Mold clamping pressure control method for injection compression molding and injection compression molding machine

A mold clamping pressure control method for injection compression molding through adjustment of the oil pressure in the mold clamping cylinder by the mold clamping cylinder control valve, wherein a resin temperature in the mold is detected by a resin temperature sensor, and detected values thereof are converted by a converter into electric signals, which are in turn compared with a series of the preset pressure changeover temperature setting values to output pressure changeover signals whenever the detected values correspond to the preset values. The pressure changeover signal selects any of a series of the preset pressure setting values to output the valve opening-shutting directing signals converted into a certain valve opening-shutting amount, thereby to control the opening-shutting amounts of the mold clamping pressure control valve.

FIELD OF THE INVENTION 
This invention relates to a mold clamping pressure control of injection 
compression molding and more particularly to an injection compression 
molding intended for accomplishing molding of quality product and 
improving accuracy through a sophisticated mold clamping pressure control 
corresponding to a variation in the resin temperature characteristics by 
correlating the pressure changeover point when the multi-stage mold 
clamping pressure changeover takes place in a compression molding 
operation by the mold clamping after injection of the resin with a 
slightly opened mold at a resin temperature detected with a temperature 
sensor mounted on the mold. 
BACKGROUND OF THE INVENTION 
Injection compression molding engineering is the technique in which the 
resin is injected into the slightly opened mold followed by a mold 
clamping operation to accomplish the molding with the following 
advantages. 
Since the cavity may be filled with an injection pressure lower by several 
fold than the pressure applied ordinarily for the injection molding, it 
will require less mold clamping pressure. 
A sufficient pressure may be applied to the molten material in the cavity 
so that relatively finer portions of the mold may be filled with a 
sufficient material to improve the transferability to the mold. 
Less residual stress and fluidity orientation may prevent deformation of 
product to improve accuracy of the shape. 
Sufficient molding pressure to the molten material with enhanced effect of 
swelling may prevent a sink mark of the thicker product. Such engineering 
is known to be preferred to manufacture the products which require greater 
thickness and high accuracy in shape, particularly optical parts such as 
plastic lens and disk, and various kinds of thicker molded products. 
In the actual compression molding operation with the specific resin 
material and the mold of the specific shape, individual mold clamping 
pressure control conditions vary depending upon the characteristics of 
resin, shape of the mold, molten resin or the temperature conditions of 
the equipment and it was impossible to determine generally how much 
clamping pressure should be applied at what point during the compression 
molding operation, for which reason in the conventional injection 
compression molding operation, the setting of the mold clamping pressure 
control condition corresponding to the individual cases has always relied 
on experience. In other words, the shrinkage characteristics of the 
applicable resin, the temperature setting of the mold temperature 
controller, the cooling effect, the temperature variation characteristics 
of the cavity section, variation characteristics of the mechanical motion 
accompanied by the temperature change of the hydraulic fluid and the like 
were empirically judged by skill to determine the status of variation in 
the mold clamping pressure during the compression molding operation. For 
example, by the timer setting a program in which the mold clamping 
pressure is varied with time is provided. 
According to the conventional injection compression molding operation which 
sets the compression molding condition for individual cases as a program, 
it was in fact impossible to cope with the variation in external 
environment including variation in operational environment particularly, 
variation in an outdoor temperature, drying extent of the resin and the 
like in addition to the problems involved in the program per se. When the 
mold clamping pressure control takes place under the preset program which 
progresses separately irrespective of the variation in phase of the resin 
having been filled in the cavity and being in transition from the molten 
condition to solidification, nonconformity occurs between the variation in 
phase of the resin and the program due to various factors, so that the 
resin is occasionally subjected not to fixed pressure intended to be 
applied under a certain condition but rather to the pressure appropriate 
for another condition. Accordingly, when the pressure rises during 
solidifying of the resin from the molten condition, the residual stress 
generates in the molded product a problem of strength and also problems of 
internal strain in the optical parts resulting in variation of the light 
refractive index. 
The conventional mold clamping pressure control for the injection 
compression molding operation requires a certain skill for setting of the 
control condition per se and gives rise to problem that may not be coped 
with in individual cases such as for the program which progresses by 
variation in an external environment and the like even when the necessary 
conditions have been provided. The aforementioned advantages of the 
conventional molding technique expected for the injection compression 
molding operation have not met with effective utilization but with 
generation of defective molded product as the case may be, for which 
reason realization of incorporating the characteristics of resin per se 
fully into the mold clamping pressure control which is not affected by 
operational conditions, external environments and the like has been 
intensively desired. 
SUMMARY OF THE INVENTION 
This invention is intended to secure a mold clamping pressure control which 
always provides a proper mold clamping pressure no matter how the resin 
temperature in the cavity varies during the injection compression molding 
operation and also always provide a fixed mold clamping pressure at the 
fixed resin temperature. 
According to the invention, there is provided a mold clamping pressure 
control method for injection compression molding through the adjustments 
of the oil pressure in the mold clamping cylinder by means of the mold 
clamping cylinder control valve, characterized in that a resin temperature 
in the mold is detected by a resin temperature sensor, detected values 
thereof are converted by a converter into electric signals, which are in 
turn compared with a series of the preset pressure changeover temperature 
setting values to output pressure changeover signals whenever said 
detected values correspond to said setting values, said pressure 
changeover signal corresponding to any of a series of the preset pressure 
setting values to output valve opening-shutting directing signals 
converted into a certain valve opening-shutting amount, thereby to control 
the opening-shutting amounts of said mold clamping pressure control valve. 
When the resin temperature is detected by the resin temperature sensor in 
which an ultrasonic wave is used, the resin temperature may accurately be 
detected without coming in direct contact with the resin in the cavity, 
which should be ideal. 
According to the invention, there is provided an injection compression 
molding machine comprising a mold clamping pressure control mechanism, 
characterized by comprising a mold clamping pressure control valve for 
adjusting the oil pressure in the clamping cylinder to control the mold 
clamping pressure of the said injection molding machine, a resin 
temperature sensor for detecting the resin temperature, a converter for 
converting the detected resin temperature values into electric signals, a 
pressure changeover temperature setting device for setting a series of the 
pressure changeover temperature setting values corresponding each to one 
of a series of the electric signals, a comparative computation section for 
comparatively computing the detected resin temperature values converted 
into the electrical signals and said series of the pressure changeover 
temperature setting values to output a pressure changeover signal whenever 
both values are conformed to each other, a pressure setting device for 
setting a series of the pressure setting values one corresponding to each 
electric signal and a mold clamping pressure control station for inputting 
said pressure changeover signals each of which corresponds to one of said 
series of the pressure setting values and for converting the corresponding 
pressure setting values into the appropriate valve opening-shutting amount 
to output the electrical signal directing said mold clamping pressure 
control valve. 
The progressive degree of variation in phase of resin being in transition 
from the molten condition to solidification is reflected by the resin 
temperature at that time. In other words, once the resin temperature 
during variation in phase is detected, the status of the resin at that 
time could be determined. The strength of appropriate mold clamping 
pressure applied during progressive of variation in phase relies 
essentially on the molten-solid condition of the resin and an error made 
in this setting results in defective molding. Nevertheless, the variation 
in condition of the resin from the molten condition to solidification will 
always occur predeterminably and the physical property constant of the 
resin halfway through the variation in condition may be measured through 
experiments on all resins available. An assumption of providing a proper 
value of the mold clamping pressure for a certain resin under a certain 
condition using such measured values, therefore, may be available. In 
other words, the resin temperature at a certain point during variation in 
condition and the magnitude of the mold clamping pressure to be applied at 
that point are substantially correlated. 
The temperature interval beginning from the vicinity of the resin filling 
temperature to the temperature lower than the resin solidifying 
temperature is divided into many temperature intervals, and the mold 
clamping pressure most suited for such temperature interval is provided 
for each interval. When the number of divisions is increased, it gradually 
comes close to continuous control and comes close to an ideal condition, 
notwithstanding the number of division in practice ought to be determined, 
taking the essential effect into consideration. When the value of the mold 
clamping pressure is changed over from the pressure value corresponding to 
the temperature interval prior to transition into the pressure value 
corresponding to the temperature interval after transition the momentary 
resin temperature detected by the temperature sensor has transmitted from 
the range of a certain temperature interval to the range of the next 
temperature interval, there is provided the mold clamping pressure most 
suited for the molten-solidifying condition of resin reflected by the 
resin temperature. 
In the course of solidification, the resin temperature gradually goes down 
and when setting a series of pressure changeover values beforehand 
beginning from the vicinity of the resin filling temperature to the 
temperature lower than the resin solidifying temperature and, on the other 
hand, setting a proper mold clamping pressure in each temperature interval 
as a series of pressure setting values and making such configuration that, 
whenever the resin temperature detected by the temperature sensor is 
conformed with a series of the pressure changeover temperature setting 
values while gradually changing and that it changes over a series of 
pressure setting values one after another, unlike the conventional mold 
clamping pressure control which may not provide any mold clamping pressure 
anticipated to vary away from the predetermined condition of the resin 
temperature, it allows to control the compatibility of a certain resin 
temperature and the value of the mold clamping pressure at that 
temperature constantly no matter how the resin temperature varies 
throughout the compression molding. 
While further detailed description is given below relating to the example 
of embodied configuration of this invention by referring to the attached 
drawings, this invention is not confined to the example of embodiments 
described.

PREFERRED EMBODIMENTS OF THE INVENTION 
Prior to running the injection compression molding, a series of pressure 
changeover temperature setting values, for instance, T.sub.1, T.sub.2, 
T.sub.3, T.sub.4, T.sub.5, and T.sub.6 are set by a pressure changeover 
temperature setting device 1 as shown in FIG. 1, and a series of pressure 
setting values, for instance, P.sub.0, P.sub.1, P.sub.2, P.sub.3, P.sub.4, 
P.sub.5, and P.sub.6 are set by a pressure setting device 2. When a high 
pressure oil is fed from a port 10 to a booster ram 12 shown in FIG. 2, a 
mold clamping ram 14 moves forward at a high speed and operates mold 
closing of a mold 16. Since the gap between the booster ram 12 and the 
mold clamping ram 14 is not completely sealed, it allows a part of the oil 
to flow into an inside 18 of the mold clamping cylinder. As the advancing 
speed of the mold clamping ram 14 is sufficiently high, it causes a 
negative pressure state in a mold clamping cylinder 18 to suck oil from an 
oil tank and the mold clamping cylinder is filled with the oil. A prefill 
valve 22 moves forward by the actions of the increased pressure when the 
high pressure oil is fed to the booster ram 12 and of the negative 
pressure in the mold clamping cylinder 18, and the port 20 is closed at 
the position where the prefill valve 22 stops and shuts off the oil in 
port 20 and the oil in the mold clamping cylinder 18. At the position 
where the prefill valve 22 stops after moving forward, the oil in the port 
10 and the oil in the mold clamping cylinder 18 pass convergently through 
a through hole 24 arranged in the prefill valve 22. The mold clamping 
pressure is controlled by the mold clamping pressure control valve 
connected through the port 10 (not illustrated in FIG. 2). 
After a molten resin is injected into a cavity 26 which is filled with the 
molten resin, the compression molding is carried out. The mold clamping 
pressure control valve is controlled and an initial mold clamping pressure 
is regarded as P.sub.0 (FIG. 3). A resin temperature sensor 28 of a 
general type such as the temperature sensor illustrated in FIG. 2 which 
uses a thermocouple or an infrared sensing type temperature sensor is so 
installed in the mold that it comes in direct contact with the resin. A 
general type temperature sensor proves more effective when it is installed 
on the part of resin which is as thick as possible. In case it is not 
desired to leave any trace of the sensor on the appearance of the molded 
product, an ultrasonic wave applied temperature sensor 30 is so installed 
that it does not come in direct contact with the resin. The ultrasonic 
wave applied temperature sensor 30 may directly measure the resin 
temperature from the external side of mold. 
As illustrated in FIG. 1, whenever using the converter 4, an electric 
signal obtained by converting the resin temperature detected by the resin 
temperature sensor 3 conforms to one of a series of electric signals 
corresponding respectively to a series of the preset pressure changeover 
setting values (T.sub.1, T.sub.2, T.sub.3, T.sub.4, T.sub.5, and T.sub.6) 
as a result of comparative computation, the comparative computation 
section 5 provides an output of the pressure changeover signal 6. Whenever 
input of the pressure changeover signal is provided, the mold clamping 
pressure control section 7 provides an output of a series of electric 
signals one by one sequentially which directs a make and break amount of 
the valve corresponding respectively to a series of the preset pressure 
setting values (P.sub.1, P.sub.2, P.sub.3, P.sub.4, P.sub.5, and P.sub.6) 
to vary the make and break amount of the mold clamping pressure control 
valve 8, whereby the mold clamping pressure is varied. As illustrated in 
FIG. 3, when the resin temperature goes down, the mold clamping pressure 
varies stepwise at a boundary of the preset pressure changeover 
temperature, and the pressure is increased in the place where the resin 
temperature is sufficiently high and the pressure is decreased as the 
resin temperature comes closer to the solidifying temperature, and when 
the resin temperature is below the solidifying temperature, little or no 
pressure need may be applied. 
The mold clamping pressure changeover point is set as the preset 
temperature during the injection compression molding process and as a 
result of the running pressure control over the mold clamping pressure is 
varied in multi-stage, since the propagation of pressure is good under the 
conditions of the higher resin temperature in the mold cavity and the 
higher fluidity of resin, the effect of injection compression pressure is 
improved to decrease the factors concerning the defective molding 
including sink marks and the like and the inner deformation in the molded 
products may be prevented to decrease the mold clamping pressure at the 
time when the pressure is not propagated when the resin temperature goes 
down and the resin is solidified. Particularly, in case the injection 
compression molding is carried out to produce engineering molded products, 
which may be defective as the permeability of light varies resulting from 
the inner deformation in the product, an occurrence of the defective 
products may be completely prevented if the mold clamping pressure control 
system according to the present invention is used.