Abstract:
In one embodiment, the invention is directed to an injection molding tool that has improved grounding. The injection molding process creates significant amounts of static electricity in the molten thermoplastic. This static electricity can be more adequately dissipated from the system through the addition of one or more paths to ground. In this manner, a significant reduction in charge passing between the stamper and the mirror block can be achieved. By reducing or eliminating the amount of charge passing between the stamper and the mirror block, erosion of the mirror block can be reduced or avoided.

Description:
TECHNICAL FIELD 
   The invention relates to injection molding of optical components such as optical data storage disks. 
   BACKGROUND OF THE INVENTION 
   Injection molding tools can be used to create high quality optical components such as optical data storage disks. Optical data storage disks have gained widespread acceptance for the storage, distribution and retrieval of large volumes of information. Optical data storage disks include, for example, audio CD (compact disc), CD-R (CD-recordable), CD-ROM (CD-read only memory), DVD (digital versatile disk or digital video disk) media, DVD-RAM (DVD-random access memory), and various types of rewritable media, such as magneto-optical (MO) disks and phase change optical disks. Some newer formats for optical data storage disks are progressing toward smaller disk sizes and increased data storage density. Injection molding tools are also used to create other molded components such as optical components having a high quality reflective surface. 
   The process of creating high quality injection molded components begins with a mastering process. For example, the creation of optical data storage disks involves making a master disk that has a surface pattern that represents encoded data on the master disk surface. The surface pattern, for instance, may be a collection of grooves that define master pits and master lands. The mastering process is a relatively expensive process in which a photoresist is coated on a master substrate, exposed to electromagnetic radiation to develop a photoresist pattern, and then removed in the non-developed regions. The mastering process may be used not only for pre-recorded disks but also rewritable disks that carry a pattern on the substrate, such as servo tracking or pre-recorded information. 
   After creating a suitable master, that master can then be used to make a stamper. The stamper has a surface pattern that is the inverse of the surface pattern encoded on the master. For example, to create a stamper suitable for use in an injection molding process, a thin nickel layer can be deposited on the patterned master surface. Then, in an electroplating process, a nickel structural layer can be uniformly plated onto the nickel coated surface of the master. When the electroplating process is complete, the nickel layers can be collectively removed from the master to form a nickel stamper. 
   The stamper, then, can be inserted within an injection molding tool, and the injection molding tool can be used to injection mold large quantities of molded components such as molded replica disks. The injection molding tool typically includes mirror blocks positioned on both sides of a mold cavity. The stamper can be vacuum drawn against one of the mirror blocks and hot molten thermoplastic can be injected into the mold cavity. The molten thermoplastic conforms to the mold cavity, defining an inverse of the pattern preserved on the stamper. The mirror blocks provide extremely flat surfaces to support the back side of the stamper and to define the non-stamper side of the mold cavity. In addition, the mirror blocks typically include a coolant system to control cooling of the molten thermoplastic. 
   Upon cooling, the molded component may contain the pattern originally encoded on the master and preserved in the stamper. For example, molded replica disks may contain the data and tracking information that was encoded on the master. Alternatively, the process can be used to create other types of molded components such as high quality reflective components. In that case, the mastering process would define the high quality reflective surface of the molded component that is ultimately created in the injection molding process. 
   SUMMARY 
   In general, the invention is directed to an injection molding tool that has improved grounding. The injection molding process creates significant amounts of static electricity in the molten thermoplastic material. This static electricity can be more adequately dissipated from the system through the addition of one or more conductive paths to ground. In this manner, a significant reduction in charge passing between the stamper and the mirror block can be achieved. By reducing or eliminating the amount of charge passing between the stamper and the mirror block, erosion of the mirror block can be reduced or avoided. Reduction or avoidance of mirror block erosion can significantly improve the longevity of the mirror block and maintain the quality of optical components produced by a mold that incorporates the mirror block. 
   In one embodiment, the invention is directed to an injection molding technique that includes grounding a molding tool so as to reduce charge dissipation through an air interface between a stamper and a mirror block of the molding tool. The method may also include creating a molded component using the molding tool. 
   In other embodiments, the invention is directed to a molding tool. For example, the molding tool may include a moving side and a non-moving side. In addition, the molding tool may include a substantially non-resistive path to ground coupled to the moving side. For example, the substantially non-resistive path may comprise an electrically conductive ground strap coupled to the moving side. The molding tool may form part of an automated molding system that includes a control unit coupled to the molding tool to control a molding process of the molding tool. A robotic arm may be coupled to the control unit, and controlled to receive molded components from the molding tool. 
   The invention is capable of providing several advantages. For example, the invention can increase the number of molding cycles that can be performed before a mirror block requires resurfacing. In addition, the invention may also prolong the life of stampers used in molding tools. In both cases, by increasing the number of molding cycles that can be performed before the molding tool requires maintenance, the per-component cost of injection molding can be significantly reduced. 
   Additional details of these and other embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages will become apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional side view of an exemplary molding tool according to an embodiment of the invention. 
       FIG. 2  is a cross-sectional side view of a molding tool that has a mirror block on a moving side that is grounded in accordance with an embodiment of the invention. 
       FIG. 3  is a cross-sectional side view of a molding tool that has a stamper that is grounded in accordance with an embodiment of the invention. 
       FIG. 4  is a cross-sectional side view of a molding tool that has two mirror blocks that are grounded in accordance with an embodiment of the invention. 
       FIG. 5  is a cross-sectional side view of a molding tool that has two stampers that are grounded in accordance with an embodiment of the invention. 
       FIG. 6  is a block diagram of a molding system incorporating one or more aspects of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a cross-sectional side view of a molding tool according to an embodiment of the invention. Molding tool  10  is an injection molding tool for injection molding thermoplastic material into high quality optical components. In the description that follows, many aspects of the invention are described in the context of an injection molding process for creating optical data storage disks. However, it is understood that the invention is not limited to the types of components being molded. Rather, the invention is readily applicable to injection molding tools which are customized to make high quality optically molded components including components having a high quality reflective surface or other components which require precision flat surfaces of mirror blocks within the injection mold. 
   Molding tool  10  includes a moving side  12  and a non-moving side  14  that define a cavity  16 . Non-moving side  14  includes a mirror block  20  and a stamper  22  positioned adjacent mirror block  20 . Mirror block  20  may include vacuum ports  24 A and  24 B to hold stamper  22  in place. In other words, a vacuum force can be used to hold stamper  22  next to mirror block  20 . Mirror block  20  may be fixed to a support structure  26 . In operation, molten thermoplastic material is injected into cavity  16  through injection aperture  18  of non-moving side  14 . The molten thermoplastic then cools, and moving side  12  retracts to open cavity  16  so that the molded component can be removed. 
   Moving side  12  also includes a mirror block  30 , but may not include a stamper. In that case, the molded component being created may be an optical data storage disk having information stored on only one side, i.e., the side corresponding to non-moving side  14  of molding tool  10 . Mirror block  30  may be fixed to a support structure  36 . 
   Moving side  12  also includes a moving unit  38  such as a computer controlled hydraulic system. The moving unit  38  can be used to open and close mold cavity  16  by moving the moving side  12 . Moving unit  38  includes various hydraulic components  40  that connect to support structure  36  and possibly mirror block  30 . Moreover, the hydraulic components  40  are typically greased to reduce friction during movement and generally improve the ability to move moving side  12 . The greased hydraulic components  40 , however, also provide a relatively large amount of electrical resistance between moving unit  38  and the other components of moving side  12 . 
   The molten thermoplastic material is typically injected through injection aperture  18  at extremely high pressures. For example, in creating molded optical storage disks, molten thermoplastic material may be injected through injection aperture  18  at pressures on the order of 25,000 to 30,000 pounds per square inch (PSI). In that case, mold cavity  16  can be filled in approximately 0.2 to 0.3 seconds. A cooling system within mirror blocks  20  and  30  can control the cooling of the thermoplastic material, and once cooled, the mold cavity  16  may open by movement of moving side  12 . The cooling system within mirror blocks, for example, may comprise water filled coils that can be used to control cooling of the molten thermoplastic material. 
   The high pressure at which the thermoplastic material is injected into molding tool  10  causes shearing in the thermoplastic material. In other words, frictional strain in the material caused by the high pressure injection can cause significant static electricity to be created in the injected material. Thus, once mold cavity  16  is filled with thermoplastic material, the material may possess a significant static charge. For example, static charge on the order of 100,000 volts may be created inside mold cavity  16 , assuming the pressures mentioned above. This static charge dissipates through molding tool  10 , following the least resistive path to ground. 
   In accordance with the invention, moving side  12  of molding tool  10  is coupled to a substantially non-resistive path to ground (as indicated at numeral  42 ). For example, a ground strap can be attached from support structure  36  of moving side  12  to any adequately grounded structure. In this manner, static charge may dissipate from mold cavity  16  through mirror block  30  and support structure  36  and ultimately to ground via the substantially non-resistive path indicated by numeral  42 . 
   The ground strap may be any substantially conductive wire electrically coupling the moving side  12  to any grounded reference voltage. In one specific example, the ground strap is a braided copper wire having a length on the order of a meter. Other conductive wires of various lengths could also be used. The ground strap may be firmly connected to the moving side via any suitable attachment mechanism such as a screw, a clamp, welding, or the like, to provide the substantially non-resistive path to ground  42 . In one case, the grounded reference voltage may comprise the non-moving side of the molding tool. 
   The substantially non-resistive path to ground  42  provides significant improvement to the life cycle of molding tool  10 . Without the substantially non-resistive path  42 , first side may not be suitably grounded because the hydraulic components  40  include a grease barrier. Thus, without non-resistive path  42 , the static charge substantially dissipates through non-moving side  14 , which can cause significant erosion to mirror block  20 . Erosion to mirror block  20  is a significant concern because of the cost and time it takes to resurface mirror blocks. Moreover, even a small amount of erosion can undermine the creation of optical data storage disks or other high quality optical components because of the precision flat surface required for mirror blocks. 
   As mentioned, stamper  22  is vacuum drawn to mirror block  20  via vacuum ports  24 A and  24 B. Thus, there remains a small air gap between stamper  22  and mirror block  20 . When charge passes across this small air gap, a small spark is created which can cause erosion in mirror block  20 . The invention can reduce or eliminate this erosion by providing the static charge within cavity  16  an alternative path to ground via the ground strap. 
   EXAMPLE 
   Sumitomo CD-RW Molding System, model number SD 40, commercially available from Sumitomo Plastics-Machinery of Norcross, Ga. was used to mold optical data storage disks. A molding tool within the Sumitomo CD-RW Molding System operated in a manner similar to the description above with reference to  FIG. 1 . However the molding tool originally did not include the substantially non-resistive path to ground  42 . 
   In that case, the life cycle of the mirror block adjacent the vacuum drawn stamper was only approximately 75,000 molding cycles. After 75,000 cycles, the mirror block required resurfacing because of erosion. Resurfacing of a mirror block may take approximately four to six weeks and typically costs approximately $4000. Given the number of molding cycles, erosion in the mirror block was adding more than $0.05 of cost to each molded data storage disk. 
   The erosion pattern of the eroded mirror block was examined under a microscope. The microscopic inspection revealed that the erosion in the mirror block looked similar to metallic surfaces finished using Electric Discharge Matching (EDM) processes. For this reason, it was hypothesized that the static charge within the mold cavity was dissipating between the stamper and the mirror block causing sparks that would systematically erode the mirror block in a manner similar to EDM processes. 
   A ground strap was installed on the molding tool of the Sumitomo CD-RW molding system to connect the non-moving side of the molding tool to a grounded structure. The molding process was then re-initiated. In that case, when the molding tool conformed to the illustration in  FIG. 1 , erosion in the mirror block adjacent the vacuum drawn stamper was significantly reduced. Specifically, more than 900,000 molding cycles were performed before the mirror block required resurfacing. Thus, with a ground strap installed in the molding tool, erosion in the mirror blocks accounted for less than $0.005 of the cost of each molded data storage disk, which was more than a 10-fold reduction in costs relating to resurfacing of the mirror blocks. 
   Different mirror blocks were tested with the ground strap installed. Mirror blocks having titanium nitride coatings and diamond like carbon coatings both proved very useful. For example, titanium nitride (TiN) coated mirror blocks lasted more than 500,000 molding cycles after installing a ground strap, and diamond like carbon (DLC) coated mirror blocks lasted more than 900,000 cycles with the ground strap installed. In the case of titanium nitride (TiN) coated mirror blocks, a number of molding cycles had already been performed prior to installing the ground strap. Thus, it is likely that significantly more than 500,000 molding cycles can be achieved with the ground strap installed on a molding tool using a titanium nitride (TiN) coated mirror block on the non-moving side. 
   Additional benefits of grounding the moving side of a molding tool include the extended life of the stamper. In other words, in addition to significantly extending the life of the mirror block on the non-moving side of the molding tool, stamper life may also be extended when a ground strap is installed as described above. Without a ground strap installed, the useful life of a nickel stamper was approximately 25,000 molding cycles. With the ground strap installed, the useful life of a nickel stamper was greater than 50,000 molding cycles. 
     FIGS. 2–5  are cross-sectional side views of exemplary molding tools configured to reduce charge dissipation through an air interface between a stamper and a mirror block. For simplicity, various features such as injection apertures and moving units are not illustrated. In particular,  FIG. 2  illustrates molding tool  200  in which mirror block  202  of moving side  204  is connected to ground as indicated at numeral  208 . Molding tool  200  may also include a non-moving side  210  that includes a mirror block  212  and a stamper  214  adjacent mirror block  212 . 
   In  FIG. 3 , stamper  314  on the non-moving side  310  of molding tool  300  is connected to ground as indicated by numeral  308 . In that case, by connecting stamper  314  directly to ground, static charge within cavity  316  will not substantially dissipate across the air gap between stamper  314  and mirror block  312 . 
     FIGS. 4 and 5  illustrate how the principles of the invention may be extended to two sided molding tools, i.e., molding tools configured with stampers on both sides of a mold cavity. Two sided molding tools are desirable for creating many types of optical components including data storage disks having information encoded on both sides. 
   In  FIG. 4  molding tool  400  includes a moving side  404  and a non-moving side  410 . Both moving side  404  and non-moving side  410  include mirror blocks and stampers. For example, moving side  404  includes a first mirror block  402  and a first stamper  420  and non-moving side  410  includes a second mirror block  412  and a second stamper  414 . Both mirror blocks  402  and  412  can be coupled to ground as indicated at  422  and  424 . Although the configuration of  FIG. 4  may not eliminate charge dissipation across air gaps between the stampers  420 ,  414  and mirror blocks  402 ,  412 , the configuration may be used to ensure that static charge within mold cavity  416  dissipates substantially equivalently across the two air gaps between the stampers and  420 ,  414  and mirror blocks  402 ,  412 . 
     FIG. 5  illustrates a two sided molding tool  500  similar to that illustrated in  FIG. 4 . However, in  FIG. 5 , both stampers  520  and  514  are directly coupled to ground as indicated by numerals  522  and  524 . This configuration ensures that static charge within mold cavity  516  will not substantially dissipate across the air gaps between the stampers and  520 ,  514  and mirror blocks  502 ,  512 . 
     FIG. 6  is a high level block diagram of a molding system. In accordance with the invention, molding system  600  includes a molding tool  602  configured to reduce charge dissipation through an air interface between a stamper and a mirror block as described herein. Molding tool  602  may be coupled to a control unit  604  that automates and controls the injection molding process. Molding system  600  may also include a robotic arm  606  coupled to the control unit  604 . 
   In an automated injection molding process, control unit  604  controls the injection molding process within molding tool  602 . Once the molten thermoplastic has been molded within molding tool  602  and cooled by a sufficient amount, control unit  604  directs molding tool  602  to open and drop the molded component onto robotic arm  606 . Robotic arm  606  may include pressure sensors to sense the molded component. Once robotic arm  606  has received and sensed the molded component, robotic arm  606  can move the molded component to a location for further cooling and processing, such as a conveyor belt (not shown) of molding system  600 . Control unit  604  can then direct the molding tool  602  to perform another molding cycle. Because molding tool  602  is configured to reduce charge dissipation through an air interface between a stamper and a mirror block, the number of molding cycles that molding system  600  can perform before molding tool  602  requires maintenance can be significantly increased. 
   Various embodiments of the invention have been described. For instance, a technique of grounding a molding tool so as to reduce charge dissipation through an air interface between a stamper and a mirror block of the molding tool has been described. A number of exemplary structures for achieving adequate grounding have also been described. Nevertheless, various modifications may be made without departing from the scope of the invention. For example, other structures or configurations could be designed to adequately ground a molding tool so as to reduce charge dissipation through an air interface between a stamper and a mirror block of the molding tool. In addition, although various aspects of the invention have been described in terms of molding optical data storage disks, it is understood that the principles of the invention are not necessarily limited to types of molded components being created. Accordingly, other embodiments are within the scope of the following claims.