Injection compression molding method for compact discs or the like

An injection compression molding method for compact discs or the like, includes the step of changing 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.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an injection compression molding method for 
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. 
2. 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. 
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 per unit area 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.

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 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 per unit area 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 at 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 per unit area 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.