A MIRROR BLANK

Abstract: A mirror blank including a shell (1) of glass or ceramics, the shape of which substantially corresponds to that of the finished mirror, and a support structure which forms a three-dimensional network which is fastened to the shell and fills the latter. The novelty of the invention consists in the support structure including fibres (3) of a material the coefficient of thermal expansion of which is adapted to that of the shell and a binder which firmly connects the fibres with each other and with the shell. By using fibres the coefficient of thermal expansion of which is greater than that of the shell the mirror blank may have an initial stress imparted to it.

Description: A mirror blank The present invention refers to a mirror blank, and more particularly to a mirror blank of the type which includes 'a shell the shape of which substantially corresponds to the shape of the finished mirror and a support structure which is fastened to the shell ' and which fills the latter. A mirror blank of this type is known from my Swedish patent application No. 79.04584-5 which is referred to for a more thorough understanding of the shell and the way in which this is formed. The support structure of the prior mirror blank consists of micro spheres which are caused to adhere to each other and to the shell. In my internation¬ al application PCT/SE80/00151 a support structure is described including foam glass which, if desired, may be mixed with micro spheres. The present invention refers to a new material for the support structure, more precisely a fibre material the coefficient of thermal expans-ion of which is adapted to that of the shell. The support struct¬ ure forms a three-dimensional network of fibre and binder which fixedly connects the fibres with each other and with the shell so that the entire support structure is fastened • to the shell. According to a preferred embodiment of the invention the filled shell shape is covered with a flat or convex plate of glass or a mat of fibres . Particularly in ccnnection with the use of the finished mirror as a sun collector a silver-plated thin glass-plate is glued on to the concave or flat surface thus obtained. For more ordinary optical applications the plate of glass covering the support struct- ure may be ground, polished and have a reflecting coating applied to it directly. By reinforcing the support structure with long, continuous glass fibres orientated in the direction along which the mirror is subjected to stresses the dimensional stability of the finished mirroris increased. By choosing the long, continuous glass-fibres of a material the coefficient of thermal expansion of which is greater than that of the shell, then after heat treatment of the fibre material and the binder of the shell and after a controlled cooling a contraction strain in the said long, continuous glass fibres and thereby a compressive stress in the shell is obtained, said compressive stress being direct¬ ed in the direction of the long, continuous fibres. Thereby it is possible to increase the bending strength of the finished mirror in the direction of the stresses. Not only the long fibres but all fibre material of the shell may have said greater coefficient of thermal expansion. Where long, continuous glass fibres are used as reinforcem- ent the support structure may consist, instead of fibre material, of micro spheres coated with a dust of sealing glass, lead glass or ceramics, or else the support struct¬ ure may consist of foam glass or of mixtures of micro spheres and foam glass. The invention will be described more in detail below with reference to the attached drawing which shows a cross-sec¬ tional view of a mould for the production of a mirror blank according to the present invention, and of a mirror blank according to the invention. A shell 1 of a normally solid material mouldable in heat, for example glass or ceramics, has been produced in a master die 2 in the way described in either of the patent applic- ations mentioned above. It may suffice here to mention that . O ?l the shell 1 is produced from a flat plate which b ' y heat treatment and with the use of vacuum or nitrogen has been formed under overpressure against the master die 2. After controlled cooling the finished shell 1 is obtained which, according to the invention, is filled with fibres of a material the coefficient of thermal expansion of which is adapted to that of the shell. Preferably, the coefficients of the thermal expansion of the fibre material and the shell are chosen to be as equal as possible. In the drawing the fibres are designated by the reference numeral 3, and they may either be short and have an unarranged orientation in the shell 1 or else they may be present in the form of fibre mats which fill the shell 1 in several layers. The fibres 3 are of glass, polymer material or any other material which has the suitable coefficient of thermal expansion. A binder not shown in the drawing firmly connects the fibres with each other to form a firm stiff three-dimensional net- work. If the fibres 3 are of glass the binder is a dust, i.e. very small particles, of glass or ceramics the melting temperature of which is lower than that of the fibres. The fibres are provided with- coating of this dust before they are introduced into the shell 1. The fibres thus dusted are allowed to fill the shell mould 1 with good measure, where¬ upon they are compressed by placing a mould part 4 having a moulding surface 5 which may be convex or flat over and in contact with the master die 2. After the compression or simultaneously therewith the fibres are subjected to a heat treatment up to the melting temperature of the dust serving as binder which then melts and wets the fibres as well as the inside surface of the shell mould 1. The whole assembly is then subjected to a controlled cooling, a three-dimensio¬ nal network being thereby formed the knots of which consist of the contact point between two fibres crossing each other. The support structure produced in this way imparts dimen¬ sional stability, i.e. stiffness, to the shell mould. If the mirror blank is to be used for sun collectors a sil¬ ver-plated thin glass plate not shown is thereupon glued on to the concave or flat surface of the support structure. Instead of compressing the fibres 3 with the moulding sur¬ face 5 of the mould part 4 a thin plate 6 may be placed between the fibres and the moulding surface 5 in which case the plate compresses the fibres 3. If desired, the plate 6 may be releasably fixed to the moulding surface, for example by vacuum which is applied through channels not shown in the mould part 4. The thin plate 6 is preferably of the same material as the shell 1 and the plate is either flat or preformed to the desired curved form. The compression may be carried out before or simultaneously with the heat treat¬ ment of the dusted fibres. The thin glass plate fused-on in this way may be ground, polished and have a reflecting coating applied to it for simple optical applications of the mirror. ' The fibres may be provided v/ith the said coating with ceram¬ ics or glass -powder dust either by being mixed together with the dust in a drum, so called tumbling, or by emulsifying the fibres in a liquid together v/ith the powder dust, where¬ upon the liquid is removed. Instead of filling the shell mould 1 v/ith fibres v/hich are orientated at random in the shell mould the fibres 3 may consist of mats of glass fibres, said mats being provided with a coating of the said dust. The shell mould filled in this way with several layers of mats is heat treated to the melting temperature of the glass dust v/hich then melts and wets the fibres of the mats. After a controlled cooling the said three-dimensional network is obtained, which may be regarded as a closed framework construction which imparts optimum dimensional stability to the shell 1. Also a mixture of mats of glass fibre and unarranged short- er fibres may occur, it being presupposed that the mats and the fibres, respectively, are provided with said dust coating. Where fibre mats or fibre mats mixed with shorter fibres, respectively, are used it is suitable that the uppermost material layer of the shell mould consists of a fibre mat on to which the thin plate 6 is either glued or fastened by melting during the heat treatment. Instead of using dust of glass or ceramics with low melting temperature as a binder, glue may be used, for example UV-hardening glue, which is allowed to wet the fibres and the fibre mats, respectively. In case the finished mirrors are to be used in an environ¬ ment where they are subjected to bending stresses it is suitable to reinforce the support structure with long fibres which are placed in close connection with at least one of the two principal surfaces of the finished mirror. The long fibres are orientated such that their direction agrees with the direction in which the stresses act. These long fibres are preferably of the same material as the rest of the fibre material of the support structure. The long fibres may either be provided with a coating of said dust of glass or ceramics powder or be wetted with said glue, and they are baked into the support structure together with the other fibres of the support structure in connection with the three- -dimensional network being produced. To increase still more the capacity of the finished mirror to resist stresses, more particularly bending stresses, for example by wind load in case the finished mirror is a sun collector, the said long fibres to v/hich the said orienta¬ tion is imparted consist of a material the coefficient of thermal expansion of which is greater than that of the shell. These long fibres are placed near the principal surfaces of the shell., i.e. the external surfaces, v/hcre owing to the wind forces the stress is greatest. After the heat treatment of the long fibres and of the. other fibre mater¬ ial of the shell, the whole assembly is subjected to a controlled cooling, a contraction strain being then created to a particularly marked degree in the long fibres since their coefficient of thermal expansion is greater than that of the shell. In other words, the long fibres shrink and thereby cause a tensile stress in the fibre and a corresp¬ onding compressive stress in the shell along its principal surfaces. Thus, each principal surface is compressed in the direction in which the long fibres are orientated. If now the mirror prestressed in this way is subjected to a deflec¬ tion the shell of the mirror will not experience any detrimental tensile stress until the deflection becomes so great that the compressive stress inherent in the shell of the mirror has been overcome. By orientating the long fibres the coefficient of thermal expansion of which is greater than that of the shell the finished mirror is prestressed in each of these directions. In case long glass fibres v/ith the said great coefficient of thermal expansion are used for prestressing the shell the rest of the support structure may, instead of fibres or glass fibre mats or glass fibre felts, consist of micro spheres of the type mentioned in the said patent applic¬ ations which have been provided in the abovementioned v/ay with a coating of dust of glass the melting temperature of which is lower than that of the shell 1, or of foam glass produced in the way described in the said patent applic- ations . The embodiments of the invention described above may be modified and varied in many ways within the scope of the basic idea of the invention.

Claims: Claims 1. A mirror blank including an open shell (1) of a normally solid materialirtoulσable in heat, for example glass or ceramics, the shape of v/hich substantially corresponds to that of the finished mirror, and a support structure which is fastened to the shell and fills the latter, characterized in that the support structure includes fibres (3) of a material the coefficient of thermal expansion of which is adapted to that of the shell and a binder which firmly connects the fibres with each other to form a three-dim¬ ensional network the knots of v/hich consist of the points of contact between two fibres crossing each other, said binder also connecting the support structure with the shell. 2. A mirror blank according to claim 1, characterized in that the free support structure is v/orked to the desired form and has a reflecting coating applied to it. 3. A mirror blank according to claim 1, characterized in that the free support structure is covered with a plate of glass or ceramics which is attached to the support structure. 4. A mirror blank according to claim 3, characterized in that the plate has a reflective coating applied ro it. 5. A mirror blank according to claim 3, characterized in that an additional plate of glass or ceramics having a reflective coating applied to it is attached to the said plate. 6. A mirror blank according to claim 1, characterized in that the fibres are glass fibres and that the three-dimen¬ sional network is produced from a dust of glass which ' serves as a binder and which is" present as a coating on the fibres and by heat treatment of the fibres (3) contained in the shell (1) , if desired carried out in combination with a compression of the fibres (3) , is caused to melt to wet the fibres and the shell, whereupon the fibres and the shell are subjected to a controlled cooling for stabilizing the said three-dimensional netv/ork. 7. A mirror blank according to claim 1, characterized in that the fibres are glass fibres or fibres of a polymer material and that the binder is a glue. 8. A mirror blank according to claim 1, characterized in that the fibres are present in the form of felts or mats of either polymer material or glass, the binder in the first case being a glue and in the last case being in the form of a coating on the mats or the felts, respectively, of dust of a glass or ceramics the melting temperature of v/hich is lower than that of the other fibre materials of the support structure. 9. A mirror blank according to claim 1, characterized in that the fibres are cut into short lengths. 10. A mirror blank according to claim 1, that the support structure is reinforced by long fibres orientating in the direction or directions along which the finished mirror is subjected to stresses. 11. A mirror blank according to claim 10, characterized in that the said long fibres which reinforce the support structure are of a material the coefficient of thermal expansion of which is greater than that of the shell. 12. A mirror blank according to claim 11, characterized in that the support structure, in addition to said long fibres of a material the coefficient of thermal expansion of which is greater than that of the shell, exclusively includes fibres with the said greater coefficient of thermal expansion. 13. A mirror blank according to claim ' 10, characterized in that the said long fibres reinforce a support structure of micro spheres or of foam glass or of mixtures thereof. 14. A mirror blank according to any of claim 11 to 13, characterized in that the reinforced fibres are melted firmly in place by means of sealing glass or lead glass.

Inventors: RENNERFELT GUSTAV B
Applicants: RENNERFELT GUSTAV B
Application year: 1981
Main classification: G02B1
IPC codes: C03C14, C03C17, C03C27, G02B5