Back pressure control for an injection molding machine

Details Back pressure control for an injection molding machine Back pressure control for an injection molding machine

1999-07-01

2002-04-16

Heitbrink, Tim (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Control means responsive to or actuated by means sensing or...

Feed control of material en route to shaping area

C264S040700

Reexamination Certificate

active

06371748

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a back pressure control method for an injection molding machine and a back pressure control apparatus therefor.
Injection molding machines are classified into hydraulic injection molding machines and motor-driven injection molding machines. Each hydraulic injection molding machine uses hydraulic actuators as driving sources for an injection device and a clamping device while each motor-driven injection molding machine uses servomotors as the driving sources. Hereinafter, although the description will be made on the premise that the injection molding machines are the motor-driven injection molding machines, the invention is also applicable to hydraulic injection molding machines; the hydraulic injection molding machines and the motor-driven injection molding machines are similar in structure and operation to each other except that there is a difference between the hydraulic actuators and the servomotors. Now, the description will proceed to operations of an injection device using the servomotors as the actuators in brief.
(1) In the plasticization and metering process, a screw is rotated by a screw-rotation servomotor. The screw is located within a heating cylinder. Resin powder is fed, as fed resin, from a hopper to a rear portion of the screw in the heating cylinder. Rotation of the screw results in feeding the fed resin having a predetermined metered amount to a nose portion of the heating cylinder with the fed resin melted by the heating cylinder to form molten resin. During this time duration, the screw is retracted due to pressure (back pressure) of the molten resin which is trapped in the nose portion of the heating cylinder.
The screw has a rear end portion which is directly connected to an injection shaft. The injection shaft is rotatably supported on a pressure plate through a bearing. The injection shaft is driven in an axial direction by an injection servomotor which is supported on the pressure plate. The pressure plate moves forward and backward along a guide bar in response to the operation of the injection servomotor through a ball screw. In the manner which will later become clear, the above-mentioned back pressure of the molten resin is detected by using a load cell and controlled with a feedback control loop.
(2) Subsequently, in a filling process, the pressure plate is advanced by means of driving the injection servomotor. The screw has a nose portion which serves as a piston to fill a mold with the molten resin.
(3) At the end of the filling process, the molten resin fills the space within a cavity of the mold. At this point, the advancing motion of the screw has a control mode which is switched from a velocity control mode to a pressure control mode. This switching is referred to as a “V-P switching”.
(4) After the V-P switching, the resin within the cavity of the mold is allowed to cool under a predetermined pressure. This process is called a dwelling process. In the dwelling process, the resin has pressure which is controlled in a feedback control loop in the similar manner which is described in conjunction with the above-mentioned back pressure control.
Subsequently, operation of the injection device returns back to the plasticization and metering process set forth in (1) after the completion of the dwelling process set forth in (4).
On the other hand, in a clamping device, an eject operation for ejecting a solid product out of the mold is carried out in parallel with the plasticization and metering process set forth in (1). The eject operation involves in opening the mold to remove the solid product from the mold by means of an ejector mechanism and thereafter in closing the mold for the resin filling set forth in (2).
In the manner which will later be described in conjunction with
FIG. 3
, a conventional back pressure control method comprises the steps of setting a plurality of fixed back pressure command values to form a back pressure pattern having the fixed back pressure command values that discontinuously vary with respect to a screw retreat position or a screw retreat time instant and of controlling back pressure so that a back pressure detected value coincides with the back pressure pattern. However, with the conventional back pressure control method, it is impossible to sufficiently appreciate a specific characteristic of resin to be molded and to carry out a multi-stage back pressure control (for example, three-stage or four-stages) unless an injection molding machine operator has rich experiences. Inasmuch as the fixed back pressure command values are mutually related to a screw retreat position, molding work must be carried out by repeatedly changing the fixed back pressure command values until a combination of the fixed back pressure command values is suitable for switching positions depending on the screw position. As a result, this setting work of the conventional injection molding machine requires work both long in time and high in skill.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a back pressure control method for an injection molding machine, which is capable of carrying out near ideal back pressure control by simple setting work.
It is another object of the present invention to provide a back pressure control apparatus for an injection molding machine, which is suitable to the above-mentioned back pressure control method.
Other objects of this invention will become clear as the description proceeds.
According to a first aspect of this invention, a back pressure control method is of controlling back pressure in an injection molding machine comprising an injection device. The injection device comprises a heating cylinder for heating resin powder therein to melt the resin powder into molten resin, a screw disposed in the heating cylinder for feeding the molten resin in the heating cylinder forward to meter the molten resin, and an injection actuator operatively coupled to the screw for driving the screw along an axial direction to inject the molten resin metered in the heating cylinder forward. The back pressure control method comprises the steps of setting a back pressure control level with respect to the back pressure, of automatically determining, on the basis of the back pressure control level, a back pressure pattern having a variable back pressure command value which continuously changes with respect to a retreat position of the screw, and of controlling the back pressure in accordance with the back pressure pattern.
According to a second aspect of this invention, a back pressure pattern preparing method is of preparing a back pressure pattern for controlling back pressure in an injection molding machine comprising an injection device. The injection device comprises a heating cylinder for heating resin powder therein to melt the resin powder into molten resin and a screw disposed in the heating cylinder for feeding the molten resin in the heating cylinder forward to meter the molten resin. The back pressure pattern preparing method comprises the steps of setting a back pressure control level with respect to the back pressure and of automatically determining, on the basis of the back pressure control level, the back pressure pattern having a variable back pressure command value which continuously changes with respect to a retreat position of the screw to prepare the back pressure pattern.
According to a third aspect of this invention, a back pressure control method is of controlling back pressure in an injection molding machine comprising an injection device. The injection device comprises a heating cylinder for heating resin powder therein to melt the resin powder into molten resin, a screw disposed in said heating cylinder for feeding the molten resin in the heating cylinder forward to meter the molten resin, and an injection actuator operatively coupled to the screw for driving said screw along an axial direction to inject the molten resin metered in the heating cylinder forward. The back pressure control method comprises the steps

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