Abstract:
A method for fabricating a mold tool for molding optical elements is taught which comprises heating a mold tool blank made from a vitreous material to a temperature above the glass transition temperature of the vitreous material; generating an axial viscosity gradient in the mold tool blank; pressing a punch into an optical quality mold surface of the mold tool blank, the punch including a pressing surface with a predetermined geometry for forming an optical feature; cooling the mold tool blank to a temperature below the glass transition temperature of the material; and removing the punch from the mold tool blank thereby creating the optical feature in the optical quality mold surface. The axial viscosity gradient is achieved by creating an axial thermal gradient. Multiple optical features can be formed in the mold surface of the blank using a single punch such that the pressing, cooling and removing steps are repeated with the punch or the blank being translated to a different position between the last removing step and the next pressing step. In such manner, a high temperature glass mold tool can be formed which can be used to mold glass optical elements either individually or in arrays.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to molding of glass optics and, more particularly, to the fabrication of molding tools by coining methods at elevated temperatures, and the use of such tools in the molding of glass lenses and lens arrays. 
     BACKGROUND OF THE INVENTION 
     Coining methods have long been used to reproduce features onto surfaces for a variety of applications. For example, U.S. Pat. No. 4,243,618 to Van Arnam describes a method for forming retroreflective sheeting having a plurality of retroreflective cube-corner prisms distributed over one of its surfaces such that the prisms are disposed in a planar array. The method comprises clamping a plurality of pins together such that the ends of the pins form a substantially planar surface, in scribing the planar surface for forming thereon a continuous pattern of solid trigonal pyramids with internal dihedral angles of ninety degrees, releasing the bundle of pins and rotating the individual pins for changing the angular orientation of the formed trigonal pyramids on adjacent pins re-clamping the pins together and using the inscribed surface of the bundle of pins for forming a mold, containing cube-corner prism cavities, and producing prismatic retroreflective sheeting by embossing, molding or casting in such mold. Van Arnam focuses on eliminating problems associated with orientation and the creation of a planar surface from the pre-assembled array. The materials being molded here are either monomer or polymer in nature, and the embossing is done at or near room temperature (20° C.). Suitable mold tool materials listed are “copper, brass, aluminum, hard plastic, hard rubber, and the like.” The coining tool is an array of individual tools, each one having an optical surface machined onto its surface. The act of holding a large group of pins together to maintain a planar surface of any accuracy that is determined by aligning the vertex points from a number of individual spheres presents a considerable problem. Also, the need to reliably machine identical precision features into a number of tools adds considerable cost and effort to the process. 
     U.S. Pat. No. 5,623,368 to Calderini describes a method for the manufacture of a micolens sheet in which a plate of deformable optical material is pressed against an undeformable furrowed surface in such a manner that neither the convex surfaces of the microlenses nor the surface of the plate of optical material that is opposite the one that bears the microlenses enter into contact with surfaces able to alter them. In essence, the lens surfaces are free formed in this stamping operation which controls only the overall size and shape of the lens. This manner of forming the lens surface precludes the formation of complex shapes such as aspheres, torics, or other desired geometries. The mold tool itself is manufactured by conventional engraving and masking techniques. 
     U.S. Pat. No. 5,298,366 to Iwasaki et al. describes a method of producing a microlens array and the necessary tooling, and in particular, the inverted master tool used in the process. The mold tool is made of a resist material and the optical surfaces are formed by heating an intermediate material and thereby smoothing the surfaces of the projections. The mold tool is then used to stamp out the finished lens arrays. Again, the surfaces are formed by inexact methods and rely on surface tension between the material and its surroundings to generate the optical form. This optic can only be spherical at best, and the materials used would not endure very high temperatures. 
     The above patents state either directly or implicitly that their intended use is with plastic materials or a suitable low melting point glass. This is also evidenced by their choice of tooling materials, most of which could not withstand the high temperatures encountered in molding high temperature glasses without experiencing some sort of degradation. Also, in each of these methods where coining or stamping is used to produce the mold tool, a problem arises that is symptomatic to every stamping operation. A basic physical law is that of conservation of mass, which here suggests that when a volume of material is displaced from one region of an object, a comparable volume of material must appear in another region. For homogeneous materials of constant stiffness, this naturally occurs at a point near the displacement, which in the case of forming small microlenses, is evidenced by a ridge or mushroom effect around the circumference of the impression. This may be overcome by pressing down to a flat portion on the coining mandrel and exerting a high level of force to displace the material away from the feature. The problems here are twofold. First, the force needed to planarize the piece may be excessive and cause other problems such as high internal stresses to develop in the tool. Also, the set up of such a tool is very costly if the depth of the feature is to be held with any precision, as is generally required in optical applications. One way to overcome this mushrooming problem is to planarize the mold tool through a secondary operation after the coining is done, but this is expensive as well and may cause material to flow back into the feature upon machining. Therefore, an effective and economical method for manufacturing mold tools with complex optical features is needed that will withstand the harsh environmental conditions associated with high temperature glass molding and will replicate without flaw the precise geometries required. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for the fabrication of a molding tool that contains high precision optical features for molding arrays of optical elements. 
     It is a further object of the present invention to provide a method for the fabrication of a molding tool that can be used for molding high temperature glass optics. 
     Yet another object of the present invention is to provide a method for the fabrication of a mold tool by coining which obviates mushrooming of the surface of the mold tool. 
     Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon the reading of the description, claims and drawings set forth herein. These features, objects and advantages are accomplished by forming a molding tool blank out of a suitable glass, providing an optical quality polished surface on the face of the molding tool blank whereon the optical features are to be impressed, forming an indenter tool or punch with a predetermined optical surface geometry which is the negative of the optical features/elements to be formed with mold tool, coating the optical surface of the indenter tool or punch with a release coating, creating an axial viscosity gradient in the tool blank by heating the mold tool blank in order to generate an axial thermal gradient therein, pressing the indenter tool or punch into the heated mold tool blank to thereby form a desired optical feature in the surface of the mold tool blank, cooling the tool blank and removing the indenter tool or punch from the mold blank. The process steps of creating an axial viscosity gradient in the tool blank by heating the mold tool blank, pressing the indenter tool or punch into the heated mold tool blank to thereby form a desired optical feature in the surface of the mold tool blank, cooling the tool blank, and removing the indenter tool or punch from the mold blank are practiced in a non-oxidizing environment. As mentioned above, each indenter tool is fabricated to have the negative of a predetermined optical surface geometry. That geometry may be spherical, aspherical, or an otherwise complex geometry. If it is desired to produce a mold tool for molding an array of integrally formed optical elements, the indenter tool or punch or the tool blank is repositioned and the axial viscosity gradient in the tool blank is re-established. The indenter can then be pressed again into the surface of the tool blank. In such manner, a plurality of identical optical features can be produced in the surface of the tool blank. Or, alternatively, different indenter tools can be used to form an array of individual and/or different optical features in a single mold tool surface. In either case, a mold tool formed with the method of the present invention can be used to mold integral arrays of optical elements which can then be cut into individual optical elements as desired. 
     The method of the present invention uses a vitreous material for the mold tool. The viscosity of the vitreous material is dependent on some other parameter that can be regulated. By controlling the viscosity gradients in the vitreous material, the shape of the feature being generated can be closely controlled without adding secondary operations such as planarizing after the forming process in order to provide a planar surface where the forming process has caused deformation to occur. This is usually done by grinding and polishing of the glass, which can add considerable cost to the tool. The present invention uses differential heating of the mold tool blank to thereby control its viscosity along an axial direction and consequently eliminate some of the problems noted above. With the present invention, the displaced material is caused to flow away from the impression point and to a region where the viscosity is low enough to permit fluid movement. This is achieved by causing the temperature at the base of the tool to be higher than the temperature at the insertion point. The viscosity must be low enough at the point of insertion so as to allow replication of the glass without chipping, yet high enough to cause the material to flow away from the lens-forming region. However, if the viscosity is too low at the insertion point, the material will sag or be pulled in by adhesive forces as the glass encounters the coining tool, which also results in unacceptable deformation in the region immediately surrounding the desired features. The present invention again differs from the aforementioned works since it can be performed at an elevated temperature (≡1000° C.) depending on the glass used. Proper material choice is essential to success, such that the transformation temperature of the mold tool is sufficiently greater than the molding temperature of the finished product. Also of great importance is the application of appropriate coatings to the mold tool that will adhere to the tool at high temperatures while also acting as a release coating to the glass being molded into lenses. 
     The method of the present invention is intended for fabricating mold tools for molding micro-optical elements. The term “micro-optical elements” as used herein is intended to mean optical elements such as lenses having a maximum diameter of not more than about one millimeter. Thus, the indenter tool is used to form depressions in the mold tool preform where the individual depressions have a maximum diameter of not more than about one millimeter. In addition, the depressions formed should have a depth/diameter ratio of not more than about 0.2. For example, if a depression is formed having a diameter of about 350 microns, then the depth of the depression should be no more than about 70 microns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the mold assembly used in the process of the present invention for forming the optical features into the mold tool blank. 
     FIG. 2 is a cross-sectional view of the mold assembly used in the process of the present invention for forming the optical features into the mold tool blank and includes an induction heating coil as the heat source. 
     FIG. 3 is a cross-sectional view of the mold assembly used in the process of the present invention for forming the optical features into the mold tool blank and includes a resistance heater as the heat source. 
     FIG. 4 is a cross-sectional view of the mold assembly used in the process of the present invention for forming the optical features into the mold tool and includes a radiant heater as the heat source. 
     FIG. 5 is a graph plotting the log of the viscosity versus the temperature for an exemplary glass having a “long” temperature/viscosity curve. 
     FIG. 6 is a graph plotting the log of the viscosity versus the temperature for an exemplary glass having a “short” temperature/viscosity curve. 
     FIG. 7 is a cross-sectional sketch of an optical feature formed with a mushrooming defect in the mold surface about the periphery of the optical feature resulting from molding with a constant axial viscosity. 
     FIG. 8 is a profilometer trace of an optical feature impression made in a mold tool blank that possessed the desired axial viscosity gradient. 
     FIG. 9 is a perspective view of mold tool formed with the process of the present invention. 
     FIG. 10 is a cross-sectional view of an alternative mold assembly from that shown in FIG. 1 which can be used in the process of the present invention for forming the optical features into the mold tool blank. 
     FIG. 11 is a cross-sectional schematic of an apparatus which employs mold tools formed with the method of the present invention to mold micro-optical elements therewith. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning first to FIG. 1, there is shown a cross-sectional view of the mold assembly used in the process of the present invention for forming the optical features into the mold tool. The mold assembly includes an indenter tool or punch  10  made of a sufficiently hard material and possessing a fine grain structure, such as CVD silicon carbide or tungsten carbide. Other materials that can be used for punch  10  include molybdenum, sapphire, CrNi steel, silicon, and hard carbon sometimes referred to as amorphous diamond. The indenter tool or punch  10  includes a flange portion  12 , a shank portion  14 , and an optical quality pressing surface  16 . The optical quality surface  16  is formed to a desired and predetermined high precision profile such as by diamond turning and polishing, or by diamond grinding and polishing. As those skilled in the art will recognize, the method used to achieve the desired and predetermined high precision profile is a function of the material from which punch  10  is made. The optical quality surface  16  of the indenter tool or punch  10  is coated with an appropriate thin film to protect the base material and to serve as a release agent from the hot glass encountered during molding. A typical coating may be of the titanium aluminide family, such as TiAlN, boron nitride (BN), platinum (Pt), tantalum (Ta), rhenium (Re), osmium (Os), or hafnium (Hf) based alloys, or composites formed with these materials which have been applied with a physical vapor deposition (PVD) or other appropriate process. 
     The indenter tool or punch  10  is supported in an upper mold support  17 . Upper mold support  17  includes a flange bore  18 , a shank bore  20 , and a preform bore  22  which are all substantially cylindrical and co-axial with one another. Depending upon the dimensions of flange bore  18  and flange  12 , the backup spacer element  23  may be used to retain indenter tool or punch  10  in a fully inserted position in upper mold support  17 . There is a lower mold support  24  residing beneath upper mold support  17 . Lower mold support  24  has formed therein a preform bore  26  and the support plate bore  28 . Slidably residing in support plate bore  28  is support plate  30 . Slidably residing within preform bore  26  is mold tool blank  32  with the mold tool blank  32  being supported on support plate  30 . The mold tool blank  32  includes a mold surface  40  into which indenter tool or punch  10  is inserted as will be described more fully hereinafter. There are means (not shown) for driving upper mold support  17  toward lower mold support  24 , or for driving lower mold support  24  toward upper mold support  17 , or both. The optical quality surface  16  of the indenter tool or punch  10  extends through preform bore and beyond the bottom of upper mold support  17 . Preform bore  22  ensures that that when indenter tool or punch  10  is driven into contact, no portion of the upper mold support  17  contacts mold surface  40 . 
     The upper and lower mold supports  17 ,  24  are used in conjunction with a heat source. A variety of different heat sources can be used. For example, as depicted in FIG. 2, the heat source can be an induction heating coil  34 . Alternatively, as depicted in FIG. 3, the heat source can be a resistance heater module  36 ; or, as depicted in FIG. 4, the heat source can be a radiant heater  38 . The heat source is used to generate an axial thermal gradient in the mold tool blank  32 . With the induction heating coil  34 , an RF generator (not shown) is used to create a magnetic field. The induction heating coil  34  acts as a solenoid creating a magnetic field within its boundaries. The magnetic field strength varies radially but is constant through all angles for any given radius. The magnetic field strength also varies axially and it is this property of the solenoid that allows for the differential heating of the mold tool blank  32 . 
     In the case of the resistive heaters and radiative heat sources, the heat source acts upon the cylindrical mold tool blank  32  from one end only, thereby producing the axial temperature gradient. The amount of thermal gradient is determined, to some degree, by the duration of time that the heat is allowed to influence the system. As the soak time is increased, the thermal gradient decreases, until a steady state condition is achieved and the thermal gradient remains constant. This steady state condition may or may not be desirable in the practice of the process of the present invention, as the specific molding conditions are heavily dependant on the geometry of the tools. In other words, with the heat source acting on the cylindrical mold tool blank  32  from one end, there would always exist a thermal gradient in the material due to the thermal conductivity of the mold tool blank  32  and the heat lost to its surroundings. However, since the mold tool blank  32  in this case is usually surrounded by some support fixtures, which also heat up and act as heat sources to the mold tool blank  32 , the thermal gradient would be changed, and possibly eliminated given enough time. Further, long slender mold tool blanks  32  with properly chosen support structures would exhibit different characteristics than those evidenced by short, stubby mold tool blank  32  in the same surroundings, and so tool geometry and support structure materials will affect the process. 
     Upper and lower mold supports  17 ,  24  are used to facilitate mounting of the mold tool blank  32  in the heater structure and to act as a magnetic susceptor for the mold tool blank  32 , since glass is a dielectric material. These mold supports  17 ,  24  should be made of materials having good thermal conductivity properties, and which can also withstand the high molding temperatures encountered in the practice of the process of the present invention. When the heat source is an induction heating coil  34 , the mold supports  17 ,  24  should be made of materials that are also electrically conductive (such as carbon graphite). Similarly, when the heat source is an induction heating coil  34 , the support plate  30 , directly below the mold tool blank  32 , should also be made of materials (such as carbon graphite) that have good electrical and thermal conductivity properties, and can also withstand the high molding temperatures encountered in the practice of the process of the present invention. The support plate  30  again functions as a susceptor in the magnetic field and adds to the creation of the axial viscosity gradient by serving as a heat source at the base of the mold tool blank  32 . 
     The length of the mold tool blank  32  and its axial placement in the coil  34  determine the amount of temperature variation to be produced. The magnetic field is strongest in the center of the coil  34  and may vary considerably when measured near the ends of the coil  34 , where fringing fields dominate. 
     In the practice of the method of the present invention the indenter tool or punch  10  is pressed into the mold tool blank  32  at an elevated temperature and in a non-oxidizing environment. Understanding that the mold tool blank  32  made with the process of the present invention is ultimately used to mold optical elements such as lenses in subsequent operations, the material from which the mold tool blank  32  is made must be able to withstand the high temperatures required to mold glass lenses with its surface. This means that the viscosity of the mold tool made from mold tool blank  32  should be at least about 10 14  poise at the temperature where the lens molding is performed. This is because the annealing temperature for glass, defined as the point at which internal stresses are relieved in about 15 minutes, occurs when the viscosity reaches 10 13  poise. At this temperature, glass has a very low fluidity and its dimensional stability is good enough for many purposes. For most glass molding applications, it is desirable that the glass possess a “long” temperature/viscosity curve. This is evidenced when a relatively large temperature change produces a small viscosity change, and allows the molder a greater range in which to work successfully. An exemplary glass having a “long” temperature/viscosity curve is shown in FIG. 5, which is a graph plotting the log of the viscosity versus the temperature. In contrast to this, a graph of a “short” glass, that is, a glass having a “short” temperature/viscosity curve is shown in FIG.  6 . This type of glass is difficult to work with given its strong viscosity/temperature profile, although it will still perform well if the proper controls are maintained. The glass used for this present invention was from the aluminosilicate family and was characterized by a viscosity/temperature curve similar to that shown in FIG.  5 . The preferred glass for mold tool blank  32  is an yttria aluminosilicate glass. Other glasses such as, for example, alumino-silicate glasses with a higher than normal levels of silica can also be used in the practice of the present invention. 
     In the practice of the method of the present invention the viscosity of the mold tool blank  32  at the mold surface  40  thereof needs to be soft enough to allow impressions to be formed therein while still being firm enough to limit deformation which would result from sagging of the material. The viscosity needed in this region can be determined empirically or by the size and depth of the impression to be made in mold surface  40  with indenter tool or punch  10 . For this invention, a viscosity of about 10 9.17  poise was found to be sufficient. It is believed that a viscosity proximate the mold surface  40  in the range of from about 10 8.98  to about 10 9.35  will be adequate for the purposes of practicing the method of the present invention. If the viscosity of the mold tool blank  32  is higher than necessary, the optical feature  44  will be formed with mushrooming defects  43  in the mold surface  40  about the periphery of the optical feature  44  as shown in FIG.  7 . If the viscosity level is higher yet, cracking and catastrophic failure may occur. When the viscosity is maintained at the proper level, an optical feature will be formed in the surface  40  without defects as evidenced by the profilometer trace of such an optical feature  46  as shown in FIG.  8 . In order to accomplish this the temperature at the base of the mold tool blank  32  must be greater than the temperature at the surface  40  of the mold tool blank  32 . In other words, achieving the desired axial thermal gradient in mold tool blank  32  produces the desired viscosity profile. 
     By way of example, the method of the present invention was successfully demonstrated using a mold tool blank  32  having a length of 15 mm wherein the forming surface  40  of the mold tool blank  32  was 35 mm from one end of a coil  34 . The induction coil  34  was a six turn induction coil made from 6 mm diameter copper tubing spaced at a 10 mm pitch for a total coil length of 56 mm. An RF generator was used at a frequency of 154 kHz to create the magnetic field and was sufficient to produce the desired result. A viscosity of about 10 9.17  poise was achieved at the mold surface  40  while simultaneously producing a viscosity of about 10 8.26  poise at the base of mold tool blank  32 . It is believed that a viscosity proximate the an end of the mold tool blank  32  opposite the mold surface  40  in the range of from about 10 8.07  to about 10 8.44  will be adequate for the purposes of practicing the method of the present invention. The viscosity proximate the mold surface  40  is, of course greater than the viscosity proximate the an end of the mold tool blank  32  opposite the mold surface  40  as a result of achieving the desired viscosity gradient in the mold tool blank  32 . This viscosity profile allowed the displaced glass to flow at a location away from the impression (resulting in the formation of optical feature  46 ) since the base of mold tool blank  32  was more than 8 times more fluid than the mold surface  40 . 
     It is necessary during the molding process to always maintain a mold tool blank  32  viscosity above the softening point, which is the temperature at which glass articles begin to deform under their own weight. That temperature is defined as the temperature at which the glass attains a viscosity of 10 7.6  poise. The glass should also be free from restrictions in order to allow it to flow at some remote location, and therefore, an annular channel  50  is provided about the base of the mold tool blank  32  for that purpose. Annular channel  50  is defined by the difference between the height of the support plate  30  and the support plate bore  28 . When the tooling is actuated to drive the punch  10  into the mold surface  40  of mold tool blank  32 , there is no relative movement between the lower mold support  24 , support plate  30 , and mold tool blank  32 . A small flange may be evidenced along the base of the mold tool after forming optical feature(s)  46  in the mold tool blank  32  if sufficient glass has been displaced. This results from the differential heating of the glass (producing an axial thermal gradient) and the material displaced by the indenter tool or punch  10 , particularly if a substantial number of optical feature(s) are formed. 
     Although the method of the present invention can be practiced with multiple punches  10  to simultaneously form multiple optical feature(s)  46  in a mold tool blank  32 , it is preferable that a single forming tool or punch  10  be used in the manufacture of an exemplary mold tool  52  (see FIG. 9) having an array  54  of optical feature(s)  46  formed therein. This will ensure consistency of form between the optical feature(s)  46  and therefore, between the lenses molded with the mold tool  52 . Further, using a single indenter tool  10  to manufacture a mold tool  52  having an array  54  of optical feature(s)  46  allows for adaptations and changes in the array pattern with out the expense of costly tooling changes. 
     The method of the present invention is intended for use with lens glasses that possess a high working temperature, but is suitable for all optical quality glasses. It is not entirely clear where the distinction is made between low and high temperature glasses in the prior art literature, but most people familiar with the art would agree that lens glasses having viscosity curves where the viscosity reaches 10 4.0  poises at or above 750° C. are considered to be in the high temperature regime. When working in this temperature range, material choice is paramount to realizing a successful and robust manufacturing process as many materials begin to break down in some fashion or another. Many materials that are readily coined at room temperature, such as nickel, will suffer degradation at elevated temperatures. Alternately, many materials that perform well at high temperatures, such as silicon carbide, do not lend themselves to coining at room temperature or at elevated temperatures. In the present invention, the forming operation is performed at an elevated temperature slightly above the transformation temperature of the glass since the glass cannot be pressed at room temperature without sustaining severe damage. 
     In the practice of the present invention, the amount of soak time employed during heating of the mold tool blank  32  can also be controlled. This allows the viscosity to be controlled in a radial direction as well during formation of the optical features  46  in the mold tool blank  32 . As a heat source is continually applied to an object over time, the temperature gradient that is initially formed in the object decreases to some minimum value based on properties integral to the material. By controlling the amount of soak time, the proper molding temperature can be achieved for any location on or within the mold tool blank  32 . This parameter also has a direct effect on the final shape of the formed optical features, and is important when forming multi-element arrays whose elements may not be equidistant from the heat source. 
     In an alternative embodiment of the method of the present invention, an intermediate ring  60  is used in the molding operation to surround the mold tool blank  32  (see FIG.  10 ). The intermediate ring  60  (which is a cylindrical structure) is fabricated from a material that can be machined to a good quality finish and possesses a low thermal conductivity, such as SiO 2  or a ceramic. This intermediate ring  60  serves to insulate the mold tool blank  32  from energy fluctuations generated by the heat source. 
     Turning to FIG. 11, there is shown a cross-sectional schematic of an apparatus which employs mold tools formed with the method of the present invention to mold micro-optical elements therewith. The apparatus  100  is described in greater detail in U.S. application Ser. No. 09/354,219 filed Jul. 15, 1999 which is hereby incorporated herein by reference. The apparatus  100  includes an upper mold tool  102  and a lower mold tool  104 . Lower mold tool  104  is one example of mold tool  52  shown in FIG.  9 . The upper mold half  102  includes an upper mold surface  106 . Upper mold surface  106  is depicted as being plano but may include other optical geometries of such as concave or convex features. The lower mold tool  104  includes an array of optical feature(s) or micro-lens cavities  110  formed in mold surface  108 . The optical feature(s) or micro-lens cavities  110  are spaced apart from a central nesting cavity  112  which provides residence for a preform  114  which is depicted as being spherical. Surrounding upper and lower mold tools  102  and  104  is induction heating coil  116 . In operation, a preform  114  is placed in central nest cavity  112  and through actuation of induction heating coil  116 , the temperature of the upper and lower mold tools  102 ,  104  and preform  114  is raised to at least the glass transition temperature of the preform  114 . Then the preform  114  is pressed between the upper and lower mold tools  102 ,  104  causing the preform  114  to deform and flow generally radially outward. As the preform flows radially outward, it fills the optical feature(s) or micro-lens cavities  110 . Compression is performed to a positive stop at which point the mold tools  102 ,  104  and the preform  114  are allowed to cool to below the glass transition temperature and preferably to below the annealing point of the glass. In such manner, an integrally formed array of lenses or micro-lenses (not shown) is formed which can then be removed from the molding apparatus  100 . It should be understood that upper and lower mold tools  102 ,  104  are not necessarily directly heated by induction. Rather, upper and lower mold tools  102 ,  104  preferably reside in a mold body (not shown) fabricated from a conductive material such as graphite or molybdenum. The mold body is heated by the induction field and the upper and lower mold tools  102 ,  104  are heated indirectly by conduction and radiant heat transfer. 
     Although preform  114  is depicted as being spherical, it is well known to those skilled in the art that preforms can have other geometries. Those other geometries are generally necessitated by the final geometry of the optical element to be formed therefrom. Thus, for example, if it is desired to form a double concave lens, then it will likely be desirable to use a plano preform. 
     It should be recognized that the preferred method of the present invention of using a single tool to press features while relying on external means to properly place the tool onto the mold eliminates the high set up costs associated with the prior art. In addition, using a single movable indenter tool allows for varying the spacing between lenses to accommodate different designs without generating new tools. 
     It should also be appreciated that the method of the present invention does not bring the glass of the mold tool blank to the melting point but rather to just the softening point of the glass. This is important since it eliminates concerns about devitrification of the glass, or the effects of forming an incomplete or inconsistent interface. 
     From the foregoing, it will be seen that this invention is one well adapted to obtain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the apparatus. 
     It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     Parts List 
       10  indenter tool/punch 
       12  flange portion 
       14  shank portion 
       16  optical quality pressing surface 
       17  upper mold support 
       18  flange bore 
       20  shank bore 
       22  preform bore 
       23  back up spacer element 
       24  lower mold support 
       26  preform bore 
       28  support plate bore 
       30  support plate 
       32  mold tool blank 
       34  induction heating coil 
       36  resistance heater module 
       38  radiant heater 
       40  mold surface 
       43  mushrooming defects 
       44  optical feature 
       46  optical feature(s) 
       50  annular channel 
       52  exemplary mold tool 
       54  array 
       60  intermediate ring 
       100  apparatus 
       102  upper mold tool 
       104  lower mold tool 
       106  upper mold surface 
       108  mold surface 
       110  micro-lens 
       112  central nesting cavity 
       114  preform 
       116  induction heating coil