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BACKGROUND O~ THE INVENTION
The present invention relates to an injection
molding method with compression and an apparatus therefor,
and more particularly to an injection molding method and
apparatus with compression suitable for molding a product
such as, for example, an optical lens which requires high
molding accuracy and uniformity in the composition thereof.
In order to obtain a precise molded product by
using thermoplastic resin, the injection molding method with
compression has lately attracted attention.
Thermoplastic resin such as, for example, PMMA
(methacrylic resin) increases its dynamic rigidity to harden
by cooling the thermoplastic resin from the melted state
having a high temperature, and the volume of the resin is
reduced with decrease of the temperature.
More particularly, the thermoplastic resin, when
cooled, hardens to form a solid product, while only simple
cooling and hardening causes failures in the molded product
such as shrinkage and warpage due to decrease of the volume
in the solid state thereof as compared with that in the
melted state thereof.
Accordingly, a compression margin corresponding to
the contraction rate in the molding is generally provided in
the joint surface of the mold. Then, after injection of the
melted thermoplastic resin into the mold, the thermoplastic
resin is cooled to harden in the applied state with mold
clamping force.
Various process control metho~s for the injection
molding with compression have been proposed heretofore.
Basically, the temperature of the moLd is previously set in
the vicinity of the extraction temperature in order to
increase the eooling efficiency and the mold is pressurized
immediately after completion of the injection so that the
capacity in the mold is equal to the volume of the molded
prodllct under the normal temperature and pressure. Then, the
pressurized force is controlled to be gradually reduced with
the contraction due to the cooling.
Namely, in the conventional process control, in
order to improve the inferiority in the shape and dimension
due to the contraction by the cooling, the pressure applied
to the mold is controlled so that the thermoplastic resin to
be molded maintains the volume under the normal temperature
and pressure in the whole range of temperature in the
cooling. Thus, a satisfactory molded product can be obtained
with stability of the shape and dimension.
The dynamic rigidity of the thermoplastic resin
does not increase uniformly from the melted state thereof of
a hi~h temperature to the solidified sta-te of the normal
temperature and suddenly increases and hardens from a
certain temperature (the glass transition point Tg).
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Acc~rdingly, if any deviation in the temperature
occurs in each portion of the thermoplastic resin upon
exceeding the glass transition point in the cooling,
partially solidified portions and partially melted portions
are mixedly produced in the thermoplastic resin in the
molding space due to the deviation in the temperature. If
the solidified portions and the melted portions are
continuously pressurized uniformly, the solidified portions
are apt to be subjected to plastic deformation and the inner
composition of the molded product is liable to lack
uniformity.
It is a matter of course that if the temperature
of the mold is previously set to a high temperature and the
mold is cooled for a sufficient time, since the temperature
deviation in each portion of the resin is reduced, the above
problems are alleviated to a certain extent. In the case of
the molded product having a large size in the dimension or a
partially large difference in thickness, if the resin in the
mold is cooled slowly so that the temperature deviation does
not occur, the operation efficiency is extremely
deteriorated.
SUMMARY OF THE INVENTION
It is an object of the present invention to
provide a novel injection molding method and apparatus with
0~
compression which can produce a molded product with
excellent uniformity in the inner composition without large
temperature deviation in each portion of resin even if a
time required to harden the resin, particularly a time
required to exceed the glass transition point is shortened.
As descrihed above, thermoplastic resin such as
PMMA possesses a property that the dynamic rigidity thereof
is increased as the resin is cooled so that the resin
hardens and particularly the dynamic rigidity is suddenly
increased upon exceeding the glass transition point.
Further, the thermoplastic resin of this ~ind possesses a
property that the dynamic rigidity thereof is increased and
the glass transition point is shifted to a higher
temperature side by increasing a pressurized force thereto
even if the temperature of the resin is the same.
This means that the thermoplastic resin such as
PMMA hardens relatively in a high temperature (for example,
125C in which the resin is in the melted state under the
normal pressure) when the thermoplastic resin is pressurized
with high pressure.
The present invention has been made by utilization
of such property that the resin hardens in a high
temperature when pressurized.
More particularly, in the present invention, the
melted thermoplastic resin injected in the molding space is
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pressurized to be hardened so that the resin is
substantially completely hardened in the range of slight
variation of temperature and consequently the temperature
deviation in each portion of the resin in hardening is
reduced.
Generally, the present invention is premised on
the injection molding method with compression in which a
predetermined amount of melted thermoplastic resin is
injected into a metal mold formed with a molding space and
the resin is cooled to obtain a molded product while
controlling a pressure applied to the injected thermoplastic
resin in the metal mold.
Metal mold means comprises a stationary metal mold
and a movable metal mold opposed to each other. The movable
metal mold can move among a first position in which the
movable mold cooperates with the stationary mold to form a
molding space substantially identical with the volume of the
product, a second position in which the movable mold
cooperates with the stationary mold to form a molding space
larger than the volume of the product and a third position
in which the movable mold is separated from the stationary
mold.
Prior to the injection operation, the movable mold
moves to the second position to form the molding space
larger than the volume of the product. Further, the
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temperature of the metal mold is initially set to a
temperature higher than a temperature at which temperature
the thermoplastic resin begins to harden under the normal
pressure.
The melted thermoplastic resin is measured by
measuring and injecting means and is injected by a
predetermined amount thereof into the molding space through
gate means. The injected resin is rapidly cooled to the
initially set temperature of the mold by heat exchange with
the mold.
Pressure control means increases the pressure
applied to the thermoplastic resin in the metal mold hefore
the injected thermoplastic resin is cooled to the
temperature at which temperature the resin begins to harden
under the normal pressure. Since the temperature of the
metal mold is previously set to the temperature higher than
the temperature at which temperature the resin begins to
harden under the normal pressure, the thermoplastic resin in
the mold is maintained in the melted state in the beginning
of application of the pressure and accordingly the
pressurizing force applies to the whole of the thermoplastic
resin in the mold uniformly.
As described above, the thermuplastic resin
possesses the property that the dynamic rigidity thereof is
increased so that the resin is hardened when the
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pressurizing force is increased even without reduction of
the temperatllre of the resin.
Accordingly, if the thermoplastic resin is applied
with the pressurizing force capable of obtaining the dynamic
rigidity larger than that of the glass transition point
under the temperature condition in the beginning of
application of the pressure, the thermoplastic resin is
hardened from the beginning of application of the pressure
without reduction of the temperature (or with slight
reduction of the temperature) and the dynamic rigidity
thereof becomes larger than that of the glass transition
point so that the resin is hardened.
~ urther, in this manner, when the thermoplastic
resin in the melted state is hardened by application of the
pressure, the deviation of the pressurizing force and the
deviation of the temperature in each portion of the resin in
the hardening process are extremely small.
The thermoplastic resin hardened by application of
the pressure is cooled to the temperature at which
temperature the dynamic rigidity under the normal
temperature and pressure is obtained while maintaining the
pressurizing force.
Since the thermoplastic resin hardened in the
pressurized state has a temperature at which temperature the
resin is still melted under the normal pressure, the dynamic
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rigidity thereof is reduced and the resin is softened if the
pressurizing force is reduced. However, as described above,
the thermoplastic resin of this kind possesses a property
that the dynamic rigidity thereof is increased as the resin
is cooled.
Accordingly, temperature control means reduces the
temperature of the mold gradually to cool the thermoplastic
resin hardened by application of the pressure to the
extraction temperature and the pressure control means
reduces the pressurizing force so that increase of the
dynamic rigidity due to cooling is canceled. Thus, the
thermoplastic resin is molded while maintaining the dynamic
rigidity under the normal temperature and pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a characteristic diagram showing a
relation of temperature and dynamic rigidity of PMMA of an
example of thermoplastic resin for parameter of pressure;
Fig, 2 is a sectional view of an injection molding
apparatus with compression of a direct pressure type
according to an embodiment of the present invention;
Fig. 3 is a sectional view of a compression margin
adjustment cylinder of an example of molding space expansion
means;
Fig. 4 is a sectional view showing an example of a
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valve mechanism for driving the compression margin
adjustment cylinder shown in Fig. 3;
Fig. 5 is a sectional view showing an example of a
plasticization device and a measuring and iniecting device;
Fig. 6 is a circuit diagram showing an example of
a control system of the present invention;
Fig. 7 is a sectional view of gate means;
Fig. 8 is a graph showing a control characteristic
in molding; and
Fig. 9 is a flow chart showing operation of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention is now
described in detail with reference to drawings.
Fig. 1 shows a relation of temperature and dynamic
rigidity of PMMA of an example of thermoplastic resin for
parameter of pressure.
In Fig. 1, curves a, b, c. d, e and f show
relations of temperature and dynamic rigidity at 1, 200,
400, 600, 800 and 1000 bars, respectively. The curve showing
a relation of the temperature and the dynamic rigidity of
PMMA is shifted to the higher temperature side by 0.025C
each time the pressure increases by 1 bar.
Further, in Fig. 1, points a1, bl, c1, d1, el and
ns
fl are points in which PMMA begins to harden at each of
corresponding pressures, points a2, b2, c2, dz, e2 and fz
are points in which PMMA hardens to a state before the glass
transition point at each of corresponding pressures, points
a3, b3, c~, d3, e3 and f3 are points in which PMMA harden to
a state exceeding the glass transition point at each of
corresponding pressures, and points a~, b4, C4, d4, e4 and
f4 are points in which PMMA hardens completely at each of
corresponding pressures.
Under the condition of pressure of 1 bar, PMMA is
in the completely melted state in the temperature range
exceeding 130C, it begins to harden at about 125C when
cooled, the dynamic rigidity just before the glass
transition point is obtained at 120C, the dynamic rigidity
exceeding the glass transition point is obtained at 115C,
and PMMA hardens completely at about 100C.
On the other hand, under the condition of pressure
of 1000 bars, PMMA is in the completely melted state in the
temperature range exceeding 155C, it begins to harden at
about 150C when cooled, the dynamic rigidity just before
the glass transition point is obtained at 145C, the dynamic
rigidity exceeding the glass transition point is obtained at
140C, and PMMA hardens completely at about 125C.
The thermoplastic resin such as PMMA completely
hardens relatively even under a high temperature when the
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resin is in a high pressure state. Accordingly, the present
invention has been made by utilization of the property of
the thermoplastic resin that the resin is hardened by
application o~ pressure.
In the present invention, the temperature of the
metal mold is initially set to a temperature higher than a
temperature at which temperature the thermoplastic resin
begins to harden under the normal pressure and the
thermoplastic resin is maintained in the melted state upon
completion of iniection.
Aft~r completion of the injection, the
thermoplastic resin is pressurized to be hardened in a high
temperature and the resin hardened by application of
pressure is cooled. At the same time as the cooling, the
pressure applied to the thermoplastic resin is controlled so
that the dynamic rigidity of the thermoplastic resin during
cooling is maintained constant. Namely, the temperature and
the pressure are controlled to cool the thermoplastic resin
so that the dYnamic rigidity of the resin under the normal
temperature and pressure is maintained.
~ ig. 2 is a sectional view of the injection
molding apparatus with compression according to an
embodiment of the present invention.
The injection molding apparatus according to the
present invention is formed with a frame 1 in the form of a
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box in a whole configuration and the frame 1 is divided into
chambers 2, 3, 4 and 5 by partition walls.
The chamber 2 constitutes an oil tank which is
filled with oil and in which an oil hydraulic pump 6 is
contained.
The chamber 3 contains a motor 7 for driving the
oil hydraulic pump 6, which is coupled with the motor 7
through a through hole formed in a partition wall 8 between
the chambers 2 and 3. The oil hydraulic pump 6 and the motor
7 eonstitute a known oil hydraulic unit to feed oil to all
oil hydraulic devices.
A mold clamping device 9 is mounted above the
chamber 4.
A stationary die-plate lO is fixedly mounted on
the upper wall of the chamber 4. A eylinder fixing plate 13
on whieh a mold clamping eylinder 12 is mounted is fixedly
mounted at upper ends of tie-bars 11 fixed vertically in
four corners of the stationary die-plate 10. A movable die-
plate 15 is mounted to a piston rod 14 of the mold elamping
eylinder 12 to be able to move up and down along the tie-
bars ll.
Further, a stationary metal mold 16 is
exehangeably mounted on the stationary die-plate 10 and a
movable metal mold 17 is also exehangeably mounted to the
movable die-plate 15.
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Compression margin forming cylinders 18 which form
a predetermined compression margin in the joint surface of
the mold between the stationary and movable Molds before the
injection operation are provided in each of the tie-bars 11.
The compression margin means a gap formed previously in the
joint surface of the mold in consideration of reduction of
the volume of the resin which occurs when the melted resin
is compression molded.
Fig. 3 is an enlarged sectional view showing the
compression margin adjusting cylinder 18, in which the same
elements as those of ~'ig. 2 are given the same numerals.
A bolt or thread lla is formed at the lower end of
the tie-bar 11 and penetrates a through hole lOa formed in
the stationary die-plate 10. A nut 18a which is fitted onto
the bolt lla is formed in the upper surface of the
compression margin adjusting cylinder 18. The bolt l]a is
turned tightly into the nut 18a so that the tie-bars 11 and
the adjusting cYIinder 18 are fixed to the stationary die-
plate 10.
A chamber 18b is formed in the lower por~ion of
the adjusting cylinder 18 and a piston ]8c is disposed in
the chamber 18b so that the piston 18c can move vertically.
A lower opening of the chamber 18b is closed by a cap 18e
having a through hole which a piston rod 18d penetrates
slidably. Numeral 18f denotes a port of the adjusting
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cylinder 18.
A bottomed cylindrical case 18g includes a nut 18h
formed 3n the central bottom of the case 18g. The nut 18h is
fitted onto a bolt 18i formed in the lower end of the piston
rod 18d.
A pin 19 is inserted into a guide hole lOb formed
in the stationary die-plate 10 vertically movably and a
lower end of the pin 19 is supported on an upper end surface
of the case ]8g.
A spacer ring 20 is mounted around the tie-bar 11
so that the ring 20 can move up and down along the tie-bar
11. Accordingly, when oil is fed through the port 18f to
move back the piston 18c into the chamber 18b, the piston
rod 18d, the case 18g, the pin 19 and the spacer ring 20 are
integrally moved up.
On the other hand, cylindrical intermediate
members 21 are fixedly mounted in the lower surface of the
movable die~plate 15 to cover the outer periphery of the
tie-bars 11. Thus, when the spacer ring 20 is pushed up as
described abovep the movable die-plate 15 is also pushed up
together with the intermediate members 21 ~o form a
compression margin. The intermediate members 21 have a
length sufficient not to prevent the mold clamping operation
when the spacer ring 20 is lowered and to form the
compression margin when the spacer ring 20 is moved up.
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The compression margin adjusting cylinder 18 and
its associated mechanism serve to set a precise compression
margin in order to obtain a precise molded product.
More particularly, the piston rod 14 is advanced
to minimize the compression margin to zero. At this
condition, when the compression margin adjusting cylinder 18
is moved against the mold clamping cylinder 12, a
predetermined compression margin is formed. After the
compression margin reaches a set value, thermoplastic resin
is injected into the mold. After completion of the
;njection, the pressure applied to the adjusting cylinder 18
is reduced and the piston rod 14 is advanced so that the
mold clamping operation is performed.
The present embodiment is characterized in that an
oil hydraulic circuit communicating with the port 18f of the
adjusting cylinder 18 is completely closed at the timing
when the compression margin reaches the set value so that
the adjusting cylinder 18 functions substantially in the
same manner as a so-called mechanical lock to thereby fix
the compression margin precisely.
Accordingly, a valve mechanism for operating the
compression margin ad~usting cylinder 18 is required to be
able to control a small amount of flow and have a high speed
response upon closure.
Fig. 4 is a sectional view showing an example of
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the valve mechanism for operating the compressiun margin
adjusting cylinder 18. The valve mechanism 22 discharges oil
stepwise in response to pulses produced from a pulse
oscillator 23.
The valve mechanism 22 shown in Fi~. 4 includes
flow ways 22c and 22d formed in parallel with each other
between an inlet 22a and an outlet 22b and is adapted to
pass oil by opening the flow ways 22d and 22d alternately
for a short time in synchronism with the pulses produced
from the pulse oscillator 23.
More particularly, a drive pin 22e made of soft
magnetic iron is swingably supported by an axis 22f in a
magnetic field. Each time the pulses of the oscillator 23 is
applied to coils 22g and 22h wound around the drive pin 22e,
the polarity of the drive pin 22e is reversed so that the
pin is swung.
In the state shown in Fig. 4, a ball 22k in a
pilot valve 22j is pushed uP together with a pin 221 by
pilot pressure applied from a pilot pressure source 22i and
the pilot pressure is applied to pressure chambers 22m and
22n.
Accordingly, since a poppet 220 opens a valve seat
22p while a poppet 22q closes a valve seat 22r, the flow way
22c is closed as a whole. Further, since a poppet 22s opens
a valve seat 22t while a poppet 22u closes a valve seat 22v,
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the flow way 22d is closed as a whole. Oil does not flow
from the inlet 22a to the outlet 22b.
In this statc, when the polarity of the pulses
produced from the pulse oscillator 23 is reversed, the drive
pin 22e is rotated clockwise in the fi~ure since the right
and left polarities of the drive pin 22e are reversed.
Accordingly, a ball 22x in a pilot valve 22w is pushed up
together with a pin 22y by pilot pressure applied from the
pilot pressure source 22i and the pilot pressure is applied
to pressure chambers 22A and 22B.
Thus, the pilot pressure applied to the pressure
chamber 22A causes the poppet 220 to close the valve seat
22p and the poppet 22u to open the valve seat 22v while the
pilot pressure applied to the pressure chamber 22B causes
the poppet 22s to close the valve seat 22t and the poppet
22q to open the valve seat 22r. However, since an orifice
22D is provided between the pilot valve 22w and the pressure
chamber 22B, a time delay occurs from the opening of the
valve seat 22v by the poppet 22u to the closure of the valve
seat 22t by the poppet 22s, oil flows through the flow way
22d from the inlet 22a to the outlet 22b.
When the polarity of the pulses produced irom the
pulse oscillator 23 is reversed again, the pilot pressure is
applied to the pressure chambers 22m and 22n again and the
valve mechanism 22 is returned to the state shown in Fig. 4.
1~
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However, since an orifice 22E is provided between the pilot
valve 22j and the pressure chamber 22n, a time delay occurs
from the opening oI the valve seat 22p by the poppet 220 to
the closure of the valve seat 22r by the poppet 22q. The
flow way 22c is opened during the time delay and accordingly
oil flows through the flow way 22c from the inlet 22a to the
outlet 22b.
In this manner, since the valve mechanism shown in
Fig. 4 causes oil to flow from the inlet 22a to the outlet
22b only during the time delay set by the orifices 22D and
22E each time the polarity of the pulses produced from the
oscillator 23 is reversed, the rate of flow is controlled
exactly as a whole in accordance with a frequency of the
pulse and the oil hydraulic circuit extending from the inlet
22a to the outlet 22b is completely closed by means of the
poppets by ceasing the pulse oscillator 23 to thereby
satisfy the reguirement of the present invention.
In Fig. 2, numeral 30 denotes a measuring and
injecting device which includes a nozz]e directed upward.
Numeral 40 denotes a plasticizing device which plasticizes
resin of raw material to feed it to the measuring and
injecting device 30. The measuring and injecting device 30
is disposed in the chamber 4 and the plasticizing device 40
is disposed in the chamber 5. The device 30 is coupled with
the plasticizing device 40.
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In the embodiment, the plasticizing device 40 is
put on a truck 40a. The plasticizing device 40 moves
together with -the measuring and injecting device 30 along a
rail 40b and rotates about an axis 40c on a vertical plane.
A nozzle 31 is positioned in accordance with the movement of
the truck 40a and is coupled with a sprue bush lOc of the
stationary die-plate 10 by the rotation about the axis 40c.
The measuring and injecting device 3Q, the
plasticizing device 40 and the associated mechanism thereof
are required to be able to measure the resin melted in the
proper temperature exactly and inject the melted resin. Fig.
5 shows in section an actual example of the measuring and
injecting device 30 and the plasticizing device 40.
In Fig. 5, the same elements as those described
above are given the same numeral as that of the elements
described above.
A plunger 33 is inserted into a lower opening of
an injection cylinder 32 and is moved up and down by oil
hydraulic cylinder 34.
A check valve 35 is provided in a tip o-E the
injection cylinder 32.
An inner diameter of a lower inner periphery 32a
of the injection cylinder 32 positioned near the oil
hydraulic cylinder 34 is smaller than that of an upper inner
periphery 32b thereof positioned near the nozzle 31 and a
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step is formed between the lower inner periphery 32a and the
upper inner periphery 32b of the injection cylinder 32. The
lower inner periphery 32a of the injection cylinder 32 and
an outer periphery 33a of the plunger 33 are sealed each
other. A gap is formed between the up~er inner periphery 32b
and the outer periphery of the plunger 33 and the
plasticized resin flows through the gap into the injection
cylinder 32.
The plunger 33 is moved by a difference between a
pressure of the resin in the injection cylinder 32 and a
pressure of the oil hydraulic cylinder 34.
Numeral 36 represents a heater for heating the
injection cylinder 32, TS1 represents a temperature sensor
which detects a temperature of the resin in the injection
cylinder 32, and PSl represents a pressure sensor which
detects a pressure in the injection cylinder 32. Numeral 37
represents a position sensor of, for example, an optical
type which detects a backward distance of the plunger 33. An
amount of resin injected at one injection operation is
measured on the basis of outputs of the temperature sensor
TS1, the pressure sensor PSl and the position sensor 37. As
far as the position sensor 37 can detect the backward
distance of the plunger 33, the disposition thereof is not
limited to the position shown in Fig. 5.
The plasticizing device 40 serves to melt the
21
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resin of raw material fed into a plasticizing cylinder 42
through a hopper 4t by heating it by the heater 43. A screw
44 is rotated by an oil hydraulic motor 45.
A poppet 46 is disposed in the flow path of the
resin extending from the plasticizing cylinder 42 to the
injection cylinder 32 and is driven by a cylinder 47 to open
and close the flow path between the plasticizing cylinder 42
and the injection cylinder 32.
The mechanism described above is controlled by a
system as shown in Fig. 6, for example.
In Fig. 6, the elements described above are given
the same numeral as those described above and description
thereof is omitted. Only elements which are not described
above are described.
Numeral 50 denotes a valve mechanism including a
~nown servo~valve of an electric~to-hydraulic conversion
type and a known pressure control valve. The valve mechanism
50 is coupled with ports 12a and 12b of the mold clampine
cylinder 12. A piston rod 14 is advanced by -feeding oil to
the port 12a through the valve mechanism 50.
Similarly, numeral 51 denotes a valve mechanism
including a known servo-valve of an electric-to-hydraulic
conversion type and which is coupled with ports 34a and 34b
of the injection cylinder 14. The injection operation is
performed by feeding oil to the port 34a through the valve
22
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mechanism 51.
Numeral 52 denotes a directional control valve of
the solenoid control type which is employed to Ieed oil to
the cylinder 47, which is coupled with any of a pressure
source or a drain in accordance with a condition of the
directional control valve 52.
Numeral 53 denotes a shutoff valve for shutting
off the oil hydraulic circuit of the compression margin
forming cylinder l8.
Numeral 54 denotes a photointerrupter constituting
an example of a sensor for detecting the compression margin.
Pulses produced from the photointerrupter 54 in accordance
with backward movement of the movable die-plate 15 is
supplied to a controller 56.
Numeral 55 denutes a heater which is used to
determine a temperature of the resin in the metal mold. The
composition and the shape of the heater 55 is different
depending on the shape of the metal mold. Numeral 57 denotes
a known input device, 58 a memory and 59 an au~iliary
memory.
TS2 is a temperature sensor for detecting a
temperature of the resin in the mold, PS2 is a pressure
sensor for detecting a pressure of the resin in the mold,
and TS3 is a temperature sensor for detecting a temperature
of the resin in the plasticizing cylinder 4~.
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Further, numeral 60 denotes a solenoid valve which
seals a gaLe provided in the mold 16. A more actual
configuration is shown in an enlarged view of Fig. 7.
The solenoid valve 6~ is coupled with a cylinder
61. The cylinder 61 advances a rod 62 in response to
excitation of the solenoid valve 60 to seal the gate and the
cylinder 61 move the rod 62 back in response to
deenergization of the solenoid valve 60 to open the gate.
Operation of the embodiment is now described with
reference to the foregoing.
The operation of the injection molding apparatus
with compression according to the present embodiment is
divided into ~1) the plasticizing operation of resin by the
plasticizing cylinder 42, (2) the compression margin forming
operation by the mold clamping cylinder 12 and the
compression margin forming cylinder 18, (3) the injection
operation by the iniection cylinder 32 and (4) the
compression molding operation by the mold clamping cylinder
12 and the metal molds 16, 17. The operation is described in
the order described above. The flow chart shown in Fig. 9
would facilitate the understanding of the operational
sequence.
~ irst of all, various data such as, for example, a
temperature of resin before injection, an amount of resin
injected by one molding operation, an injection pressure,
24
~?.t9~l6()8
an injection speed, the magnitude of the compression margin,
a series of data concerning temperature in the metal mold, a
series of data concerning the mold clamping force and the
like are inputted from the input device 5~ and stored in the
memory 58 and the auxiliary memory 59.
The heater 43 is set to 190C which is the
temperature of PM~A which is to be injected.
In the initial state, the controller 56 releases
excitation of the directional control valve 52 so that the
cylinder 47 is coupled with the drain through the
directional control valve 52.
Accordingly, the poppet 46 opens to connect
between the plasticizing cylinder 42 and the injection
cylinder 32.
The PMMA fed from the hopper 41 is heated by the
heater 43 and is melted.
When the temperature sensor TS3 detects that a
temperature of the PMMA in the cylinder 42 has reached
190C, the controller 56 drives the motor 45 ~o rotate the
screw 44.
At this time, the valve mechanism 51 keeps the
balance of pressure between the ports 34a and 34b of the oil
hydraulic cylinder 34. Accordingly, the plunger 33 can move
up and down freely in the jnjection cYlinder 32 in
accordance with the external pressure.
~.~'J9~L~;Ot~3
In the initial state, since the solenoid valve 60
is excited and the rod 62 seals the gate, the melted PMMA
flows from the plasticizing cylinder 42 intu the injection
cylinder 32 in response to the rotation of the screw 44 and
the plunger 33 is moved back.
As the injection cylinder 32 is filled with PMMA,
the plunger 33 is moved back and the backward amount thereof
is detected by the position sensor 37. When the controller
56 judges that a predetermined amount of PMMA is stored in
the injection cylinder 32 by the fact that the output of the
position sensor 37 reaches a predetermined reference value,
the controller 56 excites the directional control valve 52.
The cylinder 47 is coupled with the pressure
source in response to the excitation of the directional
control valve 52 and the shutoff valve closes between the
plasticizing cylinder 42 and the iniection cylinder 32.
Thus, the predetermined amount of PMMA is stored in the
injection cylinder 32.
As described above, in the embodiment, it is
detected by the output of the positiDn sensor 32 that the
predetermined amount of PMMA is filled in the injection
cylinder 32, while the volume of PMMA varies slightly
depending on temperature and pressure.
Accordingly, in the embodiment, the condition for
operating the cylinder 47 is corrected by the output of the
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0~3
pressure sensor PS1 and the temperature sensor TSt.
More particularly, the volume of YMMA is reduced
as the pressure applied thereto is increased. Accordingly,
the controller 56 attains correction so that the reference
value with respect to the detected output of the position
sensor 37 is reduced as the detected pressure of the
pressure sensor PSl is increased while the reference value
with respect to the detected output of the position sensor
37 is increased as the detected pressure of the pressure
sensor PSl is decreased.
Further, the volume of PMMA is reduced as the
temperature thereof is lower. Accordingly, the controller 56
attains correction so that the reference value with respect
to the detected output of the position sensor 37 is reduced
as the detected temperature of the temperature sensor TSl is
lower while the reference value with respect to the detected
output of the position sensor 37 is increased as the
detected temperature of the temperature sensor TSt is
higher.
When the predetermined amount of PMMA is filled in
the injection cylinder 32 in the manner described above, the
controller 56 controls the compression margîn forming
operation.
ln the embodiment, since the compression margin
adjusting cylinder 18 forms the compression margin against
27
the mold clamping cylinder 12, the valve mechanisms 50 and
22 are adjusted so that a relation of P1-Al<P2-A2 is
satisfied and a difference of the relation is extremely
small where the inner diameter of the mold clamping cylinder
12 is A1, the oil pressure thereof is Pl, the inner diameter
of the compression margin adjusting cylinder 18 is A2, and
the oil pressure thereof is P2.
Further, the shutoff valve 53 is deenergized to
open the port 18f of the compression margin adjusting
cylinder 18. The pulse oscillator 23 ceases the oscillation
operation thereof.
In this state, the controller 56 operates the
valve mechani~m 50 to feed oil to the port 12a. At this
time, since the pressure in the compression margin adjusting
cylinder 18 is reduced, the piston rod ]4 is advanced and
the compression margin s between the movable mold 17 and the '
stationary mold 16 is minimized to zero so that the force of
P1-Al is applied to the joint surface between the movable
mold 17 and the stationary mold t6.
When the compression margin is minimized to zero,
the controller 56 excites the shutoff valve 53 and operates
the pulse oscillator 23 and receives pulses produced from
the photointerrupter 54.
When the oscillator 23 produces the pulses, the
valve mechanism 22 discharges oil stepwise in response to
28
()8
each edge of the pulses. Further, the shutoff valve 53 is
excited to be closed. Accordingly, the oil discharged by the
valve mechanism 22 is fed to the port ]8f of the compression
margin adjusting cylinder 18.
A relation of P1-Al<P2-A2 is formed between the
force Pl-A1 produced by the mold clamping cylinder 12 and
the force P2-A2 produced by the compression margin adjusting
cylinder 18. Accordingly, the piston 18c shown in ~ig. 3 is
moved back into the chamber 18b. Since the case 18g is moved
up while pushing up the pin 19, the spacer ring 20, the
intermediate member 21 and the movable die-plate 15, the
compression margin s is increased.
The photointerrupter 54 produces pulses in
accordance with the moving up of the movable die-plate 15.
The controller 56 adds the pulses produced from the
photointerrupter 54 to obtain the current value of the
compression margin s. When the current value of the
compressian margin s reaches the set value of the
compression margin s stored in the memory 58, the pulse
oscillator 23 is ceased.
As described above, When the valve mechanism 22 is
not applied with the pulses from the oscillator 23, since
the flow ways 22c and 22d between the inlet 22a and the
outlet 22b are complctely cut off by the poppet mechanism
and the shutoff valve 53 is also shut off, the path of
29
retreat of the oil fed in the compression margin adjusting
cylinder 18 is completely intercepted.
Since the mold clamping cylinder 12 adds the force
of Pl-Al to the compression margin adjusting cylinder 18,
the adjusting cylinder 18 fixes its length completely while
satisfying the relation of Pl-Al=P2'-A2, and the compression
margin s is also fixed.
At this time, the pressure of the adjusting
cylinder 18 varies from P2 to Pz', while the compression of
oil in the adiusting cylinder 18 is in the numerical range
in which the compression can be neglected substantially as
compared with the compression margin s.
In this manner, when a proper compression margin
is set, the controller 56 controls the injection operation
of PMMA.
The temperature of the heater 55 for the metal
mold is adjusted to 125C which is an example of a
temperature before PMMA begins to harden under the normal
pressure.
The temperature of the heater 36 for the injection
cylinder 32 is adjusted to 1~0C which is an example of a
temperature at which temperature PMMA does not begin to
harden under any pressure.
Accordingly, PMMA in the injection cylinder 32 is
completely melted.
~9~i08
The controller 56 deenergizes the solenoid valve
60 shown in Fig. 7 to release the sealing of the gate.
Thereafter, the cuntroller 56 operates the valve mechanism
51 to feed oil to the port 34a of the oil hydraulic cylinder
34. Accordingly, the plunger ~3 is advanced in the injection
cylinder 32 so that the melted PMMA is injected into the
molding space formed by the movable metal mold 17 and the
stationary metal mold 16.
The pressure in the adjusting cylinder 18 is
slightly varied by the injection pressure at this time.
However, the pressure in the cylinder 18 is a reactive force
against the force exerted between the stationary die-plate
lO and the movable die-plate 15 and does not depend on the
external oil hydraulic circuit. ~'urthermore, since the
injection pressure is extremely small, the compression
margin s is not almost varied.
After completion of the injection, the solenoid
valve 60 is excited to seal the gate. l'hereafter, the
shutoff valve 53 is deenergized to reduce the pressure of
the adjusting cylinder 18 so that compression molding of
PMMA can be attained.
The resin injected into the metal mold is rapidly
cooled to 125C that is the initially set temperature by the
heat exchan~e with the metal mold.
The detected value of the temperature sensor TS2
31
~?-,~lÇ;O !3
in the metal mold rises at a heat by injecting PMMA heated
to 190C, while the detected value of the sensor TSz is
decreased again by cooling the PMMA.
As described above, PMMA does not harden under the
pressure of 1 bar until the temperature thereof is decreased
to about 125C, while the PMMA begins to harden at about
150C under the pressure of 1000 bars.
In the embodiment, when the temperature sensor TSz
detects the fact that the temperature of the PMMA reaches
the temperature at which temperature the PMMA does not begin
to harden sti]l under a lower pressure but the PMMA begins
to harden under application of the pressure, the valve
mechanism 50 is controlled to suddenly increase the mold
clamping force of the cylinder 12 so that the PMMA in the
metal mold is pressurized at a heat to obtain the dynamic
rigidity larger than that of the glass transition point with
slight reduction of the temperature.
A curve shown by thick line of Fig. 8 shows an
example of a control curve of the temperature and the
pressure upon hardening.
When F'MMA is injected into the metal mold, the
temperature of PMMA is reduced to 145C after a while from
the gate sealing. When the size of the product is large, a
case where the temperature of the resin is partially reduced
to 145C before the gate sealing is considered, while in
O~
this case the injection speed is increased or the initial
temperature of the metal mold is set to a temperature higher
than 125C to control so that each of PMMA is cooled to
about 145C uniformly.
When PMMA has been cooled about 145C under the
normal pressure, PMMA has been completely melted and
accordingly pressure is applied to each portion of PMMA
uniformly.
In the embodiment, while PMMA is in the range of
temperature in which PMMA maintains its completely melted
state, pressure applied to PMMA is suddenly increased so
that the dynamic rigidity thereof is suddenly increased
during slight reduction of temperature of P~MA.
~ hen the detected temperature of the sensor TSz
reaches 145C (point p~ of Fig. 8), the controller 56
controls the valve mechanism 50 to increase the pressure
applied to the port 12a of the mold clamping cylinder 12 so
that pressure of, for example, 600 bars is applied to PMMA
in the metal mold.
Thus, the dynamic rigidity of PMMA is increased by
the applied pressure thereto as described above and the
dynamic rigidity reaches ExlO (dyn~cm2) which is a state
just before the glass transition point when PMMA is cooled
to 135C (point P2 of Fig. 8). E is a coefficient different
depending on resin.
~9~
When the temperature sensor TS2 detects that PMMA
has been cooled to 135C, the controller 56 contr~ls the
valve mechanism 50 to increase pressure applied to the port
12a of the mold clamping cylinder 12 so that pressure of
1000 bars is applied to PMMA in the metal mold.
This application of pressure further increases the
dynamic rigidity of PMMA and when PMMA has been cooled to
133C (point p3 of ~ig. 8) the dynamic rigidity exceeds the
glass transition point completely. When cooled to 130C
(point p~ of Fig. 8) PMMA has been almost completely
hardened and when cooled to 125C (point p5 of Fig. 8) PMMA
is completely solid.
With the control as described above, since the
dynamic rigidity of PMMA exceeds the glass transition point
while PMMA is cooled in a small temperature range of 2C
from 135C to 133C, the whole cooling time is extremely
short and the operation efficiency is improved even if the
cooling speed in the small temperature range i6 made slow
sufficiently so that any deviation in temperature does not
occur in each portion of PMMA while the glass transition
point is exceeded.
~ t is needless to say that the suitable cooling
speed in exceeding the glass transition point is different
depending on the temperature conductivity of resin, a shape
of a product and the like. When a molded product having a
3~
iQ8
large size or- deviation in thickness is obtained, the
temperature of the metal mold is set to a higher temperature
or the temperature of the metal mold is reduced stepwise so
that deviation in temperature of each portion of the resin
is small.
PMMA is completely solidified at the temperature
of 125C in the applied state of pressure of 1000 bars as
described above. However, as apparent from Figs. 1 and 8,
the temperature of 125C is a temperature at which PMMA
begins to harden slightly under the normal pressure.
Accordingly, when PMMA being in the applied state of
pressure is returned to the normal pressure state, the PMMA
is softened again. Therefore, in order to extract the molded
product, PMMA must be cooled to the temperature ~that is,
100C) at which PMMA is completely solid under the normal
pressure.
Accordingly, in the embodiment, the set
temperature in the metal mold by the heater 55 is reduced
stepwise to coo} PMMA to 100C which is the extraction
temperature in the mold clamping state in which the pressure
is applied to the port 12a of the mold clamping cylinder 12
by the valve mechanism 50, while if the high pressure of
1000 bars is continuously applied to PMMA in the solid
state, plastic deformation occurs in the molded product.
As described above, when PMMA has a certain
~5
dynamic rigidity, the temperature of PMMA is varied by
0.025C each time the applied pressure is varied by 1 bar.
Accordingly, the controller 56 controls the valve
mechanism 50 while watching the detected temperature in the
metal mold by the temperature sensor TSz. Thus, each time
the temperature of PMMA is reduced by 1C, the pressure
applied to PMMA is reduced by 40 bars so that the molded
product is obtained while the dynamic rigidity of PMMA is
maintained uniform.
The control of temperature and pressure as
described above is performed by a known command value
following control.
As described above, in the case where the pressure
is reduced by 40 bars each time the temperature is reduced
by 1C, the pressure is reduced to 800 bars at 120~C, 600
bars at 115C, 400 bars at ]10C, 200 bars at 105C and the
normal pressure at 100C.
When the molding operat;on is completed as
described above, the controller 56 controls the valve
mechanism 50 to ~eed oil to the port 12b of the mold
clamping cylinder 12 so that the movable metal mold 17 is
lifted and the molded product is extracted by a known
ejector mechanism not shown.
The essence of the present invention resides in
that resin which is in the melted state under the normal
36
pressure but is set to the temperature at which the resin
begins to harden by application of pressure is applied with
pressure and is hardened to exceed the glass transition
point in the slight reduction of temperature. However, it is
needless to say that the temperature of resin before
injection, the initial set temperature of the metal mold,
the temperature in the beginning of application of pressure,
the applied pressure and the like are different depending on
the kind of resin, a size of a molded product, a shape of
the product and the like and the molding time is also
different depending on these condition. It is necessary to
decide the optimum molding conditions for each molded
product.
In the above embodiment, reference is made to the
actual example concerning the measuring and injecting device
and the plasticizing device, while the configuration thereof
is not limited as far as the temperature of resin before
injection can be controlled properly and the amount of resin
can be also measured exactly.
Further, the above embodiment makes reference to
the mechanism for forming the compression margin, while the
configuration thereof is not also limited thereto as far as
the accuracy of adjusting the compression margin can be
satisfied.
As described above, in the present invention,
37
ÇiQ~
since, when the injected resin has been cooled to the
temperature at which the resin is maintained in the melted
state under the normal pressure and begins to be hardened by
application of pressure, the resin is pressurized to be
hardened to obtain the dynamic rigidity larger than that of
the glass transition point, the resin exceeds the glass
transition point while the temperature thereof is reduced
slightly.
Accordingly, since the temperature reduction
necessary to exceed the glass transition point in the
molding of resin is extremely small, even if the cooling
speed is made slow so that the temperature deviation does
not occur in each portion of the resin during this process,
the time required to exceed the glass transition point is
small~
According to the present invention, since the
initial set temperature of the metal mold is set to a
temperature higher than that at which resin begins to be
hardened under the normal pressure and the resin befure the
beginning of application of pressure is completely melted
state, the pressure is applied to each portion of the resin
uniformly and solidified portions and melted portions are
not mixedly produced in the resin in the molding.
Accordingly, it is hard to produce partially plastic
deformation and lack of uniformity of the inner co~position
38
o~
and the molded product superior to the uniformity of the
inner composition can be easily obtained.
Further, in the present invention, since the
temperature of the resin and the pressure applied to the
resin are controlled so that increase of the dynamic
rigidity due to the temperature reduction and decrease of
the dynamic rigidity due to the pressure reduction are
canceled each other while the resin which has been hardened
is cooled to the temperature for extraction, the resin in
the cooling maintains a constant dynamic rigidity and the
property of the molded product is not deteriorated in the
cooling.
39
