Note: Descriptions are shown in the official language in which they were submitted.
1 16~3d~8
The present invention relates to a manufacturing
process for curved and bonded glass panes, especially suit-
able for use as windshields or other safety glass items in
motor vehicles or the like. More precisely, the present
invention relates 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.
Processes for forming and assembling two curved
glass panes having different physical-chemical properties
and/or different thicknesses, especially suitable for wind-
shields and other safety glass items for motor vehicles and
the like, are already known, such processes comprising the
consecutive steps of ~1) loading the glass panes on a rigid
horizontal mould with concave surface facing upwards, (2)
curving of the two glass panes simultaneously in a furnace,
~3) trimming the glas~ panes to shape and, if required,
sub~equent by temperlng the glass panes, (4) placing a sheet
of plastic material between the matched glass panes, and (5)
lastly bonding the panes an~ plastic togethe in an auto-
clave under special temperature and pressure conditions.
The applicant proposes, in order to eliminate the
faults occuring in the above-described prior processes, that
of the two glass panes to be bonded, the one with the smaller
degrée of curvature, be placed in direct contact with mould
itself, and that during the assembly phase, the position of
the two glass panes be inverted. Hence the glass pane
originally placed in direct contact with the mould will go
to form the inner part, that is, the concave part of the
finished product.
Furthermore, the Applicant suggests that the glass
pane of the two to be bonded having the smaller degree of
curvability should ascribe such property wholly or mainly to
a higher softening temperature or a higher radiant heat
-- 1 --
,. ~
1 16~348
transmission coefficient (in relation to the radiation
chiefly present in the furnace) or yet again to a greater
thickness of the actual glass pane.
In order to understand the progress achieved by
the process in accordance with the invention, it is first of
all necessary to consider in more detail the procedures and
limitations of manufacturing processes for bonded and curved
glass panes already known until 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, two operations are per-
formed: the glass panes are curved in the furnace and
trimmed to shape. These operations can be perforned accord-
ing to two different technologies. In one technology, the
two operations are performed in the order indicated above, the
proce~s being known as an expan~ion process. ~n the other
technology, the operations are performed in the reverse order,
the process being known as a 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 expel most
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 bonding operation is carried oùt
under controlled temperature and pressure conditions.
For successful results in the manufacturing pro-
cess, the glass panes must be curved in such a way that the
surfaces in contact properly match each other or, i.e., there
must be no appreciable differences in local curvature between
the two surfaces to be bonded.
,
,,,, . , ... .. ,, _ ., _, ~ ,
3A~
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 a?ter the final result, very
different is the case, for the purposes of achieving proper
bonding, 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 inter-
ruptions occurring) along the edges of the sandwich and/or
inside the sandwich.
Adequate matching of the curvatures of the two
surfaces to be assembled is still today generally accom-
plished by simultaneous curving in a furnace of the glass
panes to be subsequently bonded together. For this purpose
the glass panes are placed, during the curving phase, on a
horizontal mould with concave surface facing upwards, in
th~ same po~ition that they will as~ume in the finished
product,
For example, in the manufacture of a windshield
with two layers of glass having the same physical-chemical
properties and the same thickness, the pair of glass panes
to be curved is loaded on the horizontal mould with concave
surface facing upwards, whereby the glass pane which will
form the outer part of the windshield ~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
windshield. 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 Con-
sequently the glass panes assume, thanks to the effect of
temperature and gravity, the final mould shape. More par-
ticularly, the lower surface of the lower glass pane in
direct contact with the mould, tends to assume the shape of
' 1' '
~ - 3 -
1 16~3~8
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 ls 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 per-
mits 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 quan-
tity of heat provided 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 moul~ 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 defor-
matlons), or else it can be of the ar~iculated type (that is,
suitable hinge arrangements which permit the mould geometry
to change inside the furnace). The rigid type moulds are
mainly used for parts with small curvature, while the articu-
lated type is mainly used in the other cases.
After curving, the glass panes undergo a suitableannealing 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 and bonding a pair of glass
panes with the same physical-chemical properties, the above
described curving process certainly gives rise to good
results on glass panes of the samé thickness ~symmetrical
,~; ,.~, -- _
,,,,. ~ .
1 16i3~
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 o~ 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 su~sequent del~mination more difficult Analogous
difficultie~ can arise, in both symmetrical an~ assymetrical
bonding, when the glass panes to be bonded have different
physical-chemical properties. More particularly, 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 cor-
rectly lay down this pane on the lower pane by using conven-
tional technology. In fact, assuming equal quantities ofheat are 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
- 5 -
~ .,
l 16~3~8
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 carry-out suit-
able 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 (vis-
cous) reached by the pane within a certain time under given
ambient furnace conditions.
The degree of curvability of a glass pane therefore
is a measure of its ability to conform more or less easily to
the constraining geometry and substantially depends on the
geometry of the pane ltself and on the physical-chernical prop-
erties 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 coeffi-
cient relative to the wavelength range of the radiation.
For a given part (that is for a given contour con-
figuration), 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, the
higher is its radiant heat transmission coefficient, the
3~ higher is the softening temperature of the glass of which it
is made.
In order to solve the dificulties in curving met
with in the previously described cases, the ~pplicant first
of all carried out experiments involving the separate curving
- 6 -
1 16i3~8
of the glass panes to be bonded. However, this process gave
poor yualitative results in the assembly phase and therefore
a higher number of rejects.
In those cases where -the smaller degree of cur-
vability of the glass pane essentially depends on its highertotal 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, particu-
larly by increasing the heat transfer component by convectionat the expense of that by radiation. However, this entails
appreciable and troublesome modifications to the glass pane
curving furnace.
With thé manufacturing process in accordance with
the present invention, the Applicant has finally succeeded
in overcoming the difficulties in manufacture encountered in
the previ~usly descri~ed case~.
The process in question is applied, as stated in
the title and in the background portion of this specification,
particularly to the manufacture of curved and bonded glass
panes whose physical-chemical 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 of
that during the assembly phase, as it was surprizingly found
that this is sufficient to offset the difficulties encoun-
tered in the previous above listed processes.
Hence, more especially with the process in accord-
ance with the invention, the glass pane with the lesserdeyreé of curvability, that is, the glass pane with the higher
softening temperature, or the ~lass pane with the greater total
radiant heat transmission coefficient (in relation to the
radiation mainly present in the furnace) or the thicker glass
116-13d~
pane is placed in direct contac-t with the mould during the
curving phase, when it is to be subsequently placed in the
concave part, that is the inner part of the finished product.
~f s~lch factors influencing the degree of curvability are
present, but are not common to the same glass pane, obviously
it will be the pane with the lesser degree of curvability to
be placed in direct 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
lS during the curving phase.
Accordingly, the invention is broadly claimed here-
in as a method for forming and bonding at least two curved
~lass panes includlng first and second curved glass panes
having diferént physical-chemical properties and/or differ-
ent thic~.ness, especially suital~le for use as a windshieldor other safety glass item for a motor vehicle or the like,
the first pane having a lesser degree of curvability than the
second pane when loaded on a concave surface of a horizontal
mould and curved in a furnace, the method comprising the
steps of:
(1) loading the first and second panes on a hori-
zontal mould having a first concave surface which faces up-
wards; the first pane being in direct contact with said first
- concave surface, the second pane being loaded on said first
pane;
~ 2) after step ~1), curving the first and second
panes in said furnace on said mould to form first and second
curved panes, respectively, each having an inner concave
surface and an outer convex surface;
-- 8 --
', i1
~,
.
1 16~3J~8
(3) trimming the first and second panes;
(4) placing the first curved pane over the second
curved pane, with a plastic sheet sandwiched therebetween
such that one side of said plastic sheet is in direct con-
tact with said concave surface of the second curved pane andthe c~ther side of said plastic sheet is in direct contact
with said convex surfacé of the first curved pane; and
(5) following step (4), bonding the first and
second curved panes and said plastic sheet in an autoclave.
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 applica-
tion described below.
TABLE 1 (Average composition)
Glass pane no. 1 Glass pane no. 2
SiO2 70 - 74 % -50 - 70 ~
CaO 8 - 10 ~ 0.5 - 1.0 %
MgO 2 - 4 % 2 - 4 %
A123 0 1 - 1 5 ~ 5 - 25
Fe2O3 0 10 - 0 60 % 0 02 - 0 6
TiO2 0 05 - 0 06 % 0 05 - 0 2
Alkalies 12 - 15 % 12 - 15
EXAMPLE 1
A glass pane of mainly silica-lime composition,
designated by the letter A, in Fig-~res 1 and 2, is to be
bonded with a glass pane with a mainly silica-alumina com-
position, designated by the letter B.
The two glass panes, initially flat and of the same
thickness, before the assembly phase, must first undergo a
forming phase, as they are to bc used as a curved windshield
for a motor vehicle
The silica-lime glass pane A is to form the outer
g _
., . ..... ~, .. .... ..
'
1 16~348
part of the windshield, 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 windshield (the con-
cave 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 expla-
nation only in figure 1. It can be seen from the figure that
the glass pane B has, at the same temperature, a higher vis-
cosity 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
vaiue (for example n=108 poise). Figure 2 gives, also by
way of explanation, the monochromatic radiant heat transmis-
sion 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 ~ for the differ-
ent wavelengths mean~ that the total radiant heat transmission
coefficient (in relation to the radiation mainly present in
the furnace) of the silica-alumina 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 on a mould which
will than 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 sur-
- face faces upwards), and glass pane A will be laid on glass
pane B.
The mould-pair of gl~ss pancs assembly will then
enter the heating tunnel and glass pane A will, also on
account of its lower radiant heat transmission coefficient,
7~' - 10 -
"~
1 1613~8
reach its softening temperature (which is, among other
thinys, lower than that of glass pane B) before the under-
lyinc3 ylass pane 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 con-
forms to the curve forming mould.
Curving of ylass pane B, besides being favoured by
the force of its own weight, will also be favoured by the
weiyht of the overlying glass pane A which, having reached
its softening temperature first, will be fully resting on
glass pane B.
Heating of g~ass pane 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 the required annealln~ treatment is performed,
the gla5s pané~ 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-
limé glass pane A will go to the outer position (convex partl
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 windshields
with small or medium curvature, this slight difference in
` curvature of surfaces in contact with the sheet is such as
not to alter the successful result of the bonding process.
In fact, in such a case, the différence between the radii of
, ",
... ..
I 16~3~8
.
curvature is in the region of 0.1 - 1% of the ideal radius
of curvature.
It has also be found that the operation according
to the invention does not impair the good quality of the
bondi.ng, even in the case of windshields with an appreciable
degr~e of curvature, provided that the mould shape is properly
corrected.
EXAMPLE 2
The same glass pane of mainly silica-lime composi-
tion, according to the precedin~3 example, which will be
designated by the letter A, is to be bonded to a thinner
- glass pane, of mainly silica-alumina composition, which will
be designated by the letter B.
The two initially flat glass panes, before the
assembly phase, must first undergo a forming phase as they
are to be used as a curved windshield for a motor vehicle.
Furthermore, the silica-alumina glass pane, which
will form the inner part of the windshield (concave part of
the finished product) must unde~go, before assembly, a che-
mica~ tempering treatment.
This treatment is designed to impart greater
mechanical strength to the inner part of the windshield, and
above all, to impart a greater degree of passive safety in
the event of fracture caused by impact.
The physical-chemical properties of the two glass
panes correspond to those already given in the preceding
example 1, with the possible exception that the monochromatic
radiant heat transmission coefficient of glass pane B can be
- even higher in certain ranges of the radiation wavelengths
because of the smaller thickness of glass pane B.
It has been found experimentally than, in spite 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 deyree of curvability which,
r,4~ ~ 12
.
1 16~ 8
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 resistance to bending does not
lead in practice to deformations in the assembly phase even
for windshield of appreciably large curvature and consequently
the mould shape does not have to be significantly correct.
EXAMPLÉ 3
Two glass panes ~ the same composition ~for
e~ample silica-alumina) and having the same physical-chemical
properties but different thickne~s, are to be curved and
bonded in such a manner that the thinner glass pane, desig-
nated hy letter A, will form the outer part of the windshield
(convex part), while the thicker glass pane, designated by
the letter B, will form the inner part of the windshield
(concave part).
- The two glass panes are places 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 gl~ss pane A
will be laid down on glass pane B.
The glass panes assembled on the mould will then
enter the heating tunnel and glass pane A, on account of its
lower heat capacity, will reach its softening temperature
before the underlying glass pane B. In spite of this, glass
~ '
- 13 -
1 16~3~
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 con-
forms 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 the required annealing treatment is per-
formed, 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 sensé 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 posi*ion (concave part).
It has been found that sUch an operation does not
impair the good ~uality of the bonding, even in the case of
windshields with appreciably large curvature ana without
significant modifications to the mould shape because of the
lower rigidity of the thinner glass pane with respect to the
thicker glass pane.
It will be understood that what has been described
and illustrated above is only by way of example and that
numerous variations and modifications may be effected in
carrying out the invention without departing from the true
scope of the invention. Such modifications and variations
will therefore fall within the scope of the claims.
''!' ': -- ], 4
~ .
