Method of and device for curving glass sheets

A method of and device for curving a glass sheet, in which the glass sheet is brought through a reheating furnace by a conveyor defining a substantially horizontal transportation plane, and is then taken over by a tool, by means of which the glass sheet is curved and/or transferred to a curving and/or discharge device. The effective position of the glass sheet is detected and the taking-over tool is repositioned as a function of the effective position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the techniques of curving glass sheets, in which 
the glass sheets are brought to the curving tools or molds by a conveyor 
which defines a substantially horizontal transportation plane for the 
glass, on which the glass sheets travel one after another, heated above 
their softening temperature. More specifically, the invention concerns the 
problem of positioning the glass sheets with respect to the tools such as 
the curving molds. The invention can be applied, for example, to the 
industrial manufacture of automobile glazings. 
2. Description of the Related Art 
The proper control of the quality of the output from a production line for 
curving glass sheets requires that it shall be possible to guarantee that 
each glass sheet is correctly positioned relative to the curving tool at 
the instant at which the latter takes the sheet over. This operation is 
fairly easy to achieve when it is performed cold, which is the case, for 
instance, for the majority of curving lines intended for producing 
laminated glazing panes, in which from the time of entry into the furnace 
the glass sheets are placed on their curving frame by settlement into 
position. In these cases, the referencing is typically achieved by sets of 
stops mounted on the curving tools. Much more strict precautions must, 
however, be taken when this taking-over is performed with a glass sheet 
that has already been reheated to its curving temperature and possibly 
toughening temperature, and therefore at a time at which its plasticity 
and consequently its sensitivity to shocks and friction are at the 
maximum. 
When cold, on entering the reheating furnace, the glass sheet is charged 
onto the substantially horizontal conveyor in accordance with a very 
precise position. But its path along this conveyor, which is generally 
composed of a bed of rollers, is somewhat uncertain at least in 
consideration of the fact that in the most difficult cases, for example, 
an accuracy of positioning of the glass sheet of the order of one-tenth of 
a millimeter may be required. The discrepancies in trajectory may be 
analysed into slidings perpendicular to the axis of the conveyor and 
rotations about this axis. 
It is possible to remedy perpendicular sliding by means of straight-edges 
or lateral stops moving in synchronism with the movement of the glass 
sheets; an example of such a system of lateral positioning is described in 
European Patent Application EP-A-367 670. These systems can operate very 
gradually and gently, and for this reason do not leave any imprint on the 
final plane. But these systems do result in wear of the roller facings. 
Moreover, in roller furnaces, it is generally found that the glass sheet 
maintains the lateral position acquired from the instant at which its 
temperature exceeds the softening point of the glass, which generally 
occurs at latest after it has completed two-thirds of its travel through 
the furnace. 
In contrast, angular positioning systems are much less satisfactory, even 
though it has been possible to make considerable improvements to them in 
the past. In fact, the problem does not really reside in the systems 
themselves, but in the fact that the tendency to an axial deviation 
persists very late, virtually up to the taking-over by the shaping tool. 
This tendency can be explained, notably, by slipping on the rollers, which 
cannot be entirely eliminated, because it is absolutely imperative to 
prevent the rollers from marking the glass sheet in a permanent manner. 
To minimize the consequences of these angular deviations, it is therefore 
imperative to perform the corresponding centering virtually at the same 
instant as taking-over, which is a nuisance from several aspects. The 
first reason is that the operation takes place at the instant at which, as 
indicated above, the sensitivity to marking of the glass is at its 
maximum; now by definition the centering of a glass sheet requires that it 
shall be acted upon and touched. Systems are indeed known which tend to 
eliminate the harmful consequences of this contact, for example systems 
which do not stop the glass sheet but only slow it down (EP-A-389 316) or 
which retract again as rapidly as possible (EP-A-389 317). But the expense 
of these systems frequently becomes higher the more efficient they are and 
in any case their sophistication increases the risks of incorrect 
functioning. 
The second point is that all these systems are to some extent 
self-regulating or, more precisely, they act upon the glass sheet in a 
manner and/or for a period which depend upon the position of the sheet 
when it enters their field of action. The operator in charge of monitoring 
the production line has no means of estimating if this action has been 
performed deliberately. He will, most certainly, when this is not the case 
find out that the panes produced are not entirely in conformity with the 
requirements established but this non-conformity may possibly be explained 
by other reasons, such as incorrect setting of the pressing frame, for 
example, or more generally of the curving machine, with the result that 
there is a risk of not immediately identifying the source of the problems 
and of upsetting everything by a series of inappropriate actions. 
Another harmful aspect of late centering is that, in a certain number of 
cases, the taking-over zone is already largely encumbered by the relevant 
tools and other devices; moreover, centering systems must be retracted, 
for example to enable the glass sheet to be pressed, and this again 
increases the number of possible sources of problems. Finally, since the 
positioning system stops or at least slows down the glass sheet, it leads 
to an increase in cycle times, which runs contrary to the objective of 
high output rates, particularly for relatively simple products. 
SUMMARY OF THE INVENTION 
The present inventors have chosen to circumvent this problem of angular 
positioning to a great extent by renouncing extreme accuracy at this 
level. They propose to curve a glass sheet by bringing it through a 
heating furnace by a conveyor which defines a substantially horizontal 
transportation plane and then by having it taken over by a tool, by means 
of which it is curved and/or transferred to a curving and/or discharge 
device, the invention including in the detection of the effective position 
of the glass sheet and the repositioning of the taking-over tool as a 
function of this effective position. The detection may consist of 
measuring the angle .alpha. of difference between the effective direction 
followed by the glass sheet before it is taken over and the longitudinal 
axis of the conveyor and in rotating said taking-over tool through this 
angle .alpha. so as to center it exactly on the effective position of the 
glass sheet. This detection may also consist in measuring the transverse 
offset, relative to the longitudinal axis of the associated furnace, and 
in translating the taking-over tool in a manner corresponding to this 
transverse offset. Depending upon the case, the repositioning of the 
taking-over tool will therefore be of the rotation and/or translation 
type. 
The point of view is, therefore, entirely reversed here and it is no longer 
the glass sheets which are centered relative to the curving machine, but 
it is this curving machine which is repositioned appropriately for each 
glass sheet. By definition, this process is entirely harmless for the 
glass sheets, which are left free in their movements in the transportation 
plane. Furthermore, the accuracy of the centering may be improved, because 
the initial position of the taking-over tool is known with great 
exactness, either because the preceding position was memorized or more 
simply because the taking-over tool is replaced between two successive 
glass bodies to a reference position corresponding, for example, to a 
perfect alignment. Furthermore, this taking-over tool may certainly have a 
position which is mechanically and possibly very rapidly servo-controlled. 
To leave the glass sheet entirely free in its movements in the transporting 
plane of the conveyor does not signify absence of any measure intended for 
controlling its position; the invention does not exclude, notably, a 
correct positioning during charging into the furnace and lateral centering 
within the furnace. If necessary, a frontal centering may also be 
performed, but solely within the first part of the reheating furnace, that 
is to say at a time at which the centering cannot yet lead to marking of 
the glass. In the last part of the furnace, the glass sheet travels freely 
and can deviate from its trajectory by an angle .alpha., which can be as 
much as about 10.degree.. The possible transverse offsetting is, for its 
part, of the order of a few millimeters. 
The invention also has as its object a device for curving a glass sheet, 
comprising a heating furnace through which passes a conveyor defining a 
substantially horizontal transporting plane and a taking-over tool for the 
glass sheet at the exit from its passage on said conveyor, a device for 
measuring the position of the glass sheet, for example the angle .alpha. 
between the effective direction followed by the glass sheet and the 
longitudinal axis of the conveyor and/or the transverse offset relative to 
this longitudinal axis, and means for repositioning said taking-over tool, 
for example for causing it to rotate through this angle .alpha. or causing 
it to execute a translatory movement. 
According to a first form of embodiment of the invention, the taking-over 
tool is composed of a curving machine comprising a shaping bed, 
constituted of shaping rods disposed along a path having a curved profile 
in the direction of travel of the sheets, and equipped with a 
repositioning device, notably for pivoting and/or translatory movement. 
This type of machine is described, notably, in French patents 2 242 219, 2 
549 465 and 2 604 992. The curving machine is situated directly in the 
continuation of the conveyor passing through the heating furnace. As the 
glass sheet continues its advance through this machine, an angular 
deviation of several degrees at entry to the shaping bed results in a 
deviation of several centimeters at the exit, which at the limit would 
render inoperative the tilting devices provided for transferring the glass 
sheets to a cooling conveyor (see FR-A-2 549 465, EP-A-346 197 and 
EP-A-346 198) and which above all tends to give a twisted shape to the 
glass sheet. Now although this type of machine is generally used for 
shapes that are considered as relatively simple because they are 
essentially cylindrical, an extreme accuracy of the curvature is 
nevertheless required, because the product panes are frequently intended 
for the doors of automobiles and must therefore slide in a slender slot to 
allow the window winding mechanism to operate. 
Since each curving machine corresponds to a given radius of curvature, it 
is generally arranged for the machine to be moved on a frame mounted on 
wheels and immobilized by supports on the ground when in the working 
position. Each curving machine of this type may, therefore, be easily 
converted into a machine suitable for carrying out this invention by the 
simple addition of a motor-driven pivoting and possibly translatory 
mechanism, mounted on the frame or fixed on the ground, and mechanisms for 
freeing the floor supports. It is also possible to use a revolving bridge 
equipped with rails, by means of which the curving machine is positioned, 
remaining fixed relative to a floor which pivots it. 
According to a second form of embodiment of the invention, the taking-over 
tool is constituted of an upper seizing and possibly also shaping element, 
which we will hereinafter designate by the term "upper curving mold", 
suspended from a raising and lowering device which is also movable in a 
horizontal plane, notably by rotation about a vertical axis. In a manner 
well known in these curving techniques, the upper curving mold will seize 
the glass sheet brought by the conveyor, to deposit and/or press it onto a 
lower element, which we will hereinafter designate generally by the term 
"curving frame". The accuracy of the position of the glass sheet with 
respect to this upper curving mold conditions the succeeding operations, 
whether this involves lowering the glass sheet onto a curving frame (in 
the case of a plane or slightly curved upper curving mold) or pressing the 
glass sheet, the upper curving mould in this case fulfilling a dual role 
as a preshaping mold and male press. Contrary to the preceding case, there 
is certainly no reason to fear that the discrepancies will accumulate 
progressively as the curving operation advances, but these processes are 
nevertheless much less tolerant: a sheet placed across the curving frame 
has every chance of breaking or at least of becoming marked and therefore 
rejected, whereas in the case of a curving machine having a shaping bed 
composed of revolving elements, the only risk is that of the curvature 
obtained not being correct. 
As indicated earlier, the upper curving mold is always associated with a 
curving frame. If the shaping comprises a pressing stage, it is necessary 
to provide means for centering this curving frame. These means may be 
similar to those with which the upper curving mold is equipped, or more 
simply they may be indexing means, adapted for centering the frame 
relative to the upper curving mold. With advantage, the means described in 
the patents or patent applications U.S. Pat. No. 4,781,745 and EP-A-398 
315 may be used. 
The curving device according to this invention comprises a device for 
measuring the angle .alpha., characteristic of the offset between the 
direction effectively followed by the glass sheet and its set-point 
direction, the longitudinal axis of the conveyor passing through the 
heating furnace. This device must be capable of performing a measurement 
with sufficient speed for the rotation of the taking-over tool to be 
completed when the glass sheet arrives at the position of this tool. 
Moreover, it has been shown above that it is important for this 
measurement to be performed as late as possible. 
Several types of measuring device may satisfy these conditions. It is 
possible, for example, to use video means which, by mathematical 
morphological analysis, will compare the image of the glass sheet--or of a 
characteristic part of this sheet, notably a corner--with the memorized 
image of a correctly positioned glass sheet. These video means are 
particularly suitable in the case in which the glass sheets have a 
straight front edge which is not, however, perpendicular to the 
longitudinal axis of the conveyor, or have a curved, broken or other front 
edge. 
It is also possible to operate by means of several contactless detectors, 
for example two pneumatic detectors analogous to those described in patent 
application EP-A-348 286, associated with a time counting device and a 
computer unit.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 makes it possible to illustrate the application of the method 
according to this invention to a horizontal curving line, comprising an 
upper curving mold which cooperates with a curving frame. Very 
schematically, the steps of the curving process are as follows: the glass 
sheet is charged onto a conveyor, which passes through a reheating furnace 
where the temperature of the glass is raised to the curving temperature 
and possibly toughening temperature, that is to say to the region of 
650.degree.-700.degree. C. and which conveys the sheet beneath an upper 
curving mold. There, the glass sheet is stopped and is displaced upwards, 
preferably by a force of a pneumatic nature which presses the glass sheet 
against said upper curving mold. This upper curving mould is, possibly, 
curved for preshaping the glass sheets. After the application phase, the 
glass sheet is released onto a curving frame composed of a ring open at 
its center, by means of which the glass sheet is shaped to its final form. 
Carried by the same frame, or after having been transferred onto a special 
frame, the glass sheet is finally brought into a cooling device, for 
example of the thermal toughening or quenching type. 
In the case shown schematically in FIG. 1, the glass sheet 1, displaced 
along the line by a conveyor 2 having rollers 3, is brought into contact 
with the upper curving mold 4 by an ascending hot gas current issuing from 
the lower duct 5 and escaping through the chimney 6. The details of the 
curving line and, more specifically, of the hot air circuit are known from 
the patent EP-169 770. The invention is, of course, in no way limited to 
this given type of method and the raising of the glass sheet may also be 
achieved by means of a suction through the upper mold (EP-3 391) or by a 
sub-pressure generated at its periphery (EP-210 418 or EP-241 355). 
The rollers 3, for example of silica or other material compatible with the 
curving and toughening temperatures for glass sheets, have a core sleeved 
onto metal ferrules 7 which, in principle, run without slip on endless 
belts 8, driven in rotation at the desired movement speed. In the case of 
wear of certain components, the driving of one roller may be slightly 
different from that of the other rollers, which results in a slight 
skewing of the glass sheet if this sheet does not have the axis of the 
line as its axis of symmetry. Furthermore, these rollers may be faced--at 
least over their contact area with the glass sheets--with woven or knitted 
fabrics of refractory fibers intended for preventing pitting of the glass 
sheet. Here again, a worn facing or one fitted slightly incorrectly may 
lead to skewing. It is true that it is always possible to monitor the wear 
of the materials and the correction of interventions, and on the other 
hand other drive systems for the rollers exist, which are less subject to 
the problem of wear referred to above, but whatever the precautions 
adopted, since the first priority is given to the optical quality of the 
panes, practice shows that all the glass sheets do not follow the nominal 
trajectory corresponding to a direction of movement indicated by arrow F, 
which in the case illustrated would signify a front edge moving while 
remaining rigorously parallel to the axes of rotation of the rollers. At 
the approach to the shaping zone, the glass sheet is, for example, 
orientated in a direction defined by the arrow F', corresponding to an 
angular deviation through an angle .alpha., which will certainly vary from 
one sheet to another. The glass sheet may, moreover, be slightly shifted 
transversely. 
The invention consists, at a first time, of detecting the effective 
position of the glass sheet, which in the present case is done by a high 
speed camera 9 mounted above the conveyor. To prevent the highly 
temperature-sensitive electronic components from being damaged (the camera 
9 is disposed in the heating furnace) they are preferably insulated and 
connected to the optical part by suitable optical fibres. The images are 
immediately processed by a computer unit 10 which, for example, will 
operate by comparison with memorized images or by using any other analysis 
means appropriate for supplying a signal corresponding to this angle 
.alpha. and/or to the transverse shift, it being understood that this 
analysis must be performed at a sufficiently high speed for the curving 
tools to be orientated when the glass sheet comes to a stop beneath the 
upper curving mold. This reorientation cannot, nevertheless, be performed 
until after the preceding glass body has freed the taking-over tool. In 
the case of a curving operation with an upper mold, this freeing is 
achieved when the glass sheet is transferred onto the curving frame; in 
the case of a curving on a shaping bed, this freeing is achieved as soon 
as the preceding glass sheet is no longer in contact with the feed 
conveyor. 
The upper curving mold is mounted on a pivoting device 11, which receives a 
control signal from the computing unit 10 with the result that the upper 
curving mold 4 rotates so as to be situated exactly vertically above the 
glass sheet, whatever the angular shift of the latter. It may also be 
provided with means adapted for providing a translation of the mold, in 
order to compensate for the transverse shift. If the glass sheet is then 
simply dropped onto a curving frame without pressing, the orientation 
alone of the upper curving mold may be sufficient for obtaining a pane of 
perfect curvature; if, on the other hand, a pressing operation is 
intended, or if the accuracy demanded requires it, it is possible to 
provide an orientation of the frame. It is also possible to bring the 
upper mold systematically back to a reference position corresponding to an 
exact alignment with respect to the pressing frame. 
This frame is composed of a continuous rail 12 carried by a carriage 13, 
the wheels 14 of which move on rails 15 extending parallel to the rollers 
3. The fixing of the frame 12 on the carriage 13 is achieved by means of 
pins 16. For orientating the frame, these pins 16 are mounted on 
articulations. In the case illustrated schematically here, one of these 
articulations is adjusted by a pivot mechanism governed by the computing 
unit 10. The frame may also be self-centering, that is to say equipped 
with indexing means cooperating with elements fixed to the curving mold. 
After the sheet has been curved, it is conducted to a cooling station, 
generally of the thermal toughening type, not shown here. The invention 
assures the production of panes of a particularly high optical quality, 
because the glass sheet is not subjected to interventions intended for 
recentering it in the shaping zone. Moreover, since the glass sheet is 
always well positioned relative to the curving tools, this good optical 
quality is associated with a good quality of the shaping operation proper, 
and therefore reproducibility of the shapes produced. 
FIG. 2 illustrates another form of embodiment of the invention, which in 
this case is used within the context of a curving line, the curving tool 
of which is essentially composed of a portion of the conveyor defining a 
shaping bed curved in the direction of travel of the glass sheet (see, for 
example, FR-2 442 219, FR-2 549 465). In this case, it is the whole of 
this portion 17 of the conveyor which will be realigned relative to the 
direction effectively followed by the glass sheet 18 in proximity to the 
exit from the furnace 19. This orientation affects, however, only the 
portion 17 and there is no reason to reorientate the secondary cooling 
conveyor 20, which in any case receives only glass sheets already 
toughened, the temperature of which therefore excludes any possibility of 
further deformation. 
It should be noted that, if the shaping is cylindrical, the lateral 
displacement is not harmful, and therefore only the rotation is necessary 
for a correct orientation of the curving machine. In the case of FIG. 2, 
the value of the angle .alpha. is not determined by means of a camera but 
by two contactless detectors 21, 22, for example similar to the pneumatic 
detectors described in patent application EP-A-348 266. As soon as a first 
detector "sees" the front edge of the glass sheet moving past at a speed 
V, measured by a tachometer with which one of the rollers of this 
detection zone is equipped, it initiates the incrementation of a computer. 
This incrementation, performed at a counting pitch p of, for example, 5/10 
000 of a second, is stopped when the second detector in turn sees the 
front edge. If P steps have been counted, the distance along the 
longitudinal axis between the two front edges detected is therefore equal 
to L=P.p.V. In the case in which the glass body has a straight front edge 
intended to be parallel to the generatrices of the rollers, this distance 
may also be expressed as L=d+e tan .alpha., where .alpha. is the angle 
which the front edge makes with a generatrix of a roller, e is the 
distance along the transverse axis between the two detectors, and d is the 
distance along the longitudinal axis between the two detectors. The value 
of the angle .alpha. is therefore given by the calculation: 
##EQU1## 
The curving machine must therefore be rotated through the value of this 
angle if L is greater than d and in the opposite direction in the contrary 
case. 
The method of orientating the machine can be more especially seen from FIG. 
3, where the machine is shown schematically from the side. In this Figure, 
reference 19 again indicates the reheating furnace, through which a roller 
conveyor 23 passes. The glass sheet 18 is detected by the detectors 21 and 
22, and the speed of the conveyor is measured by the tachometer 24. The 
curving machine corresponding to the movable portion 17 is entirely 
mounted on a frame 25 displaceable by means of wheels 26. This frame 25 
carries two curved members 27 and 28 curved in a circular arc, which 
define a curvilinear track, the curvature of which the glass sheet 18 
follows. The lower curved member 27 serves as a support for the assembly 
of lower rollers 29 driven in rotation by a chain 30. In the case shown, 
the first roller 29' is exactly aligned with the last roller 23' of the 
furnace 19, and the entry of the glass sheet 18 into the curving machine 
17 is thus performed without any shock. The curving zone proper 
corresponds to the first seven rollers. Beyond this, the glass sheet 
penetrates into the toughening zone provided with four lower chests 31 and 
four upper chests 32. In this toughening zone, the advance of the glass is 
assisted by upper holding elements 33. After the toughening zone, the 
curved and toughened glass sheets 18' are taken over by a tilting device 
34, which deposits them on the secondary cooling conveyor 20. 
The orientation of the machine to the angle .alpha. measured by the 
detectors 21, 22 and the computing unit is achieved by means of a pivoting 
mechanism 35, with which is associated a drive mechanism 36 for the 
pivoting. In the case shown here, the pivoting mechanism 35 is mounted on 
the frame 25, but it may also be seated on the floor. However, the 
frame-mounted solution will be systematically chosen when translation is 
intended. Furthermore, the pivoting mechanism 35 and drive mechanism 36 
also operate blocking and release means for the floor supports of the 
curving machine.