Abstract:
An optical disk exhibiting no detrimental thickness increase (edge wedge effect) that arises at the outer diameter of an optical disk substrate during an injection molding manufacturing process, and an apparatus and method for making such a disk. The optical disk of the present invention is designed for use with an optical disk player, especially where the data on the optical disk is stored air incident. This optical disk includes a disk substrate made from a molded polymeric material. The disk substrate has a first major surface, a second major surface, and an outer edge. The first major surface of the optical disk includes a data region having an intermediate portion and an outer portion. The outer portion extends to the outer edge of the optical disk. The disk substrate has a thickness defined by the distance between the first major surface and the second major surface. The optical disk also includes an information layer covering the data region. In the present invention, the thickness of the intermediate portion of the data region is substantially equal to the thickness of the outer portion of the data region such that the outer portion of the data region is capable of being used by the optical disk player.

Description:
This application is a divisional application of U.S. Application Ser. No. 09/326,935, filed Jun. 7, 1999, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to the field of manufacture of optical data storage disks, and in particular, to a method and assembly for reducing or eliminating an increased thickness that occurs at the outer edge of an optical disk substrate as a result of the substrate molding process (otherwise known as the “edge wedge” effect). 
     BACKGROUND OF THE INVENTION 
     Data storage disks are produced using a disk replication process. A master disk is made having a desired surface relief pattern formed therein. The surface relief pattern is typically created using an exposure step (e.g., by laser recording) and a subsequent development step. The master is used to make a stamper, which in turn is used to stamp out replicas in the form of replica disk substrates as part of a disk molding process. As such, the surface relief pattern, information and precision of a single master can be transferred into many inexpensive replica disk substrates. 
     Conventional mold assemblies typically include a fixed side and a moving side. The stamper is typically attached to either or both sides of the mold assembly for replicating a desired surface relief pattern (i.e., lands, grooves and/or pits) into the replica disk substrate. A movable gate cut may be provided for cutting a central opening in the replica disk substrates. The stamper may be secured to the moving side using an inner holder and outer holder, wherein the inner holder and outer holder fit over the stamper. Several more tooling parts may be located at the center of the mold assembly to assist in ejection of the component. 
     During the disk molding process, a resin, typically optical grade polycarbonate, is forced in through a sprue channel into a substrate cavity within the mold assembly to form the replica disk substrate. The surface relief pattern or formatted surface is replicated in the replica disk substrate by the stamper as the cavity is filled. After filling, the gate cut is brought forward to cut a center hole in the replica disk substrate. After the replica disk has sufficiently cooled, the mold assembly is opened and the gate cut and a product eject may be brought forward for ejecting the formatted replica disk substrate off of the stamper. The inner holder and outer holder may be removable to allow changeout of the stamper. 
     In injection-compression molding, while the resin is forced into the substrate cavity of the mold assembly by the molding press, injection pressure overcomes clamp force causing mold to open a small amount (commonly termed “mold blow”). Pressure is then increased to the mold assembly to clamp the mold shut, forcing the resin into the microscopic surface relief pattern of the stamper (which contains the reverse image of the desired replica disk surface relief pattern). Thus, the above process is commonly termed “injection compression” or “micro-coining”. 
     For disk formats utilized in flying head applications, as disk capacity increases the design tolerances for the desired surface relief pattern become more critical. For high capacity disks the flying heads may be required to pass closer to the disk substrate, requiring tighter disk specifications, including a reduction or elimination of disk surface geometry imperfections. 
     One such disk surface geometry imperfection is the thickness increase that has been consistently seen at the outer edge of a typical polymeric optical disk substrate. This phenomenon has been given the name “edge wedge” or “ski jump” effect. This “edge wedge” is shown schematically in prior art FIG.  1  and FIG.  2 . “Edge wedge” causes problems in a hard disk-type system where a read/write head is designed to fly as close as possible (i.e., on the order of 1-5 micro-inches) to the surface of the media substrate. For example, one typical polycarbonate disk substrate has an average thickness of about 2 mm (as shown at T1), and a radius of 65 mm. The “edge wedge” effect is primarily seen at the outer radius region of the polycarbonate disk between 62 mm and 65 mm, where the maximum substrate thickness (i.e., bump height) T2 at radius 65 mm is approximately 10-20 microns thicker than the substrate thickness at radius 63 mm. When the bump height differential (T2−T1) divided by the average thickness (T1) exceeds 0.01 (1 percent), read/write flyability problems are often encountered. 
     The “edge wedge” phenomenon can be attributed to many factors. During cooling of the disk substrate in the mold, the plastic “freezes” at different rates in different radii of the part. The outer edge of the disk substrate freezes through the thickness faster due to its contact with the cold outer holder. Other factors include the tendency of the disk substrate material molecules to be in substantial radial alignment near the center of the disk substrate, and relatively misaligned near the outer edge due to the mold filling process. All of these factors result in the outer edge of the disk substrate exhibiting a greater thickness than the remainder of the disk substrate. 
     The “edge wedge” phenomenon can be further described as follows. When the optical disk substrate is molded in the micro-coining process described earlier, the densification that is associated with the cooling plastic is accommodated through a corresponding reduction in the mold cavity size (as opposed to reduction in mold cavity pressure as in conventional injection molding). During the filling phase, the mold halves are forced slightly apart by the fluid pressure applied from the injection unit. As the plastic in the mold cools, it shrinks, and the mold halves translate into closer proximity as the press maintains a constant clamp force or pressure on the solidifying melt. The part will freeze through the thickness at slightly different rates at different radii. Regions that are frozen fully through early will do so while the mold is blown to a greater extent or whilst the cavity z-dimension is larger in the earlier phases of the micro-coining molding process. These fully frozen regions will then strain due to clamp force in an elastic fashion (meaning that the solid material will spring back upon release of applied force). Regions that remain liquid at the center will strain in a viscous fashion (this is non-recoverable strain) and will continue to shrink in size or density as they more slowly solidify and eventually take on a thickness of a smaller cavity dimension from later in the coining/cooling process. Therefore, after the clamping force is removed, the early-freezing regions (outer circumference areas) will spring back to a larger thickness than those areas that froze completely through later in the process (the inner disk area). 
     For a traditional optical disk, where information is stored “substrate incident” the “edge wedge” effect does not present a major problem. In substrate incident applications, a transparent protective layer covers the information layer of the disk. An optical disk player including a laser light source positioned away from the disk surface, focuses a laser beam through the protective layer at the information layer to access (i.e., read) the data stored on the disk. However, for “flying head” applications where information is stored on a disk surface (i.e., where information is stored “air incident”), a read/write head is flying 1-2 micro-inches above the substrate surface. The “edge wedge” phenomena is associated with a loss of flyability of the read/write head where the outer edge of the head comes into contact with the rising surface of the media substrate, resulting in a “head crash” if the head were allowed to fly over the outer portion of the disk. The outer edge of the disk is unusable for data storage, since the curvature of the surface becomes too great to provide a functional air bearing between the head and the surface of the disk. This limits the capacity, functionality and robustness of the disk data storage system. 
     Unfortunately, the outer circumference of the disk substrate where the “edge wedge” effect occurs is also the most desirable area for data storage. This outer circumference provides a large area for data storage since the data tracks are larger. Therefore, the need exists to eliminate the “edge wedge” to prevent disk crashes and to increase the useable area of the disk. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an optical disk exhibiting no detrimental thickness increase (edge wedge effect or curvature) that arises at the outer diameter of an optical disk substrate during a typical injection molding manufacturing process, and an apparatus and method for making such a disk. 
     The present invention provides an optical disk for use with an optical disk player, where the data on the optical disk is stored air incident. This optical disk includes a disk substrate made from a molded polymeric material. The disk substrate has a first major surface, a second major surface, and an outer edge. The first major surface of the optical disk includes a data region having an intermediate portion and an outer portion. The outer portion extends close to the outer edge of the optical disk. The data region is defined by a plurality of lands and grooves, where the disk substrate has a thickness defined by the distance between the lands and the second major surface. The optical disk also includes an information layer covering the data region. In the present invention, the thickness of the intermediate portion of the data region is substantially equal to the thickness of the outer portion of the data region such that the outer portion of the data region is capable of being used by the optical disk player. 
     In a preferred embodiment of the present invention, the molded polymeric material is a polycarbonate or a polycarbonate blend. In order to prevent a flying read/write head from crashing on the surface of the optical disk, the thickness of the intermediate portion and the thickness of the outer portion of the optical disk varies less that 0.10 micron per millimeter proceeding radially from the center axis of the disk substrate. In one embodiment of the present invention, the outer portion of the optical disk extends radially from approximately 5 millimeters in from the outer edge of the disk substrate to the outer edge of the disk substrate, where the disk substrate has a diameter of between 120 and 130 millimeters. 
     The present invention also provides a disk molding apparatus for forming an optical disk in a disk molding process, wherein the apparatus reduces the edge wedge effect in the molded optical disk. In a first embodiment, the disk molding apparatus includes a disk substrate cavity for forming a disk substrate. The disk substrate cavity has a first major surface, a second major surface which opposes the first major surface, and an outer edge. The disk molding apparatus also includes a channel mechanism connected with the disk substrate cavity for allowing disk molding material to enter the disk substrate cavity. The disk molding apparatus further includes a stamper located on one side of the disk substrate cavity for forming a formatted surface relief pattern in the disk substrate. Finally, the disk molding apparatus also includes a thermal inhibiting mechanism located around the outer edge of the optical disk. This thermal inhibiting mechanism inhibits heat flow from the disk substrate during the cooling of the disk molding material to form the disk substrate. 
     The thermal inhibiting mechanism of the first embodiment includes an outer holder, wherein the outer holder removably secures the stamper to the first major surface. In one preferred embodiment, the outer holder is made of low thermoconductivity titanium. In another preferred embodiment, the outer holder has two ring members, wherein a low thermoconductivity ceramic member is retained between the two ring members, and a portion of the ceramic member extends from the two ring members for retaining the stamper against the first major surface. 
     In one embodiment, the thermal inhibiting mechanism of the present invention is a heating mechanism, where the heating mechanism heats the outer holder during the disk molding process to a temperature sufficient to create a smaller temperature differential between the disk substrate and the outer holder, reducing the heat transfer between the disk substrate and the outer holder. This heating mechanism of the present invention has several embodiments, including: a resistive heater placed within a channel of the outer holder; heated water circulating through the channel of the outer holder; heated oil circulating through the channel of the outer holder; a film resistive heater coupled to an outer surface of the outer holder; and an induction heater positioned external to the outer holder. 
     In a second embodiment of the present invention, the disk molding apparatus has a disk substrate cavity which includes a defined wedge containment area where the wedge is directed into during the injection molding process. The disk substrate cavity incorporating the wedge containment area has a surface area less than the surface area of a conventional disk substrate cavity without the wedge containment area. By directing the wedge into a confined area, the area of the disk substrate affected by the edge wedge effect is reduced, resulting in a greater usable data storage area within the optical disk. 
     In a third embodiment of the present invention, the disk molding apparatus includes a disk substrate cavity for forming a disk substrate. The disk substrate cavity includes a first major surface, a second major surface opposite the first major surface, and an outer edge. The disk molding apparatus also includes a channel mechanism in fluid communication with the disk substrate cavity which allows disk molding material to enter the disk substrate cavity. Finally, the disk molding apparatus includes a stamper, having an information surface and a back surface. The information surface of the stamper forms the first major surface of the disk substrate cavity, and produces a formatted surface relief pattern in the disk substrate during the molding process. Also, during the molding process, the stamper forms a shape which counters the molding edge wedge effect. 
     In order to counter the edge wedge effect during the molding process, the back surface of the stamper is electroplated with a nickel lip around the outside perimeter, such that as pressure is applied to the back surface of the stamper, the stamper flexes in a concave fashion producing a disk substrate cavity thickness which is narrower at the outside perimeter and wider in all other areas, thus creating an anti-wedge region within the disk substrate cavity. In a preferred embodiment, the nickel lip around the outside perimeter of the back side of the stamper is approximately 3 mm wide, and approximately 15 microns thick. The resultant anti-wedge region within the disk substrate cavity is approximately 15 microns narrower than all other areas of the disk substrate cavity. 
     The present invention also discloses methods for forming an optical disk in a disk molding process which reduces the edge wedge effect in the molded optical disk. The first such method begins by injecting molding material into a disk substrate cavity via a channel mechanism. The disk substrate cavity includes a first major surface, a second major surface opposite the first major surface, and an outer edge. Next, a thermal inhibiting mechanism located about the outer edge of the disk substrate cavity inhibits the escape of heat in the radial direction from the disk substrate during the cooling of the disk molding material. The thermal inhibiting mechanism includes a low thermal conductivity outer holder. In one preferred embodiment the outer holder is constructed of titanium. In another preferred embodiment, the outer holder includes a ceramic member which contacts the disk substrate during the molding process. In yet another embodiment, the outer holder includes a heating mechanism, wherein the heating mechanism heats the outer holder during the disk molding process to a temperature sufficient to create a smaller temperature differential between the disk substrate and the outer holder, thus reducing the heat transfer between the disk substrate and the outer holder. 
     The present invention also discloses a second method for forming an optical disk in a disk molding process which reduces the edge wedge effect in the molded optical disk. In this second method, the disk molding process utilizes a disk substrate cavity having a defined wedge containment area located at the outer perimeter of the disk substrate cavity. Initially, disk molding material is injected into a disk substrate cavity via a channel mechanism. Next, the disk molding material in the disk substrate cavity is compressed such that the disk molding material at the outer perimeter of the disk substrate cavity flows into the defined wedge containment area. The disk molding material is then cooled such that the optical disk is formed within the wedge containment mold. In the resultant disk, the unusable surface area is minimized. 
     The present invention also discloses a third method for forming an optical disk in a disk molding process which reduces the edge wedge effect in the molded optical disk. This third method utilizes a disk substrate cavity having a first major surface, a second major surface opposite the first major surface and an outer edge. This method also utilizes a stamper having an information surface and a back surface. The information surface of the stamper forms the first major surface of the disk substrate cavity. The back surface of the optical stamper is electroplated with a nickel lip at the outside perimeter. 
     This third method begins by injecting molten disk molding material into the disk substrate cavity. Next, the disk molding material is compressed in the disk substrate cavity. As pressure is applied to the stamper, a formatted surface relief pattern is formed in the disk substrate from the information surface of the stamper. Also, as pressure is applied to the back surface of the stamper, the stamper flexes to form an anti-wedge region in the disk substrate cavity which counters the molding edge wedge effect during the disk molding process. In a preferred embodiment, the nickel lip around the outside perimeter of the back side of the stamper is approximately 3 mm wide and 15 microns thick. The resultant thickness of anti-wedge region in the cavity is approximately 15 microns less than in all other areas of the cavity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principals of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein: 
     FIG. 1 is a side view of a prior art optical disk substrate; 
     FIG. 2 is an enlarged, cross sectional view of the outer perimeter region of a prior art optical disk substrate showing an “edge wedge” effect; 
     FIG. 3 is a plan view of an optical disk in accordance with the present invention; 
     FIG. 4 is a side view of an optical disk in accordance with the present invention including illustrating of a coordinate frame of reference; 
     FIG. 5 is an enlarged, cross sectional view of the outer perimeter region of an optical disk substrate in accordance with the present invention illustrating the absence of an “edge wedge” effect; 
     FIG. 6 is a cross sectional view illustrating one exemplary embodiment of an injection molding apparatus used in the manufacture of an optical disk in accordance with the present invention; 
     FIG. 7 is an enlarged partial cross-sectional view of the outer perimeter of the injection molding apparatus of FIG. 6, illustrating the engagement of a stamper by an outer holder. 
     FIG. 8 is a top view of one exemplary embodiment of an outer holder component of the injection molding apparatus in accordance with the present invention; 
     FIG. 9 is an enlarged, partial cross-sectional view of the one-piece titanium outer holder component of FIG. 8, illustrating the interaction between the outer holder and the stamper component; 
     FIG. 10 is a top view illustrating another exemplary embodiment of outer holder used within the injection molding apparatus of the present invention, wherein at least one piece of the outer holder is a low thermoconductivity component; 
     FIG. 11 is an enlarged, partial cross-sectional view of the outer holder of FIG. 10; 
     FIG. 12 is an illustration of one exemplary embodiment of a resistive heater used for heating the outer holder of the injection molding apparatus in accordance with the present invention; 
     FIG. 13 is an illustration of an outer holder incorporating the resistive heater of FIG. 12; 
     FIG. 14 is an illustration of one exemplary embodiment of a hollow coil which contains heated, recirculating water used for heating the outer holder of an injection molding apparatus in accordance with the present invention; 
     FIG. 15 is an illustration of an outer holder incorporating the heated recirculating water coil of FIG. 14; 
     FIG. 16 is an illustration of one exemplary embodiment of a hollow coil which contains heated, recirculating oil used for heating the outer holder of an injection molding apparatus in accordance with the present invention; 
     FIG. 17 is an illustration of an outer holder incorporating the heated recirculating oil coil of FIG. 14; 
     FIG. 18 is an illustration of one exemplary embodiment of an outer holder of an injection molding apparatus in accordance with the present invention wherein the outer holder is heated inductively by an external heat source; 
     FIG. 19 is an illustration of one exemplary embodiment of an outer holder of an injection molding apparatus having a film dispersed on the outer surface of the outer holder, wherein the outer holder is heated thermoelectrically by the film element; 
     FIG. 20 is a cross sectional view of a prior art optical disk substrate molded using a conventional mold cavity, wherein the “edge wedge” effect is seen at the outer edges of the optical disk; 
     FIG. 21 is a cross sectional view of an improved optical disk with an enhanced useable surface area molded using the outer diameter wedge containment mold of the present invention; 
     FIG. 22 is a cross sectional view of a molded optical disk substrate exhibiting the “edge wedge” effect at the outer edges of the substrate; 
     FIG. 23 illustrates an anti-wedge stamper of the present invention, wherein an electroplated nickel lip is added to the outer perimeter of the back side of the stamper; 
     FIG. 24 illustrates the anti-wedge stamper of the present invention, wherein the stamper flexes during the injection molding process such that the outer edges of the stamper deflect upwardly in an anti-wedge configuration; and 
     FIG. 25 illustrates a molded substrate formed from the anti-wedge stamper, wherein the “edge wedge” effect has been eliminated. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 is plan view of one exemplary embodiment of an optical disk  20  in accordance with the present invention. The optical disk may be a read only or a writable optical disk. Optical disk  20  is round or generally “disk shaped”, and may include an opening  22  centrally located and extending therethrough. The optical disk  20  includes a disk substrate  24  having information layer  25 . The disk  20  includes an information area  26  and a central region  28 . The central region  28  may be located between the information area  26  and the opening  22 . 
     In one exemplary embodiment shown in FIG. 3, data is stored air incident (i.e., on the disk surface) within the information area  26  of optical disk  20 . The information area  26  includes a surface relief pattern which can be defined as a series of grooves bored within a plane of higher “lands” indicated at  29  (shown enlarged for clarity). The microscopic grooves are formed on the surface of the plastic disk during an injection molding process in accordance with the present invention which is described in detail further in the specification. The grooves on optical disk  20  are arranged in a spiral track originating at the beginning of information area  26 , adjacent central region  28 , and ending at the disk outer edge  30 . 
     Data (i.e., information) is stored in/on the information layer  25  covering (e.g. deposited) over the disk surface. In one preferred embodiment, the information layer  25  comprises magnetizable material. The data is stored as magnetic transitions within the information layer and readable by an optical disk player. Conventionally, the spiral track can be defined as a plurality of generally concentric tracks, wherein each generally concentric track is a cycle of the spiral track. Alternatively, the information area may consist of a plurality of concentric tracks. Similarly, for writable optical disks, such as magneto optical disks or phase change optical disks, the data is encoded within the readable material arranged in a spiral track. In particular, the writable disks may include a spiral or concentric track formed in the disk substrate, wherein the data is encoded in the writable material located in the regions between the spiral track cycles (e.g., on top of the lands). 
     The central region  28  can include a hub  32  positioned at opening  22 . The hub  32  is employed to aid in engagement or mating of the optical disk within an optical disk player (in particular, the drive spindle located within the optical disk player) for retention and rotation of the disk curing operation of the optical disk player. Further, hub  32  is centered on optical disk  20  using a centering process. In particular, the concentricity of the formatted information located within the information area  26  can be specified relative to the hub center hole  33 . Typically, the center hole  33  serves to center an optical disk player drive mechanism to the formatted surface (information) on the disk (or more particularly, the generally concentric tracks). An information layer is deposited on the disk substrate surface. 
     FIG. 4 is a side view of optical disk  20  showing a coordinate frame of reference. The coordinate frame of reference is used for the purposes of discussing embodiments of the invention disclosed herein. In this coordinate frame of reference, a first vector  35  projects vertically from the center of optical disk  20  in a z-direction, while a second vector  33  projects horizontally from the center of optical disk  20  in an redirection, toward an outer perimeter of optical disk  20 . Angle θ, shown at  37 , is formed between first vector  35  extending vertically and second vector  33 , extending horizontally. 
     In one preferred embodiment, disk substrate  24  of optical disk  20  is formed using a disk molding process. Disk substrate  24  is typically made of a moldable polymeric material or polymer blend which in one preferred embodiment is polycarbonate or a polycarbonate blend. Other known disk substrate materials include polymethylmethacrylate (PMMA), polymethylpentene, co-polymers, or blends of polycarbonates or other polymers, “polymer z”, etc. Other suitable disk substrate molding materials will become apparent to those skilled in the art after reading the disclosure of the present invention. 
     FIG. 5 is an enlarged, cross sectional view of the outer perimeter region of an optical disk  20  in accordance with the present invention, illustrating the absence of an increased thickness at outer edge  30  or “edge wedge” effect. In the illustrated exemplary embodiment, optical disk  20  has a radius 49 of approximately 65 mm. Proceeding horizontally from the center of optical disk  20  in the r-direction from radius 0 mm to 65 mm, the thickness of optical disk  20  remains substantially uniform at approximately 2.0 mm (i.e., thickness T1). In other embodiments, the thickness T1 of other optical disks  20  ranges from about 1 mm to about 2.5 mm. In one application, in order to maintain head flyability (of the disk player), the thickness of optical disk  20  cannot vary more than 1 micron per millimeter proceeding radially from the center of optical disk  20 . This is in contrast to a prior art optical disk (FIG.  2 ), where proceeding horizontally from radius 63 mm to radius 65 mm results in a thickness of approximately 10-20 microns greater than the average thickness T1 (i.e., maximum thickness T2). In other embodiments the thickness differential (T2−T1) between the maximum thickness (T2) of optical disk  20  and the average thickness (T1) of the optical disk varies from 0.01 millimeter to 0.025 millimeter. When (T2−T1)/T1 exceeds 0.01, flyability of the read/write head may be adversely effected. By eliminating the “edge wedge” effect found in prior art molded optical disks, an air incident, flying read/write head can now traverse the entire surface of optical disk  20  without crashing. Also, since the entire surface of optical disk  20  can now be traversed by the flying read/write head, substantially more surface area is available for data storage on the surface of optical disk  20 . 
     One method for reducing the “edge wedge” effect found at the outer diameter of optical disk  20  is to mold an oversized optical disk  20 , then cut away the region at the outer edge  30  of the disk where the “edge wedge” effect occurs. As an example, an oversized optical disk of approximately 134 mm is molded by the injection molding apparatus illustrated in FIG.  6 . As described above, the “edge wedge” effect primarily occurs at the outer 2 mm of the outer diameter of the optical disk  20 . 
     In a preferred embodiment, a cylindrical cutting guide having a diameter of approximately 130 mm is centrally positioned on the oversized optical disk. Cylindrical cutting guide positions a cutting apparatus such that a 130 mm diameter optical disk may be cut from the oversized 134 mm diameter optical disk. Cylindrical cutting guide is tightly positioned on the surface of the oversized 134 mm diameter optical disk such that excess material produced by the cutting apparatus is isolated from the resultant 130 mm diameter optical disk. The cutting apparatus removes substantially all of the edge wedge effect found at the outer diameter of the oversized 134 mm diameter optical disk, producing a normal sized 130 mm diameter optical disk with minimal a unusable outside diameter which corresponds to the width of the cutting guide. 
     FIG. 6 is a cross sectional view of an injection molding apparatus  42  used in the manufacture of an optical disk  20  in accordance with the present invention. The injection molding apparatus  42  is used for molding replicas of optical disk  20  in a disk molding process. The injection molding molding apparatus  42  is part of a complete optical disk molding manufacturing process, which can be a process for manufacturing optical disk made from a molded polymeric material (e.g., CD-ROM, DVD, MO, or phase change optical disks) in accordance with the present invention. The injection molding apparatus  42  generally includes a fixed side  44  and a moving side  46 . The fixed side  44  is movably coupled to the moving side  46  to form a disk substrate cavity  48 . A sprue channel  50  is provided for allowing material for forming the substrate  24 , such as a polycarbonate resin, to enter disk substrate cavity  48 . 
     The moving side  46  includes a sprue eject  52 , a gate cut  54 , a product eject  56 , a rod cover  58 , an inner holder  60 , a stamper  62  and an outer holder  68 . Sprue eject  52  is utilized for ejection of sprue  50  during opening of the injection molding apparatus. Gate cut  54  is utilized for cutting the opening  22  within the optical disk  20 . Project eject  56  is utilized for ejecting the finished product replica optical disk  20  from the injection molding apparatus. Inner holder  60  and outer holder  68  are removable for changing out and securing stamper  62 . Rod cover  58  is stationary within the moving side  46  to constrain the positions of the adjacent movable parts product eject  56  and the inner holder  60 . Stamper  62  is utilized for forming the formatted surface on surface relief pattern  29  into optical disk substrate  24 . 
     Injection molding apparatus  42  further includes a thermal inhibiting mechanism  69 , which in the exemplary embodiment shown, includes outer holder  68 . Thermal inhibiting mechanism  69  operates to inhibit heat flow from the outer edge of the disk substrate during cooling of the disk molding material, thereby reducing or eliminating the “edge wedge” effect. In one preferred embodiment, thermal inhibiting mechanism  69  has a low thermal conductivity rating and is made of low thermal conductivity materials. In one preferred embodiment the thermal inhibiting mechanism has a thermal conductivity in the range of 0.1 to 2 BTU/hr/ft/F, and more preferably is less than 5 BTU/hr/ft/F. Alternate embodiments of thermal inhibiting mechanism are described in detail later in the specification. 
     The process for molding optical disk substrate  22  in accordance with the present invention includes filling the disk substrate cavity  48  with a disk molding material, such as polycarbonate resin, through the sprue  50  channel (indicated at  63 ). After the resin is forced into the disk substrate cavity, but before cooling of the resin, the gate cut  54  is operated forward, indicated by arrow  64 , to cut opening  22  within the optical disk substrate. After cooling of the resin within the disk substrate cavity  48 , the formatted surface  34  has been embossed in optical disk substrate  22 , and the injection molding apparatus  42  is opened. The sprue eject  56  is operated forward (indicated by arrow  61 ). At the same time, the product eject  56  is operated to remove or eject the molded disk substrate  24  from the injection molding apparatus  42  surface (specifically, the surface of the moving side  46 ), indicated at  65 . During this process, the rod cover  58  remains stationary. The above process is repeated for the manufacture of each additional optical disk (or replica optical disk) substrate. Optical disk substrate  22  then passes through a finishing process for forming additional layers over the disk substrate to form information layer  26 , such as reflective or recording layers, and in the case of CD-ROM, protective layers, depending on the type and use of the optical disk. 
     FIG. 7 is an enlarged partial cross-sectional view of one exemplary embodiment of the outer perimeter of the mold disk substrate cavity  48  of FIG. 6, showing one exemplary embodiment of a thermal inhibiting mechanism  69  in accordance with the present invention, which includes outer holder  68  made of a low thermal conductivity material. Outer holder  68  is positioned at the outer perimeter of stamper  62  such that outer holder  68  securely holds stamper  62  in place during the disk substrate molding process. Data holder  68  is removable for changeout of stamper  62 . Outer holder  68  is also positioned such that an edge  72  of outer holder  68  contacts the molten polycarbonate forming optical disk  20  during the injection molding process. 
     Outer holder  68  plays a critical role in the formation of optical disk  20 . As mentioned above, in prior art disk molding apparatus, the “edge wedge” effect  41  present on optical disk  20  is attributable to several factors present during the injection molding process as previously described herein. One such factor is that the molten polycarbonate forming optical disk  20  freezes at different rates at different radii of the part during the cooling time process in mold cavity  48 . The outer edge of the optical disk  20  substrate freezes through the thickness first of all due to its contact with the colder outer holder. Thereby, the outer edge of optical disk  20  exhibits a greater thickness than the remainder of the optical disk  20  surface. The outer holder  68  in accordance with the present invention operates to eliminate or reduce the edge wedge effects. 
     FIG. 8 is a top view of a single piece outer holder  68  of the injection molding apparatus  42 , in accordance with the present invention, and FIG. 9 is an enlarged, partial cross-sectional view of a single piece outer holder  68 , illustrating the interaction between outer holder  68  and stamper  62  wherein outer holder  68  is made of a material exhibiting low thermal conductivity properties. 
     Outer holder  68  illustrates one exemplary embodiment of a specific implementation of the generalized outer holder  68 , as described in FIG.  7 . In one preferred embodiment, outer holder  68  is formed of titanium. Titanium outer holder  68  has a plurality of mounting holes  78  formed therethrough. Mounting holes  78  accommodate fastening devices, such as screws, which affix outer holder  68  to injection molding apparatus  42 . Outer holder  68  has a lip  84  which rests over the top surface of stamper  62  to hold stamper  62  in place. Lip  84  of outer holder  68  also provides a contact surface  85 , which contacts the molten polycarbonate during the injection molding process. 
     In the illustrated embodiment, outer holder  68  is designed to limit the heat flow from the molten polycarbonate forming optical disk  20  to outer holder  68 . This is accomplished by using a low thermoconductivity tooling material in the construction of outer holder  68 . In the illustrated embodiment, this material is titanium. Table 1 gives the thermal conductivity for a number of tooling steels: 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 CONDUCTIVITY OF SELECTED MATERIALS 
               
             
          
           
               
                   
                 Material 
                 Conductivity (BTU/hr/ft/F) 
               
               
                   
                   
               
             
          
           
               
                   
                 Aluminum 
                 119 
               
               
                   
                 Copper 
                 222 
               
               
                   
                 Hardwood 
                 0.10 
               
               
                   
                 Tool Steel 
                 21 
               
               
                   
                 Titanium 
                 3.9 
               
               
                   
                 AREMCO Macor Ceramic 
                 0.97 
               
               
                   
                   
               
             
          
         
       
     
     As can be seen from Table 1, titanium has a conductivity that is considerably lower than that of typical tool steels used in components of this nature. The lower thermal conductivity of outer holder  68  made from titanium causes the molten polymeric material (preferably polycarbonate) to conduct heat away to the outer holder at a rate that is approximately 30 percent lower than typical tooling steels. Thus, by using an outer holder  68  made of low thermoconductivity titanium, “edge-wedge” effect  41  in optical disk  20  can be reduced or eliminated. Other low thermal conductivity materials useable for forming outer holder  68  will become apparent to those skilled in the art after reading the disclosure of the present invention. 
     FIG. 10 is a top view of a three piece outer holder  68  used within the injection molding apparatus  42  of the present invention, wherein one piece of the outer holder is a low thermoconductivity ceramic component  94 . FIG. 11 is an enlarged, partial cross-sectional view of the three-piece outer holder  68 , wherein one piece of the outer holder is a low thermoconductivity ceramic component  94 . 
     Three-piece outer holder  68  of the illustrated embodiment is a specific implementation of the generalized outer holder  68 , as described in FIG.  7 . Three piece outer holder  68  has a low thermoconductivity ceramic insert  94  nestled into a pocket formed by an upper ring member  90  and a lower ring member  92  (i.e., retained between member  90  and member  92 ). Outer holder  68  has a plurality of mounting holes  78  formed therethrough. Mounting holes  78  accommodate fastening devices, such as screws, which affix outer holder  68  to injection molding apparatus  42 . Ceramic insert  94  forms a lip which rests over the top surface of stamper  62  to hold stamper  62  in place. Ceramic insert  94  of outer holder  68  also provides a contact surface  95  which contacts an edge of the molten polycarbonate during the injection molding process. 
     In the illustrated embodiment, three-piece outer holder  68  is designed to limit the heat flow from the molten polycarbonate forming optical disk  20  to outer holder  68 . This is accomplished by using a low thermoconductivity tooling material in the construction of outer holder  68 . In one preferred embodiment shown, this material is AREMCO Macor ceramic. Ceramic is typically not found in injection molds as it is relatively harder to work with to create precision shapes. In the present invention, this difficulty is surmounted by integrating a simple ring shaped ceramic insert  94  into three-piece outer holder  68 . As the outer perimeter of optical disk  20  is formed from the molten polycarbonate within injection molding apparatus  42 , the molten polycarbonate encounters ceramic insert  94 , and heat flow in the redirection (as shown in FIG. 2) is vastly decreased. 
     FIG. 12 is an illustration of a resistive heater  100  and FIG. 13 is an illustration of the outer holder  68  incorporating the resistive heater of FIG.  12 . By heating outer holder  68  during the injection molding process, a smaller temperature differential is created between optical disk  20  and outer holder  68 , thereby reducing the heat transfer between optical disk  20  and outer holder  68 . Thus, raising the temperature of outer holder  68  by only a few degrees has a beneficial effect towards reducing the “edge wedge” effect. 
     In the illustrated embodiment, the present invention incorporates a resistive heater  100  within an interior channel  107  of outer holder  68  to reduce heat transfer. In one preferred embodiment the resistive heater is made of copper, and more preferably is a CalRod resistive heater. The resistive heater  100  is electrically coupled to an electrical power source  104  via interface  102 . The resistive heater  100  of the present invention provides a user a new degree of control in the optical disk injection molding process, as the resistive heater  100  can be controlled, or switched “on” and “off” during the molding process to facilitate better overall performance. 
     In another embodiment, a heated fluid may be used to heat outer holder  68 . FIG. 14 is an illustration of a hollow coil  108  which contains heated, recirculating water, and FIG. 15 is an illustration of outer holder  68  incorporating the heated recirculating water coil  108  of FIG. 14 within an interior channel  107 . 
     In the illustrated embodiment, the heated recirculating water coil  108  serves the same purpose as the resistive heater (FIG.  12  and FIG. 13, element  100 ), namely to reduce heat transfer from the outer perimeter of the optical disk  20  to the outer holder  68 . In this embodiment, the heated recirculating water coil  108  is connected to water pump  112  via interface  110 . A heat source  114  serves to heat the water in the water pump reservoir before it is recirculated through interior channel  107  of outer holder  68 . As with the resistive heater embodiment shown in FIGS. 10 &amp; 11, the heated recirculating water coil  108  of the present invention provides a user with a new degree of control in the optical disk injection molding process, as the temperature of water passing through the heated recirculating water coil  108  can be precisely controlled during the molding process to facilitate better overall performance. 
     FIG. 16 is an illustration of a hollow coil  108  which contains heated, recirculating oil, and FIG. 17 is an illustration of outer holder  68  incorporating the heated recirculating oil coil  108  of FIG. 16 within an interior channel  107 . 
     In the illustrated embodiment, the heated recirculating oil coil  108  serves the same purpose as the resistive heater (FIG.  12  and FIG. 13) and the heated recirculating water coil (FIG.  14  and FIG.  15 ), namely to reduce heat transfer from the outer perimeter of the optical disk  20  to the outer holder  68 . In this embodiment, the heated recirculating oil coil  108  is connected to oil pump  112  via interface  110 . A heat source  114  serves to heat the oil in the oil pump reservoir before it is recirculated through interior channel  107  of outer holder  68 . As with the heated recirculating water coil embodiment shown in FIGS. 14 and 15, the heated recirculating oil coil  108  of the present invention provides a user with a new degree of control in the optical disk injection molding process, as the temperature of oil passing through the heated recirculating oil coil  108  can be precisely controlled (e.g., using a control mechanism) during the molding process to facilitate better overall performance. 
     FIG. 18 is an illustration of outer holder  68 , wherein outer holder  68  is heated inductively by an external heat source  134 . In this embodiment, outer holder  68  is heated through indirect, non-contact heating, as shown at  136 . Unlike the other forms of heating described above, inductive heating may not be precisely directed to heat only outer holder  68 . Rather, inductive heating may raise the temperature of additional components within the injection molding apparatus  42 , and also molding cavity  48 . As with the other heated outer holder  68  embodiments described above, this embodiment raises the temperature of outer holder  68 , thus decreasing the temperature differential between the molten polycarbonate and outer holder  68 . This serves to reduce the edge wedge effect in the resultant optical disk  20 . 
     FIG. 19 is an illustration of outer holder  68  having a surface mount, film resistive heater  138  dispersed on the surface of outer holder  68 , wherein outer holder  68  is heated thermoelectrically by film resistive heater  138 . Film resistive heater  138  is electrically coupled to an electrical power source  140  via interface  139 . Resistive heater  138  may comprise a relatively “thick” film or “thin” film resistive heater. As with the other heated outer holder  68  embodiments described above, this embodiment raises the temperature of outer holder  68 , thus decreasing the temperature differential between the molten polycarbonate and outer holder  68 . This serves to reduce the edge wedge effect in the resultant optical disk  20 . 
     FIG. 20 is a cross sectional view of a prior art optical disk  20  molded using a conventional mold cavity, wherein the “edge wedge” effect  188  is seen at the outer edges of the optical disk  20 . As mentioned in detail above, edge wedge is a phenomenon that occurs on the surface of optical disks  20  such that the thickness of optical disk  20  increases near the outside edge of the optical disk, as shown at  188 . The wedge starts approximately 10 mm from an outside edge  186  of optical disk  20  and continues to increase in height until outside edge  186  of optical disk  20  is reached. At outside edge  186  of optical disk  20 , the width of the disk is approximately 20 microns greater than the width at an interior region  185  of optical disk  20 . Head crashes occur when a read/write head is in the region of the “edge wedge” effect  188 . For the head not to crash, it must stay out of the wedge region. This makes the “edge wedge” region  188  of optical disk  20  unusable for data storage and thus limits data capacities of optical disks  20 . 
     FIG. 21 is a cross sectional view of an improved optical disk  190  having an enhanced surface area, molded using an outer diameter wedge containment mold of the present invention. The wedge containment mold addresses the problem of “edge wedge” by providing rectangular areas at the edges of the wedge containment mold for the wedge to “flow” into, as shown at  194 . Unlike the gradually sloping characteristic of the conventional “edge wedge” effect  188 , the wedge containment mold moves the “edge wedge” effect closer to the edge of optical disk  190 , in a steeper slope profile, as seen at  194 , thus providing more usable surface area on optical disk  190 . The total surface area of the wedge containment mold is less than a conventional containment mold, thus reducing the effective area in which the “edge wedge” effect can be exhibited. Improved optical disks  190  formed from wedge containment mold allow the read/write head to fly much closer to the outer edge of optical disk  190  and thus allow for increased data capacities. 
     FIG. 22 is a cross sectional view of a prior art molded optical disk substrate  20  exhibiting the “edge wedge” effect  156  at the outer edges of substrate  20 . In the process of molding optical disk substrate  20 , molten polycarbonate is injected into substrate cavity  48  formed by stamper  62  on one side and fixed mirror block on the other side, as shown in FIG.  6 . Heated polycarbonate is injected at gate  63  located at the center of the substrate cavity  48 , as also shown in FIG.  6 . Typical optical substrates  20  are molded to 120-130 millimeters in diameter and 1.2-2.0 mm in thickness. Ideally, the information surface of stamper  62  and fixed mirror block  44  are parallel. However, when measurements of the thickness of the optical disk substrate  20  are made, the outside edge of the disk is 15 to 25 microns thicker than the thickness of the disk 3 millimeters in from the outside edge. The edge thickness change depends on substrate thickness as well as molding parameters, in particular, mold temperature. As stated earlier, even though the thickness can be influenced by molding parameters and disk size, all injection molded optical disks  20  have the edge thickness (“edge wedge”) effect  156 . 
     FIG. 23 illustrates an anti-wedge stamper  62  of the present invention, wherein lip  166 , preferably an electroplated nickel lip, is added to the outer perimeter of the back side  167  of a stamper  160 . Stamper  62  is initially plated to a thickness of 280 microns. Stamper  62  is then removed from the electroplating bath and a series of masks from 127 mm to 130 mm are adhered to back side  167  of stamper  162 . In a preferred embodiment, the best results are obtained using a laser cut 130 mm mask. After the mask is adhered to the plated nickel back side  167  of stamper  62 , stamper  62  is reinserted into the electroplating path and the non-masked area is plated with an additional 15 microns of nickel, thus producing a 3 mm wide lip  166  around the outside perimeter of back side  167  of stamper  62 . Stamper  62  is then polished, punched to size and cleaned as normal. 
     FIG. 24 illustrates anti-wedge stamper  62  of the present invention, wherein stamper  62  flexes during the injection molding process such that the outer edges of stamper  62  deflect upwardly in an anti-wedge configuration. 
     After lip  166  has been formed on the back side  167  of stamper  62 , stamper  62  is inserted into the injection molding apparatus (FIG. 6, element  42 ). Under the pressure of injecting polycarbonate  171  into molding cavity  48 , back side  167  of stamper  62  is pressed tightly against moving side mirror block  46 . However, back side  167  of stamper  62  adjacent to lip  166  cannot be pressed tightly to the bottom mirror block, and forms an arc or shape which counters the “edge wedge” effect created by the injection molding process, as shown at  169 . Mold cavity  48  is shaped by fixed side mirror block  44  and an information surface  173  of stamper  62 . With the anti-wedge effect of stamper  62  now in effect, molding cavity  48  is narrower at the outside by approximately 15 microns. This reduction in cavity thickness at the outside edge of molding cavity  48  counters the inherent “edge wedge” effect, resulting in a molded optical disk having a top surface  154  that is flat to the edge, as shown in element  180  of FIG.  25 . The “edge wedge” has now been substantially reduced or eliminated, as shown at  182 . 
     Numerous characteristics and advantages of the invention have been set forth in the foregoing description. It will be understood, of course, that this disclosure is, and in many respects, only illustrative. Changes can be made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention. The invention scope is defined in the language in which the appended claims are expressed.