Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Positioning of a mold part to form a cavity or controlling...
Reexamination Certificate
2000-12-26
2003-12-16
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Positioning of a mold part to form a cavity or controlling...
C425S145000, C425S149000
Reexamination Certificate
active
06663804
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for controlling an injection molding machine. In particular, this invention relates to a method and an apparatus for controlling mold internal pressure in order to reduce variations in weight of molded products.
Referring to
FIG. 1
, a motor-driven injection molding machine will be described focusing on an injection unit. The motor-driven injection molding machine has the injection unit which is driven by a servomotor. In such an injection unit, rotation of the servomotor is converted into linear motion by a ball screw and a nut, thereby moving a screw forward and backward.
In
FIG. 1
, the rotation of an injection servomotor
11
is transmitted to a ball screw
12
. A nut
13
is fixed on a pressure plate
14
and is moved forward and backward by rotation of the ball screw
12
. The pressure plate
14
is movable along four guide bars
15
and
16
(only two 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 to a position corresponding to the rear portion of the screw
20
. Rotating motion of a servomotor
24
for rotating the screw
20
is transmitted to the injection shaft
19
via a connecting member
23
which may be a belt, pulleys, etc. In other words, the servomotor
24
rotates the injection shaft
19
which in turn rotates the screw
20
.
In a plasticizing/measuring process, the screw
20
rotates and moves backward in the heating cylinder
21
so that 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 caused by pressure due to gradual increase in the amount of molten resin stored in front of the screw
20
.
In a filling and injecting process, the forward movement of the screw
20
in the heating cylinder
21
is caused by a driving force from the injection servomotor
11
, so that the molten resin stored in front of the screw
20
is forced into and is pressurized in a metal mold. In this case, the force for pressing the molten resin is measured by the load cell
18
as an injection pressure. The measured injection pressure is amplified by a load cell amplifier
25
and is fed into a controller
26
. The pressure plate
14
has a position detector
27
for measuring the amount of movement of the screw
20
. The measuring signal outputted from the position detector
27
is amplified by a position detector amplifier
28
and is fed into the controller
26
.
The controller
26
outputs current (torque) instruction values corresponding to the respective processes and based on some values preset by a display/setting unit
33
via a man-machine controller
34
. The current instruction values are fed to a drive
29
and a drive
30
. The drive
29
controls a current for driving the servomotor
11
to control an output torque of the servomotor
11
. The drive
30
controls a current for driving the servomotor
24
to control the number of revolutions of the servomotor
24
. The servomotor
11
and the servomotor
24
comprise encoders
31
and
32
, respectively, for measuring the number of revolutions. The number of revolutions detected by the encoders
31
and
32
are fed to the controller
26
. In particular, the number of revolutions detected by the encoder
32
is used to determine the number of revolutions of the screw
20
.
If an injection molding machine is a hydraulic injection molding machine, a hydraulic injection cylinder may be used to replace the conversion system for converting a rotary movement into a linear movement, which is effected by means of the ball screw and the nut involved in the injection apparatus. In this case, the oil pressure within the injection cylinder is measured. Since the construction of the hydraulic injection molding machine is already well known in the art, it will not be illustrated and described in this specification.
However, one subject associated with the above-described injection molding machine is to manufacture, in a shortened time period and at reduced production cost, a large number of molded products of uniform quality. Here, one factor that has an influence on the quality of the molded products is their weights.
In order to manufacture molded products of uniform weight, there has been suggested a mold internal pressure feedback control system. In this mold internal pressure feedback control system, a mold internal pressure sensor is provided in the metal mold for measuring resin pressure in the metal mold. In this way, the injection servomotor or oil pressure in the hydraulic injection cylinder may be controlled in accordance with a difference between a mold internal pressure measured by the mold internal pressure sensor and a preset value of the mold internal pressure given as a target value, with the control itself being effected by reducing the difference to zero.
FIG. 2
is a block diagram showing the above-mentioned mold internal pressure feedback control system.
However, the mold internal pressure feedback control system shown in
FIG. 2
has been found to have the following problems. That is, it is difficult to stabilize the pressure within the metal mold, merely by measuring the pressure variations within the metal mold since there are some limits to the response time of the overall control system. As a result, it is difficult to stabilize the mold internal pressure, hence making it difficult to manufacture molded products of uniform weight.
On the other hand, as discussed above, the mold internal pressure has a significant influence on the quality of the molded products, especially on the weights thereof. In view of this, there have been suggested some other control methods for stabilizing the mold internal pressure, which methods are different from that shown in FIG.
2
. For example, a method has been suggested which requires that a mold internal pressure control system be connected in cascade with an injection pressure control system, a preset value of the mold internal pressure and a measured value of the same are compared with each other, so that a preset value of an injection pressure may be changed so as to adjust the injection pressure.
In the use of the above method, it is necessary that the injection pressure and the mold internal pressure be set at the same time when setting operation conditions. However, since the response of the mold internal pressure has an extremely large time lag in the injection pressure feedback system, it is not easy to adjust the injection pressure, nor is it easy to set the mold internal pressure, hence making it difficult to set operation conditions.
In fact, the mold internal pressure often changes in the manner shown in FIG.
3
. In order to obtain molded products of a good quality, it is necessary that the mold internal pressure be set so that it can change in accordance with the graph shown in FIG.
3
. This, however, makes it more difficult to set operation conditions.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a control method for controlling an injection molding machine, which is effective for producing molded products of a uniform weight.
Furthermore, another object of the present invention is to provide a control apparatus for controlling an injection molding machine, which is capable of stabilizing a mold internal pressure by an easy operation.
The control method of the present invention is suitable for use in an injection molding machine including a mold internal pressure sensor for measuring a mold internal resin pressure as a measured mold internal pressure value. The measured mold internal pressure value is fed to a nozzle internal resin pressure feedback control system. A nozzle internal pressure pres
Sato Hiroshi
Suumen Hiroyoshi
Heitbrink Jill L.
Squire Sanders & Dempsey L.L.P.
Sumitomo Heavy Industrie's, Ltd.
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