Method of forming sagging mold from a glass ceramic

A relatively thin, smooth surfaced mold of low thermal expansion material which exhibits non-sticking qualities is disclosed for contouring glass articles, such as by sagging a preform to the contour of the mold. In addition, a method of forming such molds with a desired surface contour is disclosed.

BACKGROUND OF THE INVENTION 
The use of molds for sagging glass articles has been well known in the 
prior art, however such know molds have not been entirely satisfactory due 
to the cost and complexity of manufacturing the same, and further due to 
the undesirable physical characteristics exhibited thereby during heat-up 
and cool-down. That is, metal mold materials generally are not easily 
formed to exact contours and normally exhibit high thermal expansions 
which result in the warping of the mold during the hot temperature thermal 
cycling required for sagging glass, and accordingly an accurate work 
surface cannot be maintained. Further, the high thermal expansion not only 
tends to distort the precise curvature formed on the mold surface, but 
also laterally displaces the desired curvature relative to the glass 
article being sagged thereon thus producing dimensional problems in the 
glass being formed. In addition to the problem of high thermal expansion, 
metals have the further disadvantages of having a relatively low 
mechanical strength to thickness ratio at required sagging temperatures, 
the surface has a tendency to oxidize and deteriorate at such 
temperatures, and expensive finishing operations are required to generate 
plane or curved surfaces. 
Ceramic molds of conventional materials do not provide a durable smooth 
surface required for sagging, since such molds tend to spall with thermal 
cycling. In addition, such refractory materials must be of a substantial 
thickness in order to provide the required strength and handling 
characteristics. Such thickness requirements, however, necessitate the use 
of longer thermal cycling periods in view of the heat absorbed by the 
ceramic mass of the mold. 
Accordingly, the present invention has overcome the problems of thermal 
expansion, surface deterioration, low mechanical strength to thickness 
ratios, and complex machining requirements associated with the molds of 
the prior art, by providing an improved mold material which is not only 
easily formable to a desired surface contour, but which has a low thermal 
expansion. Further, an improved method of forming a plurality of such 
molds having virtually identical surface contours is set forth which 
overcomes the manufacturing problems heretofore encountered in the prior 
art. 
SUMMARY OF THE INVENTION 
In its simplest form, the present invention is directed to sagging molds 
for sagging glass articles to desired contours, and to an improved method 
of forming such molds with precise contours which maintain their integrity 
during thermal cycling. 
The invention has particular application to the sagging of glass sheets to 
complex curvatures such as plano-curved surfaces required by automobile 
windshields, however the invention has equal application for utilization 
in sagging glass sheets to a planar surface or to a surface forming a 
portion of a surface of revolution, such as might be utilized for sagging 
skirtless television panels. 
The mold of the present invention is formed from a refractory glass-ceramic 
having a relatively low coefficient of thermal expansion. Such materials 
provide for the formation of excellent glass-ceramic shell molds for 
sagging glass articles since such materials can be easily formed to 
complex shapes with smooth surfaces by heating sagging, and when once 
formed they are thermally stable and do not detrimentally change size or 
shape during repeated cycling, and finally because of their strength they 
can be made relatively thin in cross sectional areas which facilitates 
fast thermal cycling when sagging glass articles to a desired contour. 
In view of the fact that sagged glass articles such as windshields or TV 
panels become a part of a subassembly and must cooperate with other 
members in a predetermined manner, it is imperative that all of the 
glass-ceramic sagging molds for a given article be virtually identical, so 
that the sagged product from any such mold may be interchangeably utilized 
in the final assembly. Thus, according to the present invention a master 
mold is formed from which a plurality of glass-ceramic shell molds are 
formed having virtually identical contours for sagging articles into 
conformity therewith. 
It thus has been an object of the present invention to overcome the 
problems of thermal expansion, surface distortion and deterioration, and 
low strength to weight ratios encountered with glass sagging molds of the 
prior art, by means of easily formable smooth surfaced molds of low 
thermal expansion made of a crystalline glass-ceramic material. 
It is a further object of the invention to provide a process for repeatably 
producing glass-ceramic sagging molds with identical contours such that 
glass articles sagged thereon may be used interchangeably within 
subassemblies of which they become a part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In forming the sagging mold of the present invention, a sheet of glass 
material which is crystallizable into a crystalline glass-ceramic material 
is ground to a desired thickness and surface flatness while in its "green 
glass-ceramic" state and before being crystallized into a glass-ceramic 
material. That is, a shell mold blank is cut from a sheet of green glass 
ceramic material with a predetermined size which will be greater than the 
size of the glass article to be sagged thereon, but not larger than the 
master mold upon which the sagging shell mold is to be formed. While in 
its flat state, the blank is ground on both sides to a desired thickness 
and surface flatness, and the edges are ground with an edging wheel which 
gives a rounded edge with a nominal radius. 
The ground blank of green glass-ceramic material is then placed upon a 
master mold having a predetermined surface contour. The master mold and 
the green glass-ceramic sheet are positioned within a kiln and fired so as 
to sag the sheet into conformity with the predetermined contour of the 
master mold and to ceram (i.e. transform the material from a predominantly 
glassy state to a predominantly crystalline state through heat treatment) 
the sagged sheet into a crystalline glass-ceramic sagging or shell mold 
having a surface contour virtually identical with that of the master mold. 
The above-described process is schematically shown in the drawings, wherein 
a shell mold blank or sheet of green glass-ceramic material 20 is 
positioned upon a master mold 10 having a previously predetermined surface 
14 of the desired contour. The master mold 10 and the green glass-ceramic 
sheet 20 are positioned within a suitable kiln and subjected to a heat 
treatment cycle such that the shell mold blank 20 while in its green state 
is sagged to conform to the contour of surface 14 and then is cerammed 
into a rigid glass ceramic shell or sagging mold 30. The surface 34 of the 
shell mold 30 is identical with and conforms to the prescribed contour of 
surface 14 of master mold 10. As will be readily apparent, a plurality of 
shell molds or sagging molds 30 having identical contoured surfaces 34 may 
be formed from a single master mold 10 merely by repeating the sagging and 
ceramming cycle with consecutively presented shell mold blanks. 
It should be recognized that a key feature of the overall process is the 
ability to grind the required surface finish on the sheet while it is in 
its green state and while in the flat sheet form, and the ability to 
maintain such surface through the forming and ceramming operation. That 
is, the grinding of a flat surface, particularly when the material is in 
its glassy or green state, is relatively easy to perform and merely 
involves standard grinding equipment. 
The thickness of the sagging mold 30 may vary over a relatively wide range 
as desired, with the minimum thickness being determined by the necessary 
structural strength of the shell mold for supporting the article to be 
sagged, and the maximum thickness being limited by the amount of lost 
efficiencies which can be tolerated during the sagging cycle due to 
greater heat retention in the mold requiring longer time periods to 
dissipate the applied energy. Satisfactory results have been obtained 
utilizing molds having a uniform thickness of form about one-fourth to 
about one-half inch and flatness variations over a twelve inch span of not 
greater than about .+-.0.001 inch, with compositions similar to those 
shown in Table I of U.S. Pat. No. 3,582,371. 
When sagging and ceramming the green glass-ceramic blank 20, it is 
necessary to control the temperature cycle so that the sagging of the 
sheet to the conformity of surface 14 of master mold 10 is completed 
before sufficient nucleation and crystallization take place which result 
in the ridigification of the sheet. In other words, the ceramming 
temperature cycle is so controlled such that the temperature of the master 
mold and blank is elevated at a fairly rapid rate to effect sagging of the 
blank to the master mold prior to excessive nucleation. The cycle 
temperature then is again elevated to effect further nucleation, and 
finally the cycle is raised to form a rigid crystalline structure which is 
then cooled at a controlled rate to room temperature. For example when 
utilizing a sheet of Corning Code 0329 green glass-ceramic of a 
composition within the context of the disclosure of U.S. Pat. No. 
2,920,971, the following schedule was used: 100.degree. C./hour to 
600.degree. C., hold for 2 hours, 100.degree. C./hour to 725.degree. C., 
hold for 2 hours, 60.degree. C./hour to 1075.degree. C., hold for 1 hour, 
and allow the resulting sagged and cerammed mold to cool for approximately 
36 hours within the firing kiln. 
Although it will be understood that a variety of glass-ceramic compositions 
may be utilized to form the shell type sagging mold of the present 
invention, it is necessary for operability of the mold that it be 
thermally stable and highly refractory so as to withstand sagging cycles 
of up to at least about 730.degree. C without failing and that it have a 
relatively low coefficient of thermal expansion so that it will remain 
dimensionally stable during the heat-up and cool-down of repeated sagging 
cycles. A relatively low coefficient of thermal expansion of less than 
about 20 .times. 10.sup.-7 /.degree. C. such as shown in Table II of U.S. 
Pat. No. 3,582,371 is deemed sufficient to provide the necessary degree of 
dimensional stability. Generally, a highly crystalline glass-ceramic 
normally results in a relatively low coefficient of thermal expansion. 
Although not limited thereto, a preferred glass-ceramic composition is one 
comprising a beta-spodumene solid solution dispersed within a glassy 
matrix with a crystallinity of at least about 80%, such as those 
compositions disclosed in Table I of U.S. Pat. No. 3,582,371. 
The master mold may be formed either by a casting process or by machining a 
master mold blank, When it is necessary to provide the sagging mold with a 
rather complex curvature such as that incorporated within an automobile 
windshield, a suitable pattern representing the desired curvature but 
compensated for the shell mold thickness is made from a easily formable 
material such as wood. A female plaster mold may then be cast from the 
compensated pattern. Finally, the master mold may then be cast from the 
plaster cast female mold, which then represents the desired curvature and 
contour for forming the shell mold upon which the windshield blanks are to 
be sagged. The master mold may be formed of any suitable easily castable 
but durable and and stable material such as castable petalite or alumina 
materials. When, however, the desired surface contour is spherical, the 
master mold surface can be generated on a conventional grinder such as is 
used to generate lenses, and accordingly no pattern or special equipment 
is required to produce extremely accurate spherica surfaces. A suitable 
material which may be utilized to form such a machined master mold is a 
beta-spodumene containing low thermal expansion ceramic such as disclosed 
in U.S. Pat. No. 3,600,204 which has been layed up in a honeycomb fashion 
as disclosed in U.S. Pat. No. 3,112,184, and sold by Corning Glass Works 
under the trademark CERCOR.RTM.. Such material has good thermal stability 
up to about 1200.degree. C., has a low thermal mass, and machines easily 
to a surface which is satisfactory for making glass-ceramic shell molds. 
A blank of such material is ground to the desired shape and many identical 
shell molds may be produced from a single master mold without the need of 
subsequent finishing. 
As a specific example, but by no means limiting in nature, a shell mold 
blank was cut from an as-drawn sheet of green glass ceramic material 
having a thickness of approximately 0.3 inch and having a composition 
similar to that of Example 1 in Table I of U.S. Pat. No. 3,582,371. The 
blank was cut with a size larger than that of the glass article to be 
sagged, but smaller than the master mold. While in its green glass-ceramic 
state, and while flat, the blank was ground on both sides to a thickness 
of about 0.25 inch with both sides being substantially parallel. In 
addition, the edges were ground with an edging wheel to provide a rounded 
edge with a nominal radius of about 0.125 inch. The blank was placed upon 
a cast petalite master mold and the master mold and green glass-ceramic 
blank were run on a programmed temperature cycle in a kiln so as to 
preheat the mold and green glass-ceramic, sag the green glass-ceramic to 
the contour of the master mold, and ceram or crystallize the glass-ceramic 
material without changing the contour match with the master mold. A 
representative cycle for sagging and ceramming such material is to 
initially raise the temperature at a rate of about 300.degree. C./hour to 
between about 750.degree. C. and hold at such temperature for two hours, 
and then raise the temperature to about 1125.degree. C. and hold for two 
hours, and finally cool to room temperature with a hold of about 5 hours 
at 600.degree. C. 
Thus, the present invention not only sets forth a new and novel sagging 
mold for sagging glass articles, but also discloses a method for forming 
such molds. The utilization of shell molds as disclosed in the present 
invention provides the following advantages: 
1. The molds are lightweight, and accordingly overall handling is easier. 
2. Because the molds are lightweight, they have a low thermal mass, which 
reduces the heat requirement for use in the kiln. 
3. The molds are relatively inexpensive to make. 
4. The molds are durable in that they do not dent or scratch easily. 
5. The molds are stable over a wire range of temperature variations, in 
that they do not distort, oxidize, warp or spall off. 
Although the now preferred embodiments of the invention have been 
disclosed, it will be apparent to those skilled in the art that various 
changes and modifications may be made thereto without departing from the 
spirit and scope thereof as defined in the appended claims.