INJECTION MOLDING OF COMPACT DISCS

Abstract: A method for the injection of molding of compact discs at the very small thickness of 0.6 mm employing a critical sequence of steps during and after the injection of molten plastic into the mold cavity, including allowing the injected plastic to force the mold cavity to partly reopen for a predetermined time (or distance) before closing the mold to its final dimensions.

Description: INJECTION MOLDING OF COMPACT DISCS Fjelfl Of The invention This invention relates to methods for the manufacture of compact discs by an injection molding processes. Background Of The Invention The invention is concerned with injection molding of 0.6 mm thick optical discs. More specifically an injection molding machine operating under a unique process is utilized to produce 0.6 mm optical discs suitable for lamination to either blank or information side discs. Injection molding methods and apparatus are known in the art. Injection molding can be generally described as a replication process in which a molten material (usually a polymer) is injected into a cavity and after cooling forms the geometry of the cavity. Injection molding has been used for the mass production of 1.2 mm thick optical discs. Based on inadequate quality of the result- ant product and their slow cycle time the techniques used for mass production by injection molding of 1.2 mm optical discs are not suitable for 0.6 mm discs. The quality requirements for such discs may be expressed in terms of the tilt of the disc (viz. its deviation from planarity) , the jitter (viz. the standard deviation of the pit lengths from their predetermined values), and its birefringence (viz. The stress induced property of polymers to split an incident light beam depending upon its direction through the polymer) . Brief Description Of The Invention Variations of the injection molding process are used in the replication of optical discs such as CD audio, CD ROM and laser discs. The processes used are suitable because these optical disc formats are all of a 1.2 mm or greater thickness. A new 0.6 mm optical disc has been developed which offers several advantages over the 1.2 mm format, particularly in the field of high data density optical discs in which data is recorded in a spiral track of alternating pits and lands. Laser light returned to a reading head by such pits and lands is viewed through an optically flat side of the optical disc. Some bending of the light received from a pit or land occurs at the surface of the polycabonate surface of the optical disc. A thinner optical disc enables the reading laser to employ a smaller spit size and the lenses of the reading head to have a higher numerical aperture when used to readout a pattern of pits and lands. The readout the data from such thinner optical discs uses laser light with a smaller wavelength than was used for the 1.2 mm discs. This has entailed a wavelength change from 780 nm to approximately 650 nm or less. Thus, the readout of a higher density of pits and lands, and therefore of more data per disc, is possible with the thinner optical discs. The typical improvement is from 650 mega-bits of data to a 5 giga-bits. A key feature of the present invention is a method for the injection of molding of com- pact discs at the very small thickness of 0.6mm. In particular it has been discovered that the requirements of such discs in terms of their planarity, birefringence, and jitter can be met by a critical sequence of steps during and after the injection of molten plastic into the mold cavity, including allowing the in- jected plastic to force the mold cavity to partly reopen for a predetermined time (or distance) before closing the mold to its final dimensions. Brief Description Of The Drawings Fig. la-Id is a schematic drawing of the mold chamber of an injection molding machine during the different stages of the process of the present invention. Fig. 2 is a graph of the injection molding machine parameters, i.e. mold position, screw position, and clamp pressure during the preferred process of the present invention. Fig. 3a-b is a graph of a graph of an un- acceptable alternative set of parameters demonstrating the criticality of the present invention. Fig. 4 is a graph of the birefringence of the disc produced by the present invention. Fig. 5a-b is a histogram of the jitter of the disc produced by the present invention. Detailed Description Of A Preferred Embodiment The process for injection molding of 0.6 mm discs from can be described in four steps performed on a commercial plastics injection molding machine where the plastic is an optical grade polycarbonate. As shown in Fig. 1, the mold 1 is a cavity one face of which 3 . is optically flat, and the other face 5 . is a nickel impression of a compact disc. A channel 2 is filled with a molten polycarbonate which is driven into the chamber 9, of the mold by an advancing screw .11.. The faces of the mold, 3 . and 5 . may be moved closer together by a driving mechanism (not shown) . The dimensions of the mold are exaggerated in Fig. la-Id. The typical diameter of the face 5_ is 120 mm, while the mold opening 9 has an separation of the faces of less than 1600 μ. The polycarbonate 13 is not forced into the mold cavity until the mold cavity reaches an interim position, at which point, as shown in Fig. lb, the molten polymer 12 is injected into the mold cavity by an injection force due to the advance of the screw 11. As the injection force exceeds the closing force, the mold cavity is opened slightly, as shown in Fig. lc. As the closing force exceeds the opening force, the closing of the cavity is brought to the final dimension as in Fig. Id. It has been discovered that a particular set of parameters guiding the injection process are particularly suited to the production of a 0.6 mm compact disc having less than about 0.6 degrees of tilt and sufficiently minimal deviation in pit lengths (jitter). In particular, the preferred embodiment of this invention for a disc having a radius of approximately 60 mm is as follows, as depicted in Figure 2. The mold is open 900 μ. It is then closed to an interim position of 100 μ and injection of the polymer is begun. Before the full charge of polymer is injected, the injection pressure exceeds the clamp pressure and the mold begins to open. When the injection is completed, and during the next 0.07 seconds, the mold continues to be forced open to 300 μ and then begin to close. At the end of the positive 0.07 seconds delay the mold is clamped with full force. In order to demonstrate the criticality of these parameter values, the figures show the result of delays of positive seconds and negative 0.25 seconds, both of which result in unplayable discs emerging from the injection molding process. The tables show the results of the three examples of the patent. While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention.

Claims: What is claimed is: 1. A process for the injection molding of a compact disc made from a polymer and having a thickness no greater than 0.6 mm comprising opening the mold, closing the mold to an interim position of lOOμ and commencing the injection of the polymer, such that before the full charge of polymer is injected, the injection pressure exceeds the clamp pressure and the mold begins to open, delaying the compression of the mold for 0.07 seconds, during which the mold continues to be forced open to 300μ and then begin to close, clamping the mold with full force. 2. A process for the injection molding of a compact disc made from a polymer and having a thickness no greater than 0.6 mm comprising opening the mold, closing the mold to an interim position and commencing the injection of the polymer, such that before the full charge of polymer is injected, the injection pressure exceeds the clamp pressure and the mold begins to open, delaying the compression of the mold for 0.07 seconds, during which the mold continues to be forced open and then begin to close, clamping the mold with full force. 3. A compact disc formed by the injection molding of a polymer and having a thickness no greater than 0.6 mm, said disc having a birefringence less than 150 and a tilt less than 0.6 degrees formed by the process of opening the mold, closing the mold to an interim position of lOOμ and commencing the injection of the polymer, such that before the full charge of polymer is injected, the injection pressure exceeds the clamp pressure and the mold begins to open, delaying the compression of the mold for 0.07 seconds, during which the mold continues to be forced open to 300μ and then begin to close, clamping the mold with full force.

Inventors: MUELLER WILLIAM, PICKUTOSKI EDWARD, MUELLER, WILLIAM, PICKUTOSKI, EDWARD
Applicants: WEA MFG INC, WEA MANUFACTURING, INC.
Application year: 1997
Main classification: G11B7
IPC codes: B29C45, G11B7