but may be difficult to reach due to the moisture in the ambient air.In order to produce the cellulose screw cap 1, the pre-form 2 is inserted into the forming mould 3. A schematic forming mould 3 is shown in Fig. 1 a, where the forming mould comprises a first male part 4 and a second female part 5. The first part 4 is a circular mandrel and is in the shown example provided with a threaded section 6 at the lower end of the mandrel. The outer diameter of the mandrel corresponds to the inner diameter of the second part 5, which is provided with a hollow shape that corresponds to the outer shape of the final cellulose screw cap 1. The pre-form 2 is inserted into the second part 5 of the forming mould 3.Fig. 1b shows a cut view of the pre-form 2 and the forming mould 3. The mandrel 4 is lowered towards the second mould part 5, and in Fig. 1c, the mandrel starts to press on the pre-form blank. In Fig. 1 d, the mandrel has reached its lowermost position, and an upper stop surface of the mandrel bears on the second lower mould part. In this way, a predefined volume of the cavity is created, in which the liquid-like cellulose fibres can float and which will allow the complete cavity to be filled. The amount of cellulose fibres compressed in the mould will at least partly determine the density of the cellulose screw cap. For a given volume, more cellulose fibres will create a higher density of the cellulose screw cap. Since the cellulose material assumes liquid-like properties during the moulding process, the shape and size of the forming mould may be designed to compensate for a slight flexibility of the pressed cellulose material, since the cellulose material may spring back some when the forming pressure is released. The amount of cellulose fibres in the cellulose pre-form blank together with the dimensions of the forming mould is selected such that the density of the cellulose screw cap is greater than 1,30 g / cm3 or more The difference of the side wall thickness may be at least 200%, and may be up to 300-400%.The forming pressure is now at least 100 MPa, and may be up to 200 MPa or more, depending on the required parameters of the cellulose screw cap 1. During moulding of the cellulose HD-DMF screw cap, the pre-form blank is exposed to the high forming pressure, and the cellulose fibres will become pseudo-plastic due to the high forming pressure. The cellulose fibres will displace in the forming mould and will fill the forming mould completely since the cellulose fibres will assume liquid-like properties. The pseudo-plastic behaviour will allow the cellulose fibres to fill the forming mould evenlyIt is also possible to use a forming mould where the volume of the mould cavity is not predefined, i.e. where the upper mould part is not provided with a stop surface. In this case, the forming pressure will set the actual volume of the cellulose screw cap. A given amount of cellulose fibres and a given forming pressure will thus give a desired density of the cellulose screw cap. In this way, the forming pressure can be used to alter the actual density of the cellulose screw cap if similar cellulose screw caps with differing densities are to be produced.The high forming pressure and the fact that the cellulose fibres must be displaced some from the position in the pre-form to the position in the cellulose screw cap, thus creating some shear forces on the cellulose material, adds to the pseudo-plastic state of the cellulose fibres. The high pressure and the shear forces acting on the cellulose fibres thus creates the pseudo-plastic state that secures that an even density of the cellulose screw cap is reached. After a specified holding time, the cellulose screw cap is ready and can be removed from the forming mould.In Fig. 1e, the cellulose screw cap 1 is removed from the forming mould by raising the mandrel 4 from the second part 5. The cellulose HD-DMF screw cap 1 with internal threads is removed from the threaded section of the mandrel by rotation, e.g. as is known from injection moulding of plastic screw caps. At the same time, the outer surface of the screw cap has been finalized, since the inner surface of the second mould part is provided with the desired shape and look of the screw cap.Figs. 2a-d show schematically another method for producing a cellulose HD-DMF screw cap 1 from a cellulose material. The cellulose material may have different shapes and densities. Flere, the cellulose material is a pre-form blank 2 formed from pre-compressed cellulose fibres and some additives, where the cellulose material and the additives are mixed to a homogenous mixture. The pre-formed blank may have a dryness that is mainly corresponding to the ambient humidity in the atmosphere surrounding the cellulose material. Additional water may be added to the cellulose material, such that a water content of between 6 to 20% by weight is reached. A lower water content is possible but may be difficult to reach due to the moisture in the ambient air. In Fig. 2a, a pre-formed blank 2 of cellulose material is inserted into the forming mould 3. The cellulose material may also be air-laid directly in the required size and shape.The forming mould comprises a first male mould part 4 and a second female mould part 5. The first mould part 4 is a circular mandrel and is in the shown example provided with a threaded section 6 at the lower end of the first mould part. The first mould part 4 is further provided with a vibration device 8, which is adapted to induce vibrations to the cellulose fibres through the first mould part 4. The vibrations may be induced in an axial direction, parallel with the pressing direction of the forming mould and parallel with the centre axis 7. The vibration device 8 is here shown as a device mounted on the first mould part, but the vibration device may be arranged at any position of the forming press acting on the first mould part. The outer diameter of the first mould part 4orresponds to the inner diameter of the second mould part 5, which is provided with a hollow shape that corresponds to the outer shape of the final cellulose screw cap 1. The cellulose material 2 is inserted into the second mould part 5 of the forming mould 3.The vibrations may also be induced in a transverse direction, perpendicular to the centre axis, or in a rotational direction, such that the first mould part is rotated around the centre axis 7 of the forming mould 3. The rotations may range over e.g. 5-30 degrees. A combination of vibrations in several directions is also possible. The vibration frequency may be relatively low, such as 100-300 Hz, or may be higher, such as ultrasound above 20 kHz. The amplitude is dependent on the frequency and on the required energy.Fig. 2b shows a cut view of the cellulose material 2 and the forming mould 3. The first mould part 4 is lowered towards the second mould part 5, and starts to press on the cellulose material. In Fig. 2c, the first mould part has reached its lowermost position. In this example, an upper stop surface of the first mould part bears on the second mould part. In this way, a predefined volume of the cavity is created, in which the cellulose fibres can flow and which will allow the complete cavity to be filled. The amount of cellulose fibres compressed in the forming mould will determine the density of the cellulose screw cap. For a given volume, more cellulose fibres will create a higher density of the cellulose screw cap. Since the cellulose material will flow during the moulding process, the shape and size of the forming mould may be designed to compensate for a slight flexibility of the pressed cellulose material, since the cellulose material may spring back some when the forming pressure is released.The forming pressure is now at least 100 MPa, and may be up to 200 MPa or more, depending on the required parameters of the cellulose screw cap 1. During moulding of the cellulose HD-DMF screw cap, the cellulose material 2 is exposed to the high forming pressure and to the vibrations of the vibration device 8. The high pressure and the vibrations will induce shear forces to the cellulose fibres, which will bring the cellulose fibres to flow. Heat will increase the forming of hydrogen bonds between the cellulose fibres during the pressing action. The cellulose fibres will displace in the forming mould and will fill the forming mould completely. This will allow the cellulose fibres to fill the forming mould evenly.It is also possible to use a forming mould where the volume of the mould cavity is not predefined, i.e. where the upper mould part is not provided with a stop surface. In this case, the forming pressure will set the actual volume and thus density of the cellulose screw cap. A given amount of cellulose fibres and a given forming pressure will thus give a desired density of the cellulose screw cap. In this way, the forming pressure can be used to alter the actual density of the cellulose screw cap if similar cellulose screw cap with differing densities are to be produced. This may be of advantage since the exact amount of cellulose material must not be used. A slight variation of the amount of cellulose material will give cellulose screw cap with the desired density but with a small variation in volume. In a forming mould having a closed predefined volume, a slight variation of the amount of cellulose material will give cellulose screw cap with varying density but with an equal volume.By exposing the cellulose material to vibrations by vibrating at least one of the forming mould parts during the pressing action, a cellulose screw cap having required properties can be achieved with a reduced forming pressure, or a cellulose screw cap having increased properties can be achieved with the same forming pressure. The vibrations may be axial in the pressing direction, parallel to the centre axis 7, where the vibration device 8 is comprised in one of the forming mould parts of the forming mould, here in the first mould part 4. The vibrations may also be rotational around the centre axis, transversal perpendicular to the centre axis, or a combination of axial, transversal and / or rotational vibrations may be used.For relatively low frequency vibrations, typically below 100 Hz, a servohydraulic or electro-hydraulic device can be used. For vibration frequencies typically between 1 Hz to 2000 Hz, an electrodynamic device can be used. In one example, the vibrations have a relatively low frequency, in the range between a few Hertz up to 100 Hertz. The vibrations are superimposed on the regular forming pressure, where the initial forming pressure preferably is above 100 MPa or more. The waveform of the vibrations is not crucial, and a sinusoidal waveform or a triangular waveform may be used. The stroke of the vibration device may be relatively short, from parts of a mm up to a few mm. The total energy of the vibrations is a combination of pressure, frequency and stroke length. The vibrations are preferably induced when the forming pressure is relatively high, e.g. exceeding 100 MPa. The vibrations may be induced during the closing stroke of the forming mould, preferably at the end of the closing stroke, and / or when the forming mould is closed.It is also possible to position a vibration device at the lower end of the first forming mould part such that it can act directly on the cellulose fibres, e.g. a piezo device. Such a device is capable of producing vibration frequencies of up to 20 kHz or more. The used vibration frequency and the used amplitude will depend on the size and shape of the cellulose screw cap. A higher frequency and / or higher amplitude may e.g. be required for cellulose screw caps having thinner side walls. The vibrations may be introduced to the cellulose fibres during the closing stroke of the pressing action and / or when the forming mould is closed. The vibrations may continue during the holding time of the pressing cycle but are shut off during the opening stroke of the pressing action. The direction of the vibrations may also vary, and may be axial, rotational, translational or a combination of these.A vibration device may be arranged to induce rotational vibrations to the cellulose fibres through the first mould part. The vibration device will vibrate with a vibration frequency in the range between a few Hz up to 2000 Hz, and in one example below 100 Hz. The vibration device will induce angular rotational vibrations ranging over a rotational angle a, where a is in the range between less than a degree up to e.g. 10 degrees. The rotational angle is also dependent on the frequency, a small rotational angle allows for a higher frequency and a larger rotational angle requires a lower frequency.The vibrations introduced to the forming mould are in one example created by a vibration device integrated in one of the mould parts of the forming mould, in the shown example in the first mould part. The vibration device may be integrated directly into the forming mould part, may be positioned in the holder plate for the forming mould part or may be arranged at the forming press, e.g. by controlling the hydraulic press cylinder. By positioning the vibration device in the holder plate or at the forming press, the same vibration device can be used for different forming moulds, where a forming mould part is attached to the holder plate. A further advantage of positioning the vibration device at the holder plate or at the forming press is that the vibration device must not be exposed to the heat of the forming mould. There is an insulation between the holder plate and the forming mould. A further advantage is that forming moulds of different sizes and shapes can be attached to the same holder plate, which reduces the need for several vibration devices.The high pressure and the shear forces acting on the cellulose fibres due to the induced vibrations allows the cellulose fibres to flow in the forming mould. This may be referred to as burst flow. After a specified holding time, which may be very low, the cellulose screw cap is ready and can be removed from the forming mould.In Fig. 2d, the cellulose screw cap 1 is removed from the forming mould by raising the first mould part 4 from the second part 5. The cellulose HD-DMF screw cap 1 with internal threads is removed from the threaded section of the first mould part by rotation, as is known from injection moulding of plastic screw caps. At the same time, the outer surface of the screw cap has been finalized, since the inner surface of the second mould part is provided with the desired shape and look of the screw cap.Figs. 3a-e show schematically an example of a method where the cellulose HD-DMF screw cap is made from a paper sheet, such as a cardboard sheet or a pulp sheet, as a starting material. The starting material is formed as smaller pre-formed rolls 9 of paper that may be cut from a roll of paper. The paper sheets may be sprayed with water by a spraying nozzle in order to control the desired humidity of the pre-formed roll. Additional water may be added to the paper, such that a humidity of between 6% to 20% by weight is reached.The weight of the paper sheet is in one example 700 GSM, which means that three turns of such a strip gives a pre-formed roll of approximately 2000 GSM. The width of the strip, i.e. the height of a pre-formed roll is determined based on the desired cellulose fibre content in the final cellulose HD-DMF screw cap. Since the pre-formed roll does not have a bottom part, the height or the thickness of the pre-formed roll must compensate for the lack of a bottom, such that the required amount of cellulose fibres in the final cellulose HD-DMF screw cap is obtained.In Fig. 3a, the pre-formed roll 9 is inserted into the forming mould 3. The forming mould may be the same as described above, where a cellulose FID-DMF screw cap is moulded from an air-laid cellulose pre-form blank, or may be adapted to the paper roll method. But since the final cellulose HD-DMF screw cap is the same, a similar forming mould may be used.The forming mould comprises a first male part 4 and a second female part 5. The first part 4 is a circular mandrel and is in the shown example provided with a threaded section 6 at the lower end of the mandrel. The outer diameter of the mandrel corresponds to the inner diameter of the second mould part 5, which is provided with a hollow shape that corresponds to the outer shape of the final cellulose screw cap 1. The pre-formed roll 98 is inserted into the second mould part 5 of the forming mould 3.Fig. 3b shows in a cut view the pre-formed roll 9 and the forming mould 3 where the mandrel 4 is lowered towards the second mould part 5, and where the mandrel starts to press on the pre-formed roll. The pre-formed roll will displace and parts of the pre-formed roll will be pushed down, towards the bottom of the second mould part 5. This is shown in more detail in Fig. 3c, where most of the pre-formed roll has been compressed. The mandrel is pushed down with a pressing force F until the mandrel has reached its lowermost position and an upper stop surface of the mandrel bears on the second lower mould part, as is shown in Fig. 2d. With a sufficient amount of cellulose fibres in the mould, the forming pressure will be at least 100 MPa, and may be up to 200 MPa or more, depending on the required parameters of the cellulose HD-DMF screw cap 1. During moulding of the cellulose HD-DMF screw cap, the pre-formed roll is exposed to the high forming pressure, and the cellulose fibres will become pseudo-plastic due to the high forming pressure and possible vibrations and the cellulose fibres will displace in the forming mould, filling the forming mould completely since the cellulose fibres will assume liquid-like properties. The pseudo-plastic behaviour will allow the cellulose fibres to fill the forming mould evenly.The high forming pressure and possible vibrations and the fact that the cellulose fibres are displaced from the position in the pre-formed roll to the position in the final cellulose HD-DMF screw cap adds to the pseudo-plastic state of the cellulose fibres. The high pressure and the shear forces acting on the cellulose fibres thus creates the pseudo-plastic state that secures an even density of the cellulose screw cap. After a specified holding time, the cellulose HD-DMF screw cap is ready and can be removed from the forming mould.In Fig. 3e, the completed cellulose HD-DMF screw cap is removed from the forming mould by raising the mandrel 4 from the second part 5. The cellulose HD-DMF screw cap with internal threads is removed from the threaded section of the mandrel by rotation, as is known from injection moulding of plastic screw caps. At the same time, the outer surface of the screw cap has been finalized, since the inner surface of the second part is provided with the desired shape and look of the screw cap.In a further example, shown in Figs. 4a-d, the cellulose HD-DMF screw cap 1 is produced from a granular cellulose material as a starting material. The granular cellulose material may e.g. be cellulose granules or other smaller cellulose particles, such as cellulose pellets, cellulose fluff, saw dust, flakes from bale pulp, or separate cellulose fibres which may be more or less precompressed in order to be easier to handle. The granular material may have a dryness that is mainly corresponding to the ambient humidity in the atmosphere surrounding the granules. Additional water may be added to the granular material, up to a water content of between 6% to 20% by weight.In Fig. 4a, the granular material 10 is inserted into the forming mould 3. The schematic forming mould 3 shown in Fig. 4a comprises a first male part 4 and a second female part 5. The first part 4 is a circular mandrel and is in the shown example provided with a threaded section 6 at the lower end of the mandrel. The outer diameter of the mandrel corresponds to the inner diameter of the second part 5, which is provided with a hollow shape that corresponds to the outer shape of the final cellulose screw cap 1. The granular material is inserted into the second part 5 of the forming mould 3.When the granular material has been inserted into the second mould part of the forming mould, the mandrel 4 is lowered towards the second mould part 5, as is shown in Fig. 4b, and the mandrel starts to press on the granular material, which is compressed. In Fig. 4c, the mandrel has reached its lowermost position and an upper stop surface of the mandrel bears on the second lower mould part. The forming pressure is now at least 100 MPa, and may be up to 200 MPa or more, depending on the required parameters of the cellulose screw cap 1. During moulding of the cellulose screw cap, the granular material is exposed to the high forming pressure, and the cellulose fibres will become pseudo-plastic due to the high forming pressure, possible vibrations and the displacement of the cellulose fibres in the forming mould, filling the forming mould completely since the cellulose fibres will assume liquid-like properties. The pseudo-plastic behaviour will allow the cellulose fibres to fill the forming mould evenly.The high forming pressure, possible vibrations and the fact that the cellulose fibres are displaced from the position in the granular cellulose material to the position in the cellulose screw cap adds to the pseudo-plastic state of the cellulose fibres. The high pressure and the shear forces acting on the cellulose fibres thus creates the pseudo-plastic state that secures an even density of the cellulose screw cap. After a specified holding time, the cellulose screw cap is ready and can be removed from the forming mould.In Fig. 4d, the cellulose screw cap is removed from the forming mould by raising the mandrel 4 from the second part 5. The cellulose HD-DMF screw cap with internal threads is removed from the threaded section of the mandrel by rotation, as is known from injection moulding of plastic screw caps. At the same time, the outer surface of the screw cap has been finalized, since the inner surface of the second part is provided with the desired shape and look of the screw cap.Fig. 5 shows an example of a screw cap 1 for a bottle. The screw cap 1 comprises a bottom section 11 and a circular side wall 12 having an outer surface 14 and an inner surface 13. The inner surface is in the shown example provided with a threaded section 6 comprising one or more protruding elements formed as the threaded section. It would also be possible to produce a cap without a threaded section, such as a cap comprising a snap lock having a rim section that is arranged to snap to a rim of a container. A cap is a product well suited to be produced with the inventive method, since it is relatively small and has a relatively difficult shape that requires varying wall thickness of the product.Tests have shown that when forming a cellulose HD-DMF screw cap, a suitable forming pressure level is at least 100 MPa and may be up to 200 MPa or more, depending on the desired properties of the actual cellulose HD-DMF screw cap. A suitable moulding temperature level is in the range of 100°C to 300°C.It will be appreciated that the above description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. While specific examples have been described in the specification and illustrated in the drawings, it will be understood by those of ordinary skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure as defined in the claims. Furthermore, modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular examples illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out the teachings of the present disclosure, but that the scope of the present disclosure will include any embodiments falling within the foregoing description and the appended claims. Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand.REFERENCE SIGNS1: Cellulose screw cap 2: Pre-form blank 3: Forming mould 4: First mould part 5: Second mould part 6: Threaded section 7: Centre axis8: Vibration device 9: Pre-formed roll 10: Granular material 11: Bottom section 12: Side wall13: Inner surface14: Outer surface

Claims

1. A method for producing a cellulose High Density Dry Moulded Fibre screw cap (1) from a cellulose material (2; 9; 10) wherein the method comprises the steps of;heating a forming mould (3) to a forming temperature in the range of 100°C to 300°C,arranging the cellulose material in the forming mould (3); and forming the cellulose screw cap (1) from the cellulose material in the heated forming mould (3), by pressing the cellulose material (2; 9; 10) with a forming pressure to obtain a density of the cellulose screw cap (1) greater than 1,30 g / cm3.

2. A method according to claim 1,wherein the forming pressure is at least 100 MPa.

3. A method according to claim 1 or 2,wherein the forming pressure is at least 150 MPa.

4. A method according to any of claims 1 to 3,wherein the forming pressure is at least 200 MPa.

5. A method according to any of claims 1 to 4,wherein the method further comprises the step of vibrating the forming mould in an axial direction.

6. A method according to any of claims 1 to 4,wherein the method further comprises the step of vibrating the forming mould in a transverse direction.

7. A method according to any of claims 1 to 4,wherein the method further comprises the step of vibrating the forming mould in a rotational direction.

8. A method according to any of the preceding claims,wherein the cellulose material (2; 9; 10) contains less than 20% water.

9. A method according to any of the preceding claims,wherein the cellulose material is a dry-formed cellulose pre-form blank (2) formed in a dry-forming process where cellulose fibres are carried and formed to the dry-formed cellulose pre-form blank (2) by air as carrying medium.

10. A method according to any of claims 1 to 8,wherein the cellulose material is a paper sheet (9).

11. A method according to any of claims 1 to 8,wherein the cellulose material is a granular material (10) comprising cellulose fibres.

12. A method according to any of the preceding claims,wherein the cellulose material comprises at least 90% cellulose fibres by dry weight.

13. A cellulose High Density Dry Moulded Fibre screw cap (1) formed from a cellulose material (2), characterized in that the cellulose screw cap (1) has a density greater than 1,30 g / cm3.

14. A screw cap according to claim 12, wherein the density is greater than 1,40 g / cm3.

15. A screw cap according to claim 12 or 13, wherein the wall thickness of the cellulose screw cap (1) varies with at least 200%.