Plastic article or earthenware shaping or treating: apparatus – Control means responsive to or actuated by means sensing or... – Molding pressure control means responsive to pressure at...
Reexamination Certificate
2002-11-26
2004-11-02
Heitbrink, Tim (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Control means responsive to or actuated by means sensing or...
Molding pressure control means responsive to pressure at...
C264S040500
Reexamination Certificate
active
06811387
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electric injection molding machine for injecting molten resin into a mold to obtain a molding.
BACKGROUND ART
An electric injection molding machine according to the related art will be explained with reference to
FIG. 3
disclosed in Japanese Patent Application Publication (KOKAI) No.Hei.4-334430 hereunder.
In
FIG. 3
, an electric injection molding machine that pours resin
3
into a mold
1
to obtain an object having a predetermined shape has an injection molding machine
10
having a screw
7
in a cylinder
5
, a pressure sensor
12
for sensing the pressure in the cylinder
5
based on the pressure applied to the screw
7
, a servo motor
14
coupled/fixed to the screw
7
, a position/speed sensor
16
for sensing the position and the speed based on the rotation angle of the servo motor
14
, a current sensor
18
for sensing the current flowing through the servo motor
14
, and a control portion
20
for driving/controlling the servo motor
14
based on sensed values of the pressure sensor
12
, the position/speed sensor
16
, and the current sensor
18
and command values.
The control portion
20
has a pressure subtracter
22
for calculating a pressure deviation between a pressure command Pr of the screw
7
and a pressure sensed value Pa of the pressure sensor
12
, a pressure amplifier
24
for getting a speed command Nr by multiplying the pressure deviation by a dwell pressure control gain, a speed subtracter
26
for calculating a speed deviation between a speed command Nr and a speed sensed value Na, a speed amplifier
28
for getting a current command Ir by multiplying the speed deviation by a speed control gain, a current subtracter
30
for calculating a current deviation between the current command Ir and a current sensed value Ia, and a current amplifier
32
for getting a power command Wr by multiplying the current deviation by a current control gain.
An operation of the electric injection molding machine constructed as above will be explained with reference to
FIG. 3
hereunder.
First, assuming that an injection mode for injecting the molten resin
3
into the mold
1
is completed and then the operation is shifted to a dwell pressure mode for holding the pressure in the mold
1
at a predetermined value.
In such dwell pressure mode, the pressure command Pr in the mold
1
and a position command (not shown) of the screw
7
are input, the pressure subtracter
22
calculates the pressure deviation between the pressure command Pr and the pressure sensed value Pa from the pressure sensor
12
, and the pressure amplifier
24
calculates the speed command Nr by multiplying the pressure deviation by the dwell pressure control gain and applies the speed command Nr to the speed subtracter
26
.
The speed subtracter
26
calculates the speed deviation between the speed command Nr and the speed sensed value Na of the position/speed sensor
16
. The speed amplifier
28
obtains the current command Ir by multiplying the speed deviation by the speed control gain. The current subtracter
30
calculates the current deviation between the current command Ir and the current sensed value Ia of the current sensor
18
. The current amplifier
32
obtains the power command Wr by multiplying the current deviation by the current control gain and drives the servo motor
14
to proceed the screw
7
until the pressure applied to the shaft of the screw
7
become equal to the command value, so that the pressure is controlled at a predetermined value. Then, the operation is shifted to a back-pressure mode in which the material is supplied to the inside of the cylinder
5
.
On the other hand, the input to the injection molding machine
10
as the controlled object is a rotation amount (an integrated value of the speed N) of the servo motor
14
. This rotation amount is a moving amount Ls of the screw
7
and is in proportion to a volume Vs of a moving portion of the screw
7
in the cylinder
5
.
On the other hand, since the output of the injection molding machine can be expressed by the pressure p in the cylinder
5
, the transfer function Ga of the injection molding machine
10
is given as the following equation.
Ga=p/Vs
(1)
Where
Vs: changed volume (m
3
) of the cylinder
5
,
p: pressure (kg/m
3
) in the cylinder
5
.
Therefore, an overall block diagram of a control system in the electric injection molding machine is given as shown in FIG.
4
(
a
). In FIG.
4
(
a
),
Gp: dwell pressure control gain, G
N
: speed control gain, G
1
: current control gain, H
I
: current feedback gain, H
N
: speed feedback gain, Hp: pressure feedback gain, K
t
, K
1
: constant, J: moment of inertia (kg/m
2
) of the servo motor
12
, A: sectional area (m
2
) of the cylinder
5
, S: Laplace operator, &ohgr;: angular velocity of rotation (rad/s) of the servo motor
12
, and Vs, p: mentioned above
Since the speed feedback gain H
N
is sufficiently larger than the gain G
1
in a dotted line, the transfer function G
0
in the dotted line can be assumed as 1/H
N
. Thus, the block diagram shown in FIG.
4
(
a
) can be simplified as shown in FIG.
4
(
b
).
In this FIG.
4
(
b
), if the pressure feedback gain Hp is assumed as 1, the open-loop transfer function Gs from the pressure command Pr to the pressure feedback p can be given as the following equation.
Gs=p/Pr
=(
Gp/H
N
)×(
Kc/S
)×(
p/Vs
) (2)
Where
Kc: constant,
S: Laplace operator.
Here, the electric injection molding machine controls p/Vs in above Eq. (2) as a constant in the dwell pressure mode.
However, this p/Vs is not constant since such p/Vs is given by the relational curve between the pressure and the volume of the cylinder
5
, as shown in FIG.
5
.
The relational curve of
FIG. 5
can be calculated as follows. That is, the ideal condition that the resin
3
in the cylinder
5
can be changed by the compression under the conditions that the temperature is constant and the entropy is constant is assumed and also the pressure of the resin
3
is assumed totally uniform in the cylinder
5
.
Since the compression ratio &khgr; of the resin
3
in the cylinder
5
can be regarded as a constant according to such assumptions, the following equation can be satisfied.
&khgr;=−(1
/Vs
)×(
dVs/dp
) (3)
Where
Vs: volume (m
3
) in the cylinder
5
,
p: pressure (kg/m
3
) in the cylinder
5
.
Based on above Eq. (3), the volume Vs in the cylinder can be given as the following equation.
Vs=V
so
×&egr;
−P&khgr;
(4)
Where
V
so
: initial value of the volume (m
3
).
Eq(4) gives the relational curve between the pressure and the volume in the cylinder
5
, as shown in FIG.
5
.
Accordingly, if the pressure is controlled based on the volume in the cylinder
5
(the moving amount Ls of the screw
7
) in order to control the pressure in the cylinder
5
, there is the following problem since the relationship between the volume and the pressure is nonlinear, as shown in FIG.
5
.
In the dwell pressure mode, as shown in
FIG. 5
, under the dwell pressure control gain of the constant value that is multiplied by the pressure deviation, the volume V
1
in the cylinder
5
is high if the pressure in the cylinder
5
has the low pressure value P
1
, and therefore P
1
/V
1
has the small value. Thus, if it is tried to maintain the open-loop transfer function shown in FIG.
4
(
b
) at the constant value, a set value of the dwell pressure control gain of the pressure amplifier
24
must be increased.
Under the dwell pressure control gain value set in this manner, the volume in the cylinder
5
is reduced like V
2
if the pressure value of the cylinder
5
is increased to the pressure P
2
, for example, and therefore P
2
/V
2
becomes large. Thus, the pressure value of the screw
7
is overshot at the pressure P
2
against the pressure command Pr.
However, if the dwell pressure control gain value is set in a situation where the pressure in the cylinder
5
is high, there is a problem that the response to the pressure
Kobayashi Yukihiko
Tsutsumi Seisuke
Heitbrink Tim
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
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