Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BAC~GROU~lD OF THE I~'VE~TIO~
Field of the Invention
This invention relates generally to the manufacture of
glass optical elements, and, in particular, to methods of
molding glass optical elements such as lenses and prisms.
Description of the Prior Art
In manufacturing glass elements, it is generally
necessary that the glass element meet certain criteria in order to
be suitable for its intended use. This is particularly true for
optical elements. For instance, in selecting a lens intended for
use in photographic apparatus where goo~ image-forming qualities
are necessary, the nature of the lens surface must be considered.
The characteristics of a surface which are important in this
regard are known in the art as surface quality and surface accura-
cy. Surface quality is related to the occurrence of defects such
as scratches, digs, pits, voids, "orange peel", etc. on the surface
of an element. An element is said to be of "high surface quality"if
the number of such defects is sufficiently low so that the element
is suitable for its intended use. For instance, in the cas~ of
a lens to be used in photographic apparatus, the number of such
defects must be low enough so that image forming qualities of
the lens are not impaired. It is understood that the number of
such defects which can be tolerated depends on the particular
element being considered and its intended use.
Surface accuracy, which is usually specified in terms
of the wave length of light of a specific color, refers to the
dimensional characteristics of the surface, i.e. the value and
uniformity of the radius of curvature of the surface. The surface
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accuracy is generally determined by an interferometric comparison
of a surface of the element with a test plate gauge, by counting
the number of Newton's rings, and by e~amining the regularity
of the rings. The surface accuracy of an element is often
referred to as its fit. The fit of an element is e~pressed
in terms of its power (the number o~ Newton's rings which
are counted) and its irregularity (the difference between
the number of rings when counted in perpendicular directions
across the fringe pattern). The lower the values of the
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' 10 power and irregularity, the better the lens, or in other words,
the higher its accuracy. Therefore, "high surface accuracy",
or l'precise fitll refers to a surface which has dimensional
characteristics that are extremely close to their design value
and are very uniform. For instance, the surface accuracy of
a lens to be used in a photographic apparatus is frequently
considered high when it has a power of less than si~ rings
` and an irregularity of less than three rings.
` The manufacture of glass optical elements has
conventionally required a series of co~ple~ and expensive
steps, including molding, grinding, and polishing operations.
For instance, in the conventionalmanufacture of a glass lens, a
rough molded glass lens blank is first made by heating a chunk
of glass to a softened state and pressing the glass to the
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desired shape in a metal mold. In some cases, the glass
may adhere to the molding surfaces. To prevent this
adherence, the mold temperature may be reduced below
the glass temperature during pressing. However, this
i technique may produce an irregular surface called "chill
~rinkle", when the hotter glass comes into contact with
the cooler mold surface. Another technique is to heat
the chunk of glass on a hearth plate prior to molding.
A thin layer of a parting agent may be used to prevent ;
the glass from sticking to the hearth plate. The
parting agent may become embedded in the rough molded
glass surface. When formed with these techniques, a rough
molded lens blank does not possess the high surface quality
and high surface accuracy necessary for an image-forming
lens. Hence, it is necessary to mold a lens blank which is
larger than the intended lens element to allow for the remov-
~of meterial during the subsequent griIding and polishing
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operations needed to render the lens suitable for use.
Spherical lens surfaces can be generated by rotating
the lens blank in a vacuum chuck and grinding the lens blank
with a rotating annular tool whose a~is is at an angle to the
chuck a~is. The tool has an abrasive surface in^luding diamond
chips. The geometry of this arrangement causes a sphere to be
generated wherein the radils is determined by the angle between
the a~es of the chuck and of the rota~ing generating tool, and
by the effective diameter of the tool. The thickness is governed
by the distance the work is advanced into the tool. The surface
of the lens blank may be refined further by grinding operations ;
performed with loose abrasive in a water slurry and cast iron
grinding tools.
After the grinding operation has been concluded, the
lens element can be polished by a process similar to the grinding
process. The polishing tool is lined with a layer of pitch and
the polishing compound is a slurry of water and rouge (iron-oxide) ~`
or cerium oxide. Polishing is continued until substantially all
` of the grinding pits and scratches are removed from the surface ;
1 20 oE the lens. T ~n, the lens shape is checked and corrections
are made to assure the proper shaping of the lens.
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Following the polishing operation, the lens is centered
by grinding the rim of the lens, so that its mechanical axis
(defined by the edge of the lens) coincides with the optical axis
(the line between the centers of curvature of the two lens
surfaces). Lens centering can be done either by known visual
methods which are very accurate or by more economical mechanical
methods.
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It is considerably more complicated to produce non-
spherical lens surfaces. The manufacture of precise aspheric
lens surfaces requires a combination of exacting measurement
and skilled hand correction. One method involves the difficult
operation of working a lens blank between centers on a lathe.
Aspheric lenses can be made in small production quantities, where
high precision is not req~aired, by means of a cam guided grinding
rig for generating the lens surface. Thereafter, the troublesome
operations of grinding and polishing the aspheric lens surface
are performed, the problem being that these operations can easily
destroy the basic shape of the lens. Where precise aspheric
surfaces are required, it is necessary to make grinding adjustments
manually with the concomitant requirements of great delicacy and
finesse, the shortcomings of which are apparent.
The expense of e~isting methods for fabricating glass -
lenses has led to the limited use of plastic lenses. Plastic has
several advantages as a lens material, namely, it is light,
shatterproof, and moldable. However, presently available plastics
which are practical for use as lens materials such as polystyrene,
polycyclohexyl methacrylate, and polymethyl methacrylate, are
relatively soft and scratch easily. Moreover, the latter plastic
tends to be frequently hazy and sometimes yellowish. Also, plastics
usually soften within the range of 60 to 80C. and their indices -~
of refraction may change in time. Most plastics absorb water and
are subject to dimensional change, the latter charactertistic
being due to their tendency to cold flow under pressure and to
their hlgh thermal expansion coefficient which is almost ten times
that of glass. In addition, the high thermal e~pansion of the
plastics causes changes in the indices of refraction of the
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plastics to an e~tent ten times that of glass~ thus severely
hampering the optical performance of the lens.
Thus, glass is clearly a more desirable lens material
than plastic, but plastic lenses are sonsiderably easier and
cheaper to manufacture than glass lenses because they can be
mass produced by molding. However, conventional molding methods
have not been found suita~le for clirectly making glass lenses that
; do not require further preparatory operations. One reason for this
is the tendency of heated glass to adhere to some materials and
~iO for the glass to remain a~hered to the materials after cooling.
Thus, one of the prerequisites for producing a suitable lens ~-
directly from a mold is that the glass being molded does not
` permanently adhere to the molding s~lrface. Non-adherence alone,
however, is not sufficient, because it has been found that glass
I will replicate the surface of materials to which the glass does
j not ahere. For e~ample, glass molded in metal dies has been .
found to reproduce the grain structure of the metal molding
surfaces on the surface of the glass, and such lenses are unsuit~
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able for optical uses without further operations to improve the
quality and accuracy of their surfaces. U.S. Patent 3,244,497
~ discloses the use of e~tremely t~in coatings of refractory
', materials (appro~imately ~.alf wavelength) to protect a mirror ~ ;
finish metal molding surface and act as a parting agent in a ~
1 glass molding structure for producing ophthalmic lenses. But, ~;
-~ even though surface characteristic tolerances for most ophthalmic
lenses are not as stringent as for many optical elements (e.g.
100 ring power may be acceptable), it is nevertheless still
necessary to per~orm additionel polishin~ operations on lens
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blanks molded in the molding structure disclosed in U.S.
3,244,497 in order to produce even an opthalmic lens suitable for
use. Thus, although it is apparent that in order to directly mold
glass lenses the mold surfaces must be oE high quality and high
accuracy and must not be adhered to by glass, it is equally clear
that meeting these requirements does not guarantee that the lens
` produced will not require further preparatory operations, such as
polishing, in order to be rendered suitable for use. The failure
of known molding methods to directly produce glass optical ele-
ments suitable for use by molding alone has necessitated continued
reliance on the time-consuming and expensive grinding and polish-
ing operations described above.
`~ Recently, glasslike carbon materials have been developed
which have found many applications in the electronics and metal-
lurgy fields. It has been discovered, as disclosed in U.S. Patent
No. 3,900,328, issued August l9, 1975 that these glasslike carbon
materials can be used as a molding surface in a mold cavity for
directly producing glass lenses which require no subsequent
- grinding and polishing operations, wherein a heat-softened glass
is placed in the mold cavity and pressed to~form a lens having
a shape generally determined by the shape of the mold cavity.
An improved method of molding glass lenses employing these glass-
like carbon materials is disclosed in U.S. Patent 3,833,347,
` issued September 3, 1974 wherein the portion of glass to be
` molded is heated while it is in proximity to or in contact
with the glasslike carbon molding surface. Another
improved method of molding glass into optical elements
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employing glasslike carbon molding surfaces is disclosed in
U.S. Patent 3,844,755, issued October 29, 1974, wherein optical
glass in a glasslike carbon transfer chamber is heat-softened
and subjected to pressure, thereby transferring the glass through
a sprue and into a mold cavity having molding surfaces of glass-
like carbon.
While the use of glasslike carbon represents a signifi-
cant breakthrough in the art of lens fabrication, glasslike carbon
possesses several properties which make it a less than ideal
molding-surface material. Glasslike carbon is subject to oxida-
tion, is structurally weak, is subject to surface scratching,
.
has a low modulus of elasticity, has low fracture and impact
strength, and has low thermal conductivity. All of these
characteristics are undesirable in a glass molding material~
and tend to limit the usefulness of glasslike carbon molding
surfaces. It would be desirable to find other mold materials
possessing the favorable glass molding properties of glasslike
carbon, but materials which would at the same time possess
improved structural and thermal properties.
S~MMARY OF THE I~,'ENTION
It has now been found that glass elements having high
surface quality and high surface accuracy, and therefore requiring
no further preparatory operations such as grinding or polishing,
can be prepared by molding glass against a molding surface formed
, of a material comprising a mi~ture of silicon carbide and
carbon. The moldin~ surface can be formed from a solid body
of the material or from a layer of the material on a substrate.
In either case, the material must be of sufficient thickness that
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the molding characteristics of the molding surface are exclusively
attributable to the mi~ture of silicon carbide and
carbon; preferably such a layer should be at least 10 microns
thick.
In one e~bodiment of the present invention, the molding
process comprises the steps of placln~ a portion of heat-
softened glass in a mold having molding surfaces formed from a
material such as described above, pressing the glass against the
` molding surfaces until the glass conforms to the shape of the
mold, cooling the glass and mold, and removing the glass element
from the mold. In another embodiment of the invention, the
molding process comprises the steps of placing a portion of
glass in a mold having molding surfaces formed of the above-
- mentioned material, heating the mold to soften the glass, pressing
the glass against the molding surfaces until the glass conforms
to the shape of the mold, cooling the glass and mold, and removing
~; the glass element from the mold. In yet a third embodiment of
the present invention, the molding process comprises the steps
of placing a portion of glass in a transfer chamber having walls
formed of the above-mentioned mateLial, heating the chamber to
soften the glass, applying pressure so that the heat~softened -
glass is transferred through sprues into mold cavities defined
by molding surfaces formed of the above-mentioned material until
the glass conforms to the shape of the mold cavities, cooling the
glass and molding surfaces, and removing the glass element from
the mold.
BRIEF DESCRIPTION OF THE DRAWINGS
In the preferred embodiments of the invention described
below, reference is made to ~he accompanying drawings, in which:
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~)8196~l
Fig. 1 is a partially cut away perspective view of a
molding apparatus for producing glass elements according to the
invention;
Fig. 2 is a detailed view of a mold employed in the
molding apparatus shown in Fig. l;
Figs. 3A and 3B are partial cross-sectional views,
taken through the line 3-3 in Fig. 2, during different stages
of the operation of the molding apparatus of Figure 1.
~ETAILED DESC~IPTION OF THE PREFERRED E~iBODIMENTS
The present invention provides a practicable method for
molding glass elements having high surface quality and high surface
accuracy, such as, for instance, glass lenses possessing good
image-forming quallties for use in photographic apparatus. The
' molding process employs a molding surface having high surface
quality and high surface accuracy formed of a material comprising
a mi~ture of silicon carbide and carh~n. Th~ mat.~rlal
in which the molding surface is formed should be of sufficient
thickness that the molding characteristics of the molding surface
are e~clusively attributable to the material in order to produce
20 a glass element having the desired high surface quality and high ~ -
surface accuracy. The molding surface can be formed in a solid -~
body of the material or from a layer of the material which is
deposited on a suitable substrate by methods to be described
below. In the case of a layer deposited on a substrate, it is
necessary that the layer be of sufficient thickness so that, even
... .
after removal of some of the layer during finishing, the remaining
` portion of the layer is still sufficiently thick that the surface
i characteris~ics of the molding surface are e~clusively attributable
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to the layer, with no interaction between the substrate and the
molded glass. Preferably, the layer should be at least 10 microns
thick.
The present invention makes use of a mi~ture of silicon
carbide and carbon as a molding surface material because it has
been found that a mi~ture of these components possesses properties
which make it an excellent materiaL for such use. Like the
glasslike carbons of the prior art, such a mixture e~hibits minimal
chemical activity to glass at high temperatures, is impermeable to
~10 gases, water vapor and liquids, and is not permanently adhered to
by heat-softened glass. In contrast to glasslike carbon materials,
`~ however, such a mixture a~so possesses, to an extent determined in
part by the relative proportions of the components, the additional
favorable characteristics of being more resistant to o~idation,
having improved fracture and impact strength at high temperatures,
having improved physical hardness (being therefore less subject to
scratching), and having higher thermal conductivity (allo-~ing for
more rapid thermal cycling).
The mixture of silicon carbide and carbon useful in the
practice of the invention can be prepared by chemical vapor deposi-
tion, either as a solid piece or ~s a layer on a suitable substrate.
A mixture of silicon carbide and carbon produced by this method
possess the advan~ageous properties of lacking internal voids and
` having the capability of being pollshed to a continuous, highly `~
-~ specular surface. Further description of one method for producing
such a mixture of silicon carbide and carbon may be found in Kaae
& Gulden, "Structure and Mechanical Properties of Codeposited
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Pyrolytic C-SiC Alloys," Journal of the American Ceramic Society,
pages 605-609, 54, No. 12, 1971.
Several techniques may be used to produce a molding
surface useful in the practice of the invention. If the material
is in a solid pi-ece, the piece can be ground and polished to
form a molding surface that is comparable to the shape and finish
desired in t'ne molded elements. A rough shape could have been
pre-formed in the solid piece during its formation, thereby
minimizing the grinding operation. A second technique involves
generating, grinding, and polishing the mold form directly from
a deposit of mixture of silicon carbide and carbon. The deposit
, .
; must be thick enough to provide a sufficient base for support and
mounting, allowing for material to be cut away in forming the
desired curve. Another technique involves forming a relatively
thin deposit of a mi~ture of silicon carbide and carbon on a
suitable precurved substrate, such as graphite, molybdenum, or
. .
hot pressed silicon carbide. However, the deposit of the mixture
of silicon carbide and carbon must be thick enough so that even
after grinding and polishing to the desired specifications, the
molding surface characteristics are e~clusively attributable -~
to the deposited layer. A fourth technique for forming a suitable
molding surface involves the use of an inverse mold form surface
as a base for deposit of a mi~ture of silicon carbide and carbon.
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In this technique, a parting or reLease agent is first applied
to the inverse mold form, followed by the deposition of a
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relatively thick layer of the mixture. The resulting mold
surface separates at the parting layer from the inverse mold
form and can be used with little or no surface corrections.
Again, the layer must be sufficiently thick so that the molding
characteristics are exclusively at~ributable to the deposited
layer of the mixture. The molds so produced are preferably
installed as inserts on sturdy sl~pport members in order to add
:
strength and durability to the molds.
The method of molding glass elements essentially ~ ~-
requires that heat-softened glass be pressed against a molding
; surface formed in a material comprising a mixture of silicon
carbide and carbon produced as described above until the glass
conforms to the shape of the molding surface; the glass is then
cooled to below its transformation temperature while still being
pressed against the molding surfa~e; and then the glass element
- is removed from the molding surface.
In one embodiment of the present invention, a method
of molding glass elements according to the invention could utilize
,
a molding apparatus such as that shown in Fig. 1, which apparatus
" 20 is particularly adapted to the molding of gLass lenses. The
apparatus comprises a stationary upper mold assembly 1 and a
-- lower mold assembly 3. Upper mold assembly 1 is fixed in an
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; upper mount 5 whereas lower mold assembly 3 is vertically movable
, through a circular aperture 7 provided in a base plate 9. Upper
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mount 5 and base plate 9 are connected by a set of bolts 11 which
extend through a set of tubes 13. A molding chamber is defined
~ by a borosilicate glass (such as Pyrex) envelope 15 which has a
,' cylindrical shape and which encloses the molding space located
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between the upper and lower mold assemblies. Alternatively, the
envelopecould be made of metal. A port 17 (which can be closed
by a valve which is not shown) is connected to the molding chamber
; defined by enveloFel5 for exhausting gas from the chamber; a
second port 19 is similarly connected to the molding chamber for
admitting a controlled atmosphere to the molding chamber; and a
third port 21 is also connected l;o the molding chamber and serves
as an exhaust for the controlled atmosphere, there being provided
a check valve (not shown) for regulating the exhaust to thereby
~10 control the pressure of the gas in the molding chamber. A bellows
23 is disposed beneath lower mold assembly 3, and its interior is
connected to the molding chamber defined by envelopel5. The
purpose of the bellows is to permit movement of lower mold
assembly 3 upwardly while maintaining atmosphere control in `
the molding chamber.
Referring specifically to Fig. 2, upper mold assembly 1
has disposed on the lower f~ce thereof a mold member comprising an
insert 25 which is formed of a material comprising a mixture of
silicon carbide and carbon and is configured to provide the
proper shape o~ the part of the lens to be produced thereby.
A similar mold member in the form of an insert 27 and made of the
same material as insert 25 is disposed opposite to insert 25 and
is mounted on the upper face of lower mold assembly 3. The
opposing surfaces of the two mold inserts 25 and 27 cooperate
to form the opposite faces of the lens to be produced by the mold.
Preferably, a ring 29, which may be though not necessarily is
made of the same material as inserts 25 and 27, is disposed
` around insert 27 for adding the necessary thickness to the lens
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to be reproduced Ring 29 may be eliminated for open-die
molding. The mold forming surfaces are thus made of a material
comprising a mixture of silicon carbide and carbon, they are
finished to have a high surface accuracy and shaped to produce
the intended lens, and they are polished to a high quality com-
parable to that of the intended lens. Lower mold assembly 3
is displaceable towards upper mold assembly 1, and its path is
accurately maintained by a pair of guide pins 31 which travel
in appropriate inserts 33 extending into lower mold assembly 3.
The pins and inserts can advantageously be made of aluminum oxide.
A heating coil 34 is wrapped around envelope 15 so as
to surround the molding area. When the coils are activated, ring
29 and inserts 25 and 27, as well as a pair of supports 43 and
45 on which the respective inserts 25 and 27 are mounted, are
heated by induction. Heat is transmitted by conduction from
supports 43 and 45, which act as heat reservoirs, to inserts 25
and 27. While heating coil 34 is shown outside of envelope 15,
this is done for ease of assembly. Coil 34 could be positioned
inside envelope 15 and could take any form known in the art that
will provide sufficient heat to permit molding of the optical
glass in the molding chamber. Supports 43 and 45 may be made of
graphite, molybdenum or other similar material.
-~ The mold temperature is controlled by a pair of thermo-
couples 35 and 37, which are connected to inserts 25 and 27 by
means of appropriate leads 39 and 41 extending through the two
mold assemblies, as shown in Figs. 3A and 3B, In order to confine
the génerated heat to the molding vicinityj a pair of pyrolytic
graphite insulators 47 and 49 are disposed on the ends of supports
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43 and 45, opposite the inserts 25 and 27. The pyrolytic graphite
conducts heat in the horizontal direction (with reference to the
drawings) and is non-conductive in the vertical direction. Lower
mold assembly 3 can be displaced by pneumatic, hydraulic, mechani-
cal or any other appropriate means (not shown) for producing
the desired movement.
The method of molding giass lenses to be described
below requires a mold having molding surfaces which are configured
and finished to yield lenses having prescribed shapes and high
surface qualtities. It is to be understood, therefore, that
inserts 25 and 27 are dimensioned and polished to the accuracy
and quality of the lenses to be manufactured. While the molds
: .
have preferably been illustrated as inserts on relatively sturdy
support members, the support members are present for heat transfer
and to add strength and durability to the molding apparatus.
Molding of glass elements such as optical lenses can also be
achieved in a molding apparatus which does not incl-ude supports.
It has been found that glass elements molded against
molding surfaces having high surface quality and high surface
accuracy formed in a material comprising a mixture of silicon
carbide and carbon of sufficient thickness as described herein-
before will also possess high surface quality and high surface
accuracy. A suitable molding surface can be prepared by grinding
.-..,
and polishing the material in the form of a layer or solid body
until it meets the surface tolerance limits established for
the final glass element. As indicated previously, the "high
~- surface accuracy" required of lenses of the quality used in
photographic apparatus should be within six Newton rings of power
and three rings of irregularity. Surface accuracy within these
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tolerances has been achieved in the walls defining the cavities
of the ~olds made of a material comprising a mixture of silicon -
carbide and carbon. Likewise, these same mold walls must have
"high surface quality" as is required of the finished lenses,
which renders the walls substantially devoid of scratches, digs,
pits and the like, and such surface quality has also been at
least partially achieved in the ~ractice of this invention.
During these grinding and pollshing operations, the mold cavities
; may also be configured to yield a glass element of a predetermined
shape with a molded mounting shoulder and the ridges and grooves
associated with mounting seats. The walls defining the cavities
are shaped much like the predetermined shape of the lens and lens
shoulder to be produced by the mold, but provisions may have to
be made for dimensional changes occurring in the molded glass due
to temperature changes and the like during and after the molding
process, Thus, the molded element will be ready for final assembly
without subsequent grinding and polishing.
According to a preferred method of molding optical
elements by this invention, a quantity of glass is placed within
~` 20 the molding chamber. It may be set in contact with insert 27,
: :
~` as shown in Fig. 3A or provision may be made to hold the glass
out of contact with inserts 25 and 27 until heated. The glass
.
must be shaped to fit within the molding chamber but need not be
preformed to a different volume and shape for different lens
designs, althou~h preforming is preferable. Preferably, the
surface of the glass slug is fired or machine polished to a
high quality, but this may not be necessary in all cases. The
mechanical means are then actuated to move lower mold assembly
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~08196~
3 upwardly to bring the glass,ring 29 and insert 27 into the
molding chamber and into proximity with insert 25. Bellows
23 will be compressed while maintaining a vacuum seal. The
atmosphere within the molding chamber may now be evacuated
through port 17, Simultaneously, heat is introduced into the
molding chamber by means of coils 34 to outgas the molding chamber,
the surfaces of ring 29 and inserts 25 and 27 and the glass.
The desired controlled atmosphere, preferably nitrogen. may
now be introduced into the molding chamber throu~h port 19, with
pressure control by port ~ eating coils 34 are again
actuated causing continued heating of supports 43 and 45, ring
29, inserts 25 and 27 and the glass in the molding chamber until
the desired molding temperature is reached. The temperature
of inserts 25 and 27 may be monitored by thermocouples as shown,
~ by optical pyrometry equipment or by other suitable means. When
;, the desired temperature has been attained and stablized, the
glass,ring 29 and inserts 25 and 27 will be at substantially
although not necessarily exactly the same temperature. A load
is now applied to lower mold assembly 3, bringing ring 29 into
contact with insert 25 and forming a molding cavity with inserts
25 and 27 and ring 29. After a ~uitable molding time, the ~ -
temperature of the mold will be reduced gradually to bring the
temperature below the glass transformation point while still
maintaining a load on the glass, to minimize distortion of glass
as the temperature is reduced. After the glass transformation
l temperature has been reached, the load may be removed from
- lower mold assembly 3.
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A specific example of the practice of the invention
using an apparatus as described above is as follows:
inserts 25 and 27 are formed by coating a substrate in
the form of a hollow graphite ball having a diameter of 0.~50
inches and a wall thickness of 2mm with a 0.5mm thick layer of
a material comprising a mixture oE silicon carbide and carbon,
the silicon carbide being approximately 11% by weight; cutting
the coated substrate in half, to produce inserts 25 and 27;
performing any necessary polishing to secure a specular surface
having high surface quality and high surface accuracy; and
mounting inserts 25 and 27 in the molding apparatus; : :
with lower mold assembly 3 in its downward position, ~ ~:
a portion of extra dense flint optical glass is placed on insert
27, after which the mechanical means are actuated to move lower ~ :
. mold assembly 3 in an upwardly direction so as to enter the ~ :~
:; molding chamber and move into proximity with upper mold assembly l;
the molding chamber i5 now evacuated by means of port
17 to approximately 200 microns and simultaneously, the surface
of the glass,ring 29 and inserts 25 and 27 are outgassed at a
temperature between about 400F. and 570F. by means of heating
coils 34;
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` port 17 is now closed and a controlled atmosphere of ~ ~:
nitrogen gas is introduced into the molding chamber through
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~ port 19, with a slight over pressure maintained by means of
` port 21;
~:. power to coils 34 is adjusted to raise the temperature
within the molding chamber until the glass is softened sufficiently .-
. to mold and the temperature is maintained for a period of about
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1.5 minutes; a temperature of about 1050 F. for extra dense
flint optical glass is sufficient, but the necessary temperature
will vary depending on the nature of the glass being molded;
; a load is now applied to lower mold assembly 3, thereby
pressing the heat-softened glass within the molding chamber
between ring 29 and inserts 25 and 27 as shown in Fig. 3B, the
load applied being approximately 130 pounds per square inch and :
being applied for about 10 seconds, with a higher load requiring
a shorter loading time;
`~ 10heating of the mold members is now terminated while a
. load is maintained on lower mold assembly 3 until a temperature
below the transformation temperature of the glass being molded
. is reached;
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.j the load on lower mold assembly 3 can now be removed
and the temperature in the mold chamber reduced further to about
. 400 F, thereby minimizing the possibility of oxidation of
;`~ ring 29, inserts 25 and 27 and supports 43 and 45; and
` the mold may now be opened by downward movement of
~ lower mold assembly 3 and the resulting molded glass element
`~ 20 may be removed from insert 27.
In another embodiment of the present invention adapted
~; to the production of glass lenses, a molding apparatus is employed :
which utilizes molding surfaces formed of a mixture of silicon
;` carbide and carbon as previously described, but which is otherwise
similar to the apparatus disclosed in U.S. Patent No. 3,900,328,
issued August 19, 1975. Using this apparatus, the method
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comprises placing a portion of glass which has been heat-softened
outside of -the mold cavity into a mold cavity defined by the
material comprising a mixture of silicon carbide and carbon;
compressing the glass until the glass conforms to the shape of
the mold cavity; cooling the glass and mold; and removing the
glass lens, which is suitable for use without further preparatory
operations.
In a third embodiment of the invention, a molding
. apparatus is employed which utilizes molding surfaces formed
.~ 10 of a material comprising a mixture of silicon carbide and carbon
as above-described, but which is otherwise similar to the apparatus
disclosed in U.S. Patent 3,844,755, issued October 29, 1974.
An example of using this apparatus would be the molding of a
:~ glass lens by a series of steps comprising: placing an unheated
portion of glass in a transfer chamber defined by molding surfaces
formed in a material comprising a mixture of silicon carbide and
. carbon; heating the mold members, transfer chamber and glass to
` soften the glass; forcing the heated glass from the transfer
chamber through sprues into mold cavities defined by molding
20 surfaces formed in a material comprising a mixture of silicon
i carbide and carbon until the glass conforms to the shape of the
mold cavity; cooling the glass and mold members; and removing
the glass lens from the mold.
The invention has been described in detail with particu-
, lar reference to preferred embodiments thereof but it will be
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understood that variations and modifications can be effected
within the spirit and scope of the invention. In particular,
it should be noted that the apparatus shown is only exemplary
of molding apparatus for operation of the present invention and
other types of molding apparatus in which different elements
may be moved to define a mold cavity may be utilized for practice
of this invention. Moreover, the specific example of a molding
method is also only exemplary of many molding methods which can
be utilized for the practice of this invention, with selection
of the molding parameters, such as cycle times, loads and
temperatures, being dependent upon many factors, including but
not limited to the type of glass being molded as well as the
prescribed design of the element which is to be molded. Finally,
it should be possible to select molding parameters which would
permit the molding of plastics into flnished optic~l elements.
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