Abstract:
The invention provides a method of producing reflectors from glass or glass ceramics comprising the steps of: molding a reflector being open to the outside and having a closed bottom in a mold at a temperature above the transformation temperature; placing the reflector in a recess of a holder; heating up the reflector in the area of its bottom; lifting the bottom of the reflector from below using a die; punching out at least one opening from the bottom by moving at least one plunger into at least one matching opening in the die.

Description:
RELATED APPLICATION 
   This application claims foreign priority to European patent application 07 001 445.1 filed Jan. 24, 2007, the subject matter of which is fully incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a method of producing reflectors from glass or glass ceramics wherein a reflector being open to the outside having a closed bottom is molded in a mold at a temperature above the transformation temperature and an opening is then punched out from the bottom. 
   A method of that kind is known from US 2004/0264200 A1. 
   A reflector consisting of glass is initially produced in this case by molding a gob in a mold whereafter its bottom area is heated up locally using a burner so that an opening can then be punched out from the bottom, in the softened state of the glass, using a ram and a die. The reflector is then fire-polished to make the surface sufficiently smooth. 
   Alternatively, the opening in the bottom area can be produced by drilling. In that case as well, a fire-polishing step is carried out subsequently in order to produce a smooth surface. 
   It has been found that while a smooth surface is guaranteed in the bottom area of reflectors that have been produced according to the known method, the tolerances regarding the opening produced frequently cannot be maintained without a secondary treatment. 
   SUMMARY OF THE INVENTION 
   In view of this it is a first object of the present invention to disclose a method of producing reflectors from glass or glass ceramics which permits openings to be produced in the bottom region with sufficient precision. 
   It is a second object of the invention to disclose a method of producing reflectors from glass or glass ceramics which permits openings to be produced in the bottom region without any subsequent treatment. 
   It is a third object of the invention to disclose a method of producing reflectors from glass or glass ceramics which permits openings to be produced in the bottom region, wherein the method is suitable for a mass production. 
   These and other objects are achieved according to the invention by a method for producing reflectors from glass or glass ceramics that comprises the following steps:
         molding a reflector being open to the outside and having a closed bottom in a mold at a temperature above transformation temperature;   placing the reflector in an opening of a holder;   heating up the reflector in the area of its bottom;   lifting the bottom of the reflector from below using a die;   punching out at least one opening from the bottom by moving at least one plunger into at least one matching opening in the die.       

   The object of the invention thereby is perfectly achieved. 
   By lifting the bottom from below using a die before the opening is punched out it is ensured that no gap can form between the bottom of the reflector and the die. 
   It is thus ensured that during the subsequent punching operation no glass filaments will be drawn, which otherwise could occur if small gaps were left between the reflector bottom and the surface of the die. This guarantees the precision of the opening to be punched. 
   According to an advantageous further development of the invention, the reflector is supported laterally on the die in the area of its bottom while the reflector is lifted by the die. 
   This permits the position of the opening to be punched out from the reflector to be controlled even more precisely. 
   According to a preferred further development of the invention the reflector is supported laterally on holding elements that project from the die in upward direction. 
   Having the holding elements connected with the die in this way ensures in an easy way that the reflector will automatically be supported laterally and centered as it is lifted by the die from below. 
   This guarantees especially correct positioning of the opening so punched out. 
   To the extent the reflector comprises a pot-shaped bottom, the latter preferably is held laterally, during the step of lifting the reflector by means of the die, by its contact with rod-shaped holding elements mounted on the die. 
   This guarantees easy and reliable support by the holding elements. 
   Preferably, the rod-shaped holding elements used for this purpose are carbide metal pins. 
   In this way, premature wear of the holding elements under the action of glass that has been heated beyond its softening point can be largely prevented. 
   In the step of lifting the reflector bottom by the die, the latter preferably is lifted by a small amount only, namely by an amount of between 0.1 and 3 mm, preferably between 0.1 and 2 mm, more preferably between 0.1 and 1 mm, most preferably between 0.1 und 0.5 mm. 
   Lifting the die by such a small amount only guarantees precise contact of the die with the reflector bottom and counteracts at the same time any instability of the reflector that may be produced by the lifting action. 
   According to a further embodiment of the invention, the reflector is lifted off the opening of the holder while it is lifted by the die, being merely supported on the die laterally, in the area of its bottom. 
   In this case, the position of the opening to be punched out from the reflector bottom is exclusively defined by the die and the holding elements connected with it so that especially high precision is obtained with respect to the position of the openings to be punched out from the reflector bottom. 
   According to a further embodiment of the invention, a die urges the reflector against an abutment, preferably in the form of a plate, during the lifting step. 
   It is possible in this way to bring the spacing between the upper end of the reflector and the reflector bottom precisely to a nominal dimension without any need for a secondary treatment, for example in the form of a grinding step. Possible tolerances are balanced out in this case in the bottom area by a minimum amount of lateral migration of the material; this is, however, not a disadvantage as larger tolerances are allowed in a direction transverse to the longitudinal axis of the reflector. Thus, the predefined tolerances are precisely adhered to in the longitudinal direction of the reflector in spite of a simplified production process. 
   Prior to punching out the opening from its bottom, the reflector is heated up preferably in the area of its bottom using a burner with a heating-up rate of at least 100 K/s, preferably of at least 200 K/s, more preferably of at least 300 K/s, to a temperature clearly above the transformation temperature T g  at which the material is sufficiently soft to permit punching of the opening(s). 
   Preferably, the reflector is heated up locally in the area of its bottom to a temperature of 700 to 1000° Celsius, preferably in the range of 800-1000° Celsius, more preferably in the range of 850 to 950° Celsius, using a burner. 
   Depending on the composition of the glass used or the glass ceramics used, an optimum punching process is thus rendered possible without any risk of damage to the surface of the glass or the glass ceramics or any risk of glass filaments forming during the punching operation. 
   Following the production of the reflector by molding, the reflector preferably is removed from the mold in hot condition, for example by means of a handling device, and is placed into the opening of a holder. 
   Thereafter, preferably a burner, being directed against the bottom of the reflector from below, is used for heating. 
   Preferably, a H 2 O 2  burner is used for heating. Using a burner of that type, very targeted heating of the bottom area of the reflector can be achieved within a minimum of time, which on the one hand reduces the processing time and on the other hand prevents the reflector as such from being heated in the area of its optically effective inner surface and, accordingly, from becoming distorted. 
   It is understood that the features of the invention mentioned above and those yet to be explained below can be used not only in the respective combination indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will become apparent from the detailed description of preferred embodiments given with reference to the drawings. In the drawings: 
       FIG. 1  shows a schematic sketch illustrating the production of a reflector from a gob by a molding operation; 
       FIG. 2  shows a reflector in an enlarged cross-sectioned view, placed in an opening of a holder, with an associated burner intended to locally heat the bottom area; 
       FIG. 3  shows a sectioned view of the reflector illustrated in  FIG. 2 , illustrating the next step in which the reflector is lifted from below using a die; 
       FIG. 4  shows a top view of the reflector illustrated in  FIG. 3 , after a central opening has been punched out from the bottom; and 
       FIG. 5  shows a top view of an alternate design of a die with associated holding elements, designed for punching out two openings, arranged one beside the other, from the bottom of a reflector. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The method according to the invention is suited for producing reflectors from glass or glass ceramics, especially such used for illumination purposes with high-energy light sources, for example for beamers or the like. As such reflectors are exposed to high thermal loading and precise tolerance control is simultaneously required, they lately have been produced from glasses or glass ceramics which in most of the cases have a thermal coefficient of expansion in the range of approximately 30-45×10 −7 /K and offer sufficient thermal resistance of up to 600° Celsius or more. After production, the inner surface of such reflectors is coated with a suitable material, such as aluminum, and can then be used for example as light-reflecting means for high-energy light sources. 
   The invention now discloses that opening(s) to be produced in the bottom of such a reflector are made by punching them out from the softened material, with special process control guaranteeing very precise tolerances and high surface quality. 
   According to the method of the invention, a reflector of this kind is initially produced from a gob by a conventional molding operation. In principle, all glass types that meet the specifications for such a reflector made from glass or glass ceramics are suited as starting material for the reflector. Generally, the transformation temperature T g  is at least 550° Celsius. 
     FIG. 1  shows a molding device for the production of reflectors, indicated generally by reference numeral  10 , comprising a circular disk  12  in which a plurality of cavities  14  is provided and which can be rotated about a rotary axis  20 , as indicated by arrow  22 . A compression mold  16  is provided with a matching ram  18 . The compression mold  16  can be moved toward the circular disc  12 , in the direction of arrow  24 , for producing from a gob placed in a cavity  14  a glass product of the desired shape of a reflector. By rotating the circular disc  12  it is thus possible to mold a series of reflectors in the different cavities  14  of the circular disc  12 , after a gob has been placed in each of the cavities  14  using an associated handling device. 
   The molding operation is carried out in the known way at a temperature clearly above transformation temperature in a range where the respective glass matrix has sufficiently softened. 
   While during the molding operation the shape of the reflector inside can be imaged with sufficient precision so that no complex reworking of the inner surface  32  will be needed, the openings required in the bottom area cannot, however, be produced with sufficient precision during the molding operation. 
   The reflectors are therefore removed in still hot condition, immediately after their production, using a suitable handling device and are placed in a suitable holder  38 , as shown in  FIG. 2 . The holder  38  consists of a plate  38  which is held in a supporting table  36  and is provided with an opening  40  adapted to the contour of the outer surface of the reflector  30 . In a second process step, a reflector  30  placed in the holder  38  in still hot condition is then purposefully heated from below in the area of its pot-shaped bottom  34 , using a H 2 O 2  burner  42 . The burner  42  used is one with very high burner capacity and permits the reflector to be heated up again within a very short time from a temperature in the range of approximately 500° Celsius to a red heat range, i.e. to a temperature in the order of approximately 900° Celsius, in less than one second, for example in 0.8 seconds. 
   Thereafter, the supporting table  36  is moved on, and the at least one opening in the bottom  34  of the reflector  30  is produced by a punching operation in a next step explained in more detail in  FIG. 3 . To this end, a die  44  is urged against the pot-shaped bottom  34  from below so that the reflector is lifted by a small amount of, as a rule, between 0.1 and 1 mm. 
   During that operation, a plate  60  may be used as contact surface for the reflector  30  on the outside of the reflector  30 , as shown in  FIG. 3 . As the reflector  30 , being lifted by the die  44 , tends to be lifted off its support in the opening  40  of the holder  38 , precise positioning of the reflector  30  is to be achieved by holding elements  46  that support the pot-shaped bottom  34  of the reflector  30  over an area  56 , preferably over approximately one third of the bottom  34 . 
   These holding elements  46 , being preferably designed as carbide metal pins and being directly mounted on the die  44  via supporting arms  48 , serve to laterally support and to precisely position the reflector  30  as it is lifted by the die  44 . 
   This guarantees precise positioning of the opening(s) to be punched out during a subsequent punching operation. 
   The die  44  is provided with one or more opening(s), corresponding to the opening(s) in the reflector bottom to be punched out, and a punching section  54  of one or more associated rams  52  can be run down into each of such openings for punching out the opening or the openings from the bottom  34  of the reflector  30 . 
   The punching operation is effected using a die  44  of carbide metal having a sharp-edged punching section  54 . A certain play of a few tenths of a millimeter is used between the punching section  54  and the associated opening  50  of the die  44 . The contact between the upper end of the reflector  30  and the plate  60  as the die  44  is lifted from below guarantees that a precise dimension is maintained between the upper end of the reflector  30  and the lower end of the reflector bottom  34 , as the material, having been softened before, will yield laterally to balance out any deviation. At the same time, this arrangement contributes toward exactly positioning the reflector  30 . Further, pressing the die  44  against the reflector bottom  34  from below ensures that no glass threads will be formed during the punching operation. 
   If necessary, the die  44  and/or the ram  52  may be cooled in addition. 
   In the arrangement illustrated in  FIG. 3 , holding elements  46  are arranged one opposite the other so that a total of four holding elements are provided. 
   Alternatively, it is also possible, for example, to use three holding elements  46   a  arranged in triangular configuration, as shown in  FIG. 5 .  FIG. 5  shows a top view of a correspondingly designed die  44   a  in which three carbide metal pins serving as holding elements  46   a  are arranged in triangular configuration. Contrary to the die  44  shown in  FIG. 3 , the die  44   a  comprises two openings  50   a  so that two openings arranged one beside the other can be punched out from the bottom  34  of the reflector  30  in one punching operation. 
     FIG. 4  shows a top view of the reflector  30  after one opening  50  has been punched out from the bottom  34 . 
   It is understood that the form of the reflector  30  shown in the drawing is given by way of example only and that the method according to the invention can be used irrespective of the form of the respective reflector  30 . 
   Further, the method according to the invention is largely independent of the type of material used for the glass or glass ceramics, provided the operation is carried out in a suitable temperature and/or viscosity range.