DE102009010273B4 - Method and control device for controlling an electric injection device of an injection molding machine, as well as injection molding machine and computer program product - Google Patents

Abstract

A method for controlling an electric injection device of an injection molding machine, wherein the electric injection device is designed to inject a molding compound into a tool (112) in an injection phase and to exert a predetermined pressure on the molding compound in the tool (112) in a holding pressure phase, the method comprising the following steps:
Providing, with a drive controller (220) having an output stage with three transistor pairs, a first drive current to a motor (102) of the electric injection device for driving the motor (102) during the injection phase, wherein the first drive current is provided by the drive controller (220) at a first frequency;
Switching the frequency, with a device (222) coupled to the drive controller (220) for frequency switching, in order to set a second frequency in the drive controller (220) for a holding pressure phase, which is lower than the first frequency, and
Providing, with the drive controller (220), a second drive current to the motor (102) of the electric injection device for driving the motor (102) during the holding pressure phase to exert the predetermined pressure on the molding compound in the tool (112), wherein the second drive current is provided by the drive controller (220) at the second frequency such that the predetermined pressure is exerted on the molding compound in the tool (112) even though the motor (102) is not rotating at all or at a very low speed during the holding pressure phase.

Description

The present invention relates to a method and a control device for controlling an electric injection device of an injection molding machine as well as to an injection molding machine and a computer program product.

Plastic parts can be manufactured using an injection molding machine. The injection molding machine has an injection unit that injects a plastic molding compound into a tool or mold. The tool represents a negative mold of the plastic part to be produced.

The injection unit has an injection motor that can drive a spindle. The spindle is coupled to an injection cylinder that can force the molding compound into the mold. During the different phases of the plastic part manufacturing process, the molding compound can be forced into the mold at different speeds, or the pressure acting on the molding compound can be varied. To adjust the speed and pressure, the injection motor can be controlled accordingly by a drive controller.

After the molding compound has been pressed into the workpiece, pressure is continued on the molding compound during a holding pressure phase. The application of pressure during the holding pressure phase may be necessary as long as the molding compound is still soft. While the injection cylinder exerts pressure on the molding compound during the holding pressure phase, the injection cylinder moves only slightly or not at all. The movement of the injection cylinder during the holding pressure phase depends on the shrinkage behavior of the plastic. If the injection cylinder does not move or moves only slightly, this means that the injection motor does not rotate or only rotates at a very low speed. Despite the standstill or near standstill of the injection motor, it is necessary for the injection motor to provide sufficient torque so that the injection cylinder can exert the required pressure on the molding compound.

3 shows a circuit diagram of an output stage of a drive controller for controlling an injection motor 102, according to the prior art. The drive controller of the injection motor can be a pulse frequency converter. The drive controller can have six transistors I1, I2, I3, I4, I5, I6 in the output stage, which form three transistor pairs. Transistors I1, I3, I5 are connected to an "intermediate circuit +" on the input side, and transistors I2, I4, I6 are connected to an "intermediate circuit -" on the output side. The pulse frequency leads to power loss in the transistor pairs depending on the occurring edges. This power loss is distributed among the three transistor pairs during normal operation. However, if the injection motor comes to a standstill, the entire current of the output stage can be statically distributed to one transistor. For example, at a standstill, the current can flow only through transistor I6 on one side and only through transistors I1, I3 on the other side. This can lead to overheating of the power stage. To prevent such overheating, the current supplied by the drive controller to the injection motor can be reduced. However, this results in the injection motor being able to provide less force, which may not be sufficient to exert the required pressure on the molding compound during the holding pressure phase.

To both prevent overheating and provide the required pressure, larger inverters are used as drive controllers. However, this entails additional costs and space requirements.

DE 10 2004 054 212 A1 shows an injection molding or die casting machine with a motor that prevents the motor from coming to a standstill under load or from moving very slowly. DE 699 36 282 T2 Not only one motor is controlled, but several motors, namely an injection motor 345 and a dosing motor 344. DE 694 16 140 T2 teaches a continuous adjustment of a frequency and a phase using a vector control to maintain a 90° phase relationship between stator and rotor fluxes. JP 09-1 21 595 A refers to a temperature rise protection device for an inverter, but does not consider an injection molding machine.

It is the object of the present invention to provide an improved method and an improved control device for controlling an electric injection device of an injection molding machine as well as an improved injection molding machine and an improved computer program product.

This object is achieved by a method according to claim 1, a control device according to claim 8, an injection molding machine according to claim 9 and a computer program product according to claim 10.

The present invention is based on the finding that the power loss in the drive controller can be reduced by reducing the frequency of the current provided by the drive controller.

In this way, the current supplied by the drive controller during the hold-pressure phase can be increased without causing the drive controller to overheat. This results in cost savings for the machine manufacturer, as a smaller drive controller can be used in many cases.

The present invention provides a method for controlling an electric injection device of an injection molding machine, wherein the electric injection device is designed to inject a molding compound into a tool in an injection phase and to exert a predetermined pressure on the molding compound in the tool in a holding pressure phase, the method comprising the following steps: providing a first control current to the electric injection device during the injection phase, wherein the first control current has a first frequency; and providing a second control current to the electric injection device during the holding pressure phase, wherein the second control current has a second frequency which is lower than the first frequency, as described in claim 1.

The electric injection device can be at least temporarily stationary during the post-pressure phase. Thus, the predetermined pressure can be provided by the electric injection device even though an electric motor of the electric injection device is not rotating at all or is rotating at a very low speed.

According to the invention, the second drive current has a value that is large enough to enable the injection device to exert the predetermined pressure on the molding compound.

For example, the first and second drive currents may be provided by an output stage of a drive controller, and the second frequency may be small enough to prevent overheating of the output stage due to a through loss caused by the second drive current.

According to one embodiment, the first frequency can have a value between 3 kHz and 8 kHz, and the second frequency can have a value between 1.5 kHz and 3 kHz. These frequencies can ensure both rapid injection of the molding compound into the mold and pressure application during the holding pressure phase with tolerable flow losses.

The first control current can have a value of more than 300% of a nominal current, for example 4 times or 5 times the nominal current of the electric injection device, and the second control current can have a value between >0% and 200% of the nominal current of the electric injection device.

According to one embodiment, the electric injection device can comprise an electric servomotor driven by the first and second drive currents. Thus, the inventive approach can be used in known electric injection machines.

The present invention further provides a control device which is designed to carry out the steps of the method according to the invention for controlling an electric injection device of an injection molding machine.

The present invention further provides an injection molding machine having the following features: an electric injection device configured to inject a molding compound into a tool in an injection phase and to exert a predetermined pressure on the molding compound in the tool in a holding pressure phase; as described in claim 9.

Also advantageous is a computer program product with program code stored on a machine-readable medium such as a semiconductor memory, a hard disk memory or an optical memory and used to carry out the method according to the invention when the program is executed on a control device.

• 1 eine schematische Darstellung einer Spritzgießmaschine, gemäß einem Ausführungsbeispiel der Erfindung;
• 2 ein Blockschaltbild eines Steuergeräts zum Ansteuern der Spritzgießmaschine, gemäß einem Ausführungsbeispiel der Erfindung; und
• 3 ein Schaltbild einer Endstufe, gemäß dem Stand der Technik.

The invention is explained in more detail below with reference to the accompanying drawings. They show:

• 1 a schematic representation of an injection molding machine according to an embodiment of the invention;
• 2 a block diagram of a control device for controlling the injection molding machine, according to an embodiment of the invention; and
• 3 a circuit diagram of a power amplifier, according to the state of the art.

Identical or similar elements may be provided with identical or similar reference numerals in the following figures. Furthermore, the figures of the drawings, their description, and the claims contain numerous features in combination. A person skilled in the art will appreciate that these features may also be considered individually or combined to form further combinations not explicitly described here.

1 shows an electric injection molding machine according to an embodiment of the invention. The electric injection molding machine has an electric injection device that can be driven by a servomotor 102. The servomotor 102 is coupled to a screw 106 via a toothed belt 104. Granulate, which consists, for example, of solid plastic particles, can be fed to the electric injection device via a hopper 108. The screw 106 can be rotated by the servomotor 102. The rotation of the screw 106 mills the material fed through the hopper 108 and thereby liquefies it. In addition, the material is pressed towards the injection nozzle 10. In addition, heating bands are attached around the injection device to accelerate the liquefaction of the material and keep the temperature constant. The liquefied material can be pressed out of the injection nozzle 10 into a mold or tool 112. For this purpose, the electric injection device can be moved in the direction of the tool 112.

Furthermore, in 1 a spindle 114 for injection and a plasticizing motor 116 are shown.

During an injection phase, the material is injected into the tool 112. For this purpose, a corresponding injection pressure is built up by means of the motor 102. During the injection phase, the motor 102 may briefly consume a current that corresponds to a multiple of the rated current of the motor 102. A corresponding drive current for operating the motor 102 may be provided by a drive controller or a comparable device.

Once the mold 112 is filled with material, a holding pressure phase follows the injection phase. During the holding pressure phase, pressure is maintained on the material in the mold 112 by means of the motor 102. During the holding pressure phase, the motor 102 can have a current consumption that can be up to 100% of the rated current of the motor 102 or even higher.

Depending on the shrinkage behavior of the material in the tool 112, the motor 102 may be stationary or have a very low speed during the holding pressure phase. This leads to severe thermal stress on a drive controller of the motor 102. During normal operation of the motor 102, the power loss of the drive controller is distributed across all transistors of an output stage of the drive controller. Typically, the output stage has three transistor pairs. When the motor 102 is stationary, the power loss is distributed across only two or three transistors. This can lead to overheating of elements of the output stage. To avoid overheating, the current supplied by the drive controller to the motor 102 would have to be reduced to approximately 60% of the rated current of the motor 102. However, according to the invention, a higher drive current can be provided without the risk of overheating the output stage. This is made possible according to the invention by reducing the frequency of the current supplied by the drive controller. Such a frequency switching allows a higher current to be available in the drive controller at lower speeds of the motor 102.

2 shows a block diagram of a control unit for controlling an electric injection molding machine according to an embodiment of the invention. For example, the control unit can be used to 1 shown injection molding machine and in particular the electric motor 102 of the electric injection molding machine.

The electric motor is controlled by a drive controller 220. The drive controller 220 can be embodied as an inverter or converter. To control the motor 102, the drive controller 220 can provide a control current to the motor 102. The drive controller 220 is coupled to a frequency switching device 222. The frequency switching device 222 can be integrated into the drive controller 220 or embodied as a separate unit. The frequency switching device 222 can be configured to execute software by means of which the frequency switching can be carried out. It can also be a pure software solution.

According to the invention, the frequency switching device 222 is configured to set a first frequency in the drive controller 220 during the injection phase and a second frequency in the holding pressure phase. Accordingly, the drive controller 220 can provide a first drive current at the first frequency to the motor 102 during the injection phase and a second drive current at the second frequency in the holding pressure phase.

The second frequency has a lower value than the first frequency. For example, the first frequency can have a value of 4 kHz and the second frequency a value of 2 kHz. According to this exemplary embodiment, the first frequency can also have a value that lies between 3 kHz and 8 kHz and the second frequency can have a value that lies between 1.5 kHz and 3 kHz. With a correspondingly different dimensioning or design of the drive controller 220 and/or the motor 102, Other frequency ranges are possible. In general, the first and second frequencies can be selected such that overheating of the drive controller or other circuit elements does not occur during either the injection phase or the holding pressure phase, and, on the other hand, the required pressure can be built up on the molding compound by the motor 102.

According to the invention, the current in the drive controller is thus limited during the hold-pressure phase, since low speeds generally occur during the hold-pressure phase. For example, the possible output current can be significantly reduced at output frequencies below 3 Hz, which correspond to the speeds of motor 102. By switching from 4 kHz to 2 kHz, the available current of drive controller 220 can therefore be increased.

The illustrated embodiments are selected only as examples and can be combined with one another. Frequency switching can also be used in other possible operating phases of the electric injection machine. The approach according to the invention can also be used in other machines where there is a risk of the drive controller overheating due to a standstill or a slow speed of the motor controlled by the drive controller.

List of reference symbols

102

Servo motor

104

Timing belt

106

Snail

108

funnel

110

Injector

112

Tool

114

spindle

116

Plasticizing motor

220

Drive controller

222

Frequency switching

Claims (10)

A method for controlling an electric injection device of an injection molding machine, wherein the electric injection device is configured to inject a molding compound into a tool (112) during an injection phase and to exert a predetermined pressure on the molding compound in the tool (112) during a holding pressure phase, the method comprising the following steps: Providing, with a drive controller (220) having an output stage with three transistor pairs, a first drive current to a motor (102) of the electric injection device for driving the motor (102) during the injection phase, wherein the first drive current is provided by the drive controller (220) at a first frequency; Switching the frequency, with a frequency-switching device (222) coupled to the drive controller (220), in order to set a second frequency in the drive controller (220) for a holding pressure phase, which is lower than the first frequency, and providing, with the drive controller (220), a second drive current to the motor (102) of the electric injection device for driving the motor (102) during the holding pressure phase to exert the predetermined pressure on the molding compound in the tool (112), wherein the second drive current is provided by the drive controller (220) at the second frequency such that the predetermined pressure is exerted on the molding compound in the tool (112) even though the motor (102) is not rotating at all or at a very low speed during the holding pressure phase. Method according to one of the preceding claims, in which the electric injection device is at least temporarily at a standstill in the holding pressure phase. Method according to one of the preceding claims, wherein the second drive current has a value which is large enough to enable the electric injection device to exert the predetermined pressure on the molding compound. Method according to one of the preceding claims, in which the first and the second drive current are provided by an output stage of a drive controller (220) and in which the second frequency is small enough to prevent overheating of the output stage due to a through loss caused by the second drive current. Method according to one of the preceding claims, wherein the first frequency has a value between 3 KHz and 8 KHz and the second frequency has a value between 1.5 KHz and 3 KHz. Method according to one of the preceding claims, wherein the first drive current has a value of more than 300% of a nominal current of the electric injection device and wherein the second drive current has a value between 10% and 200% of the nominal current of the electric injection device. Method according to one of the preceding claims, wherein the electric injection device comprises an electric servo motor (102) driven by the first and second drive currents. Control device for controlling an electric injection device of an injection molding machine, which is designed to carry out the steps of a method according to one of the Claims 1 until 7 to carry out. An injection molding machine, comprising: an electric injection device configured to inject a molding compound into a tool (112) during an injection phase and to exert a predetermined pressure on the molding compound in the tool during a holding pressure phase; a drive controller (220) having an output stage with three transistor pairs and configured to provide a first drive current with a first frequency during the injection phase and a second drive current with a second frequency during the holding pressure phase to a motor (102) of the electric injection device for driving the motor (102) such that the predetermined pressure is exerted on the molding compound in the tool (112), even though the motor (102) is not rotating at all or at a very low speed during the holding pressure phase. and a frequency switching device (222) coupled to the drive controller (220) and configured to switch the first frequency, at which the drive controller (220) provides the first drive current, to the second frequency, which is lower than the first frequency Computer program product with program code stored on a machine-readable carrier for carrying out the method according to one of the Claims 1 until 7 when the program is executed on a control unit.

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