A MIRROR BLANK AND A METHOD AND MEANS FOR THE PRODUCTION THEREOF

Abstract: Mirror blank, method of producing the same and mould for the production of the mirror blank. The mirror blank includes a front glass (7) and distinguishes itself by a support structure (14) of porous foam glass fused on to the back of the front glass, the surface layer (15) of the support structure showing a smooth surface. The method according to the invention involves that a composition of glass powder and expanding agent is allowed by heat treatment to expand in a closed space in one mould part (1) of the mould. By the expansion pressure the support structure becomes almost smooth in its surface layer (15) while the remaining part of the support structure is porous. The mould which is used for the production of the mirror blank includes two mould parts one of which has a forming surface (8) which is complementary to the form of the front glass and the other (2) of which has a recess (3) which is surrounded by a countersink (6) in which the front glass (7) is placed.

Description: A mirror blank and a method and means for the production thereof The present invention refers to a mirror blank, a method as well as means for the production thereof. The mirror blanks according to the invention may be utilized for producing mirrors of optical quality, by which is meant that what is dealt with is a precision mirror the form correctness of which is better than about 0,001 mm. The mirror blanks according to the invention may be utilized also in other reflective systems, such as for example sun mirrors for winning sun energy, mirrors for radar purposes etc. In my Swedish patent application No. 79.04584-5 and in my international patent application PCT/SE80/00151 mirror blanks are described which permit the production of.mirrors which are of optical quality and have a low weight. These ■ prior mirror blanks are produced by so called shell forming technique, which means that a glass plate is formed into a shell or a bowl the bottom surface of which forms the front glass of the finished mirror. The shell is filled with a support structure which imparts dimensional .stability to the mirror. The forming of the shell and the filling of the shell with the support structure may take place in a single step of treatment. The present invention represents a further development of ray earlier invention, as the shell forming step and large parts of the shell mould may be omitted without appreciably impairing the dimensional stability of the finished mirror. The present invention permits the production of a particul¬ arly light mirror blank with a porous support structure which adheres to the front glass of the mirror to be and which has a surface layer which has a higher density and less porosity than the rest of the support structure and which thereby also has a higher mechanical strength. To an eye the surface layer appears smooth but is to a certain degree porous. The finished mirror lends itself particul¬ arly for use in environments where it is not subjected, for example, to moisture or large mechanical strains but where the requirement of low weightis set high. The foamed surface " of the support structure has lower strength than a homogen- • eous glass surface. The invention will be described more in detail below in connection with the attached drawing, in which:- Figure 1 shows a cross-sectional view of a mould which is used in the manufacture of a mirror blank according to the present invention with a flat front glass. Figure 2 shows a cross-sectional view of a mirror blank produced in the mould according to Figure 1, and Figure 3 shows a cross-sectional view of a mould for the production of a mirror blank according to the present invention with a concave front glass. In Figure 1 there is shown in cross-section a mould which includes two mould parts 1, 2 of porous graphite or any other similar temperature resistant material, for example a ceramic material. Many ceramic materials have good affinity towards glass and then it is necessary to coat the mould with a release agent to reduce its affinity towards glass. In the upper mould part 1 a circular recess 3 is made. The circular recess is limited by a bottom surface 4 and a peripheral surface 5 inclined in relation to the bottom surface. A countersink 6 a - of circular ring-shape surrounds the peripheral surface 5. The countersink 6 is intended to receive a circular glass plate 7 which is to form the front glass of the finished mirror. The depth of the circular ring-shaped countersink 6 corresponds to the thickness of the plate 7. The lower mould part 2 has a forming surface 8 which when the mould ' parts 1 and 2 are placed against each other supports the plate 7 everywhere. By means of fixing members which are shown diagrammatically by the dot and dash lines 9 and 10 the mould parts 1 and 2 are pressed and fixed against each other so that a tight joint 11 is formed between them. A batch 12 of a composition of a powder of a normally solid material which is mouldable when heated and an expanding agent is placed in the recess 3 and lies on the top surface of the plate 7. Preferably, the batch 12 is first laid on the bottom surface 4 of the mould part 1 which at that time occupies a reversed position in relation to that shown in Figure 1, whereupon the plate 7 is laid in the countersink 6. Then the mould part 2 is placed over the whole assembly and the fixing members 9 and 10 are tightened whereupon the entire mould is turned to occupy the orientation shown in Figure 1 with which it is introduced into an oven where the whole assembly is heated to a high temperature such that the composition swells up and completely fills the space 3 and fuses to the inner surface of the plate 7. The expan¬ ding agent produces an expansion pressure which causes the surface layer of the swollen structure, i.e. the layer next to the bottom surface 4 and the peripheral surface 5, to be compressed. The heat treatment temperature and time are dep- endant on the material of the composition and of the plate and by experiments the time and temperature are determined which are required for the surface layer to form. In any case the heat treatment is to be carried to the softening temperature of the powder composition and it is obvious that the plate must be of a material which stands this temperat- ure. When the powder of the composition is a powder of win¬ dow glass or the like and when the expanding agent cont¬ ains a lithium compound the last entioned compound lowers the softening temperature of the entire powder composition so that it becomes lower than that of window glass powder alone and of the window glass plate. After the heat treatment step the whole assembly is sub¬ jected to a controlled cooling. After the cooling being terminated the fixing members and the mirror blank are removed from the mould. Figure 2 shows the mirror blank in cross-section after its removal from the mould in Figure 1. The mirror blank may, if desired, be trimmed along the dot and dash line 13. The structure 14 fused on to the front glass is porous and imparts the desirable dimensional stability to the mirror. The surface layer 15 of the porous structure 14 is smooth and may be compared to a skin or a film. The material of the plate 7 and the material of the composition which is - solid in normal condition but ouldable when heated are chosen so that the coefficients of expansion of the materi- ials are adapted to each other. Normally, the materials are chosen so that the coefficients of expansion are as like as possible. The material of the plate is preferably of glass but cera¬ mics and plastics may also be conceivable. Equivalent cond¬ itions hold for the material of the composition. If the material of the plate is window glass the material of the composition also preferably is window glass and as expanding agent 0-2 percent by weight (counted on the glass powder) of viscosity reducing Li-,C0 3 and 2-4 percents by weight (coun¬ ted on the glass powder) of CaC0 3 are used. If borosilicate glass is used for the plate and -che powder composition other expanding agents than those mentioned are used. Figure 3 shows a mould which is suitable for the produc¬ tion of concave mirrors. The top mould part 1 is built in the same way as the corresponding mould part in Figure 1 while the bottom mould part 2 has a convex forming surface 8 which is complementary to the shape of the desired con¬ cave mirror. At the production of the concave mirror blanks it is proceeded in the same way as has been described above in connection with Figure 1. If the curvature of the desired concave mirror blank is great the plate must be preformed which may take place by well-knovm glass modelling technique. The mould is made of the same material .as used in the mould according to Figure 1. Although the invention has been described above in connec¬ tion with the production of circular mirrors, also other exterior contours of the mirror may obviously be used, such as for example elliptical, square etc. Instead of the con- cave form described the surface of the mirror may be of con¬ vex or other non-spherical form. The mirror blank may also be provided with through holes, for example for the suspen¬ sion of the mirror or for other optical purposes. The hole or holes are formed by the forcing surface having an up- right pillar in the form, for exar.ple, of a tube on to .which the preformed front glass is threaded. The trimmed mirror blank is then provided with a re iectant layer on its front surface. An alternative procedure is first to form the support struc¬ ture only, for example in a graphire r-oulc, and then, by means of a glass with a low melting ter.perature, for exam¬ ple sealing glass or lead glass, zo fix the plate which is to be the reflecting surface of the mirror by fusing. If found desirable a back may be fixed by fusing at the BAD OR same time to increase the mechanical strength of the blank. The embodiments of the invention described above may be modified and variedin many ways within the scope of the basic idea of the invention.

Claims: Claims : 1. A support structure for mirror blanks, characterized in that it includes porous foam glass with a compressed sur¬ face layer which is integral with the rest of the support structure. 2. A support according to claim 1, characterized in that the foam glass is- made from a composition of a powder of a material which is solid in normal condition and mould- able in heated condition, and an expanding agent. 3. A support structure according to claims 1 and 2, characterized in that a front glass (7) is fused on to the support structure and that the front glass and the glass powder of the comp¬ osition is window glass. 4. A support structure according to claim 3, characterized in that the support structure extends across the whole surface of the front glass of the finished mirror. 5. A method of producing a support structure according to claim 1, characterized in that a batch (12) of a composition of a powder of a material which is solid in normal condition and mouldable in heated condition and an expanding agent is introduced into a mould (1, 2) of temperature resistant material, that the entire assembly is heat treated until the composition is caused to expand and entirely fill the mould cavity and the expanding pressure of the expanding agent causes causes the surface layer of the swollen structure to be compressed. 6. A method of producing a mirror blank by utilizing the method according to claim 5, characterized in that a front glass is fused on to the support structure. 7. A method according to claim 6, characterized in that the batch is placed in a recess (3) in a first mould part (1) , that the front glass is placed in a countersink (6) made around the recess, that a second mould part (2) is placed over the first one (1) so that the forming sur- face (8) of the second mould part (2) everywhere engages the front surface of the front glass, that the mould parts (1, 2) are fixed to each other to form a tight joint (11) between them, that the mould is turned and thereupon is subjected to the said heat treatment, whereupon ' the mould is subjected to a controlled cooling. ' 8. A mould for manufacturing a mirror blank (7) according to claim 7, said mould including two mould parts (1, 2) one (2) of which has a forming surface (S) the contour of of which is complementary to the contour of the front glass of the finished mirror and the other (1) of which has a recess (3) , characterized in that the mould parts are of porous graphite or of a similar temperature resistant material, for example ceramic mater¬ ial, and that in the other mould part (1) , around the recess (3) , there is made a countersink (6) the depth of which corresponds to the thickness of the front glass (7) and the width of which is greater than the dimensions of the front glass of the finished mirror.

Inventors: RENNERFELT GUSTAV B
Applicants: RENNERFELT GUSTAV B
Application year: 1981
Main classification: G02B1
IPC codes: C03B19, C03B23, C08J9, G02B5