Abstract:
An injection molding process wherein the times of heating to melt a resin are reduced to minimize the thermal hysteresis and to decrease the deterioration of physical properties caused by the thermal hysteresis, so that high quality information recording disks may be produced. A molding process wherein the plasticating and the injection and filling of a material resin are carried out in separate cylinders, allows constant molding for a long period of time. A material resin such as polycarbonate is supplied without predrying to an injection apparatus for a first time molding. Resin is kneaded by the apparatus while volatile components are heated to be vaporized and exhausted through a vent port. An inert gas atmosphere is provided within the cylinder to prevent the material resin from being oxidized. Since predrying can be dispensed with, the thermal hysteresis will be extremely small and the deterioration of the resin caused by such heating may be suppressed. Also, there is less chance of generation of carbonized black spots caused by foreign matter, molding failures caused by hydrolysis may be eliminated, and residual monomers can be vaporized and exhausted concomitantly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of U.S. application Ser. No. 08/745,677, filed Nov. 8, 1996, entitled PROCESS FOR INJECTION MOLDING INFORMATION RECORDING DISKS, which claims priority to Japanese application No. 7-315855, filed Nov. 10, 1995, and Japanese application No. 7-315857, filed Nov. 10, 1995 now abandoned; and a continuation in part of U.S. application Ser. No. 08/745,883, filed Nov. 8, 1996, entitled: PROCESS FOR INJECTION MOLDING, which claims priority to Japanese application No. 7-315856, filed Nov. 10, 1995, all incorporated herein by reference now abandoned. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a process for injection molding information recording disks wherein the predrying of a material resin is dispensed with. It also relates to a process for injection molding thin information recording disks wherein the plasticating of a material resin and the injection and filling thereof into a mold are separately accomplished. 
     Injection molding of information recording disks, such as optical disks, magnetic disks, magneto-optical disks, or the like has conventionally utilized an in-line screw type injection molding apparatus, which is equipped with an injection screw rotatable and movable back and forth in a heating cylinder having a nozzle at the front end and a feed port at the rear. 
     Traditionally, less expensive, unpelletized resins can hardly be used as feedstock for the injection molding. The unpellitized resins might thus be pelletized into pellets of a predetermined size for use as feedstock for the injection molding. Forming the pellets from resins is called pelletization and is carried out by means of an extruder with a vent. The extruder has a heating cylinder to melt the resin. The molten resin is extruded into strand and the strand is then cut into a predetermined length. 
     Since a resin to be used for such injection molding is of small-grained, granulated sugar type and contains unreacted monomers, there arises a problem in molding such a resin. The resin is therefore melted by heating and is processed into pellets of a certain grain size before being used as a material resin. Since this resin as prepared into the pellets is hygroscopic, it must be predried to remove moisture before being used for molding. Conventionally, the molding material has a water content of from 0.005% to 0.01% by the steps of pelletizing process and preliminary drying process. These predrying steps must be carried out in the prior art, or the physical properties of the molded article are degraded due to hydrolysis. However, the moisture which may remain in the material resin can not only be the cause of molding failures such as silver streaking or yellowing, but as well result in the hydrolysis of the resin when being plasticated to cause a decrease in the strength of the molded disks. Also, an ill-managed dryer may allow fine foreign matters in the air to be absorbed, and such matters can be, for example, carbonized and surface as black spots on the molded articles, thereby rendering the articles defective. When the molded articles are information recording disks, such as optical disks, magnetic disks, magneto-optical disks, or the like in particular, extremely small and few black spots would cause disorders in their reading. 
     In addition, in a typical prior art molding process, the material resin, as such, will be subjected to at least three times of heating including pelletization, predrying, and injection molding. This extended heat history can easily cause a deterioration in physical properties, especially in strength. Transparency may also be impaired. 
     Further, when a conventional in-line screw type injection apparatus is utilized, a material resin fed into a cylinder through a feed port located at the rear of the cylinder is melted and kneaded, that is, plasticated, by the revolutions of an injection screw, the screw being moved backward by the pressure of the resin. As such, the resin will be metered within the front portion of the cylinder, and the metered material will be injected and filled by the advancement of the screw into a cavity of a mold to which a nozzle tip abuts to produce the desired information recording disks. For the purpose of improving transferability and birefringence in thin disks (approximately 1.2 mm in thickness) as well, compression molding which compresses a resin injected and filled into a mold is sometimes practiced. 
     In such molding in which the plasticating of a resin and the injection and filling thereof are carried out by a single screw, however, since the metering is accomplished by moving an injection screw backward by means of the pressure of the plasticated resin, dispersion in the metering will tend to show as uneven areas despite the use of a small injection apparatus for molding disks or the like which involves small amounts of resin. It will result in an inconstant amount of injection and filling into a cavity, possibly producing accidental no-filling or overfilling and will prevent constant molding for a long period of time. 
     In addition, unevenness of temperatures will occur throughout the plasticated resin. Moreover, an L/D ratio will change due to the backward movement of the injection screw, giving a tendency that the resin temperature at the completion of metering is lower than that at the beginning. Solutions to these problems include the adoption of a screw with mixing means or a spiral barrier type screw, etc.; however, the effects of such solutions have so far been insufficient and improvements are desired. 
     BRIEF SUMMARY OF THE INVENTION 
     In the light of the foregoing, it is one object of the present invention to provide a novel injection molding process wherein the times of heating to melt a resin are reduced to minimize the thermal hysteresis or to decrease the deterioration of physical properties caused by the thermal hysteresis, so that high quality information recording disks may be produced. 
     It is another object of the present invention to provide a novel injection molding process wherein the plasticating and the injection and filling of a material resin are carried out in separate cylinders, so that constant molding can be provided for a long period of time even for disk molding involving a small amount of resin. 
     According to a first aspect of the present invention, a material resin such as polycarbonate is supplied, predrying being dispensed with, to an injection apparatus having a vent port; the material resin is melted for a first time since polymerization and kneaded by the apparatus while volatile components such as water contained in the resin are heated to be vaporized and exhausted through the vent port out a cylinder; the melted resin is metered; and the resin is injected and filled into a mold having therein a cavity for molding information recording disks to produce thin information recording disks. 
     According to a second aspect of the present invention, the injection apparatus is a screw preplasticating injection apparatus so as to produce thin information recording disks. A material resin is melted and kneaded by a plasticating screw within a plasticating cylinder having a vent port while volatile components contained in the resin are exhausted through the vent port out the cylinder; the melted resin is forcibly fed toward and metered against the front portion of an injection cylinder having a plunger inserted therethrough; and the metered resin is injected and filled by the plunger into a mold having therein a cavity for molding information recording disks to produce thin information recording disks. 
     According to yet a third aspect of the present invention, an inert gas atmosphere is provided within the cylinder between the material feed port and the vent port of the injection apparatus, the inert gas preventing the material resin from being oxidized during the melting and kneading. According to a further aspect of the present invention, a plurality of information recording disks are produced simultaneously by a mold having a plurality of cavities therein. 
     According to the process of the present invention, since predrying by heating preceding molding can be dispensed with, the thermal hysteresis of a resin will be extremely small and the deterioration, especially the decrease in strength, of the resin caused by such heating may be suppressed. Moreover, since it allows less chance of any foreign matters entering and mixing, the generation of carbonized black spots caused by such matters, which has long been a problem in the art, will be solved. 
     In addition, molding failures caused by hydrolysis may be eliminated so that high quality information recording disks are obtained. Also, since residual monomers can be vaporized and exhausted concomitantly, the amount of mold deposits adhered on a stamper tensely provided in a mold cavity is reduced, thereby the interval of maintenance action being prolonged. Moreover, when the material resin is plasticated in the inert gas atmosphere, the yellowing of the resin caused by oxidization will be avoided and the thermal hysteresis will lessen, with a result that light transmission through a disk is increased and, especially for polycarbonate, transparency is fully exhibited so that high quality products may be obtained. 
     According to the molding process of the present invention, a preplasticating injection apparatus equipped with a vent port may also be implemented. When such a preplasticating injection apparatus is used, a material resin is melted and kneaded, that is, plasticated, by a plasticating cylinder having a vent port, which is separately provided from an injection cylinder and rotates at a generally fixed position, while the resin is moved a predetermined distance. As such, the resin having reached the front end of the cylinder shows less dispersion in temperature than when the resin is plasticated in the injection cylinder. Therefore, the temperatures throughout the resin metered into the front portion of the injection cylinder through a resin passage having heating means around it will be uniform, with a result that an extremely thin (0.6 mm) injection molded disk has no residual stress, so that birefringence is improved and a uniform transfer of pits is accomplished as well. 
     In addition, since the metering is made by moving the plunger backward by the pressure of the resin which is forcibly fed from the plasticating cylinder side, dispersion in the metering will be eliminated and accidental no-filling or overfilling which may be caused by inconstant metering will be avoided. As a result, the dispersion in weight will advantageously decrease from ±0.1 g down to ±0.02 g, and the transferability will also improve so that constant molding may be possible for a long period of time. 
    
    
     DETAILED DESCRIPTION OF THE OF THE DRAWING 
     FIG. 1 is a longitudinal sectional view of the principal part of a vent type, internal screw equipped, injection molding apparatus utilized for the information recording disk injection molding apparatus utilized for the information recording disk injection molding process according to the present invention; and 
     FIG. 2 is a longitudinal sectional view of a preplasticating injection molding apparatus utilized for the information recording disk injection molding process according to the present invention; and 
     FIG. 3 is a partial longitudinal sectional view illustrating the relationship between the preplasticating injection molding apparatus and a mold. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a vent type injection molding apparatus by which the injection molding process according to the present invention may be implemented. A heating cylinder  1  has band heaters, not shown, mounted around its outer surface. The heating cylinder  1  has an injection screw  2  inserted therethrough in such a manner that it is rotatable and movable back and forth. The rear end of the cylinder is mated with a retaining cylinder  3  and is provided with a feed port  4  which is upwardly open. 
     A vent port  5  is provided through the upper portion of the cylinder forwardly in relation to the feed port  4 . The vent port  5  and the feed port  4  are connected respectively to a vacuum suction unit  5   a  and an inert gas force feed unit  4   a.    
     In such a vent type injection molding apparatus, the injection screw  2  is rotated in a conventional manner, while an inert gas such as nitrogen gas is forcibly fed through the feed port  4  into the cylinder. A vacuum is applied inside the cylinder through the vent port  5 . 
     As such, an undried material resin, for example, polycarbonate, which has been fed in advance into a hopper  6 , will be fed through the feed port  4  into the cylinder. As the feedstock is of a free-flowing granular composition, no external feed screw is required in the hopper. Typically, granules having a particle size of 0.5 mm or less provide smooth feeding and consistent melting. The unpelletized, unextruded material resin will then be fed forward within the cylinder and compressed by the revolutions of the injection screw  2 . 
     In course of compression, the material resin will be kneaded while being melted by the heat transferred from the heating cylinder  1  and by the heat generated by the revolutions of the screw. Since such melting and kneading are accomplished in the inert gas atmosphere provided within the cylinder between the feed port  4  and the vent port  5 , oxidization by heating may be avoided. In addition, volatile components such as water will be vaporized by the heating and compression and be freed within the cylinder. In accordance with the present inventor, feedstock with a water content between 0.1% up to saturated water content can be provided. 
     These gases will be sucked through the vent port  5  which is normally under vacuum along with the inert gas and be exhausted out the cylinder, not remaining in the plasticated resin located forwardly in relation to the vent port  5 . Consequently, if a resin containing moisture was used, neither yellowing due to oxidization nor deterioration due to hydrolysis would occur in the resin metered in the cylinder against the front part thereof by the backward movement of the screw  2  caused by the pressure of the resin. 
     The metered resin will be filled by the advancement of the injection screw  2  from a nozzle  7  located at the front end of the cylinder via a hot runner  10  into a cavity  9  within a mold  8 . The cavity  9  is used for a thin information recording disk, and includes a stamper which is tensely provided on one side thereof. Thus, a disk to be molded in the mold will have, transferred onto one side thereof, pits which have been formed on the stamper and will be formed into a thin (approximately 0.6 to 1.2 mm) information recording disk made of polycarbonate. 
     As described above, no inconveniences would exist in injection molding if a resin to be used for such molding was an undried one. This resin per se is preferable in terms of physical properties because it exhibits less thermal hysteresis than a predried material resin does. Since transparency is not impaired for polycarbonate, it is feasible to injection mold high quality optical disks, magneto-optical disks, or the like which have improved birefringence as compared with conventional products. Such information recording disks as produced by the present embodiment exhibit superior light refracting and magnetic properties. Such disks, when subsequently treated for optical or magnetic recordation allow for anisotropic surface characteristics ideal for digital information storage. It also allows a reduction in molding costs because the predrying may be dispensed with. 
     A preplasticating injection apparatus which can implement another aspect of the present invention will now be described with reference to FIG.  2 . An injection apparatus  11  has a plasticating apparatus  12  installed thereabove in parallel relationship thereto, the apparatuses communicating with each other by a resin passage  13  provided across their front ends. 
     The injection apparatus  11  comprises an injection cylinder  22  having an injection plunger  20  inserted therethrough in such a manner that it is movable back and forth and a nozzle  21  located at the front end, and an injecting hydraulic cylinder  24  contiguously provided to the rear end of the injection cylinder  22  in which a piston  23  is linked to the plunger  20 . 
     The injection cylinder  22  has an inflow passage  25  from the resin passage  13  located in the upper portion at the front end corresponding to the advancement limit of the plunger. The inflow passage  25  is inclined toward the inner peripheral rim  22   a  of the injection cylinder  22  so that the plasticated resin fed through the resin passage  13  may flow along the inner peripheral surface. 
     Upper and lower resin extraction holes  26  are provided on the injection cylinder  22  rearwardly in relation to the plunger stroke of the injection cylinder  22 . Band heaters  27  are disposed around the cylinder up to a point rearward in relation to the resin extraction holes  26  to provide a longer heater zone. 
     The plasticating apparatus  12  comprises a plasticating cylinder  32  having a plasticating screw  30  inserted therethrough in a rotatable manner and an outflow passage  31  located at the front end, a retaining cylinder  33  located at the rear end for retaining the plasticating cylinder  32 , a hydraulic cylinder  34  linked to the rear end of the retaining cylinder  33  for moving a screw back and forth and a revolution drive unit  35  for moving a screw  30  attached to the rear end of the hydraulic cylinder  34 . 
     The drive shaft  35   a  of the revolution drive unit  35  is linked to a rotation axis  33   a  which has its rear end inserted through the piston  34   a  of the hydraulic cylinder  34  and is received within the retaining cylinder  33  in such a way that it is axially movable. Linked to the front end of the rotation axis  33   a  is the rear end of the screw  30 . The rotation axis  33   a  is also linked via a member  33   b  with the piston  34   a  in such a manner that it can only move in the axial direction, and moves back and forth together with the piston  34   a  so as to move the screw  30  also back and forth. The member  33   b  is provided with a stop member  33   c  which limits the forward movement of the screw  30  within a certain range. 
     Band heaters  36  are attached around the outer periphery of the plasticating cylinder  32 . A feed port  37  which is upwardly open is provided at the rear end of the plasticating cylinder  32  which is mated with the retaining cylinder  33 . A vent port  50  is provided through the upper portion of the cylinder forwardly in relation to the feed port  37 . The vent port  50  is connected to a vacuum suction unit  50   a  and the feed port  37  is provided with an inert gas force feed unit  37   a.    
     Attached to the front end of the screw  30  is a check valve  38  comprising a mushroom-shaped valve body having a cone-shaped tip. Inside the front end of the plasticating cylinder  32  where this check valve  38  is located, a recess is formed so as to correspond with the tip of the valve body, so that a flow passage communicating with the outflow passage  31  is formed around the check valve  38 . Also, an annular valve seat  39  is fitted on the cylinder side opposed to the canted rear face of the check valve  38 . 
     Depending upon the resin, the flow passage is so designed that a flow gap  31   a  of 1.0 mm to 1.5 mm may be formed between the check valve  38  and the valve seat  39  when the check valve  38  has a diameter of 36 mm and, at the same time, a sufficient flow passage may be formed between the check valve  38  and the wall of the recess. If the flow gap  31   a  measured 0.5 mm or less, heat generation within the plasticating cylinder would be too great, and when 2.0 mm or more, closing the valve would take too much time, easily resulting in incomplete prevention of the metered resin from flowing back. 
     The resin passage  13  is composed of capillary tubes  40  and has band heaters  41  around the outer periphery. It also has a required mixing member  42  within the upper portion. The capillary tubes  40  are provided at an angle with joints  40   a  and  40   b  across the inflow passage  25  of the injection cylinder  22  and the outflow passage  31  of the plasticating cylinder  32 . When the plasticated resin passes through the resin passage  13 , the temperatures throughout the resin will further be equalized. 
     The injection apparatus  11  and the plasticating apparatus  12  are linked integrally in a two-stage, upper and lower, configuration by fixing the retaining cylinder  33  on a support  28  installed on the injection cylinder  22  and are installed upon a platform  17  by placing and fixing the hydraulic cylinder  24  of the injection apparatus  11  on a slidable base block  16  linked via a nozzle touch cylinder  15  to a stationary platen  14  of a clamping device not shown. The tip of the nozzle  21  is in touch with a gate of an information recording disk mold  51  attached to the clamping device on the platform. 
     The mold  51  is composed of a fixed side part and a movable side part as is a conventional mold, with a plurality of disk molding cavities  52 ,  52  formed along the parting surface between those parts. The cavities  52 ,  52  are in communication with a mold gate through a hot runner  53  located within the fixed side part. In addition, a stamper is tensely provided on one face of each cavity  52 . 
     A process for injection molding information recording disks utilizing the above described injection apparatus will now be described. 
     Hydraulic oil is supplied into the rear chamber side of the hydraulic cylinder  34  to advance the piston  34   a  and the rotation axis  33   a  together with the screw  30 , thereby parting the canted rear surface from the valve seat  39  to open the valve. Almost simultaneously, the screw  30  is rotated with the rotation axis  33   a  by the revolution drive unit  35 , and an inert gas such as nitrogen gas is forcibly fed into the cylinder through the feed port  37 . 
     Pelletized polycarbonate is then supplied as a material resin through the feed port  37  into the plasticating cylinder  32 . The material resin is melted and kneaded, that is, plasticated, by the heating by the band heaters  36  and the revolutions of the screw  30  under the inert gas atmosphere provided between the feed port  37  and vent port  50 , while being forcibly fed progressively forward. 
     By means of the melting and kneading, the water contained in the material resin will be vaporized and the residual monomers will also be dissipated off the melted resin. These volatile gases are sucked at the vent port  50  and removed outward, not remaining in the portion of the melted resin which is forward in relation to the vent port  50 . The melted and kneaded resin will then progressively reach the front end of the cylinder, to be fed out through the outflow passage  31  which is connected with the flow gap  31   a  between the check valve  38  and the valve seat  39  and into the resin passage  13 . 
     In the resin passage  13 , the subdivision and mixing of the resin will repeatedly take place, allowing the resin to flow toward the inflow passage  25  while being further kneaded. The resin will further flow into the front end of the injection cylinder  22  through the inflow passage  25 , thereby applying pressure against the front end surface of the plunger  20  and moving the same backward, to be accumulated in and metered against the front end of the cylinder. Such metering will continue until the plunger  20  is set back to a predetermined location. 
     When the plunger  20  moves back to and stops at the location where the metering stops, the revolutions of the screw  30  will stop, temporarily discontinuing the plasticating of the resin. In addition, since the frontal area of the valve body of the check valve  38  is greater than the area of the canted rear surface, creating a difference in area, if the resin pressure during plasticating was greater at the plasticating cylinder side than at the front side of the valve body, the resin pressure acting against the front face of the valve body would be greater when the plasticating is not in progress. When the hydraulic oil acts against the front chamber of the hydraulic cylinder  34 , therefore, the screw  30  will immediately move back to where the rear surface of a the valve body abuts the valve seat  39 , unaffected by the resin pressure in the plasticating cylinder. As a result, the valve is closed within the front end of the plasticating cylinder  32 , so that the flow of the resin will surely be cut off. 
     Besides, because the valve closing as described above is made by the backward movement of the screw, no extrusion of the resin at the front side of the valve body will occur and the location of the plunger  20  where the metering stops will be constant. As a result, a predetermined amount of resin will be metered at the front portion of the plunger  22 . 
     When the plunger is moved forward with the check valve  38  closed, the metered resin in the injection cylinder will be injected through the nozzle  21  into the mold  51 . The injection pressure applied by the plunger  20  will also act on the plasticating cylinder side through the resin remaining in the resin passage  13 . Since the check valve  38  is closed, however, the resin will be prevented from flowing back to the plasticating cylinder side, so that the whole amount of metered resin will be filled from the nozzle  21  through hot runner  53  into both the cavities  52 ,  52  to be molded into thin disks and the transfer of the pits from the stamper will simultaneously be made. 
     Upon completion of the injection process confirmed, the screw  30  will be moved forward by the hydraulic cylinder  24  to open the valve, and will be rotated by the revolution drive unit  35  to start metering another portion of resin. 
     According to the molding process as described above, the plasticating of the resin and the volume of injected resin will be much more constant than when the plasticating is made also in the injection cylinder, so that error, if any, in the metering repeatedly made will be extremely small, enabling each time the injection and filling of the same amount of resin. It is therefore preferable to increase the number of cavities in a mold so as to produce a plurality of units simultaneously. This is preferable also in the sense that the metering will be more constant when a greater amount of resin is required for a batch of molding. By molding a plurality of units simultaneously, it will be able to produce more quality-constant information recording disks. It is therefore another characteristic of the present invention to produce a plurality of units simultaneously.