Glass manufacturing – Press molding machine
Patent
1988-01-06
1989-12-26
Kellogg, Arthur
Glass manufacturing
Press molding machine
653741, 6537413, 106 389, C03B 1108
Patent
active
048895482
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the use of inorganic, nonmetallic, oxide ceramic materials as compression molds for the production of elements made from glass and/or a glass-containing ceramic and having high quality and dimensional accuracy. As a consequence of their shape accuracy and surface quality, the optical elements thus produced need not be subjected to further, expensive processing, such as grinding, polishing etc.
PCT/DE 84/00,128 has already disclosed compression molds for optical elements which comprise monocrystalline, nonmetallic, inorganic materials, such as, for example, NiO, Cr.sub.2 O.sub.3, sapphire or spinell. Although these materials are suitable, in particular, for use at high temperatures with respect to their contact inertness, considerable difficulties arise during the production of the compression molds and during the pressing processes themselves carried out therewith as a consequence of the anisotropic character of the materials and as a consequence of their strength behavior.
Monocrystalline substances, ie. so-called "monocrystals", exhibit anisotropy in their physical, chemical, mechanical and thermal properties; ie. the properties mentioned here change as a function of the crystallographic orientation present in each case.
Thus, for example, the hardness, and thus the abrasion resistance, of monocrystals varies depending on the orientation. This state of affairs alone leads to various erosion rates during polishing, so that the precision of shaping, which is in the nanometer region in the case of such molds, can only be achieved with difficulty.
Since the mold must be heated to high temperatures during the actual pressing process, a material expansion caused by the heat takes place. In the case of monocrystalline materials, this is again dependent on the crystallographic orientation of the pertinent monocrystal. This means that the geometry of the compression mold which is present at room temperature changes in a fashion such that the optical elements to be shaped do not have the desired or necessary optical precision fit. This has the consequence that complicated mathematical computational operations for retrocorrection of the mold become necessary. A further disadvantage of monocrystalline materials is that the production of a spatial shape, for example a dish shape, can only be carried out if the crystal has previously been aligned precisely with respect to its crystal axes using complicated light or x-ray optical methods and clamped.
In addition to the heat-expansion anisotropy to be found in noncubic monocrystals, anisotropy of the elastic constants, which, like the hardness anisotropy discussed above and the thermal conductivity anisotropy discussed below, also applies to maximum-symmetry cubic monocrystals, presents difficulties, since the pressing process, logically, proceeds under pressure and crystal orientation-dependent elastic expansion or contraction must also be taken into account.
Anisotropy of the therml conductivity has a negative effect in that local overheating occurs both in the compression mold and in the pressed part itself, thus causing thermally induced mechanical stresses and buckling.
With respect to the strength behavior, monocrystals prove to be extremely brittle. The so-called critical stress intensity factor K.sub.ic can be regarded as a measure of the brittleness, where stress, and
For example, this value for monocrystalline aluminum oxide is merely:
For the use of monocrystal molds, this principally means an increased danger of fracture under mechanical and thermal load.
The object of the present invention is, therefore, to specify materials for compression molds for optical elements which materials do not have the disadvantages of the materials which are known for the applications mentioned and which permit substantially longer use as compression molds while retaining their respective geometrical spatial shapes under extreme physicochemical, thermal and mechanical process conditions.
This object is achieved according to the invention in that the ox
REFERENCES:
patent: 3728098 (1973-04-01), Giffen
patent: 4168961 (1979-09-01), Blair
Berichte Der Deutschen Keramischen Gesellschaft, vol. 44, Nr. 11, 1978; pp. 487-491.
"Nature", vol. 258, Dec. 25, 1975; pp. 703 and 704.
"Journal of the American Ceramic Society"; vol. 68, No. 1, Jan. 1985; C4-C5.
Bluethgen Waldemar
Franek Henning
Kriegesmann Jochen
Ernst Leitz Wetzlar GmbH
Kellogg Arthur
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