Glass manufacturing – Processes – Reshaping or surface deformation of glass preform
Reexamination Certificate
1999-07-15
2001-10-23
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
Reshaping or surface deformation of glass preform
C065S102000, C264S001100, C264S002500, C425S357000
Reexamination Certificate
active
06305194
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the compression molding of glass lenses and, more particularly, to methods and apparatus for molding arrays of lenses and microlenses.
BACKGROUND OF THE INVENTION
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Engle et al, U.S. Pat. No. 4,139,677 to Blair et al, and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of mold inserts used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the mold inserts included glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity with the mold being formed out of one of the above mentioned materials. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the viscosity of the preform into the range from about 10
10
P to about 10
6
P. Pressure is then applied to force the preform to conform to the shape of the mold cavity. The mold and preform are then allowed to cool below the glass transition temperature of the glass. The pressure on the mold is then relieved and the temperature is lowered further so that the finished molded lens can be removed from the mold.
With regard to the compression molding of near-net-shape glass optical elements it is well known that a glass preform with a precision polished surface must be pressed between the upper and lower halves of a mold. If a double positive lens (convex-convex lens) is to be molded, for example, a spherical or oblate spheroid glass preform of the proper volume is placed between the mold halves, heated until the glass has a viscosity in the range of 10
6
-10
10
Poise, and is compressed until the mold is closed then preferably cooled to a temperature below the annealing point and demolded. In such an arrangement, as shown in
FIG. 1
, the upper and lower mold halves
10
,
12
compress a spherical glass preform
14
therebetween. The radius of the spherical glass preform
14
must be less than the radius of both of the concave mold surfaces
16
,
18
. As the glass preform
14
is compressed, the glass flows generally radially outwardly from the center of the mold cavity thereby expelling any gas from the mold cavity. This results in the production of a double convex lens
20
free from distortion due to trapped gas. Such molded lenses typically have accurate and repeatable surface replication relative to the mold.
Depending on the final shape of the lens to be formed, specially shaped preforms are sometimes required to ensure that the glass flows from the center of the mold cavity to the edge as shown in
FIGS. 2-4
.
FIG. 2
schematically depicts a prior art arrangement wherein the upper mold half
22
includes a plano mold surface
24
and the lower mold half
26
includes a concave mold surface
28
. In such an arrangement, a spherical preform
30
just as with the arrangement depicted in
FIG. 1
, but in this instance to produce a piano-convex optical element
32
. However, looking at
FIG. 3
there is schematically depicted a prior art arrangement wherein the upper mold half
34
includes a convex mold surface
36
and the lower mold half
38
includes a concave mold surface
40
. In such an arrangement, it is preferred to use a plano-convex preform
42
to produce a concave-convex optical element
44
. The radius of the convex surface of preform
42
must be less than the radius of concave mold surface
40
. This ensures first contact between mold surface
40
and preform
42
substantially at the centerline of the mold apparatus thereby causing the preform to flow generally radially outwardly to prevent the trapping of gases. Similarly, the first contact between convex mold surface
36
and the piano surface of preform
42
is substantially at the centerline of the mold apparatus thereby also causing the preform
42
to flow generally radially outwardly to prevent the trapping of gases.
FIG. 4
schematically depicts a prior art arrangement wherein the upper mold half
46
includes a convex mold surface
48
and the lower mold half
50
includes a convex mold surface
52
. In such an arrangement, it is preferred to use a plano-plano preform
54
to produce a double concave optical element
56
. The plano-plano preform
54
ensures first contact between the mold surfaces
48
,
52
and preform
54
substantially at the centerline of the mold apparatus thereby causing the preform to flow generally radially outwardly to prevent the trapping of gases. Examples of such practices are cited in U.S. Pat. Nos. 5,662,951 and 4,797,144. The method outlined in these patents works well for single cavity molds where one lens is molded from one preform. When molding an array of lenses or microlenses from one preform, the above approach will trap gas causing surface distortion of the lenses. U.S. Pat. No. 5,276,538 indicates that an array of microlenses may be fabricated by pressing a plano preform between an upper plano mold surface and a lower mold surface with concave microlens cavities. This approach, however, will cause surface figure distortion of the microlens features due to trapped gas. Another method of forming an array of microlens is taught in U.S. Pat. No. 5,276,538 where micro-sized spherical preforms are placed in a plurality of cavities of the lower mold and many microlenses arc molded simultaneously. However, due to the expensive fabrication costs of the spherical performs and the production time required to place many microspheres onto a mold, this method would be cost prohibitive.
SUMMARY OF THE INVENTION
It is, therefore, the object of the present invention to provide a method and apparatus for compression molding an array of integrally formed glass lenses using a single preform.
It is a further object of the present invention to provide a method and apparatus for compression molding an array of integrally formed glass lenses which obviates surface figure distortion of the lenses in the array.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and drawing set forth herein. These features, objects and advantages are accomplished by fabricating a first mold half; fabricating a second mold half with a central nest and a plurality of predetermined negative optical surface features therein; placing a glass preform in the central nest; heating the first and second mold halves and the glass preform to at least the glass transition temperature of the glass preform; pressing the glass preform between the first and second mold halves to thereby form an integral array of optical elements, each of the optical elements being a positive of the predetermined negative optical surface features; cooling the integral array of optical elements to below the glass transition temperature; and removing the integral array of optical elements from the first and second mold halves. The apparatus used for performing the method of the present invention comprises an upper mold half that is either piano or has microlens features; a lower mold half with microlens cavities and a central nest or depression for holding and aligning a cylindrical, spherical or oblate spheroid glass preform; and a means for heating the upper and lower mold halves and the preform. In the practice of the present invention i
Bourdage Phillip D.
Budinski Michael K.
Ludington Paul D.
McLaughlin Paul O.
Nelson Jayson J.
Bocchetti Mark G.
Eastman Kodak Company
Vincent Sean
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