Method of prestressing at least one bent sheet of a sandwich type safety glass

A method of prestressing at least one of the glass sheets of a sandwich safety glass, while being bent in a furnace, the glass sheet, after passing through a bending zone, being passed by a blow system and cooled by cooling air in the furnace in a temperature zone above the transformation point but below the deformation point of the glass, and an apparatus for such prestressing comprising a travelling die supporting superimposed sheets, a bending furnace zone to soften the sheets, and a cooling zone, the cooling zone having a blow system that subjects the bent sheet or sheets to cooling air.

BACKGROUND OF THE INVENTION 
This invention relates to a method of prestressing at least one of the 
glass sheets of a sandwich safety glass, said sheets being bent in a 
bending furnace by the action of gravity, in superimposed relationship on 
a bending die having a substantially horizontal shaping surface, the die 
together with the glass sheets in superimposed relationship thereon 
passing through the furnace and the superimposed glass sheets, after 
passing through the bending zone in which the sheets are heated to the 
softening temperature and bent, being taken past a blow system which 
subjects the bent sheet or sheets to the action of cooling air and which 
is disposed in the furnace in a temperature zone above the transformation 
point but below the deformation point of the glass, and apparatus for 
prestressing at least one of the superimposed sheets of a sandwich safety 
glass, which are bent simultaneously by the action of gravity in the 
bending furnace, comprising a substantially horizontal die which travels 
through the furnace in a direction parallel to the axis of curvature of 
the sandwich, and which supports the superimposed sheets, a bending 
furnace zone through which the die travels and in which the superimposed 
sheets are heated to the softening temperature and bent, and a cooling 
zone, which follows the bending zone and is disposed in a temperature zone 
above the transformation point but below the deformation point of the 
glass, said cooling zone comprising a blow system which subjects the bent 
sheet or sheets to cooling air, suitable more particularly for performing 
the above-described process. 
In a known process and a known apparatus of the type described hereinbefore 
(DOS No. 26 50 206), the two superimposed sheets are bent at a temperature 
above the transformation point and then vertically subjected to cooling 
air so that only the edge of the sheet is covered by the streams of 
cooling air, so that compression stresses are produced there. The 
compression stresses produced at the edge of the sheet with the process 
and apparatus according to DOS No. 26 40 206 are intended to guarantee 
clear vision in the stress-free central zone in the event of breakage of 
the sandwich type safety glass, which is of use more particularly as a 
windscreen for motor vehicles. However, it has been found, particularly 
with sandwich type safety glass consisting of individual sheets of 
different thicknesses, such as are frequently used as motor vehicle 
windscreens, in which the thinner sheet of the sandwich is disposed on the 
inside of the motor vehicle, that the safety glass does not have the 
required resistance to breakage if the central zone, particularly of the 
thin inner sheet, is not prestressed or toughened but is substantially 
stress-free, and in addition it does not give a breakage pattern in the 
form of extensive cracking which is desirable as a compromise between the 
glass strength requirements, on the one hand, and low risk of injury due 
to cuts, on the other hand. 
DAS No. 10 35 328 discloses a process in which the entire surface of the 
individual sheets is subjected to cooling air in the bending furnace, so 
that they are prestressed. However, this process is not suitable for 
satisfactorily improving the properties of the individual sheets of a 
safety glass sandwich bent in superimposed relationship on the bending die 
to give slight prestressing. German patent specification No. 616 445 also 
describes the prestressing of bent sheets by cooling air nozzles, although 
here again there is no guarantee of satisfactory slight prestressing of 
the individual sheets of the sandwich bent in superimposed relationship on 
a bending die. Other prestressing systems operating with cooling air are 
described, for example, in DAS No. 10 80 742, German patent specification 
No. 11 14 993 and DAS No. 20 00 271, but all the processes and apparatus 
described therein are not specifically suitable for slightly prestressing 
or toughening at least one of the individual sheets of a safety glass 
sandwich are bent in superimposed relationship on a bending die. 
The object of the present invention is to provide a process and apparatus 
of the type indicated hereinbefore whereby at least one of the individual 
sheets of a safety glass sandwich, bent in superimposed relationship on a 
bending die by gravity, is slightly prestressed. The object of the process 
and apparatus is to give the slightly prestressed and particularly very 
thin sheet of the sandwich the properties of high-strength glass. 
SUMMARY OF THE INVENTION 
According to the invention, in a process of the type on which the invention 
is based, to this end, at least one bent sheet is subjected to cooling air 
over its entire surface. 
The apparatus of the type referred to hereinbefore is characterized 
according to the invention in that the blow system comprises at least one 
blow pipe which is disposed transversely of the direction of conveyance 
and which has cooling air holes directed towards the glass sheet for 
cooling, the blow pipe being so curved as to match the transverse 
curvature of the sheets that the cooling air holes have substantially the 
same distance from the surface of the sheet over the entire transverse 
dimension of the individual sheets. 
Particularly preferred embodiments of the process and apparatus according 
to the invention are disclosed. A particularly advantageous development of 
the apparatus according to the invention is characterized by a suction and 
support system which is disposed in the cooling zone and which is movable 
therein with the die in the direction of conveyance, for temporarily 
lifting the top sheet from the sheet therebeneath, and by at least two or 
four blow pipes, at least one of which is disposed above and at least one 
of which is disposed beneath the passing lifted top sheet. 
With the process and apparatus according to the invention it is possible 
slightly to prestress uniformly in a simple manner the individual sheets 
of safety glass sandwiches of the kind in question, more particularly the 
top very thin sheet on the die, for what is known as asymmetrical sandwich 
safety glass, so that the single sheet subjected to the cooling air is 
given the properties of high-strength glass. Consequently, not only is the 
breaking strength of the sandwich safety glass greatly increased, but in 
addition there is an improved breakage pattern with the required extensive 
cracking. According to the invention, therefore, at least the top thin 
inner sheet of an asymmetrical sandwich type safety glass for use as a 
motor vehicle windscreen is so subjected to cooling air in the bending 
furnace itself over its entire surface such that the inner sheet receives 
slight prestressing or toughening like a high-strength glass. Of course it 
is also possible to cool both the top sheet and the bottom thicker outer 
sheet of such asymmetrical sandwich type safety glass. As in the preferred 
embodiment of the apparatus according to the invention described, it is 
particularly advantageous for the top sheet to be lifted from the bottom 
sheet by a suction and support system near the blow system, the said 
suction and support system moving with the bending die as it travels 
through the furnace and depositing the sheet on the bottom outer sheet on 
the bending die as it passes the blow pipes which, in this case, are 
disposed both above and below the raised top inner sheet.

DETAILED DESCRIPTION 
Referring to FIG. 1, the exemplified embodiment of the apparatus according 
to the invention comprises a blow system consisting of pairs of blow pipes 
10, 12 and 14, 16 extending substantially transversely of the longitudinal 
direction of the bending furnace, the base of which is denoted by 
reference 18 in FIG. 1 and the roof of which is denoted by reference 20. 
The blow pipes 10, 12 and 14, 16 are arranged in pairs with vertical 
spacing as shown in FIG. 1 and are supported on a supply pipe 22 which 
extends up through the furnace roof 20 and leads to a control system 24 to 
which cooling air in the form of compressed air is fed from a source (not 
shown). The vertical positions of the pairs of blow pipes 10, 12 and 14, 
16 can be adjusted independently of one another by means of an adjusting 
system, from a position outside the furnace roof 20, FIG. 1 showing an 
adjusting ring 26 of this system. 
FIG. 1 also shows a supporting and suction system 28 which, as shown in 
FIG. 1, comprises two elongate suction strips which extend parallel to the 
longitudinal axis of the furnace and which are suspended from supports 30, 
32, which are adapted to reciprocate in the direction of the longitudinal 
axis of the furnace via rollers 34, 36 which together form a trolley, in a 
guide 38 attached to the roof 20. FIG. 1 shows the supporting and suction 
system 28 comprising supports 30, 32 and trolley 34, 36 in solid lines in 
a first position on the left in FIG. 1, and in another position farther to 
the right, which is shown in broken lines, near the blow system 10, 12, 
14, 16. 
Finally, FIG. 1 also shows in solid lines on the left a bending die 44 
which travels through the furnace on rollers 40, 42 in the direction of 
the arrow shown in the right in FIG. 1, said die having two glass sheets 
45, 46 thereon. In the position of the die 44 and glass sheets 45, 46 
shown in solid lines on the left in FIG. 1, the glass sheets have already 
been bent in superimposed relationship in a zone of the furnace 18, 20 in 
which the temperature is above the deformation point of the glass making 
up the individual sheets 45, 46, e.g. above 585.degree. C. Consequently, 
the two sheets 45, 46 of the laminated safety glass under production are 
curved (curvature not visible in the drawing) about an axis of curvature 
parallel to the longitudinal axis of the furnace. 
In addition to the position of the die 44 shown in solid lines in FIG. 1, 
in which position the glass sheets 45, 46 rest on the die, FIG. 1 also 
shows the die 44 farther to the right in broken lines, in a position in 
which the top sheet 46 has been lifted by the suction and support system 
28, from the bottom sheet 45 still resting on the die 44. FIG. 1 then 
shows a position of the die 44 in broken lines in which the top sheet 46, 
lifted from the bottom sheet 45 by the suction and support system 28, in 
the position shown in broken lines has been brought by the trolley 34, 36 
along the guide 38 into the zone of the blow system 10, 12, 14, 16. 
Finally, FIG. 1 also shows in broken lines an end position of the die 44 
in which the two sheets 45, 46 again rest in superimposed relationship on 
the die 44, thus concluding the blow operation, which will be described 
hereinafter. 
FIG. 2 shows that the blow tubes 16, 12 shown in diagrammatic section at 
right angles to the longitudinal axis of the furnace are curved to match 
the curvature of the top sheet 46 fed between them by the system 28, while 
a number of cooling air holes 48, 50 to which compressed air is fed via 
the control system 24 and the supply pipe 22 as shown in FIG. 1, direct 
jets of cooling air uniformly over the entire surface of the top sheet 46. 
The top sheet passes the blow tubes 12, 16 perpendicularly to the drawing 
plane of FIG. 2 and is lifted by the suction and support system 28 from 
the bottom sheet 45, which continues to rest on the die 44, the cooling 
air being applied both to the top outer surface and the bottom inner 
surface of the sheet 46 with respect to FIG. 2. 
FIG. 3 shows that the top two blow tubes 14, 16 (the same remarks apply to 
the bottom pair of blow tubes 10, 12) are disposed in close adjacent 
relationship, the individual cooling air holes 48 in the two parallel and 
adjacent blow pipes 14, 16 being staggered. The cooling air holes 48 may, 
as shown in the example illustrated, have spacing of 25 mm in the 
longitudinal direction of the blow pipes 14, 16, given a hole diameter of 
2 mm in each case, so that there is a cooling air hole 48 and 50 
respectively every 12.5 mm in the longitudinal direction of the pairs of 
blow pipes 14, 16 and 10, 12 respectively. 
The above-described system illustrated diagrammatically operates as follows 
by the method according to the invention. 
When the two glass sheets 45, 46 have been heated to the softening 
temperature, i.e. a temperature above the deformation point of the glass, 
and bent simultaneously in superimposed relationship on the die 44, in the 
bending zone which is not shown in FIG. 1, because it is farther to the 
left, the sheets curved in this way are transported, as shown in FIG. 1, 
by the bending die 44 travelling from left to right in respect of FIG. 1 
through the furnace 18, 20 parallel to the longitudinal axis of the 
furnace, and are brought into a temperature zone in which the temperature 
is below the deformation point, but above the transformation point, of the 
glass making up the sheets 45, 46, i.e., for example, in the range of 
525.degree. to 585.degree. C. When the die 44 is in the position shown in 
broken lines in FIG. 1, the top sheet 46 is lifted from the bottom sheet 
45 still on the die 44, by means of the suction strips which act on the 
longitudinal edges of the sheet 46 parallel to the furnace longitudinal 
axis, and which form part of the vertically adjustable support and suction 
system 28. The die 44, on the one hand, and the support and suction system 
28 suspended from the trolley 34, 36, on the other hand, then move 
synchronously in the furnace farther to the right in FIG. 1, the raised 
top sheet 46 being taken through the pairs of blow pipes 10, 12 and 14, 
16, where jets of cooling air are applied. After passing through the blow 
system comprising the pipes 14, 16 and 10, 12 respectively, the sheet 46 
is replaced on the bottom sheet 45, the final condition being shown in 
dotted lines fully on the right in FIG. 1. From here the die 44 together 
with the sheets 45, 46 of the sandwich safety glass under manufacture pass 
to the furnace exit in known manner. 
The cooling air fed by the blow pipes 14, 16 and 10, 12 respectively 
slightly prestresses or toughens the raised top sheet 46 to give a safety 
glass which gives an improved breakage pattern in the event of breakage of 
a safety glass made from the sheets 45, 46, particularly if the sheet 46 
is the inner sheet of the sandwich, because glass of this kind does not 
break with the breaks in a radiating form, but rather like a wired glass 
with an extensive cracking pattern. Glass of this kind also has a 
resistance of two to two-and-a-half times that of non-toughened standard 
glass of the same thickness so that the method according to the invention 
is particularly suitable for toughening very thin sheets 46, which are 
preferably used as the inner sheets of laminated safety glass. 
Of course, in a modification of the exemplified embodiment of the system 
according to the invention, the top sheet 46 is not lifted from the bottom 
sheet 45; instead, the two sheets resting on the die 44 are taken past a 
pair of blow pipes 14, 16 disposed above the top sheet 46. In that case, 
only the top outer surface of the sheet 46 is subjected to the action of 
cooling air, but again this gives a high-strength glass with improved 
properties as compared with standard glass. In another modification of the 
system and method according to the invention, the bottom sheet 45 may 
additionally be subjected to cooling air, in which case another pair of 
blow pipes could be provided, for example, in the zone of the blow system 
shown in FIG. 1, to subject the surface of the bottom sheet 45 to cooling 
air. 
Of course, with the method according to the invention it is also possible 
to replace the blow pipes or other kinds of air nozzles forming the blow 
system, by radiant coolers so that again one or both sheets are uniformly 
prestressed or toughened, thus providing a high-strength glass. In the 
exemplified embodiment illustrated, the suction and support system 28, 
i.e. particularly the supports 30, 32, can be taken past the blow pipes 
10, 12, 14, 16 by arranging for at least the bottom blow pipes 12, 10 to 
be pivotable. In this way the top sheet 28 can first be raised against the 
top blow pipes 14, 16 and then the bottom blow pipes 10, 12 can be pivoted 
inwardly. In another system, the bottom blow pipes could be laterally 
supported only outside the path of movement of the support and suction 
system 28 and project freely into the intermediate space between the 
sheets. 
Sandwich type safety glass comprising a thin inner plate of high-strength 
glass slightly prestressed by the method according to the invention has a 
much better flexural breaking strength as compared with sandwich safety 
glass of the same construction but consisting of standard glass. 
Experiments carried out with such glass have shown that flexural breaking 
strength improvments from 30 to 100% can be obtained with different types 
of sandwich safety glass of the kind used for different types of vehicles. 
The features of the invention disclosed in this specification, in the 
drawings, and in the following claims, are important to embodiments of the 
invention in its various forms both individually and in any desired 
combination.