NO119868B - - Google Patents

Description

Method for achieving an equal sinking speed for two glass plates lying on top of each other and placed on a simple bending form.

The present invention relates to the pre-treatment of curved glass sheets

lamination.

It is well known that when glass plates

is heated, the viscosity of the glass decreases, and

if the glass extends without support,

between storage points or is placed on a curved surface, the plate will not

supported weight deform the glass softened by the heating, and thereby

causing it to sink down and assume its form

of the curved surface. According to usual

practice is the reduction in viscosity which is necessary for deformation

of the glass while forming plates corresponding

to a particular configuration achieved

only by means of heating. Rapidly cooled glass has less viscosity than annealed glass, which. is cooled slowly. According to the invention, this circumstance is used for faster forming of glass sheets

than is possible with previously known methods. A record becomes more special

cured, and then subjected to heating

to softening temperature for deformation of

the hardened plate. Tempering is here understood to mean heating the glass to

steady temperature above the annealing point, immediately followed by rapid cooling to below the annealing point. As a result of this treatment, the interior of the glass will have a lower

viscosity after curing than if the glass is allowed to cool slowly or is annealed. This difference in viscosity, which is

a consequence of different heat treatment,

is believed to be due to a time delay between

the temperature change and the change in mole-

the cooling structure, which determines the viscosity of the glass.

An aim of the present invention is to utilize the above-mentioned phenomena to achieve a more uniform and simultaneous bending of glass sheets into complicated curved shapes, so that the sheets, which are to form the laminated, curved sheet, are shaped in exact accordance with each other before lamination. A further aim is the simultaneous bending of glass sheets of different thickness before lamination for the production of certain types of aircraft windows, and likewise to produce intricate and complex curvatures, where flat glass sheets are bent with varying curvature along two axes, which form an angle with each other.

When bending glass plates when heated to the softening temperature of the glass, the thickness of a particular plate will determine the rate of subsidence. In particular, a thin plate of a certain shape will tend to soften faster than a thick plate of the same shape, as a result of the thin plate absorbing incident heat rays more quickly.

If it is thus necessary to simultaneously bend a thin and a thick plate, and the thin plate is placed under the thick one, and both plates in this position are placed on a form for heating the glass to softening temperature, the thin plate will bend down faster than the thick during the first part of the bend, and a space is formed between the facing surfaces of the plates, as a result of this difference in the bending speed. This gap enables the deposition of hot dust particles or other foreign substances on the heated, soft glass surfaces, thereby damaging the optical properties of the glass plates.

Placed instead of the thick plate

below the thin one to bend, the bending of the thin plate will slow down, as more heat is required before the thick plate starts to bend down. The thin plate is thereby exposed to stronger heating than is necessary for the bending, and may even fuse to the other plate.

Although boards of the same thickness should

bent to compound curvatures, the overlying plate will in many cases not bend down at the same speed as the underlying plate. If such a pair of plates is laminated to opposite sides of a plastic intermediate layer, the space between the glass plates could cause breakage during the lamination.

According to the present invention, these problems are solved by means of a special method for achieving simultaneous bending of the plates.

According to the invention, there is provided

a method for achieving an equal rate of descent for two glass plates lying on top of each other and placed on a simple bending form, possibly of different thickness, which are heated at the same time

to the softening temperature, while they are arranged on the mould, particularly for the production of laminated glass sheets, the method being characterized in that the thicker and/or upper glass sheet is subjected to a hardening before bending, or at least to a stronger hardening than the thin one and/or other glass plate, whereby it has a lower viscosity than the latter plate and is more easily bent.

Such treatment of the thickest

plate before both plates are exposed to temperature, is sufficient to cause the plates to sink, makes it possible to achieve the same bending speed for the two plates. By increasing the sinking speed of the thickest plate, not only will the two plates bend faster, but the plates are kept in more intimate contact with each other during the entire bending process. This avoids the formation of air gaps as a result of different deformation rates for the plates.

According to a preferred embodiment of the method according to the invention, the thick, hardened plate is placed on top of the thin plate, whereby the sinking due to the weight of the thick plate accelerates the bending speed of the thin plate.

When two or more sheets (of the same thickness and circumference) are bent into compound curve shapes for lamination with a plastic interlayer for the manufacture of car windscreens, it is exceedingly difficult to achieve synchronous sinking of the sheets, so that they remain in intimate contact with each other during the entire bend. This difficulty is increased if the plates are to be bent with varying radii of curvature along two axes, which form an angle with each other. According to the present invention, an improvement in the production of such objects is achieved by first hardening one of the plates, after which the hardened plate is placed on top of the untreated plate in a glass bending mold, after which the plates are exposed to heating to the softening temperature of the glass. The overlying plate remains in intimate contact with the lower plate, as a result of its lower viscosity due to its prior curing. When the bending process is finished, the plates fit each other exactly despite being bent into a complex curve shape.

In order for the invention to be understood, it will be described in more detail in the following in connection with the drawings, where fig. 1 is a cross-section showing a pair of glass sheets placed on a convex bending mold to be bent on it. Fig. 2 shows a typical separation between the plates, which occurs during bending, when glass plates of different thicknesses are bent, without using the method according to the present invention. Fig. 3 shows how glass plates of different thicknesses remain in intimate contact with each other during bending, when the thickest plate is treated as indicated according to the present invention. Fig. 4 is an isometric illustration of a typical complex, laminated windscreen, which can be produced more efficiently by the method according to the invention. Fig. 5 is a longitudinal section along line V

—V in fig. 4.

Fig. 6 is a cross-section along line VI

—VI in fig. 4.

In fig. 1 shows a thick glass plate 10 and a thin plate 12, placed on a form M for bending. The plates are separated by a suitable separating material 14 of mica, diatomaceous earth or other suitable material, which reduces the danger of the glass plates fusing together during their heating to the softening temperature.

If the plate 10 is not pre-treated by hardening, the lower plate 12 will sink down faster during bending corresponding to the surface of the mold M, so that a space 16 is thereby formed between the edges of the thin and thick plate. Fig. 2 shows such a development, which is characterized for bending according to previously known methods. If, however, the thick sheet 10 is previously hardened, so that its internal viscosity is lowered sufficiently when the mold and the glass sheets 10 and 12 are subjected to heating to the softening temperature of the glass, the thick sheet 10 will soften more quickly and remain in intimate contact with the thin plate 12 during the entire bending process, as illustrated in fig. 3. In fact, the weight of the thick plate 10, acting against the unsupported parts of the thin plate 12, will accelerate the rate of bending thereof.

It is possible by controlling the degree of hardening of the thick plate 10 to achieve that in certain cases intimate contact is maintained, when the thin and thick plate are simultaneously heated over a mold, and the thin plate is placed over the thick one. However, it is preferable to place the prehardened thick plate over the thin one for bending by immersion under heating, thereby ensuring that the thick plate, which is more easily bent due to its prior hardening, remains in intimate contact with it. thin plate during the entire bend. When the plates 10 and 12 have been bent by the method described above, they are separated from each other, and a suitable plastic intermediate layer is placed there, e.g. of polyvinyl butyral, between the plates, after which they are exposed to temperature and pressure, suitable for further lamination for the production of laminated safety glass.

If glass sheets are simultaneously bent into shapes with a concave contour, the finished bent sheets will tend to be at a distance from each other, as a result of uneven bending, especially in the case that the glass is to be bent into non-uniform curve shapes. The upper plate tends to extend like a bridge over the area which is subject to the strongest curvature. However, by treating one plate so that its viscosity is reduced before it is placed over another plate for placement on a bending mold for heating to the softening temperature of the glass, the plates can be bent under continuous intimate contact. In this case, there is no space between the plates.

If the specified precautions are not taken, the plates will locally be separated from each other as a result of different bending. The manufactured laminated products will suffer from many defects, such as poor optical properties, as a result of uneven flow of the plastic intermediate layer in the areas for spaces between the glass plates, penetration of oil into the separated parts during treatment in an autoclave, and formation of air pockets, which damage the optical properties of the composite plate. In addition, breakage can easily occur during lamination, as a result of tensile stresses that occur in the separated areas during lamination.

The present invention further reduces the tendency of the bottom plate in two or more joined plates to extend like a bridge over the area of the strongest curvature in a concave glass bending shape. This is achieved by exposing the bottom plate to a partial hardening, while the top plate is subjected to an even stronger hardening. In this way, the sinking of the lower plate, corresponding to the contours of the mold, is facilitated as a result of the lower viscosity of the glass plate due to its partial hardening. The upper glass plate, which is more hardened than the lower plate, sinks down more easily than this, and therefore remains in intimate contact with the lower plate to a much greater extent during the entire bending than if both plates had been subjected to identical pre-treatments. For similar reasons, these principles can be used for bending two or more joined glass sheets for concave or convex glass bending shapes.

Fig. 4, 5' and 6 show different views of a typical complex curved windscreen, which can be produced more efficiently than hitherto by using the method according to the invention. In these figures, 20 denotes a laminated windscreen, which consists of

a glass plate 22, a thermoplastic, transparent intermediate layer 24 and another glass plateau 26, which is curved corresponding essentially to the curvature of the plate 22. The laminated plate comprises a main part 30, which is. very strongly curved near each longitudinal end forming wing sections 32, which extend in approximately parallel planes, approximately perpendicular to the tangent plane to the center of the main part 30. One side of the main part 30 is curved so that a part 34 is formed, determined to form the front part of a car roof. The roof portion 34 extends in the same direction as the wing portions 32 in a plane which is approximately vertical to the tangent plane of the wing portions. Since the inner surface of the composite plate is concave on the side facing the corners, while the outer surface is convex, the initially hardened plate can be the plate 22, if

the plate is bent into a convex shape. If, however, the plate is to be bent into a concave shape before lamination, it is the plate 26 that must be previously hardened.

It is known that when hardened sheets are subjected to heating to the softening temperature of the glass for bending the glass sheets into complex curved shapes, the tensile stresses that occur in connection with the hardening are equalized, and the glass sheet has essentially the same stress pattern after bending as if it had not been hardened before bending. The laminated plates, which are produced using the method according to the invention, are, however, less prone to breakage and optical errors than laminated plates, composed of glass plates with identical heat treatments, such as plates used in previously known methods.

Claims (1)

Method for achieving an equal sinking speed for two glass plates placed on top of each other and placed on a simple bending form, possibly of different thickness, which are simultaneously heated to softening temperature while they are placed on the form -, in particular for the production of laminated glass plates, characterized in that the thicker and/or upper glass plate before bending is subjected to a hardening in advance, or at least to a stronger hardening than the thinner and/or other glass plate, whereby it has a lower viscosity than the latter plate and bends more easily.