IL29171A - Process and apparatus for bending glass in sheet form and product obtained thereby - Google Patents

Description

mm misa n»3¾3t ns»a3 τρηπι PROCESS AND APPARATUS FOR BENDING GLASS IN SHEET FORM AND PRODUCT OBTAINED THEREBY This invention relates to a process for bending glass in sheet form.

Bent sheets of tempered glass are required for vehicle windscreens and a variety of other purposes. The known processes of production involve bending the glass at high temperature and then toughening the glass by heat-treatment known as thermal tempering.

This treatment must be performed after the glass has been bent because it is not possible to bend thermally tempered glass. In carrying out the thermal tempering there is a considerable risk of the glass becoming deformed or of its optical properties becoming impaired .

The present invention is based on the surprising discovery that sheets of chemically tempered glass can be bent to a predetermined shape quite quickly and without exposing the glass to very high temperatures involving substantial risk of spoiling the glass .

The expression "chemical tempering" refers to the production or increase of compressive stresses in an exterior layer or layers of the glass by causing atoms, molecules or ions from a contacting medium to enter such layer or layers. Chemical tempering processes, which usually involve the replacement of ions in the glass by entering ions of different size by an ion exchange mechanism, are known per se and need no detailed explanation.

It has also been found that glass in sheet form can be bent while chemical tempering is proceeding.

According to the pres'ent invention, a sheet of glasV is permanently deformed by applying bending forces to the sheet during and/or after chemical tempering of the glass 1Θ and while the viscosity of the glass is at least 1Θ -poises. ; The term ''sheet" in this definition includes any piece of flat glass of whatever size and includes part of a continuously formed ribbo of glass. The viscosity range ;> 10 poises will be hereafter referred to as "the transformation range", the invention is also of interest when using viscosities above this transformation range, but in this latter case, the time necessary for permanently bending the sheet must be longer , so much the more that temperature is lower.

The preferred viscosity range within which to subject the glass to bending forces is ΙΟ *"* - 10^*"* poises.

The invention is primarily but not exclusively concern4d with the bending of drawn sheet glass of ordinary composition e.g., ordinary soda-lime glass. The transformation range of such glasses corresponds approximately to the temperature range 600° - 400°C. The invention can also be employed however for bending other glasses, e.g., lithium silicate and more complex glasses such as lithium silico-aluminate glasses, all of which can be chemically tempered by replacing lithium ions in superficial layers of the glass by sodium and/or potassium ions.

Any type of chemical tempering can be adopted when carrying out the invention. Thus the chemical tempering rocess ma be one wherein ions in the lass are substituted substituted by smaller ions while the temperature of the 13 · 2 glass is above the annealing point (corresponding to 10 poises). In that case the viscosity of the glass can be above or below 10 poises (but preferably in the range 1010 to 101^poises) during bending. If ions in the glass are replaced by larger ions, e.g., if sodium ions are replaced by potassium ions or lithium ions are replaced by sodium and/or potassium ions, lower temperatures are required.

Preferably larger ions are substituted for smaller ions in 13 2 the glass while the viscosity of the glass is above 10 * .

In that case the sheet should be bent also while the viscosity 13.2 of the glass is above 10 * poises, however to perform quick bending of the sheet, the sheet should be bent, while 13 2 15 the viscosity is in the range 10 * to 10 poises.

The substitution of ions may be brought about by holding the glass at an appropriate elevated temperature in a bath of liquid medium comprising a salt for producing the ion exchange, either in solution or in molten condition. As a specific example, a sod$*l-ime silicate sheet glass can be tempered in a bath of molten potassium salt(s) at a temperatune towards the lower end of the transformation ' range, say a temperature of 500°C or less, sodium ionw at the surface of the glass being substituted by much larger potassium ions.

Alternatively diffusion of ions into the glass may take place from a gaseous medium, e.g., an atmosphere of hydrogen or superheated steam, in contact with the glass, or from a medium applied to the glass to form a layer nn one or each side thereof, e.g., a layer essentially comprising carbon or a metal such as silver, tin or lead, or a layer of a molten salt such that ion exchange takes place between the glass and the coating layer. Any medium used to provide the requisite ions for diffusing into the glass, which is adherent to the glass after the chemical tempering process can "be left as a coating on the glass or removed.

The coefficient of diffusion increases with increase in temperature. For example, the coefficient of diffusion of potassium into a soda-lime glass is ten times higher at 500° than at 400° C. There are advantages therefore in keeping the temperature at or near the upper end of the range permissible for the selected type of chemical tempering process. To take an example, the mechanical tensile strength of a sheet of soda-lime silicate glass, which was initially 5-10 kg/mm , increased in a few moments to a value of 100-150 kg/mm when subjected to chemical tempering in a bath of potassium salt(s) at a temperature just below 500°C. After its strength has been so increased, considerable bending forces can be applied to the glass, within the transformation range, without breaking the glass.

The bending of the glass can occur at any stage after superficial layers of the glass have been mechanically strengthened by chemical tempering, and if the glass has been allowed to cool following this tempering its temperature can be raised to a suitable value preparatory to bending, in any desired manner, e.g., in a furnace or by immersing the sheet in another heated bath. The interval of time between tempering and bending can be as great as may be required but it is preferable to reheat the glass sufficiently rapidly to avoid any chance of stress relaxation occurring.

The glass sheet can be bent while it is still immersed in a chemical tempering bath at an appropriate temperature.

The extent to which chemical tempering must take place before bending forces are applied depends on the magnitude of such forces and thus on the thickness of the sheet of glass and on the degree to which it is to be bent. It is possible to carry out some bending operations after only a shallow tempering, for instance of the order of a few microns. If this shallow tempering does not impart sufficient mechanical strength for other purposes related to the eventual use of the glass sheet, then further chemical tempering can be carried out after the sheet has been bent and this further treatment may last as long as is necessary for imparting the requisite mechanical strength. Advantages are to be gained by performing a first stage tempering at a temperature at or near the upper end of the temperature range permissible for the selected chemical tempering process, and a second stage tempering, after the sheet has been bent, at a lower temperature. Sy keeping the temperature lower at the second tempering stage there is less tendency for stress relaxation to occur than at the higher temperature, which latter however is itself desirable at the first stage in order that the mechanical strength of the glass can be quickly increased to a value permitting the glass to be bent.

As an example, a first stage tempering involving the substitution of ions in the glass by larger ions can be performed below but near to the temperature at which the viscosity of the glass is 10 13 2 poises, and a second stage tempering can be performed at a lower temperatire . In the case of a soda-lime glass with an annealing point of about 500°C the first stage tempering can be performed at a temperature just below 500°C and the second stage can be performed in the region of 50°C.

The bending forces can be applied to the glass, e.g., by pressing the sheet between shaping moulds, or by subjecting the periphery of the sheet to pressure while the sheet is supported on a shaped former. In designing the mould components or other formers, a correctional coefficient should preferably be applied to allow for delayed elasticity of the glass and to avoid unduly long treatments. The effect of delayed elasticity varies in dependence on temperature. To give some idea, the orders of magnitude of the corrections to be applied in designing and preparing moulds, vary between 5 and 10?¾ in dependence on temperature. If no correction factor is introduced the sheet must be held flexed at the elevated temperature for an appreciably longer time. For example, if a correction of 10 is allowed, the duration in the mould can be about two thirds of the duration if no correction factor is allowed for.

The fact that the entire tempering and "bending process can be performed at temperatures lower than those necessary in thermal tempering processes is very important for the production of high quality glass products. In particular, the mechanical strength of the glass is improved by the lower temperature treatment. In addition, the lower temperatures are beneficial in lessening the demands made on the shaping tools. The tools used for supporting bent glass sheets during thermal tempering are expensive; they have to withstand the high temperatures to which the glass is raised in thermal tempering and to possess high mechanical strength at those temperatures. Moreover there are considerable problems due to the tendency of the glass to adhere to the formers.

Attempts have been made to interpose materials between the glass and the formers in order to reduce adherence and impressions on the glass as far as possible, but these steps have not proved to be adequate.

Formers for use in a glass bending process according to the invention can be made from a variety of materials. In general they have only to withstand temperatures of about 500°C at the most. Various steels can be used. A good example is an austenitic 18/8 (18$ nickel; 8$ chromium) steel with a low carbon content (for instance 0.02$). A wide range of suitable materials is available however. Since the effects of scaling by oxidation are considerably reduced contact with the glass is also improved. At the lower temperatures, adhesion is not so much a problem, and the interposition of special material between the glass and the former can usually be dispensed with.

The most suitable temperature at which to bend the glass and the rate at which the glass is bent are factors which are subject to variation from case to case. For example, the bending time may be from a few seconds to a few hours, depending on the degree of deformation or the shape to be produced and on the properties of the glass sheet.

The following table gives a few test results illustrating the times necessary for producing permanent flexure of a sheet of glass measuring lm x lm x 0.Π04 m and of the same composition as the glass used in Example 1 set out subsequently herein, during immersion of the sheet in a bath of K^C^O^. The bending forces were applied by a bar running parallel with two margins of the sheet while such margins were supported on two other bars extending along the length thereof.

Temperature Flexing Moment Duration in Flexure oC . (Kgm) minutes 480 7.5 30 0.8 480 20.0 30 15.0?· 480 30.0 18 15.0$ 500 30.0 120 30.0^ In all these instances, bending occurs at a relatively low temperature in comparison with the known processes. Some glasses can be bent at very low temperature (e.g., about 250°C)by a process according to the invention.

It is well known per se in the art of chemically tempering glass that for achieving the best results the glass surfaces and/or the edges should be subjected to some preliminary treatment to remove superficial flaws and this practice can be frllowed when required, preparatory to carrying out the present invention. A suitable treatment, known per se, comprises the use of a solution of hydrofluoric acid.

The invention includes apparatus for use in carrying out the new process. Such apparatus comprise means for bending a sheet of glass to a predetermined shape, a container for holding a bath of molten salt, means for supporting a sheet of glass in the container so that chemical tempering of the glass can take place therein and means for exerting bending forces on a said sheet of glass to bend it to a predetermined shape, preferably while the sheet is immersed in a said bath. The container may be equipped with means for maintaining a body of metal salt in mrlten condition in the container, or the container may be located within a furnace. The sheet supporting means may comprise upper and lower forming dies, or a forming die and a pressing element or elements, the dies, or the die and the pressing element or elements, being mounted for guided movement in unison into and out of the container, and for movement relative to each other for exerting bending forces on a sheet located between them.

Certain apparatus according to the invention and suitable for carrying out a glass-deforming process as hereinbefore defined, are illustrated by way of example in the accompanying diagrammatic drawings which will now be referred to. In these drawings; Pig. 1 is an elevation of one apparatus according to the invention; Pig. 2 is a cross-sectional plan o^- part of the apparatus, on line II-II in Pig. lj and Pig. 3 is an elevation of an alternative apparatus .

The installation shown in Pigs. 1 and 2 comprises a monorail track 1 along which a crane trolly 2 runs by means of rollers 3. A cable 4 associated with the trolley carries a rocking lever 5 having tongs 6 for supporting a sheet of glass 7 in a bath 8 of salt, for instance potassium nitrate, contained in a tank 9 heated by electrical resistors 10.

After a suitable immersion period, the sheet 7 is removed from the bath and laid flat on a die 11 having wheels 12 which run on rails 13. This die, with a chemically tempered glass sheet thereon, can be pushed into a furnace 14, in which the mould and glass sheet are shown in broken lines and designated 11' and 7' .

The furnace 14 comprises an enclosure 15 with an inlet aperture 16 and an outlet aperture 17. Each of the apertures 16,17 can be closed by a drop gate 18 suspended from a cable 19 which runs over a pulley 20 borne by a frame 21 and has a counterweight 22 attached to its other end. Bearings 23 disposed at the four corners of the furnace support four screw-threaded rods 24, These rods are in screw-threaded engagement with horizontal "bars 25 arranged as shown "by Pig. 2. These "bars are in the form of square-section tubes and can be raised or lowered by rotating the rods 24 in one direction or the other. The rods are connected by a roller chain 27 which passes around pinions 26 on the rods and the system is actuated by a flywheel 28.

The furnace is heated by electrical resistors (not shown). Any other type of heaters could be used.

Transverse pressing elements 29 are rockably supported by end trunnions 30 between the bars 25.

The said trunnions are received in apertures in the bars 25. A series of such apertures is preferably provided to permit the positions of the pressing elements to be varied to suit glass sheets of different sizes. When a glass sheet has been brought, on the die 11, into the furnace and has been brought to a suitable temperature, the bars 25 are lowered to bring the pressing elements 29 into contact with the end margins of the sheet. Then the bars 25 are further lowered gradually over a certain period of time so that the pressing elements press down on the ends of the sheet and progressively bend the sheet to the curvature of the die. The length of time taken for this bending operation depends inter alia on the temperature of the glass sheet. Afterwards, the bars 25 and the pressing elements are raised and the die is advanced to the position 31. The bent sheet is removed from the die by means of the crane trolley and immersed in a bath 32 of molten salt in a tank 33 for the purpose of further chemically tempering the glass.

The alternative apparatus shown in Pig. 3 includes a frame comprising a rectangular platform 34 having uprights 35 at its corners. These uprights are provided with top rings 36 enabling the apparatus to be suspended via cables 37 from a crane trolley similar to that shown in Pig. 1.

The platform 34 supports a lower concave die component 38. A co-operating convex die component 39 is carried by a plate 40 in the end portions 41 of which there are apertures through which vertical guide rods 42 extend with clearance. The guide rods are carried by the platform 34.

The apparatus shown in Pig. 3 is used in conjunction with a first chemical tempering tank like tank 9 in Pig. 1. After chemically tempering a sheet of glass in that tank, the glass sheet is laid flat on the lower die component 38 as shown in Pig. 3 and the upper die component is allowed to rest on the sheet. Weights 43 are placed over the guide rods 42 and rest on the plate 40 so as to exert the required bending pressure.

The whole frame including the platform 34 and the supported dies is then lowered into a bath 44 of molten salt held in a tank 45· The sa.lt is kept in molten state by electrical resistance heaters 10. The dies and glass sheet are left submerged in the molten salt sufficiently long for the glass sheet to bend into conformity with the die surfaces.

The frame is subsequently raised and further chemical tempering of the glass sheet takes place in a third bath, the sheet being retained between the dies .

The first chemical tempering bath can be dispensed with if chemical tempering is allowed to take place in the bath 44 before the sheet is bent. To this end the lower die component with the glass sheet thereon can be lowered into the bath and the upper die component can be placed in position after the requisite degree of chemical tempering has occurred. Chemical tempering subsequent to bending can also be performed in the same bath, by simply removing the upper die component after the desired curvature has been imparted to the glass sheet and leaving the sheet in the bath for a further period of time.

Co-operating die components as shown in Pig. 3 can equally well be used for bending a glass sheet in a furnace.

The following are examples of processes according to the invention: Example 1 The process according to the invention was applied for bending a glass sheet measuring lm x lm x 0.004 m and formed of glass of the following composition, the precentages being by weight: 40$ Na O^ and 2 Li O^, the percentages being by weight.

After withdrawal of the glass sheet from the chemical tempering bath its temperature was allowed to fall to 520°C and a permanent flexure of 10$ was imparted to the sheet by progressively bending the sheet to this extent over a period of five minutes. The same curvature can be obtained at 459°C, but in this case the time during which the flexure efforts must be applied is increased to a half hour.

Example 2 A sheet of glass of the same size and composition as the sheet referred to in Example 1 was subjected to a chemical tempering process as stated in Example 1. Almost immediately after withdrawal of the sheet from the ehemical tempering bath and while the sheet was at a temperature of 560°C, a permanent flexure of 10$ was imparted to the sheet by progressively bending the sheet to this extent over a period of three minutes.

Example 3 The process according to the invention was applied for bending a glass sheet measuring 0.2 x 0.5 x 0.003 m and formed of a borosilicate glass of the following composition, the percentage being by weight: Si02 60 Na20 12% CaO 10% MgO 6% B203 6% • P2°5 1 A1203 5% The viscosities of such a glass at 450°C, 535°C and 600°C are 1015, 1013 and 10 poises respectively.

The glass sheet was subjected to chemical tempering by holding it immersed for 30 minutes at a temperature of 480°C in a bath of molten salt composed of 40% KNO^, 30 KC1 and 30% NaNO^, the percentages being by weight.

At the end of the 30 minutes period, bending forces were applied to the sheet, while it remained in the chemical tempering bath at 480°C, so as progressively to bend the sheet to a flexure of 5% over a period of 30 minutes.

By way of comparison, if a sheet of glass havin a composition as set out in any of the foregoing examples had to be bent to the extents indicated and thermally tempered, the glass would have to be bent prior to tempering and the temperature of the glass during the application of the "bending forces would have to be above 620°G.

Claims (25)

WHAT WE claim is:

1. A process wherein a sheet of glass is permanently "bent by applying bending forces to the sheet during and/or after chemical tempering of the glass and while the viscosity of the glass is in the range 1010 - 1015 poises.

2. A process according to claim 1 wherein the viscosity of the glass, while the sheet is being bent, is in the range 10 11" 5 - 1014 poises.

3. A process according to any preceding claim wherein the chemical tempering is performed so as to produce or increase compressive stresses at both surfaces of the glass sheet.

4. A process according to any preceding claim wherein the chemical tempering involves ion exchange between the glass and a contacting medium.

5. A process according to claim 4 wherein the ion exchange takes place between the glass and a bath of liquid medium in which the glass sheet is immersed.

6. A process according to claim 5 wherein said liquid medium is molten salt.

7. A process according to claim 5 wherein said liquid medium is a salt solution under a pressure sufficient to prevent vaporisation thereof at the working temperature.

8. A process according to any of claims 4 to 7 wherein the chemical tempering involves the substitution of ions in external layers of the glass by smaller ! ions, while the glass is at a viscosity below 10 /poises.

9. A process according to any of daiins 4 to 7 wherein the chemical tempering involves the substitution of ions in the glass by larger ions, while the glass is at a viscosity above 10 13.V 2 poises, and the sheet is bent while the viscosity of the glass is above 10^ poises, provided that the duration is not sufficient to allow relaxation to take place.

10. A process according to claim 9 , wherein the chemical tempering involves the substitution of potassium ions for ions Of smaller size present in the glass.

11. H i A process according to claim 10 wherein the chemical tempering involves ion exchange between the glass and a contacting medium composed of or including potassium nitrate.

12. A process according to any preceding claim wherein bending of the glass takes place while the glass is immersed in a liquid medium providing ions which diffuse into the glass.

13. A process according to any of claims 1 to 11 wherein thejbending of the glass takes place in a furnace .

14. A process according to any preceding claim wherein the glass is further chemically tempered after it has been bent.

15. A process according to claim 14 wherein the chemical tempering after the bending of the glass

16. A process according to claim 15 wherein the glass is subjected before being bent, to chemical tempering involving the substitution of ions in the glass by larger ions a little below a temperature at which the viscosity of the glass is 10 13 ' 2 poises, and is subjected after being bent to a second chemical tempering treatment at a lower temperature.

17. A process according to any preceding claim wherein the sheet is bent by means of shaped dies.

18. A process according to any preceding claim wherein the glass is a soda-lime glass.

19. A process for bending a sheet of glass substantially according to any of the Examples herein.

20. Apparatus for use in bending sheets of glass, comprising means for bending a sheet of glass to a predetermined shape, a container for holding a bath of molten salt, means for supporting a sheet of glass in the container so that chemical tempering of the glass can take place therein, means for heating the glass to reduce its viscosity to a value in the range 10"^ - lO^ poises, and means for exerting bending forces on a said sheet of glass to bend it to a predetermined shape while the glass is in said viscosity ran e .

21. Apparatus according to claim 20 wherein said heating means is arranged to heat a glass sheet while it is supported in a bath of molten salt in said container, and said means for exerting bending forces on a sheet of glass is arranged to operate while a said sheet is so supported.

22. Apparatus according to claim 20, wherein said heating means is a furnace.

23. Apparatus according to any of claims 20 to 22, wherein said supporting means comprise upper and lower forming dies or a forming die and a pressing element or elements for exerting pressure on a pa.rt at least of the periphery of a said sheet.

24. Apparatus for use in bonding sheets of glass, substantially as herein described with reference to the accompanying drawings.

25. A sheet of glass which has been tempered and bent by a process according to any of claims 1 to 19. DATED THIS 19th day COHEN ZKDE & SPISBACH P.O.BOX 1169, TEL-AVIV Attorneys for Applicants