Injection compression molding method for compact discs or the like

Abstract: An injection compression molding method for compact discs or the like, comprising the step of chang­ ing the clamping pressure or compression pressure, after the internal cavity pressure in the mold after injection has reached its peak, stepwise or continuously as the cooling proceeds, thereby making the applied pressure per unit area with respect to the effective pressure range constant. In a compact disc formed by this method, the residual stress after cooling is uniform, so that its optical properties are improved substantially.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an injection compression molding method for producing compact discs or the like, and in particular, to an injection compression molding method which copes with the problem of residual stresses, which problem occurs as the production cycle for compact discs or the like is shortened. Description of the Related Art In the production of compact discs, for example, it has been the common practice to provide a sufficient length of cooling time, approximately 6 to 8 seconds, when cooling each optical disc base with a view to mitigating the various stresses involved, i.e., filling pressure, dwell pressure, clamping pressure, compression pressure, etc. To reduce the production costs, however, it is necessary to shorten the above-mentioned cooling time. In view of this, various attempts have been made to improve the cooling efficiency. Examples of such attempts include: bringing the cooling water channel nearer to the disc surface; widening the channel so as to increase the water flow; and selecting a mold material which has a higher heat conductivity. These arrangements, on the other hand, have not been without their problems: the disc surface layer is apt to become hardened at an early stage; a difference in stress is apt to be generated between the inside and the exterior surface portion of the disc; and the optical properties of the disc, such as birefringence, are deteriorated. SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide an injection compression molding method which makes it possible to effectively eliminate or mitigate residual stresses in a compact disc even when the cooling time for the disc is as short as 4 seconds or less, without deteriorating the optical properties of the disc, such as birefringence. According to the invention this object is solved by providing a method as defined in claim 1. In accordance with this invention, the clamping pressure or compression pressure in a system which allows this pressure to be changed during clamping is changed, after the internal cavity pressure in the mold after injection has reached its peak, stepwise or continuously as the cooling proceeds, thereby making the applied pressure with respect to the effective pressure range constant. The cooling process may be controlled by controlling the time and pressure stepwise or by a ramp function control method. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the control system of this invention; Fig. 2 is a drawing showing the way in which the compact disc is cooled; Fig. 3 is a graph showing the birefringence profile when the control process according to this invention is adopted; and Fig. 4 is a graph showing the birefringence profile when the clamping force is constant as in the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS What is indispensable to this invention is a compression press injection molding machine of the type in which the clamping force can be arbitrarily changed during the period between the time when the clamping is completed and the time when the opening of the mold is started, i.e., a straight hydraulic mold clamping system. It might also be possible to adopt a toggle type mold clamping system. In that case, however, the system must be capable of generating the clamping force by hydraulic means or the like and adjusting the generated clamping force over a period of time (as with Hydrobalancer manufactured by the applicant of the present patent application), or it must be equipped with a core press type injection clamping device. Generally speaking, the cooling of a material is gradually effected, starting from the flow end portion and advancing toward the gate. In the case of the molding of a compact disc, the cooling is effected in a manner as indicated by the arrows of Fig. 2 (in which the reference numeral 1 indicates a cavity for molding compact discs and the reference numeral 2 indicates a gate). When pressed with a constant clamping pressure or compression pressure, the effective pressure range decreases gradually as the cooling proceeds, so that the pressure applied to a unit area becomes higher. If, in this condition, the disc is released from the mold without having been cooled to a sufficient degree, a difference in stress is generated between the surface portion and the inside portion of the disc, with the result that the residual stresses exist in the disc in an irregular manner, which leads to dispersion in birefringence (which is one of the optical properties of the disc) or an excessively large degree of change in the disc properties with passage of time. To solve these problems, it is necessary to improve the optical properties of the disc such as birefringence by changing the clamping pressure or the compression pressure stepwise or continuously as the cooling proceeds to make the applied pressure constant with respect to the effective pressure range thereby to remove the residual pressure or make it constant. The control operation for removing residual pressure is performed particularly during the period between the time when the internal cavity pressure is at its peak (see the reference symbol A in Fig. 1), which approximately corresponds to the termination of the filling and the initial stage of dwell pressure, and the time when the opening of the mold is started (see the reference symbol B of Fig. 1). Before that, there is no problem whether it is higher or lower than the clamping pressure or compression pressure at its peak. After the internal cavity pressure has reached its peak, it is necessary to control the time and pressure stepwise as indicated by the reference symbol a in Fig. 1, or a continuous ramp function control has to be effected (as indicated by the reference symbol b in Fig. 1). Fig. 1 shows a concrete example of the control process flow and Fig. 3 shows the birefringence profile of a molding obtained through this control process. Fig. 4 shows the birefringence profile of a molding obtained under a constant clamping force. As is apparent from these two graphs, the molding obtained through the control process of this invention exhibits a uniform birefringence profile (Fig. 3). Thus, in accordance with this invention, the clamping pressure is changed, after the internal cavity pressure in the mold after injection has reached its peak, stepwise or continuously as the cooling proceeds, thereby making the applied pressure with respect to the effective pressure range constant. By thus controlling the cooling process, the residual stress in the compact disc after cooling is removed or made uniform, thereby substantially improving the optical properties of the molding, as shown in Fig. 3.

Claims: An injection compression molding method for producing optical compact discs using an injection molding apparatus which has a mold having a mold cavity and which can exert a controllably variable compression pressure on said mold to vary internal cavity pressure in said mold cavity, comprising the steps of: introducing a moldable liquid material into said mold cavity while applying a first predetermined compression pressure on said mold; cooling said moldable liquid material while changing the compression pressure on said mold, after the internal cavity pressure in the mold after injection has reached its peak, as the cooling proceeds, such that the effective pressure applied to unhardened portions of said moldable material which remain within said mold cavity without being hardened is substantially constant, to produce an optical compact disc having generally uniform residual stress. An injection compression molding method according to claim 1, wherein said compression pressure is varied in a stepped manner as a function of time and pressure. An injection compression molding method according to claim 1, wherein said compression pressure is varied using a ramp function control method.

Inventors: KITAMURA TAKEHIKO, KITAMURA, TAKEHIKO
Applicants: SUMITOMO HEAVY INDUSTRIES, SUMITOMO HEAVY INDUSTRIES, LTD
Application year: 1990
Main classification: B29C45
IPC codes: G11B7, G11B3, B29C45, B29L17