GB2078169A - Forming and Assembly Process for Two or More Curved Glass Panes Having Different Physical-chemical Properties and/or Different Thickness - Google Patents

Abstract

A manufacturing process for a laminate of two curved glass panes; having different physical-chemical properties and/or different thicknesses, suitable for windscreens and other safety glass items for motor vehicles with an intermediate sheet of plastics material comprises the consecutive steps of loading the glass panes on a rigid horizontal mould with concave surface facing upwards, simultaneously curving said two glass panes in a furnace, trimming them to shape, placing a sheet of plastics material between the matched glass panes, and bonding the assembly in an autoclave under special temperature and pressure conditions, the glass pane with the smaller degree of curvature being placed in direct contact with the mould during the forming phase, and, during the assembly phase, the glass pane originally placed in direct contact with the mould form only the inner part, that is the concave part of the finished product.

Description

SPECIFICATiON Forming and Assembly Process for Two or More Curved Glass Panes Having Different Physicalchemical Properties and/or Different Thickness The present invention refers to a manufacturing process for curved and bonded glass panes, especially suitable for use as windscreens or other safety glass items in motor vehicles or the like. More precisely, the present invention refers to a manufacturing process for bonded and curved glass panes which can be used in cases where the physical-chemical properties and/or thickness of the glass panes to be bonded and curved are not the same.

In order to understand the progress achieved by the process in accordance with the invention it is first of all necessary to consider the procedures and limitations of manufacturing processes for bonded and curved glass panes already known up till now. In such processes, two consecutive phases can be distinguished, namely: forming and assembly, with these phases being repeated in the process concerned.

During the forming phase, the glass panes are curved in the furnace and trimmed to shape, and this can be performed according to two different technologies; that is, when said operations are performed in the order indicated above, the process is known as an expansion process, or else said operations can be performed in the reverse order in which case the process is known as contour process. The process in accordance with the invention can be carried out using either of the two forming technologies.

During the assembly phase, a sheet of plastic material is first placed between each pair of glass panes to be bonded; next the so-formed sandwich (glass plus plastic) is pressed under given temperature conditions to expell most of the air trapped between the plastic material and the glass; lastly the sandwich is placed for a sufficiently long period in an autoclave, where the true bonding operation is carried out under controlled temperature and pressure conditions.

For successful results in the manufacturing process, the glass panes must be curved in such a way that the surfaces in contact properly match each other or, rather, there must be no appreciable differences in local curvature between the two surfaces to be bonded.

While, in fact, if the manufacturing process upstream and downstream to the curving phase is properly carried out, slightly different curvatures between the two surfaces to be bonded will not alter the final result, very different is the case, for the purposes of achieving said result, when there is a great local difference of curvature on one of the two surfaces to be bonded. Such a case would lead to delamination after assembly (poor bonding with interruptions occurring) along the edges of the sandwich and/or inside it.

Adequate matching of the curvatures of the two surfaces to be assembled is still today generally accomplished by simultaneous curving in a furnace of the glass panes to be subsequently bonded together; for this purpose said glass panes are placed, during the curving phase, on a horizontal mould with concave surface facing upwards, in the same position that they will assume in the finished product.

For example, in the manufacture of a windscreen with two layers of glass having the same physical-chemical properties and the same thickness, the pair of glass panes to be curved are loaded on the horizontal mould with concave surface facing upwards, whereby the glass pane, which will form the outer part of the windscreen (convex part), is positioned in contact with the mould (that is, underneath), and then over this (that is, on top) is placed the glass pane which will form the inner part (concave part) of the windscreen. The pair of glass panes loaded on the mould will then be placed in the heating furnace where the glass will be gradually raised to the softening temperature.

Consequently the glass panes assume, thanks to the effect of temperature and gravity, the final mould shape; more especially the lower surface of the lower glass pane in direct contact with the mould, tends to assume the shape of the latter, while the lower surface of the upper glass pane tends, in turn, if suitably heated, to assume the shape of the upper surface of the lower glass pane.

It is found that, in order to achieve a good matching of the surfaces in contact, the upper glass pane must be raised to an average temperature which is higher than that of the lower glass pane; in fact, the resultant lower viscosity reached by the glass of the upper pane permits this pane to be laid down on the lower glass pane. In order to accomplish these heat conditions, the heating furnaces for glass panes of equal thickness are generally operated so that, especially in the curving zone, the quantity of heat given up to the upper glass pane mainly by radiation is greater than the quantity given up to the lower glass pane.

It should also be observed that, still in order to obtain a correct final shape of the glass pane in the curving phase, the curve forming mould can either be of the rigid type (in the sense that its geometry does not vary in the furnace apart from the inevitable elastic and thermal deformations), or else can be of the articulated type (that is, suitable hinge arrangements permit mould geometry to change inside the furnace). The rigid type moulds are mainly used for parts with small curvature, while the articulated type is mainly used in the other cases.

After curving, the glass panes undergo a suitable annealing treatment which, by lowering the glass temperature without setting up consistent states of tension in it, permits the panes to maintain their shape already reached in the curving phase.

In the case of forming a pair of glass panes with the same physical-chemical properties, the above described curving process certainly gives raise to good results on glass panes of the same thickness (symmetrical bonding) or else on glass panes of different thickness (assymetrical bonding) always provided that the thinner glass pane forms the upper part of the pair to be curved, that is, it is on the concave part of the finished product. In fact, in the latter case, the lesser thermal capacity of the upper glass pane (thinner pane) favours its reaching higher average temperatures, and therefore its laying down on the lower glass pane; the laying down of this pane is also facilitated by the greater deformability of the thinner glass pane with respect to that of the thicker glass pane.

The Applicant has noticed that if the thinner glass pane is placed, instead, in the lower part of the pair, that is in the convex part of the finished product, the greater thermal capacity and greater rigidity of the upper glass pane tends to render the correct curving and bonding without subsequent delamination more difficult. Analogous difficulties can arise, in both symmetrical and assymetrical bonding, when the glass panes to be bonded have different physical-chemical properties. More especially, as stated previously, if the glass pane placed on the concave part of the finished product has a higher softening temperature and/or higher transmission coefficient of the radiation mainly present in the curving furnace, it is not possible to correctly lay down this pane on the lower pane by using conventional technology.In fact, assuming equal quantities of heat be absorbed, the glass pane with the higher softening temperature undergoes a lesser degree of curving than that of the other glass pane, just as, for equal radiant heat flow, the pane with the higher radiant heat transmission coefficient reaches lower average temperatures than does the other pane.

As a result, in conventional technology, unless very strict control is kept over the heat flow and the method of heat transfer in the furnace, if the glass pane with higher softening temperature and/or higher radiant heat transmission coefficient is positioned in the upper part of the pair (because it must be in the concave part of the finished product) it will prove very difficult to lay it down on the lower glass pane.

The Applicant has found that after carrying out suitable experiments, the difficulties described in the preceding cases are to be ascribed to the smaller degree of curvature of the upper glass pane with respect to the lower pane, whereby the degree of curvability of a glass pane placed in a curving furnace and submitted to the force of its own weight, is understood to be the permanent deformation (viscous) reached by the pane within a certain time under given furnace ambient conditions.

The degree of curvability of a glass pane therefore is measure of its ability to conform more or less easily to the constraining geometry and substantially depends on the geometry of the pane itself and on the physical-chemical properties of the glass of which it is made.

Concerning the latter properties, of particularly importance, as already stated, is the softening temperature of the glass and its total radiant heat transmission coefficient relative to the wavelength range of the radiation.

For a given part (that is for a given contour configuration), the dependence of the degree of curvability of a glass pane on its geometry is also essentially linked to its thickness. More precisely, the degree of curvability of a glass pane decreases the higher is its thickness and/or the higher is its radiant heat transmission coefficient and/or the higher is the softening temperature of the glass of which it is made.

In order to solve the difficulties in curving met with in the previously described cases, the Applicant first of all carried out experiments involving the separate curving of the glass panes to be bonded. However, this process gave poor qualitative results in the assembly phase and therefore a higher number of rejects.

In those cases where the smaller degree of curvability of the glass pane essentially depends on its higher total radiant heat transmission coefficient (in relation to the radiation mainly present in the curving furnace), attempts were then made to increase this degree of curvability by modifying the heat transfer procedure in the furnace, more especially by increasing the heat transfer component by convection at the expense of that by radiation. However, this entails appreciable and troublesome modifications to the glass pane curving furnace.

With the manufacturing process in accordance with the present invention, the Applicant has finally succeeded in overcoming the difficulties in manufacture encountered in the previously described cases.

The process in question is applied, as stated in the title and in the background portion of this specification, more especially to the manufacture of curved and bonded glass panes whose physicalchemical properties and/or thickness are not the same.

The process is essentially characterized by the fact that during the curving phase, the order in which the glass panes are positioned on the mould is the reverse with respect to the order during the assembly phase, as it was surprizingly found that this is sufficient to offset the difficulties encountered in the previous above listed processes.

Hence more especially with the process in accordance with the invention, the glass pane with the lesser degree of curvability, that is the glass pane with the higher softening temperature, or the glass pane with the greater total radiant heat transmission coefficient (in relation to the radiation mainly present in the furnace) or the thicker glass pane is placed in direct contact with the mould during the curving phase, when it is to be subsequently placed in the concave part, that is in the inner part of the finished product. If such factors are present, some in one and the other in the other of the two glass panes, obviously it will be the pane anyhow with the lesser degree of curvability to be placed in direction contact with the mould during the curving phase, when it is to be placed in the concave part, that is, the inner part of the finished product.Likewise in the case of a product entailing the use of more than two glass panes having different degrees of curvability, it also follows that the glass pane with the lesser degree of curvability, when it is to be placed in an inside position in the finished product, will be placed in direct contact with the mould during the curving phase.

The advantages to be derived from the manufacturing process of curved and bonded glass panes in accordance with the invention will appear clear from the examples of application described below.

Table 1 (Average Composition) Glass Pane No. I Glass Pane No. 2 SiO2 7074% 5070% CaO 81 0% 0.5-1.0% MgO 24% 24% Al203 0.1-1.5% 525% Fe203 0.100.60% 0.020.6% TiO2 0.050.06% 0.050.2% Alkalies 12--1 5% 12--1 5% Example 1 A glass pane of mainly silica-lime composition, designated by letter A is to be bonded with a glass pane with a mainly silica-alumina composition, designated by letter B.The two glass panes, initially flat and of the same thickness, besides the assembly phase, must first undergo a forming phase, as they are to be used as a curved windscreen for a motor vehicle.

The silica-lime glass pane (A) is to form the outer part of the windscreen, that is it will be in the convex part of the finished product, while the silica-alumina glass pane (B) is to form the inner part of the windscreen (the concave part of the finished product).

The two glass panes have different physical-chemical properties. Their average compositions are given in Table 1. The viscosity curves covering the concerned range for the two types of glass, are given by way of explanation only in Figure 1; it can be seen from the figure that the glass pane B has, at the same temperature, a higher viscosity than that of glass pane A, and therefore it has a higher softening point. The latter can be defined as the temperature at which the viscosity assumes a certain given value (for example q=108 poise).Figure 2 gives, still by way of explanation, the monochromatic radiant heat transmission coefficient curves of the two glass panes in relation to the wavelength of the radiation in the range concerned; the higher monochromatic radiant heat transmission coefficient of glass pane B with respect to glass pane A for the different wavelengths, means that the total radiant heat transmission coefficient (in relation to the radiation mainly present in the furnace) of the silicaalumina glass pane is higher than that of the silica-lime glass pane. In certain zones of the glass curving furnaces, the ratio of the two total heat transmission coefficient is in the region of about 2.

The two glass panes are placed in a mould which will then be advanced along the glass curving furnace; in accordance with the present invention, glass pane B will be placed in direct contact with the mould (whose concave surface faces upwards), and glass pane A will be laid on glass pane B.

The mould-pair of glass panes assembly will then enter the heating tunnel and glass pane A will, also on account of its lower radiant heat transmission coefficient, reach its softening temperature (among other things, lower than that of glass pane B) before the underlying glass plane B. Inspite of this, glass pane A will start to curve only when glass pane B will also have reached its softening temperature.

Curving will then continue until glass pane B conforms to the curve forming mould.

Curving of glass pane B, besides being favoured by the force of its own weight, will also be favoured by the weight of the overlying glass pane A which, having reached its softening temperature first, will be fully resting on glass pane B.

Heating of glass plane B is favoured by its contact with glass pane A which, being more opaque to radiation from the furnace will tend to capture it more rapidly; in fact, the contact between the two glass panes favours heat transfer by conduction between them.

After performing the required annealing phase, the glass panes are separated and bonded together with an interposed sheet of plastic; in accordance with the present invention, the position of the two glass panes will be reversed in this operation, in the sense that the silica-lime glass pane (A) will go to the outer position (convex part) while the silica-alumina glass pane (B) will go to the inner position (concave part).

The position reversing operation gives rise to a slight difference in curvature between the surfaces which, during the assembly phase, will be placed in contact with the sheet of plastic.

It has been found that in the case of windscreens with small or medium curvature, this slight difference of the sheet is such as not to alter the successful result of the bonding process. In fact, in such case, the difference between the radii of curvature is in the region of 0.1--1%0 of the ideal radius of curvature.

It has also be found that the operation according to the invention does not impair the good qualitative standards of the bonding, even in the case of windscreens with an appreciable degree of curvature, provided that the mould shape is properly corrected.

Example 2 The same glass pane of mainly silica-lime composition, according to the preceding example, which will be designated by the letter A, is to be bonded to a thinner glass pane, of mainly silicaalumina composition, which will be designated by the letter B.

The two initially flat glass panes, in addition to the assembly phase, must first undergo a forming phase as they are to be used as a curved windscreen for a motor vehicle.

Furthermore, the silica-alumina glass pane, which will form the inner part of the windscreen (concave partof the finished product) must undergo, before assembly, a chemical tempering treatment.

This treatment is designed to impart greater mechanical strength to the inner part of the windscreen, and above all, to impart a greater degree of passive safety in the event of fracture through impact.

The physical-chemical properties of the two glass panes correspond to those already given in the preceding Example 1, except at the most, the monochromatic radiant heat transmission coefficient of glass pane B can even be higher in certain ranges of the radiation wavelengths on account of its smaller thickness.

It has been found experimentally that, inspite of the smaller thickness (which can be, by way of example, between two thirds and a quarter the thickness of glass pane A), glass pane B possesses a degree of curvability which, under the usual ambient conditions of the glass curving furnace, is less than that of glass pane A. This means that in this case, the influence of the softening temperature and the total radiant heat transmission coefficient is greater than that of the thickness.

Also in this case, it is convenient to adopt the process in accordance with the invention if good qualitative standards of bonding are to be achieved. Hence the forming and assembly process is identical to that of the preceding example.

It should also be noticed that in this case, if glass pane B has an appreciably smaller thickness than that of glass pane A, its lower rigidity to bending does not lead in practice to deformations in the assembly phase even for windscreens of appreciably large curvature, consequently the mould shape does not have to be significantly correct.

Example 3 Two glass panes of the same composition (for example silica-alumina) and having the same physical-chemical properties but different thickness, are to be curved and bonded in such a manner that the thinner glass pane, designed by letter A, will form the outer part of the windscreen (convex part), while the thicker glass pane, designated by the letter B, will form the inner part of the windscreen (concave part).

The two glass panes are placed on a mould which will then be advanced along the glass curving furnace; in accordance with the present invention, glass pane B will be placed in direct contact with the mould, and glass pane A will be laid down on glass pane B.

The mould-pair of glass panes assembly will then enter the heating tunnel and glass pane A, on account of its lower heat capacity, will reach is softening temperature before the underlying glass pane B. Inspite of this, glass pane A will start to curve only when glass pane B will have also reached its softening temperature.

Curving will then continue until glass pane B conforms to the curve forming mould.

Curving of glass pane B, besides being favoured by the force of its own weight, will also be favoured by the weight of the overlying glass pane A which, having reached its softening temperature first, will be fully resting on glass pane B.

After performing the required annealing phase, the glass panes are separated and bonded together with an interposed sheet of plastic; in accordance with the present invention, the position of the two glass will be reversed in this operation with respect to the curving phase, in the sense that that thinner glass pane A will go to the outer position (convex part) and the thicker glass pane B will go to the inner position (concave part).

It has been found that such an operation does not impair the good qualitative standards of the bonding, even in the case of windscreens with appreciably large curvature and without significant modifications to the mould shape thanks to the lower rigidity to bending of the thinner glass plane with respect to the thicker glass pane.

As a person skilled in the art can well understand what has been described and illustrated up till now is only by way of example, as numerous variations and modifications may be effected on carrying out the invention without departing from the true scope of the invention, which rather covers them all.

Such modifications and variations will therefore fall within the scope of the claims.

Claims (12)

Claims

1. Manufacturing process-that is to say forming and assembly-for two curved glass panes, having different physical-chemical properties and/or different thickness, especially suitable for windscreens and other safety glass items for motor vehicles and the like, comprising the already known consecutive phases of loading the glass panes on a rigid horizontal mould, with concave surface facing upwards, simultaneous curving of said two glass panes in a furnace, their trimming to shape and, if required, subsequent tempering, placing of a sheet of plastic material between the matched glass panes, and lastly subsequent bonding in an autoclave under special temperature and pressure conditions, characterized in that during the forming phase the glass pane with the lesser degree of curvability, of the two to be bonded, is placed in direct contact with the actual mould and during the assembly phase the position of the two glass panes is reversed, thus the glass pane originally placed in direct contact with the mould goes to form the inner part, that is the concave part of the finished product.

2. Manufacturing process for two curved glass panes as claimed in Claim 1, characterized moreover in that the glass pane of the two to be bonded which has a lesser degree of curvability must ascribe this characteristic wholly or mainly due to a higher softening temperature.

3. Manufacturing process for two curved glass panes as claimed in Claim 1, characterized moreover in that the glass pane of the two to be bonded which has the lesser degree of curvability must ascribe this characteristic wholly or mainly due to a higher total radiant heat coefficient (in relation to the radiation mainly present in the furnace).

4. Manufacturing process for two curved glass panes as claimed in Claim 1, characterized in that the glass pane which, of the two to be bonded, has the lesser degree of curvability, must ascribe this characteristic wholly or mainly due to a greater thickness of the glass pane itself.

5. Manufacturing process for two curved glass panes as claimed by one or more of the Claims 1 to 4 but in which an articulated mould is used for forming.

6. Manufacturing process for two curved glass planes as claimed by one or more of the Claims 1 to 5 but in which a contour process is used for the forming phase instead of an expansion phase, that is to say the glass panes are trimmed to shape and they are subsequently curved in the furnace.

7. Manufacturing process for two curved glass panes as claimed in one or more of the Claims 1 to 6 in which the two glass panes have, as a result of different physical-chemical properties, a different colour.

8. Manufacturing process for two curved glass panes as claimed in one or more of the Claims 1 to 7 in which the two glass panes have an average composition as given in Table 1.

9. Manufacturing process for two curved glass panes as claimed in one or more of the Claims 1 to 8 and as described in Example 1.

10. Manufacturing process for two curved glass panes as claimed in one or more of the Claims 1 to 8 and as described in Example 2.

11. Manufacturing process for two curved glass panes as claimed in one or more of the Claims 1 to 8 and as described in Example 3.

12. Manufacturing process for three or more curved glass panes together forming a windscreen or other safety glass items for motor vehicles, such glass panes having all or part of the physicalchemical properties and/or thickness differing from each other, as claimed in the preceding claims, characterized in that the glass pane with the lesser degree of curvability is placed in direct contact with the mould and the others are placed gradually further and further away from it the greater is the respective degree of curvability, wherein the glass panes must assume a reverse order during bonding of the finished product.