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BACKGROUND OF THE INVENTION
This invention relates to a method for injection molding of a disk shaped
substrate for an optical information record carrier formed of
polycarbonate resin, such as the magneto-optical recording medium or the
write once type recording medium.
The optical recording system can record and/or reproduce in contact-free
manner, can be handled easily and is invulnerable to demage or
contamination, while it has the recording capacity about tens or hundreds
of times larger than that of the conventional magnetic recording system.
Thus it is practically used in a compact disk having digitally recorded
audio signals or a video disk having recorded video signals, while it is
expected to be used in large capacity information record file, such as
code or image information.
Various types of the optical recording media of the optical recording
system are known in addition to the aforementioned compact disks or video
disks, such as write once type optical disk, erasable optical disk or the
magneto-optical disk. These disks are comprised of a recording layer for
the optical information formed on an transparent disk of polycarbonate
resin or PMMA (polymethethylmethacrylate) resin. Various demands have been
raised in forming the disks.
Typical of these demands are the reduced double refraction which means
phases shift between the disk signal readout incident light and reflected
light and, mainly is caused by the internal strain of the disk, optimum
transfer properties and surface smoothness, reduced contamination and
reduced deflection (skew) of the formed surface. Above all, it is required
of the magneto-optical disk to reduce double refraction caused in the
transparent disk, since it is adapted to read out as signals only minute
rotations of the plane of polarization of the irradiated laser light.
Under these circumstances, evolution in the art of injection molding is
carried out in many aspects so that it is now possible to produce the
polycarbonate resin disk which is almost completely free of double
refraction at the time of forming.
However, as a result of our further investigations, it has been found that,
while it is possible to produce the disk showing only reduced double
refraction at the initial stage after termination of the molding under the
conventional injection molding art, the double refraction when being used
was changed chronologically and, above all, shifted towards the minus
side, thus resulting in increase of the double refraction. The tendency is
outstanding in an internal area with a magneto-optical disk having a
central hub for assuring the chucking accuracy relative to the recording
and reproducing apparatus. For example, when it is used in the apparatus,
it is becoming difficult to assure a normal operation because of increased
double refraction.
In the present invention, the polarity of double refraction that is, minus
or plus, is defined in such a manner that the direction in which double
refraction on the inside area of the formed disk heated to 100.degree. C.
is changed is defined as minus, with the direction opposite thereto being
defined as plus. The double refraction is a phenomenon that takes place
when the light passes through a medium having variable refractive indices
along the directions within the same plane, such medium being anisotropic
with respect to the refractive index. For example, with the refractive
index in a prescribed direction (x-direction) as n.sub.x and with that in
a direction orthogonal thereto (y-direction) as n.sub.y, phase shift is
caused between the x-component (the light having the plane of polarization
parallel to the x-direction) and the y-component (the light having the
plane of polarization parallel to the y-direction). The phase difference
.delta. is given by
.delta.=2.lambda./.pi.. (n.sub.x -n.sub.y)d . . . (1)
wherein .delta.: wavelength of light and d: the distance in the medium
traversed by light. The direction in which the phase difference .delta. of
the transmitted light is changed chronologically or under the
aforementioned heating conditions was defined as the minus direction, with
the direction opposite thereto being defined as the plus direction.
Although it may be thought to produce the disk having double refraction
previously set to the plus side, in order to manage the aforementioned
chronological changes, the current situation is necessarily to sacrifice
other properties.
That is, in order to obtain the plus double refraction of the produced
disk, it is necessary to increase the temperature of the resin or the mold
during the injection molding. However an increase in the mold temperature
results in curved or warped disk and loss of smoothness, while an increase
in the resin temperature results in decomposition of the resin and
increased contamination. When forming under a lesser pressure, it is
possible to produce a disk having a plus double refraction. In such case,
however, transfer characteristics are lowered, while it becomes difficult
to assure a flatness of the hub attaching reference plane and skew is also
increased.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method for
molding plastic material into a disk shaped substrate for an optical
information record carrier.
It is another object of the present invention to provide a method for
molding plastic material into a disk shaped substrate having small double
refraction caused by time lapse.
It is a further object of the present invention to provide a molding method
of plastic material having superior transfer characteristics of a surface
of a stamper.
According to one aspect of the present invention, there is provided a
method for molding plastic material into a disk shaped substrate for an
optical information record carrier which comprises the steps of, injecting
molten resin into a cavity defined between a stationary mold and a movable
mold under an application of mold clamp pressure between the molds to form
a disk shaped substrate, releasing the moldclamp pressure at a
predetermined timing after the resin injection is completed and the resin
is partially solidified, holding the resin in the mold under the released
mold clamp pressure condition until the resin is solidified and taking out
the molded substrate from the mold.
BRIEF DESCRIPTON OF THE DRAWINGS
FIG. 1 is a schematic side elevation showing an example of an injection
molding machine employed in practicing the present invention.
FIGS. 2 to 4 are schematic sectional views showing the injection molding
process in the order of the process steps, wherein FIG. 2 shows the mold
closure state, FIG. 3 the molten resin charging state and FIG. 4 the mold
clamping state.
FIG. 5 is a schematic side elevation showing another example of the mold
clamp unit of the injection molding machine.
FIG. 6 is a graph showing the relation between the mold pressure releasing
timing and the initial value of the double refraction of the disk; and
FIG. 7 is a graph showing the initial value of the double refraction as
compared to the double refraction after the annealing.
FIG. 8 is a timing chart showing the mode of changes in the internal
pressure in the metal mold in the process of an embodiment of the present
invention; and FIG. 9 is a chart showing the initial values of the double
refraction in the radial direction of the disk obtained in the present
embodiment and the same values after the annealing.
FIG. 10 is a timing chart showing the manner of changes in the internal
pressure within the metal mold in a conventional process; and FIG. 11 is a
graph showing the initial values of the double refraction in the radial
direction of the disk produced disk in the prior-art example and the
similar values after annealing.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As a result of our researches into attaining the above objects, the present
inventors have arrived at a finding that the initial value of double
refraction is to be controlled by the timing of releasing the mold
clamping pressure that is released before the disk solidifies completely.
Based on this finding, the present invention resides in a method for
injection molding of a disk wherein the molten resin is injected into a
cavity defined between a stationary metal mold and a movable metal mold to
form a disk, said method comprising substantially releasing the mold clamp
pressure of said movable metal mold at a predetermined timing after the
termination of a resin injection step and until the resin is solidified,
holding the resin in this state until it is solidified and taking out the
molded disk of said resin.
When molding a disk by injecting the molten resin into a cavity defined
between a stationary mold and a movable mold, if the mold clamp pressure
applied to the metal mold since the time of termination of injection of
the resin material until the resin is solidified is released, the double
refraction especially on the inside area of the disk is shifted to the
plus side for controlling the initial value of double refraction
dependinig on the releasing timing.
Also, since the uniform pressure is applied immediately after the injection
on the overall disk surface with a prescribed mold clamp pressure,
sufficient transfer characteristics are assured. Since the mold clamp
pressure is released after the peripheral portion is solidified to some
extent, there is no risk that the transfer characteristics are badly
affected.
In addition, the metal mold is maintained in the contact state with the
disk even after releasing the mold clamp pressure, there is no risk of
disk deflection or warping.
An embodiment of the injection molding of the disk in accordance with the
present invention will be described in the sequence of the process steps
thereof and by referring to the accompanying drawings.
The construction of an injection molding machine employed in practicing the
present embodiment is firstly explained.
As shown in FIG. 1, the injection molding machine is roughly classified
into a resin injection part (1) for melting and feeding the molten resin
into the metal mold, a metallic mold part (2) for forming a cavity in
accordance with the disk shape, and a mold clamp mechanism (3) for
applying a pressure to the metal molod part (2).
The resin injection part (1) is comprised of a charging hopper (11) into
which are charged the resin pellets as the starting material, a heating
cylinder (12) provided with a peripheral heater and an inside screw for
successively feeding out molten resin, and a nozzle (13) through which the
molten resin material is ejected. The resin material supplied from the
charging hopper (11) is successively molten and the molten resin is fed
out at the end of the nozzle (13).
The metal mold part (2) is mainly composed of a movable metal mold (21) and
a stationary metal mold (22), as shown in FIG. 2, there being a stamper
(23) secured to the movable metal mold (21) by an inside stamper holder
(24) and an outside stamper holder (25). The stationary metal mold (22) is
secured to a stationary plate (26) and has a central rein injection
opening (27) coupled to a nozzle (13) of the resin injection part (1) and
having an end opening (27a). To the peripheral part of the stationary
metal mold (22), there is provided a metal mold holder (28) secured to the
stationary plate (26). The outer peripheral portion (21a) of the movable
metal mold (21) abuts on an end face (28a) of the mold holder (28) to
close the mold, such that the cavity (29) is defined as the prescribed
space between the metal molds (21) and (22).
The mold clamp mechanism (3) is provided to the metal mold part (2) to
reciprocate the movable metal mold (21). In the present embodiment, a
so-called booster ram type mold pressure mechanism is adopted as the mold
clamp mechanism (3). This booster ram type mold clamp mechanism has a
booster ram of a diameter lesser than the diameter of a main ram inserted
coaxially as the center axis of the main ram for advancing the main ram at
a higher speed, and is widely used in the medium sized injection molding
machine. A servo valve unit (31) is connected to the mold clamp mechanism
(3) and adapted to be controlled by a mold clamp pressure adjustment unit
(32). The mold clamp pressure adjustment unit (32) is connected via a
pressure sensor (33) to the aforementioned mold clamp mechanism (3). The
mold clamp pressure applied to the movable metal mold (21) by the
operation of the mold clamp mechanism (3) is sensed by the pressure sensor
(33), the resulting sensor signals controlling the mold clamp pressure
adjustment unit (32) to control the servo valve unit (31). To the mold
clamp pressure adjustment unit (32) is also connected a program setting
unit (34). The application timing of the mold clamp pressure of the
movable metal mold (21) is stored in the program setting unit (34) so that
the output of the setting unit (34) is applied to the mold clamp pressure
adjustment unit (32) to control it to actuate the servo valve unit (31) to
control the oil pressure and the operating time of the mold clamp unit (3)
thereby to change the mold clamp pressure of the movable metal mold (21)
during or after the injection process of injecting the molten resin
material into the cavity (29) between the movable metal mold (21) and the
stationary metal mold (22). It is noted that, when considering the
application to the embodiment of the present invention, digital control of
the metal clamp pressure and quick response properties (for example,
termination of the decompression within 0.5 second from 140 kg/cm.sup.2 to
zero kg/cm.sup.2) are desirable.
The so-called toggle type mold clamp mechanism shown for example in FIG. 5
as well as the aforementioned booster ram type clamp unit may be used as
the mold clamp unit (3). The toggle type mold clamp unit is designed to
enhance the force of the mold clamp cylinder (41) by a link unit (42)
called a toggle to produce a large mold clamp pressure while providing for
a high speed mold clamping. In an example shown in Fig. 5, a movable metal
mold (21) is secured to the supporting disk (43). The mold clamp cylinder
(41) is controlled by a mold clamp drive control circuit similar to one
provided to the aforementioned booster ram type mold clamp mechanism to
provide for rapid change in the mold clamp pressure of the movable metal
mold (21).
The above described injection molding machine is used to form the disk of a
polycarbonate resin. The molding method is described in detail.
For producing the disk as the substrate of the optical recording medium in
the present embodiment, the outer peripheral part (21a ) of the movable
metal mold (21) is caused to abut on the end face (20a ) of the mold
holder (28) provided to the outer periphery of the stationary metal mold
(22), as shown in FIG. 2, to maintain the metal mold in the closed state.
In this state, the molten resin material (4) of the polycarbonate resin
molten in the resin injection part (1) is injected into the cavity (29)
through the nozzle (13) and the resin injection port (27).
When injecting the molten resin material (4), the resin temperature is
preferably as low as possible, insofar as it is such as to permit uniform
kneading of the resin in the heating cylinder (12) of the resin injection
part (1). For example, it should be controlled to be lower than
330.degree. C. at the maximum. With too high a resin temperature, the
resin itself is decomposed to increase contamination.
The temperature of the movable metal mold (21) and the stationary metal
mold (22) is preferably lower than the thermal deformation temperature of
the disk in order to promote the molding cycle and to improve production
efficiency and in view of skew characteristics. Therefore, with the glass
trasition temperature Tg of the currently employed polycarbonate resin for
disks of approximately 124.degree., it is set so as to be 110.degree. to
120.degree., for example.
During the injection, the molten resin is filled into the cavity (29). In
the present embodiment, the injection charging pressure is selected to be
slightly higher than the mold clamp pressure applied to the movable metal
mold (21).
Hence, during injection of the molten resin material (4) into the cavity
(29) of the metal mold part (2) kept in the mold closure state, the
movable metal mold (21) is receded a distance .DELTA.l under the charging
pressure as shown by an arrow mark a, the distance between the two metal
molds (21), (22) being enlarged from T.sub.1 of FIG. 2 to T.sub.2 of FIG.
3.
In such manner, the charging pressure is set so as to be slightly larger
than the mold clamp pressure applied to the movable metal mold (21) to
produce a gap between the metal molds (21) and (22), in such a manner that
the pressure within the cavity (29) is leaked through the gap so that the
molten resin material (4) is filled completely in the cavity (29) to make
it possible to form the disk with a high dimensional accuracy. Since the
metal mold (21) is opened under the charging pressure, there is no risk
that the pressure in the cavity (29) is increased excessively or a
unnecessary stress is applied to the molten resin material (4), which is
favorable for improving the double refraction. However, with too small a
mold clamp pressure, transfer characteristics may be insufficient. It is
desirable to properly set the mold clamp pressure with this being taken
into account.
When the charging pressure causes the metal molds (21), (22) to be opened
as described above, molten resin material (4) may be intruded into the
small gap between the metal molds (21), (22) to form so-called burrs.
Therefore, the construction of the metal molds (21), (22) is desirable in
which the burrs are not produced even when the metal molds (21, 22) are
opened for example by about 0.5 mm. In the present embodiment, the inner
peripheral surface (25a ) of the ring-shaped outer peripheral stamper
holder (25) provided to the movable metal mold (21) in association with
the outer peripheral edge of the disk and the outer peripheral surface
(22a ) at the step corresponding to the disk diameter of the stationary
metal mold (22) are provided in intersecting and substantially orthorgonal
direction to the disk surface to minimize the burr production.
Upon termination of charging of the molten resin material (4), since the
mold clamping pressure is maintained in the movable metal mold (21) at
this stage, a predetermined uniform pressure is applied to the overall
surface on the resin material (4), the metal molds (21), (22) are
progressively clamped by contraction due to cooling, as shown in FIG. 4.
As a result, the molten resin material (4) in the cavity (29) is
press-molded into a disk of a desired plate thickness T.sub.3 while the
signals (pits) of the stamper (23) as well as the guide grooves
(pre-grooves) are transferred. At this stage, the resin material (4) is
not completely solidified and the outer peripheral part is solidified as a
skin layer, while the inner part is still in the fluid state as a core
layer.
In the present embodiment, before the aforementioned state is reached, that
is, before complete solidification of the resin material (4), the mold
clamp pressure so far applied to the movable metal mold (21) is rapidly
released to zero or to an extremely low pressure (so-called
decompression).
For rapidly releasing the mold clamp pressure, the servo valve unit (31) is
actuated through the mold calmp pressure adjustment unit (32) when using
the aforementioned booster ram type mold clamp unit, for rapidly
discharging the oil in the cylinder chamber of the main ram and the
booster ram. Similarly, in case of the toggle type mold clamp unit, the
pressure in the mold clamp cylinder is released to release the mold clamp
pressure of the link mechanism (42). When using the general-purpose
injection molding machine, the rapidly impressed pressure is released by
opening the pressure valve, for example.
Our experiments have revealed that rapid release of the mold clamp pressure
is highly effective to control double refraction.
Thus, as a result of our investigations into double refraction after
termination of molding at 2.5 cm away from the center of a disk 13 cm in
diameter, it has been shown that double refraction is changed as shown in
FIG. 7 with the timing of releasing the aforementioned mold clamping
pressure. Thus, in 3 to 7 seconds after injection of the molten resin
material (4), the pressure was released (the resin material (4) was
solidified almost completely in 7 seconds). It was shown that the initial
value of double refraction of the disk obtained in dependence upon the
releasing timing tends to become smaller on the plus side. Also, at this
disk position, double refraction shifted to the minus side to the
approximately same extent, irrespective of the extent, as shown in FIG. 7.
It was shown in the present example, the initial value of the double
refraction is indicated by the curve .sub.x depending on the pressure
release timing, whereas it is shifted to a position shown by a curve y in
the drawing after annealing at 100.degree. C. for five hours. The double
refraction was measured herein as the value for double pass. Therefore,
chronological changes of the double refraction can be suppressed by
previously grasping the degree of shifting of the double refraction that
can be expected from the chronological changes and setting the pressure
release timing so that the initial value of double refraction on the plus
side. This setting of the double refraction to the plus side means a
substantial increase of the allowable range of the chronological changes
of the double refraction in the optical disk such that it becomes possible
to maintain the value of the double refraction of the disk for prolonged
time within the prescribed range. However, when the aforementioned
pressure is released immediately after injection, the outer peripheral
portion of the resin material (4) is not solidified sufficiently such that
the disk is unusable because of sporadic double refraction. The effect of
pressure releasing is not exhibited after the resin material (4) is
solidified completely such that it becomes impossible to control the
double refraction.
In such manner, the mold clamp pressure of the movable metal mold (21) is
released at a prescribed timing to control the initial value of the double
refraction. The resin material (4) is allowed to cool completely while
maintaining this state. The mold clamp pressure only is released at this
time, with the metal molds (21), (22) being kept in a mutually contacting
state.
The metal molds (21), (22) are ultimately opened to take out the molded
disk.
When the disk thus taken out may be used directly, the double refraction
approaches to zero gradually by chronological changes. However, when the
disk is annealed, the double refraction becomes stable in the vicinity of
zero. The temperature condition for annealing is practically in the range
of 70.degree. to 120.degree. C. It is however preferred that the
temperature be in the range of 100.degree. to 120.degree. C.
FIG. 8 shows the changes in the internal pressure of the metal molds (21),
(22) through the above described process steps in a graphic form.
Thus, at the time of injection of the molten resin material (4), the
internal pressure is rapidly increased as shown at A in the drawing until
the internal pressure reaches a peak on termination of injection into the
cavity (29). Then, as shown at C in the drawing, a constant mold clamping
pressure is uniformly applied to the resin material (4) as indicated at C
in the drawing and the mold clamping pressure is rapidly released at a
prescribed timing (at a position indicated at D in the drawing, which is
determined by taking the results of FIG. 6 and the shifting of the double
refraction to the minus side by chronological changes into account. The
metal molds (21), (22) are then maintained at a low pressure approximately
equal to zero, as indicated at E in the drawing. The mold is opened when
the resin material (4), in 9 seconds herein, to take out the molded disk.
When the disk of polycarbonate resin is molded under these injection
molding conditions, the producted disk exhibits the double refraction
towards the plus side in the vicinity of the center as indicated at a
curve i in FIG. 9, with the double refraction becoming gradually smaller
towards the outside periphery when the disk is annealed for example at
100.degree. C. for five hours, the double refraction on the plus side, the
double refraction becomes stable at approximately zero from the inside
towards the outside peripheries, as shown by a curve ii in FIG. 9.
When the constant internal pressure in the metal molds (21), (22) is
continuously maintained after the injection of the molten resin material
(4), as shown in FIG. 10, the double refraction of the produced disk
initially shows a value close to zero, as shown by a curve iii in FIG. 11.
However, when the disk is subjected to an anneal test at 100.degree. C.
for 5 hours for investigating into the chronological changes, it shows a
larger value of double refraction on the minus side especially at the
inside periphery as indicated by a curve iv in FIG. 11.
As described above, the method of the present invention may be said to
consist in the separation of the transfer assuring zone and the double
refraction control zone in the molding process from each other. Therefore
it becomes possible to make an independent control of these conditions and
to produce a molding disk simultaneously satisfying the mutually
contradictory conditions, that is, double refraction and the transfer
characteristics.
The present invention is not limited to the above described embodiment. For
example, it does not matter what kind of the injection molding machine or
what shape of the metal mold is employed. The disk size or the mold
clamping pressure are the matter of design and may be changed as desired.
From the foregoing it may be seen that, according to the present invention,
the molten resin material is injected into the cavity formed between the
stationary metal mold and the movable metal mold for forming the disk, the
mold clamping pressure is released during the time since the termination
of the injection of the resin material until the resin material is
solidified, so that it becomes possible to control the initial value of
the double refraction to a prescribed value on the plus side and hence to
produce the disk suffering from only little chronological changes to the
double refraction.
Especially the produced molded disk is further subjected to annealing
whereby the disk may be produced which is only negligible in chronological
changes and high in reliability and in which the double refraction is
stable in the vicinity of zero and in which chronological changes are
small.
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