Apparatus and method for controlling stresses in laminated automotive glass

A glass sheet annealing ring includes an insulating ring juxtaposed the inboard periphery of the annealing ring for reducing the cooling rate of the glass sheet in proximity to the insulating ring thereby reducing the magnitude of net inner band tension while maintaining edge compression as the glass sheet is cooled.

TECHNICAL FIELD 
This invention relates to an improved annealing ring for supporting a glass 
sheet during cooling and improved method of cooling the glass sheet. 
BACKGROUND ART 
Glass sheets which are press formed individually for making laminated 
backlites or windshields are cooled in such a manner as to control 
stresses. This cooling is accomplished either by natural convection or by 
low level forced air cooling. Such lites develop surface stresses which 
are comparable to those of actively annealed glass sheets from 
conventional, gravity sagged windshield processes. 
However, the edge compression stress on press formed lites cooled by 
natural convection or low level forced cooling is generally higher than 
that for conventional, actively annealed lites. Edge compression is 
typically 300 to 400 Kg/cm.sup.2 and 150 to 200 Kg/cm.sup.2 for press 
formed lites cooled by natural convection or low level forced cooling and 
conventional actively annealed lites, respectively. This higher edge 
compression on the pressed formed lites represents an improvement in that 
said lites are less susceptible to edge breakage from handling. 
Inboard from the edge compression, however, is a band of tension. The 
integrated inboard tension must equal the integrated edge compression. 
Therefore, the inboard tension is necessarily higher for individually 
pressed formed and cooled lites than for conventional gravity sagged 
lites. The measurable net inner band tension (NIBT) on press formed and 
cooled lites is typically 50 to 120 Kg/cm.sup.2, where on conventional 
gravity sagged lites it is typically 25 to 50 Kg/cm.sup.2. 
Experience has shown that levels of net inner band tension in excess of 
about 60 Kg/cm.sup.2 can result in performance problems (breakage) for 
windshield lites. Specifically, laminated windshields with inner band 
tension in excess of 60 Kg/cm.sup.2 do not pass the so-called "scratch 
test" for windshields. 
In the scratch test, the laminate (or individual lite) is abraded using 80 
grit aluminum oxide sandpaper. The area typically abraded is a band about 
6 inches wide immediately inside the perimeter of the glass. If cracking 
occurs within 24 hours of the abrasion, the sample is said to fail the 
scratch test. Experience has shown that samples with NIBT in excess of 60 
Kg/cm.sup.2 usually fail the test while those with less NIBT do not. The 
test is considered a good measure of the propensity of an installed 
windshield to crack from incidental abrasion or from stone impact. 
Attempts have been made to reduce the magnitude of NIBT by reducing the 
rate of cooling of the glass sheet, especially inboard of where the glass 
sheet contacts a support ring. 
Experience has also shown that edge compression of less than about of 150 
to 200 Kg/cm.sup.2 can result in breakage of laminated glass during 
installation of said glass into its opening, typically in an automobile. 
Higher edge compression will reduce the propensity of breakage during 
installation. 
U.S. Pat. No. 5,069,703 discloses a covering for the tempering of glass 
sheets wherein a metallic fabric is used to cover a frame intended to 
support glass sheets being annealed. The metallic fabric is of low thermal 
conductivity and reduces the heat sink characteristics of a typical frame. 
U.S. Pat. No. 4,687,501 discloses lightweight bending iron heat shields for 
glass bending molds that shade glass sheets from a heat source. The heat 
shields are utilized to change the heating rate of different areas of the 
glass sheets to effect the final shape of the glasses. 
Typical glass sheets cooled by natural convection or low level forced 
cooling on conventional annealing rings have excess net inner band 
tension. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved glass sheet 
annealing ring that modifies edge stresses obtained on a glass sheet which 
is press formed and then cooled. 
Another object of the present invention is to provide an improved annealing 
ring that reduces net inner band tension on a press formed glass sheet 
cooled by natural convection or forced air. 
A further object of the present invention is to provide an improved method 
of locally retarding the rate of cooling of a heated glass sheet about its 
edge in such a way that as the stresses are being made permanent in the 
glass sheet, the temperature of the glass sheet in proximity to its edge 
is maintained above its strain point so that the inner band stresses are 
distributed over a larger area. 
Another objective of the present invention is to provide an improved method 
for maintaining the edge compression in a glass sheet developed by cooling 
with natural convection or low level forced cooling. 
In carrying out the above objects and other objects of the invention, the 
improved glass sheet annealing ring constructed in accordance with the 
present invention includes an insulating ring juxtaposed the inboard 
periphery of the annealing ring. The insulating ring reduces the magnitude 
of net inner band tension, typically by 50% by retarding the rate of 
cooling of the peripheral portion of the glass sheet vis-a-vis its central 
portion. 
Preferably, the insulating ring extends generally 7-13 cm inwardly of the 
forming and annealing ring. The insulating ring makes the inner band of 
tension or area of tension inboard of the peripheral portion of the glass 
sheet larger than that created on a conventional annealing ring. 
The inner band of tension balances the edge compression in the glass sheet 
cooled on such a ring because the summation of these forces must be zero. 
In one embodiment, the improved annealing ring also includes a support ring 
which is also mounted inboard and about the inboard periphery at the 
bottom of the annealing ring. The support ring extends inwardly of the 
annealing ring generally the same distance as the insulating ring. The 
support ring provides support for the insulating ring. 
Preferably, the insulating ring is made from an insulating material. Such 
materials include glass fiber and ceramic resin felt materials. 
The insulating ring is used to modify the edge stresses obtained on back 
lites and windshield lites which are press-formed and then cooled by 
increasing the area of inner band tension which thereby decreases the 
magnitude of inner band tension and the propensity of the glass sheet to 
break from impact. 
The above objects and other objects, features, and advantages of the 
present invention are readily apparent from the following detailed 
description of the best mode for carrying out the invention when taken in 
connection with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIGS. 1 and 2 of the drawings, an improved glass sheet 
annealing ring is generally indicated by reference numeral 10. Annealing 
ring 10 reduces the magnitude of net inner band tension, or inner band 
tension value, while maintaining edge compression, or edge compression 
value of a formed glass sheet as it is cooled on the ring. As is 
hereinafter more fully described, the improved annealing ring 10 controls 
regional cooling and allows glass sheets used as laminated automobile 
lites to be annealed to meet specifications without using an annealer, 
thereby generating substantial cost savings. 
As seen in FIGS. 1 and 2, the glass sheet annealing ring 10 includes an 
insulating ring 12 juxtaposed the inboard periphery of the annealing ring. 
Insulating ring 12 retards heat transfer from the portion of the glass 
sheet in proximity thereto to reduce the magnitude of net inner band 
tension by increasing the inboard area of forces that balance edge 
compression forces created during cooling. At the same time, insulating 
ring 12 maintains the edge compression created as the glass sheet is 
cooled. Because all the stresses over the entire glass sheet must average 
or integrate to zero, the magnitude of the inner band tension is a 
function of the area of the band. Preferably these forces integrate to 0 
at a distance of at least 6 cm inwardly from the edge of the glass sheet. 
A preferred range for integration to 0 has been formed to be 7-16 cm 
inwardly from the edge of the glass sheet. Most preferably these forces 
integrate to 0 at about 11 cm. 
Preferably, the insulating ring 12 extends generally 7-13 cm inches 
inwardly of the annealing ring to produce a wide band of reduced tension 
in the glass sheet while not substantially changing the edge compression 
of the glass sheet. Thereby, the magnitude of net inner band tension can 
be comparable to that found in conventionally annealed glass sheets used 
as windshields, while the edge compression is significantly greater. Inner 
band tension can also be, and preferably is, less than that found in 
conventionally annealed glass sheets, while the edge compression is 
significantly greater. 
Preferably, the glass sheet annealing ring 10 also includes a support ring 
14, made from thin gauge stainless steel material, mounted inboard and 
about the inboard periphery at the bottom of the annealing ring 10. The 
support ring 14 extends inwardly of the annealing ring generally the same 
distance as the insulating ring 12 and provides support for the insulating 
ring. 
Preferably, the insulating ring comprises a glass fiber matrix material 
having very low thermal conductivity. Kaowool.TM. insulating material has 
been successfully used as insulating ring 12. 
The insulating ring 12 retards the rate of cooling of the peripheral 
portion of the glass sheet in such a way that as the stresses are being 
made permanent as the glass sheet cools, the insulating ring 12 maintains 
the temperature of the glass, in proximity to it, above the strain point 
of the glass. Thereby, the stresses formed during cooling are distributed 
over a larger area than is the case when cooling occurs without the 
insulating ring 12. 
An example which follows compares the average edge compression and the 
average inner band tension between control samples of glass sheets 
annealed without an insulating ring 12 and glass sheets annealed on an 
annealing ring 10 having an insulating ring and industry standards. These 
numerical results represent average values of 4 points on each of 3 glass 
sheets formed for use as laminated windshield lites. It can be appreciated 
that the values at each point are summed and averaged. The 4 points were 
located approximately 8 inches from the vertical center line on each side 
of the center line, top and bottom of the glass sheet. Industry Standards 
are measured values from traditional gravity sagged and actively annealed 
samples. 
______________________________________ 
Avg. Edge 
Avg. Inner 
Compression 
Band Tension 
(Kg/cm.sup.2) 
(Kg/cm.sup.2) 
______________________________________ 
Industry Standards 
150 30 
Samples 
Control Samples 413 64 
Insulating Ring Samples 
410 34 
______________________________________ 
As illustrated through the example, use of such an insulating ring 12 has 
reduced inner band tension by 47% while maintaining edge compression when 
compared to the control samples. FIGS. 3 and 4 represent unmodified and 
modified stress distributions in glass sheets annealed without and with 
the hereinabove described insulating ring 12. FIG. 3 illustrates the 
unmodified stress distribution. FIG. 4 illustrates the modified stress 
distribution wherein the amplitude of the tensile stress which typically 
occurs 20 millimeters from the edge of an annealed glass sheet has been 
reduced in magnitude approximately 50% by the use of the insulating ring 
12. 
Such a reduction in the magnitude of inner band tension can also be 
accomplished in glass sheets which are gravity sag-formed on a perimeter 
ring and subsequently annealed. However, the edge compression is typically 
200 Kg/cm.sup.2 or less yielding a higher propensity to breakage during 
installation into the vehicle. The cooling rate in the active annealing 
portion of a gravity sag lehr is significantly lower than it is for 
natural convection or low level forced cooling. This reduced cooling rate 
is required to minimize warpage and breakage of the stacked glasses on the 
perimeter ring and to control net inner band tension. This lower cooling 
rate results in lower edge compression. 
While the best mode for carrying out the invention has been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.