WO2011051567A2

WO2011051567A2 - Method for machining material by a laser device

Info

Publication number: WO2011051567A2
Application number: PCT/FI2010/050857
Authority: WO
Inventors: Antti Salminen; Heidi Piili; Tuomas Purtonen
Original assignee: Lappeenrannan Teknillinen Yliopisto
Priority date: 2009-10-28
Filing date: 2010-10-28
Publication date: 2011-05-05
Also published as: RU2012122720A; FI20096110A; FI20096110A0; WO2011051567A3; EP2493651A2

Abstract

The invention relates to a method for machining material by a laser device. According to the invention the material that is used is a natural fiber based material which is machined by a laser device using optical fiber having the wavelength of the laser beam of between 200nm and 3000nm and having sufficient quality of the laser beam in order to reduce the size of the focal point and provide non-linear absorption behavior, and a threshold intensity of the laser beam produced by the laser device is determined in order to provide sufficient absorption, the size of the focal point to be used is adapted to be suitable for the machining method to be used, and the natural fiber based material is machined by directing the laser beam of the laser device thereto for the machining thereof at an intensity that exceeds the threshold intensity, and the natural fiber based material is machined in the area of non-linear absorption behavior wherein sufficient absorption of the laser beam is obtained in order to machine the natural fiber based material.

Description

METHOD FOR MACHNING MATERIAL BY A LASER DEVICE

FIELD OF THE INVENTION

The invention relates to the method defined in the preamble of claim 1 for treating a natural fiber based material by a laser device.

BACKGROUND OF THE INVENTION

Known from the prior art are different proc- esses and apparatuses for cutting and cross-cutting the edge trim of a paper web in chemical pulp, paper and paperboard machines. In addition, different methods for creasing and cutting to shape in the conversion of paperboard and paper as well as for perfora- tion in the manufacture of tissue papers are known.

Known from the prior art is the mechanical slitting of a paper web by blades, wherein the machine reel is cut into customer rolls of a desired width. The mechanical cutting of a paper web causes plenty of dust which is an impression destroying factor e.g. in printing houses. The paper dust may also cause fires in plant conditions. In addition, the cutting marks made by mechanical blades are often uneven and fringed, and there remain plenty of fibers torn by the blade on the cutting surface. In addition, a problem is that the cutting by mechanical blades causes fric- tional losses and the blades need to be maintained regularly.

In addition, known from the prior art are different machining lasers and their uses in different machining applications.

It is known from the prior art to form a permanent mark by a focused laser beam on the surface of a material to be marked by laser marking. The marking is provided by engraving the surface or changing the color or optical properties of the surface. The laser marking process demands from the laser device to be used a very good beam quality and good focusability of the laser beam. By lasermarking, both informative and decorative markings can be made in different materials.

In addition, known from the prior art is the use of carbon dioxide laser in laser machining. By carbon dioxide laser a beam having a wavelength of e.g. approximately 10600nm is delivered by mirrors, wherein each mirror reduces the power for up to 5%. The use of carbon dioxide laser is known in the cutting of paper. However, the depth of sharpness of carbon dioxide laser is not sufficient e.g. for cutting thick cores, because a sufficiently small-scale focal point is not reached by the carbon dioxide laser. As the focal point grows larger, the area over which the power is distributed increases and the depth of sharpness that is reached becomes poorer. The depth of sharpness to be reached by carbon dioxide laser is, even with a long focal distance, such as a focal distance of 190.5mm and size of the focal point of 190μπι, only a couple of millimeters, which is not enough for cutting thick cores. In addition, a problem in the use of carbon dioxide laser encompasses the occupational safety risks consequent upon the use of mirrors. In addition, in machining paper by carbon dioxide laser, a great obstacle to wider use is the yellowish to brownish color caused by cutting on the cutting edge, and the odor that arises during cutting and remains in the paper. The discoloration of the cutting edge is harmful particularly in press materials, and the odor is problematic e.g. in food applications.

Publication US 5068513 discloses the use of a water jet and a laser for cutting the long side of a paper web. Publications EP 0763627, US 2008295984, WO 200568173 and US 2008174104 disclose different carbon dioxide laser applications, such as cutting and marking of paper.

Publications US 2008219300 and US 2007147442 disclose fiber laser apparatuses. Fiber laser has not been used in the laser machining of paper.

OBJECTIVE OF THE INVENTION

An objective of the invention is to disclose a new method for providing efficient and easy machining of a natural fiber based material and to eliminate the problems referred to above. SUMMARY OF THE INVENTION

The method according to the invention is characterized by what has been presented in the claims .

The invention is based on a method for ma- chining material by a laser device. According to the invention the material is a natural fiber based material that is machined by a laser device using optical fiber having a wavelength of the laser beam of between 200 and 3000 nm and having sufficient quality of the laser beam in order to reduce the size of the focal point and thus to provide non-linear absorption behavior. The method comprises determining a threshold intensity of the laser beam produced by the laser device, which is to be determined specifically for each appli- cation of use and which must be sufficient to provide sufficient absorption of the laser beam and depends on the natural fiber based material to be machined and on the laser device to be used and the wavelength of the laser beam. In addition, the size of the focal point to be used which depends on the natural fiber based material to be machined and its dimensions is adapted to be suitable for the machining method to be used in order to provide adequate quality of the machining, e.g. depth of sharpness. The natural fiber based material is machined by directing the laser beam of the laser de- vice thereto for the machining thereof at an intensity that exceeds the threshold intensity, and the machining thereof takes place in the area of non-linear absorption behavior wherein sufficient absorption of the laser beam is obtained in order to machine the natural fiber based material. The sufficient absorption is dependent on the machining application, the material to be machined and the desired final result.

In connection with the invention it was unexpectedly discovered that natural fiber based materials can be machined by laser devices using optical fiber wherein the wavelength of the laser beam produced is between 200 and 3000 nm. In the prior art methods, with natural fiber based materials, the absorption of the laser beam in the material has been typically improved as the wavelength has been increased. However, good quality of the laser beam has not been achieved at long wavelengths. A short wavelength of less than 3000nm has only been absorbed in a natural fiber based material at a very high intensity and very short pulses. Conven- tionally, the absorption coefficient of a particular wavelength in the interaction between a natural fiber based material and a laser beam remains constant, and the absorption in the material grows in proportion as the wavelength of the delivered laser beam grows and/or the intensity rises. In connection with the invention it was unexpectedly discovered that the absorption of the laser beam in a natural fiber based material rises quickly when acting in the area of non-linear absorption behavior and when the intensity of the laser beam exceeds a specific threshold intensity. In this case, the ability of the material to absorb the wavelength of the laser beam rises to be substantially greater than what was known or expected before when the cross- sectional area of the laser power per laser beam exceeds a specific threshold value. It was unexpectedly discovered that the laser cutting rate relative to laser power grows steeply and quickly, in some embodiments exponentially, when the threshold intensity is exceeded. The threshold intensity was discovered to depend substantially on the material to be machined and on the utilized laser device and the wavelength of the laser beam. It was discovered that the threshold intensity can be exceeded when the laser beam has particular properties, i.e. sufficient beam quality in order to have a sufficiently small size of the focal point, whereby the expression of non-linear absorption behavior is provided. It was found that as the threshold intensity is exceeded and the absorption of the laser beam in the natural fiber based material becomes better, the machineability is improved. Surprisingly, it was discovered that good quality laser machining is provided in the invention by the combination of a' laser beam having a wavelength of 200 to 3000 run that may be conveyed in optical fiber, good quality of the beam, exceeding of the threshold intensity and non-linear ab- sorption behavior. This way, absence of discoloration and odor in the cutting edge were substantially achieved.

In this context a natural fiber based material means any material that is at least mainly manufactured from natural fibers, fibers of a natural origin or recycling fibers, e.g. paper material, intermediate product, end product or other further processed product manufactured from fibers or fiber pulp.

In one embodiment of the invention the method is used in the machining of paper material. In this context paper material means any material manufactured from fiber-based pulp. The fiber- based pulp may be formed from soft- or hardwood or a parallel wood plant or other botanical material and/or recycled fibers. The fiber-based pulp may be mechanical, chemical or chemi-mechanical pulp or parallel pulp or recycled pulp. Alternatively, the fiber-based pulp may be formed by any method. Paper material means in this context any paper material, e.g. paper, cardboard, paperboard, tissue paper, dried chemical pulp, dried mechanical pulp or parallel material which may be in the form of an intermediate product or an end product. The paper material may consist of one or several material layers. When the paper material consists of sev- eral material layers, at least one of the material layers may contain or consist of other than fiber-based material, e.g. plastic-based material. The paper material may be plastic-coated or pigment-coated.

In one embodiment of the invention the laser device that is used is any laser device using optical fiber wherein the wavelength of the laser beam produced is 200 to 3000 nm and wherein the laser beam produced may be conveyed in optical fiber. In one embodiment a fiber laser device is used. In one embodiment a diode laser device is used. Preferably, the laser device may be selected from short pulse laser devices wherein the pulse length is Ins up to continuous beam laser devices. The beam quality of the beam in the laser device must be sufficiently good in order to obtain a suffi- ciently small size of the focal point, thereby providing the expression of non-linear absorption behavior and sufficiently good quality of the machining.

Preferably, a laser device that is suitable for the purpose is used, wherein the power of the la- ser beam produced may vary. In one preferred embodiment the good beam quality of the laser device pro- vides for high intensity, whereby the optical properties of the natural fiber based material to be processed start to change, and the material can be machined even with very low laser power.

In one embodiment of the invention the laser machining of the natural fiber based material is selected from the group of: laser cutting, e.g. cutting of the edge trim, cross-cutting and cutting to shape, laser marking, laser welding, laser engraving, laser drilling, laser perforation, i.e. punching, laser creasing, laser watermarking and their combinations or an equivalent machining method, e.g. in different operational steps of the manufacture of chemical pulp, paper and paperboard.

In one embodiment of the invention the laser beam of the laser device is divided in at least two parts. In one embodiment the laser beam is divided in e.g. 2 to 5 parts and the divided beam can be used e.g. in slitting a natural fiber based material. Con- ventionally, 5 to 8 cutting positions per machine reel width are used in slitting a paper material in the prior art application. In one method according to the invention this can be carried out at a machine width of five meters e.g. by two laser devices.

In one embodiment the laser beam of the laser device is divided so as to produce simultaneously at least 2, e.g. 2 to 10, beams by optical fiber. The length of the optical fiber may vary and machining sites may be located in different areas of the mate- rial when machining a natural fiber based material.

In one embodiment of the invention an agent that increases absorption of the laser beam wavelength produced by the laser device is added to the natural fiber based material in order to adjust or change, e.g. reduce, the threshold intensity. In one embodiment the absorption of the laser beam wavelength may be in- creased by plastic fibers, plastic particles, pigment particles, fibrous raw materials and their combinations or parallel raw materials to be added to the natural fiber based material to absorb the wavelength in ques- tion.

In one embodiment of the invention at least one of the overlapping material layers is treated by the laser device, wherein the material layer to be treated must be laser beam absorbing. In one embodi- ment substantially all overlapping material layers are treated by the laser device, preferably substantially simultaneously. In an alternative embodiment at least one material layer that- is under the topmost material layer or layers, i.e. an intermediate layer, is treated by the laser device. The intermediate layer must be laser beam absorbing. For example, the fiber laser device lends itself to the treatment, e.g. cutting, of overlapping paper materials because, unlike the mechanical blades, the fiber laser device does not tear the paper material, and the overlapping material layers are not able to move relative to each other. By the fiber laser device, also a depth of sharpness of several millimeters is achieved with a small, e.g. a 40pm, focal point. The simultaneous cutting of several overlapping material layers reduces the cutting time needed and increases the production rate.

In one embodiment of the invention more than one laser device is used in machining the natural fibers based material. This embodiment may be used e.g. in a solution where it is desirable to machine at least one layer under the topmost material layer or layers. In an alternative embodiment the laser devices may be used in parallel in order to provide the desired machining application. This embodiment provides for the use of low-power laser devices. In one embodiment of the invention the laser machining parameters are determined, wherein the parameters are selected from the group of selection of a laser device, selection of a natural fiber based mate- rial, wavelength of the laser beam, threshold intensity of the laser beam, size of the focal point and their combinations; a parameter library is generated; the desired parameters are selected from the parameter library for machining the natural fiber based material; the natural fiber based material is machined in the area of non-linear absorption behavior by directing the laser beam of the laser device to the natural fiber based material in order to machine it at an intensity that exceeds the threshold intensity; and the machining is controlled in order to provide sufficient absorption of the laser beam in the natural fiber based material by utilizing the selected parameters.

In one embodiment the edge trims of a paper material are cut in connection with production, coat- ing and printing machines. In one embodiment the method is used for partial laser cutting of a paper material by removing material from the surface of the paper material so as to form on the surface a groove which does not extend through the material. In one em- bodiment an origin mark is made on a layered paper material by cutting an identification mark in one of the layers .

In one embodiment the wet web of a paper material is cut at the wet end of papermaking. The mechani- cal strength properties of a wet web are nonexistent, so it has not been possible to cut it mechanically. Preferably, the slitting, cross-cutting and cutting to shape of a wet paper material is possible by a fiber laser device. In one preferred embodiment the method is used for laser cutting of the core of a paper reel. In one embodiment the beam of the laser device remains stationary and the core is rotating. In one embodiment the beam of the laser device moves and the core remains stationary. In one embodiment the cutting is provided from the inside of the core. In this context a core of a paper reel means a core on which paper is reeled. In one embodiment the method is used for laser cutting of tissue papers. In one embodiment the perforated lines for tearing are manufactured in tissue papers.

In one embodiment the method is used for laser cutting of dried chemical pulp and/or dried mechanical pulp. In one embodiment the method is used for slitting, wet web cross-cutting and cutting to shape in cardboard.

In one embodiment the method is used for laser creasing of a natural fiber based material by machining the material with a laser beam, whereby the mechanical properties of the material are weakened locally and whereby the creasing and folding of the material become easier. In one embodiment the method is used in digi- tal, contact-free creasing, e.g. in providing a crease site in one or more material layers, including the intermediate layer between material layers.

In one embodiment complicated exterior and interior packages are manufactured from a natural fi- ber based material, e.g. by creasing and/or cutting.

In one embodiment the method is used for laser marking of a natural fiber based material. Laser marking means herein any mark to be produced on the surface of a material. In one embodiment the method is used for laser engraving of a natural fiber based material by removing material from the surface of the natural fiber based material and forming a groove having a specific geometry. In one embodiment an engraving is made on the surface of a natural fiber based mate- rial, wherein e.g. metal powder may be spread in the engraving and/or the engraving may be coated by e.g. a plastic layer. In one embodiment the method is used for laser drilling of a natural fiber based material by forming in the material holes which either penetrate the material or extend to a specific depth. The diame- ter of the holes may be from micrometers up to centimeters. In one embodiment the method is used for laser perforation of a natural fiber based material by forming successive holes in the material.

In one embodiment the method is used for laser watermarking of a natural fiber based material by providing in the material a mark that is parallel to a watermark.

In one embodiment the method is used for making rapid models from a natural fiber based material or from plastic-coated natural fiber material.

In one embodiment a relief is made on the surface of a natural fiber based material by the method according to the invention.

The method according to the invention pro- vides considerable advantages compared to the prior art .

Thanks to the invention, a quick and efficient laser machining method of a good quality that is applicable for many uses, saves time and energy and is maintenance-free is provided. The invention provides a method that does not set restrictions for the shape to be cut .

Thanks to the invention, a method for cutting natural fiber based materials is provided, wherein the cutting marks are sharp, light, clean, odorless and dustless, without fibers protruding from the cutting surface. In addition, no subsequent discoloration is provided in the cutting marks. In addition, high cutting rates are reached in the cutting of natural fiber based materials, thanks to the invention. The use of the method according to the invention in e.g. cutting of edge trims also reduces the amount of the edge trims and the amount of loss.

In addition, a dust-free method is provided, thanks to the invention, to be used in printing proc- esses where the excess edges are cut off after printing .

An industrially applicable easy, quick and useful manner of machining a natural fiber based material is provided by the invention. An advantage of the invention is that it is easy and quick to change the reel widths e.g. in the machining of paper material. In addition, the apparatus to be used in the method according to the invention has only a few wearing parts, which reduces the maintenance measures.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the invention will be described by detailed examples of its embodiments. Example 1

Laser cutting of different natural fiber based materials by a fiber laser device was examined in these experiments.

The materials used included copying paper, pulp board manufactured from refined mechanical pulp, pulp board manufactured from chemical softwood pulp, pulp board manufactured from unbleached chemical hardwood pulp, as well as combination board formed from bleached and unbleached chemical pulp.

The laser device used was a fiber laser and the cutting device used was a 2D cutting head having a 3mm cutting nozzle and a focusing lens having a focal distance of 190mm. The wavelength of the laser beam in the fiber laser device was 1070nm, and the diameter of the passive fiber, i.e. the beam conveying fiber, was 8pm. The utilized values of the laser power varied between 50 and 200 and the utilized machining rates varied between 0.1 and 15 m/min.

In the experiments it was discovered that all other materials except for the combination board could be easily cut by the fiber laser device. The cutting marks were clean. In addition, it was discovered that the cutting result in the material could be influenced by adjusting the intensity of the laser beam. It was discovered that each material had a threshold intensity that could be exceeded in order to provide quick and successful cutting of the material. Also the combination board could be cut, although it was more difficult by the utilized device and adjustments than with the other materials.

Example 2

In these experiments the laser marking of different natural fiber based materials by a fiber la- ser device equivalent to that of Example 1 was examined.

It was discovered that the following experimented materials could be marked: cellulose + diatoma- ceous earth, recycled corrugated fiberboard, extruded recycled paperboard, recycled paper, recycled cardboard, SC paper, tissue paper and pulp board manufactured from unbleached chemical pulp.

The method according to the invention lends itself to be used as different embodiments in different machining applications of the most different types of natural fiber based materials.

The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.

Claims

1. A method for machining material by a laser device, chara ct e r i z ed in that the material that is used is a natural fiber based material which is machined by a laser device using optical fiber having the wavelength of the laser beam of between 200nm and 3000nm and having sufficient quality of the laser beam in order to reduce the size of the focal point and pro- vide non-linear absorption behavior, and a threshold intensity of the laser beam produced by the laser device is determined in order to provide sufficient absorption, the size of the focal point to be used is adapted to be suitable for the machining method to be used, and the natural fiber based material is machined by directing the laser beam of the laser device thereto for the machining thereof at an intensity that exceeds the threshold intensity, and the natural fiber based material is machined in the area of non-linear absorp- tion behavior wherein sufficient absorption of the laser beam is obtained for the machining thereof.

2. The method according to claim 1, cha ra ct e r i z ed in that the machining method of the natural fiber based material is selected from the group of: laser cutting, laser marking, laser welding, laser engraving, laser drilling, laser perforation, laser creasing, laser watermarking and their combinations.

3. The method according to claim 1 or 2, cha ract eri z ed in that the laser device that is used is a laser device using optical fiber wherein the wavelength of the laser beam produced is 200 to 3000 nm and wherein the laser beam produced may be conveyed in optical fiber.

4. The method according to any one of claims 1 to 3, cha ra ct e r i z ed in that an agent that increases absorption of the laser beam wavelength pro- duced by the laser device is added to the natural fiber based material.

5. The method according to any one of claims 1 to 4, cha ract eri z ed in that the laser beam of the laser device is divided in at least two parts in order to machine the natural fiber based material.

6. The method according to any one of claims 1 to 5, cha ra ct er i z e d in that at least one of the overlapping material layers of the natural fiber based material is machined by the laser device.

7. The method according to any one of claims 1 to 6, cha ract e ri z ed in that more than one of the overlapping material layers of the natural fiber based material is machined by the laser device.

8. The method according to any one of claims 1 to 7, cha ra ct e ri z e d in that the laser machining parameters are determined, wherein the parameters are selected from the group of selection of a laser device, selection of a natural fiber based material, wavelength of the laser beam, threshold intensity of the laser beam, size of the focal point and their combinations; a parameter library is generated; the desired parameters are selected from the parameter library for machining the natural fiber based material; the natural fiber based material is machined in the area of non-linear absorption behavior by directing the laser beam of the laser device thereto for the machining thereof at an intensity that exceeds the threshold intensity; and the machining is controlled in order to provide sufficient absorption of the laser beam in the natural fiber based material by utilizing the selected parameters .

9. The method according to any one of claims 1 to 8, cha ract e ri z ed in that more than one laser device is used in machining the natural fiber based material.

10. The method according to any one of claims 1 to 9, cha ract e r i z e d in that the method is used in the machining of paper material.