Electricity: motive power systems – Limitation of motor load – current – torque or force
Patent
1986-12-08
1989-12-12
Hoag, Willard
Electricity: motive power systems
Limitation of motor load, current, torque or force
425145, 425149, B29C 4550, B29C 4577
Patent
active
048870129
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an injection molding machine and, more particularly, to an injection pressure control apparatus for an injection molding machine using a servo motor as a drive source of an injection mechanism.
2. Description of the Related Art
An injection molding machine in which an injection mechanism is driven by oil pressure to axially move a screw to perform injection, whereby injection pressure is controlled by hydraulic control, is known.
On the other hand, an injection molding machine in which an injection mechanism is driven by a servo motor to perform injection has also been developed. In order to control an injection pressure in an injection molding machine driven by a servo motor to perform injection, an output torque of the servo motor must be controlled. One of methods of controlling a torque of a servo motor is to perform torque limiting. In general, when torque limiting is performed during rotation of a servo motor, it is also performed during acceleration and deceleration. During acceleration, no inconvenience occurs because speed increases slowly. However, during deceleration, a sufficient damping force may not be applied to the servo motor due to such torque limiting, and an operation portion of an injection molding machine, such as a screw, may overrun by its inertia or the like. Especially when injection of a screw without molding material is performed in purging or the like, since a reaction force from the molding material is not applied to the screw unlike the case in which injection molding is performed by actually injecting a molding material to an injection mold, the screw particularly tends to overrun. Upon overrun of the screw, the distal end of the screw strikes against a heating cylinder, damaging the screw, the heating cylinder, a nozzle or the like.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an injection pressure control apparatus for an injection molding machine in which an injection molding machine is driven by a servo motor, which can prevent overrun of a screw even during injection without molding material.
In order to achieve the above object of the present invention, an injection molding machine, in which an injection mechanism is driven by a servo motor to perform an injection operation of the injection mechanism, includes instruction means for outputting a torque limit instruction which can be variably set, and torque limit means for limiting a value of a torque instruction for the servo motor to a value of the torque limit instruction from the instruction means only in an injection direction.
Thus, the present invention utilizes, in an injection molding machine in which an injection mechanism is driven by a servo motor, the fact that an injection operation of the injection mechanism is performed in one direction, i.e., the injection operation is performed by a rotation in one direction of the servo motor. The servo motor is torque-limited only in a resin injection direction of the injection mechanism. Therefore, a speed of the servo motor in the injection direction can be rapidly decelerated, a screw can be positioned at a set position without overrun even when injection without molding material is performed during, e.g., purging, and an injection pressure can be controlled at a given value.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a graph showing changes in motor current as time passes when a servo motor is driven without torque limiting;
FIG. 2 is a graph showing changes in motor rotating speed as time passes without torque limiting;
FIG. 3 is a graph showing changes in motor current when the torque is limited as in a conventional apparatus;
FIG. 4 is a graph showing changes in motor rotating speed in a conventional apparatus
FIG. 5 is a block diagram of an injection pressure control apparatus according to an embodiment of the present invention;
FIG. 6 is a graph showing changes in motor current as time passes in the embodiment of FIG. 5; and
FIG. 7 is a
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Kiya Nobuyuki
Sakamoto Keiji
Fanuc Ltd.
Hoag Willard
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