GB2139613A

GB2139613A - Method and apparatus for cutting glass

Info

Publication number: GB2139613A
Application number: GB08313287A
Authority: GB
Inventors: Clerck Georges De
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1983-05-13
Filing date: 1983-05-13
Publication date: 1984-11-14
Also published as: GB8313287D0; BE899594A; GB2139613B

Abstract

In a method of fracturing a body of vitreous material 1 through its thickness by irradiating the body with laser radiation 10 which scans along a desired path of fracture so that the material there fractures due to thermal shock caused by the absorption of such radiation, the body 1 is scanned by laser beams 10, 14 which trace the path of fracture in such manner that during such scanning, a volume increment within the thickness of the body for example at a common point of focus 12 of the two beams is simultaneously irradiated from opposite sides of the body. <IMAGE>

Description

SPECIFICATION Method and apparatus for cutting glass This invention relates to a method of fracturing a body of vitreous material by scanning the body with laser radiation.

A principal field of use for such methods lies in trimming the edges of a continuous ribbon of freshly formed flat glass as it leaves an annealing lehr and before it is cut into sheets. For this purpose a static laser beam may be focused on each margin of the ribbon a desired distance in from its edge so that a desired path of fracture is scanned as the ribbon is conveyed forwards. The edges of the ribbon break off due to thermal shock created in the glass by the absorption therein of the laser light energy. Use may also be made of a static or a moving laser beam for cutting a glass ribbon into sheets or for cutting sheets into smaller sheets.

When using prior art methods of fracturing vitreous material by laser irradiation, difficulties have been encountered in achieving a clean fracture in an efficient manner.

It is an object of the present invention to go at least some way towards remedying this defect.

According to the present invention, there is provided a method of fracturing a body of vitreous material by scanning the body with laser radiation, characterised in that the body is scanned by laser beams which trace the path of fracture in register and from opposite sides of said vitreous material.

In contrast to this, the prior art methods relied in general on directing laser radiation onto one side only of the body to be cut. This presents disadvantages overcome by the present invention. In the prior art methods, if the laser wavelength selected was strongly absorbed by the vitreous material, the energy absorbed by various strata within the thickness of the vitreous material would vary widely so that the stresses induced would be widely different across the body. This has been found to militate against clean fracture.

If, on the other hand the laser wavelength used is weakly absorbed by the glass so that the energy absorbed by the various thickness strata is more nearly equal it is of course necessary to use a much more powerful laser source in order to achieve the required energy absorption for fracture of the vitreous body to be induced. The adoption of the present invention promotes more uniform and/or more symmetrical energy absorption by the vitreous body through its thickness so that a clean fracture is achieved efficiently and more easily.

A further advantage of severing by laser radiation in accordance with the invention is the ease with which complicated fracture paths can be followed.

The laser light scanning opposite sides of the vitreous material preferably travels in directly opposed directions. This contributes to the formation of cleanly fractured edge faces.

In the most preferred embodiments of the invention the wavelength of the laser radiation and the thickness and composition of the vitreous material are such that between 20% and 70% of the radiation incident on each side of the vitreous material is absorbed within its thickness. This has been found to promote a favourable compromise between uniformity of energy absorption throughout the thickness of the vitreous body and total energy absorption by the body and it also promotes rapid cutting due to direct absorption of radiation at different levels within the vitreous material.

In some embodiments of the invention, two laser light sources are used for illuminating opposite sides of the vitreous material to be fractured. When this is done, the different laser sources may be of same or different powers and may operate at the same or different wavelength.

It is however preferred to use a single laser light source since this promotes a symmetrical energy dissipation across the thickness of the vitreous material without the need for a second laser source. Accordingly, in some preferred embodiments of the invention, a laser light beam scanning one side of said body, and passing through it, is reflected back along its path to scan the opposite side of the vitreous material. This also facilitates registration of the incident beams.

In such embodiments, it is especially preferred that the wavelength of the laser radiation and the thickness and composition of the vitreous material are such that between 40% and 60% of the radiation incident on each side of the vitreous material is absorbed within its thickness.

In other preferred embodiments of the invention, said scanning is effected by laser light from a single source which is split and directed to opposite sides of the body.

Advantageously, the wavelength of the laser radiation used lies in the range 2 ,lim to 6 ym inclusive. The absorption of radiation of such wavelengths by ordinary soda-lime glass is good for the purpose in view. In general, the longer the wavelength the greater is the coefficient of the absorption so that within that range ionger wavelengths are best used for fracturing thinner glass and shorter wavelengths for fracturing thicker glass.

In order to achieve a high energy flux density at the path of fracture, it is preferred that the laser radiation scanning each side of said vitreous material is focused at said path of fracture.

Unless steps are taken to modify a laser beam cross-section, such cross-section will, in general, be circular.

Such a beam will at any instant act to heat a substantially cylindrical portion of a body of vitreous material on which it is incident.

When a cylindrical portion alone is heated it will be apparent that the stresses induced are uniformly distributed around the cylinder and there is no preferential breaking direction.

Concentration of the stress to give a preferential breaking direction arises because the laser beam scans the vitreous material, and/or because the vitreous material is pre-scored. In order to achieve further concentration of stress to ensure fracture of the vitreous material cleanly along the desired path it is preferred that the or a laser beam is directed through an aperture in a screen to provide such beam, when incident on the path of fracture, with a cross-section having an apex, and that the body is scanned so that such apex points along the path of fracture.By adopting this preferred feature and modifying the crosssection of the laser beam incident on at least one side of the vitreous material so that it has an apex pointing along the path of fracture, the induced stresses are strongly concentrated at the apex and act to fracture the vitreous material cleanly along the desired path.

Preferably, a central portion of a beam of such radiation passes uninterrupted through said screen aperture to lie in an apical region of the modified beam cross-section. This promotes efficiency since the energy dissipated by a laser beam of unmodified cross-section has a greater flux density at its centre.

The flux density of the energy dissipated by the laser radiation incident on the vitreous body can be further increased by adopting any one or more of the following preferred features: i. the laser radiation is focused on the plane of said screen aperture; ii. an image of the screen aperture is focused on the path of fracture; iii. the screen is provided with a reflective cup surrounding its aperture which directs substantially all of said radiation through said aperture.

It is preferred that such beam cross-section is provided with one or two apexes and that the body is scanned so that the or each such apex points along the path of fracture. This avoids any problems which might arise due to stress concentrations at an apex of the beam which does not point along the path of fracture. It is particularly convenient for such beam cross-section to be provided with a single apex.

When acting in accordance with the present invention, it has been found unnecessary to pre-score the vitreous material with a cutting wheel or other scoring tool. indeed such scoring is disadvantageous because it tends to create fissures across the path of fractures which are subsequently able to act as stress raisers in the vitreous material after cutting. It is accordingly preferred that said fracture is wholly attributable to thermal shock due to the absorption in the vitreous material of laser energy.

The invention includes apparatus suitable for performing a method as herein defined and accordingly provides apparatus for fracturing a body of vitreous material comprising a support for such a body, at least one laser light source and means for causing radiation emitted thereby to scan a said body on said support, characterised in that the laser light source or sources and said scanning means are arranged to direct laser radiation from opposite sides of such vitreous material to trace the path of fracture in register.

It is preferred that the or at least one laser light source is a hydrogen fluoridedeuter- ium fluoride laser or a CO laser. This has the advantage that the wavelengths of the radiation emitted thereby are favourably absorbed by the vitreous material.

Apparatus according to the invention preferably incorporates one or more of the following optional features.

i. the scanning means is arranged to direct said laser radiation from opposite sides of such vitreous body along directly opposed directions; ii. a mirror is provided for reflecting laser light from a single source back along its path so that the direct and reflected laser light beams scan said opposite sides; iii. means is provided for splitting a laser light beam from a single laser light source and passing the split beams to said scanning means; iv. the wavelength emitted by the or each laser light source lies between 2 ,um and 6 ium inclusive; v. means is provided for focusing the laser radiation in the fracture path; vi. a screen having an aperture is located in the path of the or at least one beam of laser radiation to impart to such beam a crosssection having an apex which points along the projected path of fracture; ; vii. means is provided for focusing such beam on the plane of said screen aperture; viii. means is provided for focusing an image said screen aperture on a vitreous body on said support; ix. the screen is provided with a reflective cup surrounding its said aperture to direct substantially all of such beam through the aperture; x. said screen aperture is such as to impart to said beam a cross-section having two apexes pointing in opposite directions.

xi. said screen aperture in such as to impart to said beam a cross-section having a single apex.

xii. means is provided for altering the wavelength of the laser radiation.

The wavelength of the laser radiation may for example be altered by making use of a dye laser or frequency shifting crystals as known in the art.

A preferred embodiment of the present invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 is a graph indicating the proportion of radiation energy incident on a vitreous body which penetrates that body to a given depth, Figure 2 is a schematic diagram of apparatus according to the invention, and Figures 3 and 4 are respectively sectional and frontal views of an optional screen for use in preferred embodiments of the invention.

In Figure 1, curve A represents quanta of laser energy available at various levels in a sheet of glass of thickness T in which of the incident energy is absorbed in the glass. It will be noted that the available energy decays logarithmically from the top of the curve A to the bottom of that curve. This inequality of available energy is unfavourable for fracturing glass cleanly. In accordance with the invention therefore laser energy is directed from opposite sides of the glass sheet or other vitreous body to trace the desired path of fracture in register. If a second identical laser is directed at the opposite side of such a glass sheet, then the total laser energy available within the glass will be as represented in curve B which shows a symmetrical and much more even energy distribution.If on the other hand the emergent laser energy is reflected back along its course, the available energy at different levels due to that reflected beam will be as shown in curve C. Curve D represents the sum of curves A and C and, it will be noted, gives a more symmetrical energy distribution than either of those curves though not so symmetrical or even as that represented by Curve B. The energy distribution represented by curve D is however perfectly satisfactory in practice and avoids the necessity for an additional laser source.

In fact, provided that sufficient quantity of energy is absorbed, no matter what proportion of incident energy is absorbed by the vitreous body, clean fracturing thereof will be improved by tracing the desired path of fracture in register and from opposite sides of the body.

Figure 2 shows an embodiment of apparatus according to the invention for fracturing glass sheet or ribbon 1 as it travels along supported by a conveyor 2. A laser beam 3 emitted by a laser light source 4 travels to first and second mirrors 5 6. The first mirror 5 is a plane mirror, but the second mirror 6 is a concave mirror. The effect of these two mirrors 5, 6 is to reverse the direction of the laser beam 3 and focus it at 7 in the plane of an aperture 8 in an optional screen 9 where the cross-section of the beam is modified. The modified beam 10 continues to a second concave mirror 11 whence it is reflected through the glass sheet or ribbon 1 at a point 1 2 on the desired path of fracture. The second concave mirror 11 acts to focus the laserprojected image of the screen aperture 8 at that point.A proportion of the laser radiation which is not absorbed by the sheet or ribbon 1 passes through the glass to a third concave mirror 1 3 eg a spherical mirror located so as to reflect that radiation 14 back and bring it to focus at the same point 1 2.

The mirrors used suitably bear a front-surface reflective coating for example of gold or aluminium so that as little laser energy is absorbed by them as possible.

The aperture 8 in the optional screen 9 is shown in Figures 3 and 4. The aperture is surrounded by a reflective cup 1 5 which is targeted by the laser beam 3 so that substantially all the laser radiation energy passes directly or is reflected through the aperture 8.

As shown in Figure 4, the aperture 8 is shaped with an apex 1 6 to modify the cross section of the beam 3 to the same shape and the screen 9 is oriented so that the resulting apex of the beam points along the desired path of fracture. When the thus modified beam 10 passes through the sheet or ribbon the resultant energy absorption will stress the glass normally to the outline of the beam so that there will clearly be a high stress concentration at the laser-projected image of the apex 1 6 and the resultant stresses there will be at right angles to the desiredpath of fracture thus contributing to a clean fracture of the glass.That the image of the aperture 8 carried by the returning laser radiation 14 is reversed is not important because the image of the apex 1 6 will still be directed along the desired path of fracture though in the opposite direction.

As shown in Figure 4 the beam 3 is aimed at the reflective cup 1 5 so that a central portion 1 7 of the beam 3 passes directly through the aperture 8 in the region of its apex 1 6. Since such central beam portion 17 has the highest energy flux density this further contributes to a high stress concentration in the glass at the region of the projected image of the apex 1 6 thus further promoting a preferred fracture line in the glass along the desired path.

By this means it is possible using for example a 4 to 5 watt laser to dissipate some 300 to 400 W/mm2 in the glass being severed.

When severing ordinary soda lime glass it is recommended to use a laser emitting radiation in the 3-4 izm range when the glass is 3 mm thick. This results in absorption by the glass of about 50% of the energy incident on each side. For a similar energy absorption when severing very thin glass eg glass less than 1 mm thick it is suitable to use a laser light with a wavelength of about 5 um.

The invention is also useful for cutting along lines which are other than straight for example for cutting circular discs.

The use of the optional screen 9 for modifying the laser beam cross section so that it has one or two apexes which or each of which points along the lines of severance forms the subject matter of our co-pending application No 831 3288 filed under Agents reference 263/BERMUDA.

Claims

1. A method of fracturing a body of vitreous material by scanning the body laser radiation characterised in that the body is scanned by laser beams which trace the path of fracture in register and from opposite sides of said vitreous material.

2. A method according to claim 1 wherein the laser light scanning opposite sides of the vitreous material travels in directly opposite directions.

3. A method according to any preceding claim wherein the wavelength of the laser radiation and the thickness and composition of the vitreous material are such that between 20% and 70% of the radiation incident on each side of the vitreous material is absorbed within its thickness.

4. A method according to claims 2 and 3, wherein a laser light beam scanning one side of said body, and passing through it, is reflected back along its path to scan the opposite side of the vitreous material.

5. A method according to claim 4 wherein the wavelength of the laser radiation and the thickness and composition of the vitreous material are such that between 40% and 60% of the radiation incident on each side of the vitreous material is absorbed within its thickness.

6. A method according to claim 1,2 or 3 wherein said scanning is effected by laser light from a single source which is split and directed to opposite sides of the body.

7. A method according to any preceding claim, wherein the wavelength of the laser radiation used lies in the range 2 ym to 6 lim inclusive.

8. A method according to any preceding claim, wherein the laser radiation scanning each side of said vitreous material is focused at the path of fracture.

9. A method according to any preceding claim, wherein the or a laser beam is directed through an aperture in a screen to provide such beam, when incident on the path of fracture, with a cross-section having an apex and the body is scanned so that such apex points along the path of fracture.

10. A method according to claim 9 wherein a central portion of such beam passes uninterrupted through said screen aperture to lie in an apical region of the modified beam crosssection.

11. A method according to claim 9 or 10 wherein the laser radiation is focused on the plane of said screen aperture.

1 2. A method according to any of claims 9 to 11, wherein an image of the screen aperture is focused on the path of fracture.

1 3. A method according to any of claims 9 to 12 wherein the screen is provided with a reflective cup surrounding its aperture which directs substantially all of said beam through said aperture.

14. A method according to any of claims 9 to 13, wherein such beam cross-section is provided with one or two apexes and the body is scanned so that the or each such apex points along said path of fracture.

1 5. A method according to claim 14, wherein such beam cross-section is provided with a single apex.

1 6. A method according to any preceding claim, wherein said fracture is wholly attributable to thermal shock due to the absorption in the vitreous material of laser energy.

17. Apparatus for fracturing a body of vitreous material comprising a support for such body, at least one laser light source and means for causing radiation emitted thereby to scan a said body on said support, characterised in that the laser light source or sources and said scanning means are arranged to direct laser radiation from opposite sides of such vitreous material to trace the path of fracture in register.

18. Apparatus according to claim 1 7 wherein the or at least one laser light source is a hydrogen fluoride-deuterium fluoride laser or a CO laser.

1 9. Apparatus according to claim 1 7 or 18, wherein the scanning means is arranged to direct said laser radiation from opposite sides of such vitreous body along directly opposed directions.

20. Apparatus according to claim 1 7 wherein a mirror is provided for reflecting laser light from a single source back along its path so that the direct and reflected laser light beams scan said opposite sides.

21. Apparatus according to any of claims 1 7 to 20, wherein means is provided for splitting a laser light beam from a single laser light source and passing the split beams to said scanning means.

22. Apparatus according to any of claims 1 7 to 21, wherein the wavelength emitted by the or each laser light source lies between 2 ym and 6 ,um inclusive.

23. Apparatus according to any-of claims 1 7 to 22, wherein means is provided for focusing the laser radiation in the fracture path.

24. Apparatus according to any of claims 1 7 to 23, wherein a screen having an aperture is located in the path of the or at least one beam of laser radiation to impart to such beam a cross-section having an apex, the scanning means being so arranged that such apex points along the projected path of fracture.

25. Apparatus according to claim 24, wherein means is provided for focusing such beam on the plane of said screen aperture.

26. Apparatus according to claim 24 or 25, wherein means is provided for focusing an image of said screen aperture on a vitreous body on said support.

27. Apparatus according to any of claims 24 to 26, wherein the screen is provided with a reflective cup surrounding its said aperture to direct substantially all of such beam through the aperture.

28. Apparatus according to any of claims 24 to 27 wherein said screen aperture is such as to impart to said beam a cross-section having two apexes pointing in opposite directions.

29. Apparatus according to any of claim 24 to 27, wherein said screen aperture in such as to impart to said beam a cross-section having a single apex.

30. Apparatus according to any of claims 1 5 to 27 wherein means is provided for altering the wavelength of the laser radiation.