Manufacture of curved glass sheets

Curved glass is maufactured by supporting glass on a sag bending mould which is passed through a furnace. During initial heating of the glass on the mould, hot air is directed around the mould beneath the glass to minimize the temperature difference between the mould and the glass.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of curved glass. More especially 
the invention relates to a method and apparatus for manufacturing curved 
glass sheets which are to be used in the manufacture of vehicle 
windscreens. In particular the invention relates to a method and apparatus 
for sag-bending pairs of glass sheets which are to be laminated together 
to produce a vehicle windscreen. 
It is known to produce curved glass, such as a single glass sheet for use 
as a vehicle windscreen, or a pair of glass sheets which are to be 
laminated together to produce a vehicle windscreen, by supporting the 
glass on a sag bending mould which is passed through a furnace where it is 
heated to bending temperature and sags to conform to the shape of the 
mould. 
It has been customary to mount each mould on a carriage which may include a 
box in which the mould is located, to load each mould with one or a pair 
of glass sheets at a loading/unloading station and then to advance a 
succession of loaded carriages through heating sections of a tunnel 
furnace having roof heaters. Each carriage is stationary at each heating 
section for a pre-set time. The glass is gradually heated to a sag bending 
temperature, for example 590.degree. C. to 610.degree. C., by the time it 
reaches the hottest section of the furnace, where the glass sags to the 
shape of the mould and attains a required configuration before being 
removed for cooling. Cooling continues as the carriage is transferred back 
to the loading/unloading station, where the cooled bent glass is removed 
and glass to be bent is loaded on to the mould. 
The carriages are circulated step-wise around a continuous path which 
includes the tunnel furnace. This path may be in the form of a horizontal 
loop around which the carriages are carried back to the loading/unloading 
station. In other arrangements the carriages removed from the hot end of 
the furnace may be raised by a lift to a return run above the furnace roof 
and then lowered to the unloading/loading station, or the carriages may be 
lowered from the hot end of the furnace to a return run beneath the 
furnace and then lifted back to the loading/unloading station. 
Furnaces of this kind are described in GB-A-No. 1 299 384, GB-A-No. 1 310 
670and EP-A-No. 0 132 701. 
In the manufacture of laminated windscreens for vehicles flat glass sheets 
are cut to the required external shape to suit the styling of the vehicle. 
These glass sheets are then loaded in pairs on to sag bending moulds at 
the loading/unloading station. Each sag bending mould usually has lateral 
members with upper curved edges shaped to determine the final curvature to 
which the glass sheets sag. When the end parts of the sheets are to have a 
substantial curvature to match the styling of a vehicle, the mould has 
pivoted wing pieces which pivot upwardly under the influence of the weight 
of the sagging glass to conform the ends of the glass to the required 
windscreen shape. 
When the cold flat glass is loaded on to the sag bending mould the glass 
has a limited number of points of contact with the mould, notably the 
upper ends of the lateral mould members and the predominant points of any 
wing pieces. It has been observed that these points of contact are points 
where damage can occur to the lower glass surface. In general these points 
of contact lie near to the edges of the glass, and can cause breakage 
during the glass bending operation or during subsequent processing of the 
glass. Even if breakage does not occur there may be unacceptable flaws in 
the glass surface. The damage is due to a combination of sliding contact 
between the metal mould and the glass surface, causing minor scratching, 
and tensile stresses induced in the glass surface by the thermal gradient 
between the generally warm glass and the cold spot caused by localised 
contact with the mould. The tensile stresses tend to cause propagation of 
cracks from the minor scratches. Further damage may occur at other points 
as the hot glass sags onto the colder mould during the final stages of the 
bending. 
This invention is based on the discovery that reduction in the temperature 
differential between the mould members and the glass during heating of the 
glass, particularly the initial stages of heating of the glass, results in 
a reduced temperature differential being maintained throughout the sag 
bending process, thereby minimizing the possibility of damage to the glass 
surface where it contacts the mould members. 
BRIEF SUMMARY OF THE INVENTION 
According to the invention there is provided a method of manufacturing 
curved glass in which the glass is supported on a sag bending mould and 
passed through a furnace where it is heated to bending temperature by 
radiant heating from above the glass and sags to conform to the mould 
shape, characterised in that during the initial heating of the glass on 
the mould, hot air is directed around the mould beneath the glass to 
minimise the temperature difference between the mould and the glass. 
Usually each mould is mounted in a box on a carriage and the boxes are 
transported in sequence through a tunnel furnace having a number of 
heating sections. In one embodiment of the invention hot air is supplied 
downwardly into each box at least at a first heating section of the 
furnace, that hot air is deflected sideways beneath the mould in that box 
and air is extracted upwardly above the centre of the box. 
The hot air may be supplied at least at the first three heating sections of 
the furnace, or at all heating sections, for example five heating sections 
which constitute the heating part of the furnace. 
The invention also comprehends apparatus for manufacturing curved glass in 
which a sag bending mould is mounted on a carriage which is transported 
through a tunnel furnace having a number of heating sections, 
characterised by a hot air supply means at least in a first heating 
section of the furnace, for directing hot air beneath and around each 
mould as it supports glass in the furnace. 
Such hot air supply means may be embodied in at least the first three 
heating sections of the furnace, or at every heating section. 
In a preferred embodiment the carriage includes a box in which the mould is 
mounted and deflectors are mounted in the box at either side of the box 
for directing downward hot air flows beneath and around the mould. 
Preferably the hot air supply includes supply ducts leading to slots at 
either side of the roof of each heating section of the furnace, which 
slots direct hot air downwardly to the deflectors in a box located in that 
heating section. 
The deflectors may be inclined deflector plates fitted into the lower 
corners of the sides of the box. 
Further according to the invention the furnace roof may have a central 
extract aperture between the slots, which is connected to a fan which 
directs air extracted from around the mould through a heater to the supply 
ducts. 
The invention also comprehends curved glass, and in particular a pair of 
curved glass sheets which are to be laminated together to produce a 
vehicle windscreen, produced by the method of the invention.

DETAILED DESCRIPTION 
Referring to FIG. 1 a tunnel furnace is divided into a number of sections 
leading from a loading/unloading station 1 at the inlet end of the 
furnace. The first five sections of the furnace constitute together a 
heating part 2 of the furnace which leads to a pre-bending section 3, 
followed by a bending section 5 which is maintained at the bending 
temperature. The next section of the furnace is a transfer section 6 which 
has open sides to permit an operator to view the bending section 5. The 
final two sections together form a cooling section 7 which leads to a 
transfer station 8 at the outlet end of the furnace. 
As is customary each mould is mounted in a steel-walled insulated box 9 
which is mounted on its carriage 10 which runs on rails 11 which extend 
right through the furnace from the loading/unloading station 1 to the 
transfer station 8. At the transfer station 8 each carriage in sequence is 
lifted by a lift table to an upper return run of rails 12 along which the 
carriages run as they are pushed back to a lift indicated at 13, which 
lowers each carriage in turn to the level of the rails at the station 1 
ready for unloading and reloading. 
The carriages indexed through the furnace in sequence with a fixed 
residence time, e.g. 90 seconds, in each section of the furnace. 
FIGS. 2 and 3 illustrate the mounting of a sag bending mould in each box 9. 
In this embodiment each box 9 is an enclosed structure having a floor 14, 
side walls 15 and front and rear walls 16. The floor 14 and the side rear 
walls 15 and 16 are of double-walled stainless steel with insulation. The 
sag bending mould is mounted on the floor 14 and includes lateral mould 
members 17 and end wing pieces 18 which are shown diagrammatically and are 
of conventional design. The lateral mould members 17 are narrow strip 
members having an upper edge of curved configuration to which the glass 
conforms as it sags in the bending section 5 of the furnace. The wing 
pieces are usually conterpoised so that they can readily swing upwardly 
under the effect of gravity as the glass sags. 
A pair of glass sheets which are to be sag-bent together and are for 
eventual lamination together to form a vehicle windscreen, are indicated 
at 19. FIGS. 2 and 3 show how the glass 19 has only points of contact with 
the predominant points of the mould when the glass is loaded on to the 
mould at the station 1. Initially the glass will be at room temperature as 
it is loaded and the mould members in the box will also be at about room 
temperature, or possibly rather higher depending on the cooling effected 
during the return run of the carriage. 
Each mould in its box 9 is advanced in turn into the five heating sections 
which together form the heating part 2 of the furnace. The construction of 
each of these five heating sections of the furnace is identical and is as 
illustrated in FIGS. 2 and 3. 
The roof of each heating section of the furnace carries a pluraltiy of 
radiant heaters 20 and heat is radiated downwardly through the open top of 
the box 9 towards the upper surface of the glass 19 which is now 
stationary in the heating section. The mould members are partially 
screened from the radiant heat by the glass itself. 
In the embodiment illustrated each of the heating sections has a hot air 
supply including means for directing hot air beneath and around each mould 
supporting the glass. At either side of the furnace roof in each of the 
heating sections there are hot air supply ducts 21 leading to slots 22 at 
either side of the roof. Each of the ducts 21 leads from a heater 23 which 
is supplied with air by a fan 24 which extracts air through a central 
extract aperture 25 in the furnace roof 26 between the slots 22. 
If the air extracted through the central aperture 25 is hot enough it may 
not be necessary to provide the heaters 23. 
The slots 22 supply hot air downwardly into each box just within the side 
walls 15 of the box, and inclined deflector plates 27 are fitted into the 
lower corners of each box where the side walls 15 of the box meet the 
floor 14. These deflector plates 27 have the effect of deflecting the hot 
air flows sideways beneath the mould as indicated by the arrows 28. The 
hot air flows underneath the mould and around the mould members 17, 18 and 
outwardly around the mould members towards the front and rear walls 16 of 
the box, and then up to the roof 26 of the furnace for extraction through 
the central extract aperture 25 as indicated by arrows 29. The hot air 
heats the lower surface of the supported glass to assist thermal 
equalisation through the glass thickness, especially when two glass sheets 
are being bent together, and has the effect of minimizing the temperature 
difference between the mould members and the glass. 
In one example of operation, by employing in the first heating section of 
the furnace flows of hot air at 300.degree. C. with a rate of flow of 
0.47m.sup.3 /s, it was found that the temperature difference between the 
mould members and the glass, which otherwise might have been of the order 
of 100.degree. C., was reduced to about 45.degree. C. to 50.degree. C., 
and surface flaws on the lower surface of the glass were eliminated. 
It is desirable to maintain this reduced temperature difference which has 
been introduced in the first of the heating sections. To effect this hot 
air flows 28 around the mould may be provided in at least the first three 
heating sections. Usually the hot air flows 28 are provided in the second 
to fifth heating sections of the heating part 2 of the furnace in the same 
way as in the first heating section. 
The carriage then moves to the bending section 3 where the furnace 
temperature is of the order of 700.degree. C. From the pre-bending section 
3 the carriage moves to the bending section 5 where the glass is at a sag 
bending temperature of about 590.degree. C. to 610.degree. C. with the 
mould members at a temperature of about 530.degree. C. to 540.degree. C. 
The minimal temperature difference between the mould and the glass which 
is established in the first heating section 2 of the furnace, had been 
maintained throughout the concomitant heating of the mould members and the 
glass prior to bending. 
The indexing cycle time is such that immediately the glass has sag bent to 
the required configuration of the mould, it is removed from the bending 
section and cooled and gradually transported step-by-step back to the 
loading/unloading station 4, by which time it has cooled to about room 
temperature, or somewhat above. 
In each of the heating sections 2, the downward flow of air 28, which are 
deflected beneath and around the mould, are heated to a temperature 
commensurate with the temperature of the glass in that furnace section, 
for example 400.degree. C. in the fourth heating section and 500.degree. 
C. in the fifth heating section. 
The deflector plates 27 are, in the illustrated embooiment, the faces of 
members of triangular section which are fitted into the corners of the box 
9. Deflector plates which are at 45.degree. to the side walls 15 and floor 
14 of the box have been found to be effective. Other shapes may be 
employed, for example deflector members with conrave surfaces which modify 
the flows beneath and around the mould members. 
In another way of carrying out the invention, each box 9 may be of a 
simplified construction which is of L-shaped cross section having a floor 
and a front wall only. The boxes abut against each other, so that the 
front wall of each box in the furnace acts as the rear wall of the 
preceding box. The side edges of these simplified boxes are close to the 
side walls of the furnace sections, and at each of the sections of the 
heating part 2 of the furnace there are air supply slots in the side walls 
of the furnace, at the level of the moulds, which direct the hot air flows 
around each mould beneath the glass in similar manner to the flows 28 
described with reference to FIGS. 2 and 3. 
The convective heating which is employed in carrying out the invention has 
been found to eliminate the problem of the generation of tensile stresses 
in the glass due to thermal gradients between the generally hotter glass 
and the cooler spots caused by localised contact with the mould. Damage 
caused by the hotter glass sagging on to the colder mould during the final 
stages of the bending is also avoided. Overall heating efficiency is 
improved, which has permitted a reduction in the timing of the indexing 
cycle while still ensuring, when manufacturing pairs of glass sheets to be 
laminated, that the two sheets conform sufficiently closely to one another 
and the desired shape. 
For example it has been found that operation of the method and apparatus of 
the invention increases the rate of heating of the glass by a factor of 
between 2 and 3. It has even been possible to reduce the residence time in 
each heating section from 90 seconds to 60 seconds by employing the hot 
air flows in at least the first three heating sections to maintain a 
tolerable temperature difference between the mould and the glass.