Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SPECIFICATION
TITLE OF THE INVENTION
,
METHOD FOR CONTROLLING BACK PRESSURE IN
El.ECTRICALLY-OPERATED INJECTION APPARATUS
FIELD OF THE INVENTION
.
This invention relates to a method for controll-
ing back pressure in an injection apparatus which forms
a part of an injection molding machine used to mold a
synthetic resin and which is operated by an electric
motor to charge and inject the material.
BACKGROUND OF THE INVENTION
U.S. Patent No. 4,540,359 Specification discloses
the art in which the injection apparatus in the injection
molding machine is driven by means of an electric motor
in place of a hydraulic means. This prior art has been
developed by the present inventor alone and the control
of back pressure disclosed in said patent is carried
out by use of an electrically-operated brake.
In the aforementioned prior art in which back
pressure of the injection plunger is controlled by the
electrically-operated brake, there involves a problem
that a screw shaft for forwardly moving the injection
plunger, a plunger movable member threadedly engaged
-- 1 --
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with the screw shaft to convert the rotational force
into axial thrust of the injection plunger, and the like
are provided between the plunger and the back pressure
control device, and therefore a backward force from the
injection plunger received by the back pressure control
device is indirect and a frictional force between the
members present therebetween is generated, thus lacking
in accuracy in control of back pressure.
SUMMARY OF THE INVENTION
This invention has been achieved to solve the
problem encountered in the art wherein the aforesaid
brake means is used as the back pressure control means.
An object of the invention is to provide a new method
in which an electric motor is used as a back pressure
control means, and a feedback control and a rotating
force of the electric motor may be utilized to thereby
control back pressure in the injection plunger with
excellent responsiveness and high accuracy.
Another object of the invention is to provide
a back pressure control method which comprises electrically
measuring a strain of a part receiving a thrust generated
as a reaction of a backward force of the injection plunger
or molten resin pressure to detect back pressure, and
driving and controlling the electric motor by feedbakc
control so that back pressure is made in coincidence
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with a set value by a differential current between the
back pressure and the set back pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
PIG. 1 is a side view of a part of a molding
machine showing in longitudinal section an injection
apparatus capable of carrying out the back pressure control
method in accordance with the present invention.
FIG. 2 is a sectional view taken on line II-II
of PIG. 1.
FIG. 3 is a rear view of the injection apparatus
with a part thereof cutawayO
FIG. 4 is a sectional view of the injection
apparatus on which is mounted a resin sensor for detecting
resin pressure used for detecting back pressure.
FIG. 5 is a block diagram of a back pressure
control circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The injection apparatus comprises an injection
heating tube 2 containing an injection plunger 1 having
a screw provided in-the outer periphery thereof and a
housing 4 on a machine bed 3 which also serves to retain
the injection heating tube. The housing 4 includes therein
a plunger movable member 6 connected to the rear end
of the injection plunger 1 through a rotary shaft 5 and
support shafts 7, 7 on both sides of the movable member
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mounted over front and rear walls 4a and 4b of the housing
parallel with the rotary shaft 5. The plunger movable
member 6 is mounted on the support shafts 7 and 7 with
both ends thereof extended therethrough so that the movable
member 6 may be slidably moved in a lateral direction.
A small-diameter extending shaft 5a having
a gear 8 for rotating the plunger is projectingly provided
at the rear end of the rotary shaft 5, and the plunger
movable member 6 is connected to the end of the shaft
5a through a thrust bearing 9.
A central portion of the plunger movable member
6 is in the form of a cylindrical shape, and a thread
receiving member 10 having threads formed in the inner
peripheral surface thereof is fitted into the rear end
of said central portion, and a screw shaft 11 is screwed
into the member 10 concentric with the injection plunger 1.
The rear end of the screw shaft 11 constitutes
a shaft portion lla rotatably retained on a bearing portion
4c of the rear wall 4 of the housing by means of a thrust
bearing--12, and a gear-13 is mounted on the-shaft portion-
lla.
The aforesaid gears 8 and 13 are meshed with
gears 17 and 18, respectively, which are mounted on a
transmission shaft 14 interiorly and on the underside
of the housing 4. The transmission shaft 14 and a driving
-- 4
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shaft of a DC electrically-operated servo-motor 19 connected
to the underside surface of the housing 4 are connected
through a driving belt 20 so that the servo-motor 19
may be used as a driving source to rotate and axially
move the injection plunger 1.
Reference numeral 21 designates a strain gage
for measuring the amount of deformation of the rear wall
4b of the housing, which strain gage is mounted on a
part which receives a reaction produced when the injection
plunger 1 charges the material or it advances, that is,
on the bearing portion 4c of the rear wall 4b of the
housing on which the shaft portion lla is retained.
Reference numeral 22 designates a DC electric
motor for controlling a backward force (back pressure
of the plunger) produced when the injection plunger 1
charges the material through the shaft portion lla, which
electric motor 22 is secured to the bearing portion 4c.
While the back pressure control device in this
embodiment is a DC servo-motor, it is to be understood
that the device can be--an elec-tric motor ca~pable--of
controlling torque, for example, such as an AC servo-
motor, a hysteresis motor, a torque motor, etc.
The strain gage 21 is commercially available
as a mold type strain gage, in which the strain gage
is molded into a plastic casing and fixed to the bearing
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portion 4c by means of a screw.
Rotation and backward movement (chargi.ng of
material) of the in;ection plunger 1 in the electrically-
operated injection apparatus constructed as described
above are carried out by rotation of the gears 17 and
8 and forward movement for injection of the injection
t plunger 1 is carried out by rotation of the gears 18
and 13.
More specifically, in charging the material,
the rotary shaft 5 is rotated by rotation of the gear
8, and the injection plunger 1 is also rotated so that
the material from a hopper is transported to the foremost
end of the heating tube 2 by means of the screw.
The backward force generated as the reaction
for transporting material in the injection plunger 1
is transmitted to the thread receiving member 10 through
the plunger movab-le member 6 and further exerted on the
` screw~shaft ll to generate in the-screw shaft~ the -
rotating force in a direction for moving bac.kward the. .
injecti.o:n:plun-ger l together with.the.movable member-
6. ~t the same time, the electric motor 22 is driven
and controlled to apply a braking force to the screw
shaft 11 thereby carrying out the charging of material
while appl~ing the so-called back pressure (back pressure
of plunger) to the molten material.
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The thrust is generated as the reaction of
the back pressure in the screw shaft 11. The aoresaid
back pressure is carried on the rear wall 4b of the housing
through the thrust bearing 12, as a consequence of which
stress is applied also to the rear wall 4b of the housing,
and a deormation which is minute correspondlng to the
back pressure is produced also in the bearing portion
4c mounted on the strain gage 21, which amount of
deformation is electrically measured by the strain gage
21 and detected as the back pressure.
The aforementioned detection of back pressure
is effected by use of the backward force (thrust) of
the injection plunger 1 at the time of charging material.
On the other hand, an example shown in FIG. 4 shows the
case where pressure of molten resin is detected by a
resin pressure sensor 21a mounted on the foremost end
of the heating -tube 2, and the aforesaid molten resin
pressure is used for detection of back pressure.- ~
~ ext, the method for controlling back pressure .. .
will be.-descr-ibe-d-with ref-erence-*o-FlG. 5.-
The thus produced back pressure is measured ----
by the strain gage 21, and when it is amplified by a
strain amplifier 23, an electric current I2 corresponding
to the back pressure is produced.
A differential electic current ~I obtained
by adding an electric current Il corresponding to back
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pressure set by a back pressure setting unit 24 and the
back pressure current I2 fedback to an adder 25 is amplified
by an electric current amplifier 26, after which it is
supplied to a power converter 27. The power converter
27 is composed of an ignition control circuit which uses
a thyristor or a pulse width control circuit which uses
a transistor, and is designed so that an amature current
corresponding to an amplifying current to be inputted
may be directed to the DC electric motor 22. With this,
in the control of back pressure of the injection plunger
1, the control of driving the motor 22 is carried out
by the feedback control so as to assume the set value
of back pressure, and the back pressure is controlled
with high accuracy.
While in the present embodiment, the servo-
motor is provided for the purpose of injection and rotation,
and another electric motor is used for exclusive use ~--
of back pressure control, it is to be understood that
the servo-motor for injection-can be used also for back
pressure control,.and a motor for rotation of the plunger
can be ~sepalately provided.-
INDUSTRIAL APPLICABILITY
As described above, according to the present
invention, the electric motor is used as the control
device for back pressure and the electric motor is driven
-- 8
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and controlled by the feedback control so that back pressure
is in coincidence with the set value to control back
pressure. Therefore, the device is not affected by the
frictional force of transmission mechanism thus providing
for excellen~ responsiveness and being capable of carrying
out back pressure control with high accuracy. In addition,
¦ the back pressure control may be performed with high
accuracy according to material to be molded or articles
I to be molded, and the control of back presure can be
made with high accuracy. Therefore, the apparatus may
be applied as an optimum back pressure controlling method
for the injection apparatus using an electric motor as
a driving source.
