Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Reexamination Certificate
2001-01-02
2003-04-29
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
C264S040400, C425S140000, C425S145000, C425S148000
Reexamination Certificate
active
06555034
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling an injection molding machine and, more specifically, a controlling method that is suitable to reduce variations in weight of molded products.
Referring now to
FIG. 1
, a motor-driven injection molding machine will now be described, centering on an injection unit included therein. The motor-driven injection molding machine comprises an injection unit driven by a servomotor. In such an injection unit, rotation of the servomotor is converted into a linear motion by a ball screw and a nut, thereby moving a screw forward and backward.
In
FIG. 1
, rotation of an injection servomotor
11
is transmitted to a ball screw
12
. A nut
13
is fixed on a pressure plate
14
and moved forward and backward by rotation of the ball screw
12
. The pressure plate
14
is movable along four guide bars
15
,
16
(only two of them are shown in the figure) fixed on a base frame (not shown). Forward and backward motion of the pressure plate
14
is transmitted to a screw
20
via a bearing
17
, a load cell
18
, and an injection shaft
19
. The screw
20
is rotatably and axially movably disposed in a heating cylinder
21
. The heating cylinder
21
includes a hopper
22
for feeding a resin at the position corresponding to the rear portion of the screw
20
. Rotation motion of a servomotor
24
for rotating the screw
20
is transmitted to the injection shaft
19
via connecting members
23
such as a belt or pulleys. In other words, the servomotor
24
rotates the injection shaft
19
, which rotates the screw
20
.
In a plasticizing/measuring process, the screw
20
rotates and moves backward in the heating cylinder
21
, whereby a molten resin is stored in front of the screw
20
, that is, in the heating cylinder
21
on the side of a nozzle
21
-
1
. The backward movement of the screw
20
is effected by a pressure caused by the gradually increasing amount of a molten resin stored in front of the screw
20
.
In a filling and injecting process, forward movement of the screw
20
is effected by driving force of the injection servomotor
11
, whereby the molten resin stored in front of the screw
20
is filled and pressurized in a metal mold for molding. In this case, the force to press the molten resin is detected by the load cell
18
as an injection pressure. The detected injection pressure is amplified by a load cell amplifier
25
and fed into a controller
26
. The pressure plate
14
is provided with a position detector
27
for detecting the amount of movement of the screw
20
. The detected signal outputted from the position detector
27
is amplified by a position detector amplifier
28
and fed into the controller
26
.
The controller
26
outputs current (torque) instruction values corresponding to the respective processes based on the set values preset by a display/setting unit
33
via a man-machine controller
34
. A drive
29
controls current for driving the injection servomotor
11
to control output torque of the injection servomotor
11
. A drive
30
controls current for driving the servomotor
24
to control the number of revolutions of the servomotor
24
. The injection servomotor
11
and the servomotor
24
comprise encoders
31
and
32
, respectively, for detecting the numbers of revolutions. The numbers of revolutions detected by the encoders
31
and
32
are fed to the controller
26
. Especially, the number of revolutions detected by the encoder
32
is used to know the number of revolutions of the screw
20
.
On the other hand, a plurality of heaters
40
are disposed around the heating cylinder
21
for heating and melting the resin fed from the hopper
22
. These heaters
40
are controlled by a temperature controller
41
. The temperature controller
41
receives the temperature detecting signals fed from a plurality of thermocouples
42
disposed in the vicinity of the heaters
40
. The temperature controller
41
outputs the temperature detecting signals from the plurality of thermocouples
42
to the controller
26
as thermocouple-detected values. The temperature controller
41
also controls the plurality of heaters
40
based on the heater temperature-setting signals that represent the heater temperature setting values sent from the controller
26
.
Actually, as shown in
FIG. 2
, multiple zones are defined around the heating cylinder
21
, and the respective heaters are disposed in their respective zones around the heating cylinder
21
and independently controlled in terms of energization. Normally, multiple zones are defined in such a manner that a zone Z
0
is allocated immediately below the hopper
22
, and from there, zones Z
1
, Z
2
, Z
3
, Z
4
, and Z
5
are allocated in sequence toward the nozzle
21
-
1
.
In the injection molding machine, it is important to manufacture a large volume of products that are stable in quality in a short time at a low cost. Hereinafter, the description about the stable quality will be made limiting to the weight of the molded product. The controlling methods for obtaining a stable quality are as follows. The first method is a controlling method that can make correction for disturbances. In other words, feedback control maintains a characteristic that is considered to be an alternative to variations in weight of molded products in constant. The second method is a control method that aims at no-variation in weight by estimating variations in weight of the molded products in advance, and applying signals that cancels the estimated variations (feed forward control).
However, in the second controlling method, it is very difficult to ascertain the controlled object. Therefore, before using the second controlling method generally, many problems must be solved.
Referring now to the block diagram of
FIG. 3
, the outline of a mold internal pressure feed forward controlling method based on the second controlling method presented in the injection process will be described. In
FIG. 3
, Gc(S) represents a transfer function in the controller for controlling the injection servomotor
11
described in conjunction with
FIG. 1
, and Gp(S) represents a transfer function of the process. G
1
p(S) represents a transfer function for converting a disturbance such as variations in temperature of the heating cylinder
21
into variations in density of the molten resin. It is because variations in density of the molten resin effect on the mold internal pressure, and consequently, the weight of the molded product may vary. The disturbance is caused by various factors, for example, by variations in temperature of the heating cylinder
21
, or by the state of the molten resin such as the temperature or the pressure, and the number of revolutions of the screw. In any cases, respective sensors that correspond to the respective types of the disturbance may detect such disturbances, and the detected results are fed to a subtracter
51
. Assuming that the signal between the transfer function Gc(S) in the controller and the transfer function Gp(S) of the process is a value detected by the load cell described in conjunction with
FIG. 1
,
Gc (S)=value detected at the load cell (S)/disturbance (S),
Gp (S)=mold internal pressure/value detected at the load cell (S), are satisfied.
On the other hand, the transfer function G
1
p(S) is used for generating signals for canceling variations in amount of control caused by disturbances (in this case, the mold internal pressure that may effect on the weight of the molded product). Assuming that the amount of change in the mold internal pressure caused by the disturbance is &Dgr;p (t), the transfer function G
1
p(S) is used for generating the signal corresponding to −&Dgr;p(t).
As described above, in the current feed forward controlling method, variations in weight of the molded product is intended to be eliminated by maintaining the mold internal pressure at a intended value by detecting the disturbances that have been converted into variations in density of the molten resin and applying operational signal
Fontaine Monica A
Silbaugh Jan H.
Squire Sanders & Dempsey L.L.P.
Sumitomo Heavy Industrie's, Ltd.
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