INJECTION MOLDING MACHINE AND METHOD OF CONTROL THEREOF

Abstract: A method for controlling an injection molding machine comprises the steps of: detecting, by a screw position detector (4), a position of a screw (2) of the injection molding machine immediately before a start of injection, to produce a detecting signal; comparing in a comparator (7) the detecting value with a predetermined value for a plasticising/measurement position set in a measurement position setter (5), to calculate a deviation therebetween; and adding/subtracting the deviation in an adder/subtractor (8) to or from predetermined values representative of switching-over positions set in switching-over position setters (6), thereby to keep constant an actual screw stroke from the plasticising/measurement position to the switching-over position in every shot of the injection molding machine.

Description: The present invention relates to a switching-over control method and apparatus for controlling, for example, the injection pressure and the holding pressure in an injection molding machine. In a conventional technique, a control for switching over the pressure applied to the resin from the filling pressure to the holding pressure in an injection molding machine is conducted by setting positions of an injection screw relative to a cylinder in advance and comparing the set positional values with values detected by screw position sensors. On the other hand, a starting position where the screw is operative for resin supply is determined by the screw stop position upon completion of plasticising action or measurement operation. For that purpose, the set value of the measurement position setter and the positional signal of the position detector for the screw are compared with each other by a comparator, so that when both the values are identical with each other, an ending signal is applied to a pressure control valve, a flow valve, and/or directional valves regulating to thereby stop the movement of the screw. According to the conventional method, a signal representative of completion of the plasticising action and measurement operation is generated at a position Sm as shown in Figure 1 of the accompanying drawings. However, the above-described conventional control manner suffers from the following disadvantages. Namely, although the screw should be stopped exactly at the time of generation of the signals representative of the completion of the plasticising action and measurement operation, the screws will be rotated through a small angle due to the inertia moment thereof. It is very difficulty to keep constant such a rotation due to instability of mechanical loss or other factors. Furthermore, the pressure distribution of the molten resin in the cylinder is shown in Figure 2. Assuming the pressure at the screw tip end be expressed by P1 and the pressure on the screw flight side be expressed by P2, the relationship, P1 > P2, is established during screw rotation and after the completion of the measurement, so that the molten resin will flow to the left and will gradually move to meet the requirement, P1 = P2, to stop. During a period from P1 > P2 to P1 = P2, the screw will be moved by a distance (Sm'- Sm), by the reaction of the molten resin as shown in Figure 1 to increase the screw stroke. Namely, as shown in Figure 1, the total increment of resin amounts corresponding to the inertia rotation of the screw and the resin reaction is expressed by α. As a result, as shown in Figure 1, the deviation α is added to the stroke from a predetermined measurement position Sm to the filling pressure and/or speed change-over position Si, and the strokes to the holding pressure change-over position Sh (Sm-Si, Sm-Sh), respectively. Namely, since the amount of resin to be filled in the mold cavity is changed in accordance with the deviation α, there is a problem in obtaining a precise product. Accordingly, an object of the present invention is to overcome the drawbacks inherent in the prior art system. According to the present invention, there is provided a method for controlling an injection molding machine comprising the steps of: detecting, by a screw position detector, a position of a screw of said injection molding machine immediately before a start of injection comparing said detecting signal with a predetermined value for a plasticising/measurement position set in a measurement position setter, to calculate a deviation therebetween; and adding/subtracting said deviation to or from predetermined values representative of switching-over positions set in switching-over position setters, thereby keep constant an actual screw stroke from said plasticising/measurement position to the switching-over position in every shot of said injection molding machine. According to another aspect of the invention, there is a method for controlling an injection molding machine comprising the steps of: detecting a position of a screw by a screw position detector; setting injection speed and/or pressure change-over positions in advance; setting a rearmost position of said screw; detecting a deviation between a value representative of said rearmost position and a value representative of a position, immediately before an injection molding operation, by said screw position detector, adding or subtracting the deviation to respective values representative of injection speed and/or pressure change-over positions; storing results of calculation in the step of adding; and controlling the injection molding operation in accordance with the results. According to still another aspect of the invention, there is a method for controlling an injection molding machine comprising the steps of: detecting, by a screw position detector, a position of a screw of said injection molding machine immediately before a start of injection; calculating a deviation between said position detected by said screw position detector and a value outputted from a memory that stores a screw position of a former shot immediately before a start of injection; and adding/subtracting said deviation to or from predetermined values representative of switching-over positions set in switching-over position setters, thereby performing a switching over operation when a value corrected with said deviation is identical with the detected value from said screw position detector. Three examples of methods and of machines in accordance with the invention will now be described with reference to the accompanying drawings in which:- Figure 1 shows a positional relationship between a screw and a cylinder in a prior art injection molding machine; Figure 2 illustrates the drawbacks inherent in the prior art injection molding machine; Figure 3 is a block diagram showing a first embodiment of a machine in accordance with the present invention; Figure 4 shows a positional relationship between a screw and a cylinder in the machine shown in Figure 3; Figure 5 is a block diagram showing a second embodiment of a machine in accordance with the present invention; Figure 6 is a block diagram showing a third embodiment of a machine in accordance with the present invention; and Figure 7 shows a positional relationship between a screw and a cylinder in the machine shown in Figure 6. The present invention will now be described in detail with reference to the accompanying drawings. Fig. 3 shows a block diagram in accordance with one embodiment of the invention. Fig. 4 shows the positional relationship of the screw stroke in accordance with the system shown in Fig. 3. As shown in Fig. 3, a screw 2 within a cylinder 1 is driven by a hydraulic pressure of an injection ram 3. The actual position of the screw 2 is detected by a position sensor 4 well known per se. A signal of the position sensor is applied through an amplifier 9 to a comparator 7. On the other hand, a predetermined set value representative of the screw position upon the completion of the plasticizing and measurement operation is applied from a suckback or measure­ ment position setter 5 to the comparator 7. Then, the detected value and the set value of the screw position are compared and calculated by the comparator 7, to thereby generate a deviation α to an adder/subtracter device 8. In the device 8, the deviation α is added to or substracted from the switch­ ing-over position set values, for the filling pressure and/or speed and the holding pressure, outputted from a switching-over position setter 6, thus correcting the set values. corrected set values are applied from the device 8 to a sequencer 21, and then a pressure control valve 10 and/or flow regulating valve 12 and driven through an amplifier 9', thereby performing the switching-over control between the filling pressure and/or speed and the holding pressure. As is well known in the art, the control valve 10 is connected to a fluid passage between a pump 11 and a flow regulating valve 12, and the pressure is selectively applied to the ram cylinder to displace the ram 3. Fig. 4 shows the switching-over position between the filling pressure and/or speed and holding pressure when corrected with the deviation α. Si is the filling pressure and/or speed change-over position and Si' is the corrected position. Sh is the holding pressure change-over position set by the setter 6 and Sh' is the corrected position. So is the screw foremost position in the conventional control and So' is the position in the present invention. Namely, the relationships, Si' = Si + α, Sh' = Sh + α, and So' = So + α are established. According to the present invention, each control signal generated from the device 8 is corrected with the deviation α. Therefore, for example, the relationship, Sm - Sh = Sm' - Sh' is established, and hence the screw stroke may be kept exactly constant. The amount of the resin may be always kept exactly at the predetermined one. According to the present invention, even if the molten resin amount at the plasticizing/measurement operation is changed, the amount of the resin injected into the mold cavity is kept constant between the respective switching-over posi­ tions between the filling pressure and/or speed and holding pressure, irrespective of the change of the molten resin amount. A stable product may be always obtained. Fig. 5 shows another embodiment of the system according to the present invention. In Fig. 5, the same reference numerals are used to denote the like components shown in Fig. 3. In Fig. 5, numeral 5' denotes a rearmost retracted posi­ tion setter for setting a position where the measurement of the screw 2 is completed or a rearmost retracted position where a suckback operation or any other necessary operation is completed. In order to keep constant the screw stroke, the following operations are carried out. A position of the screw 2 immedi­ ately before the injection action in the subsequent molding cycle is detected by a screw position detector 4. A deviation between the actual screw position value detected by the screw position detector 4 and the value set by a screw rearmost retracted position setter 5' is calculated by a comparator 7' which is a deviation calculator. The obtained deviation is added or subtracted by adder/subtracter 8' to each value representative of the screw position set in advance by an in­ jection process control position switching setter 6. Each added value is stored in a corrected injection process control position switching memory 20. A designation of the screw stroke switching position for the actual injection speed is inputted into a sequencer 21 from the memory 20. Then, the sequencer 21 applies its opening signal to a pressure control valve 10 and/or flow regulating valve 12 through a driver amplifier 9'. Fig. 6 shows another embodiment of the present invention in which the same reference characters are used to designate the like components or members in the foregoing embodiments. As shown in Fig. 6, a screw 2 in a cylinder 1 is driven through a working oil. An actual position of the screw is detected by a position detector 4. The signal from the detector 4 is inputted to a comparator through an amplifier when an injection start signal 15 is generated. Also, the start signal 15 is stored in a memory 16 as a signal repre­ sentative of the screw position of the last shot immediately before the injection start. On the other hand, a signal re­ presentative of a screw position immediately before the in­ jection start of a preshot stored in a memory 16' is applied to the comparator 7. In the comparator 7, the detected value of the screw position, immediately before the injection start, of the present shot to be carried out is compared with the stored value of the screw position before the injection start of the preshot, to thereby input apply its output to an adder/­ subtracter 8, where the deviation ΔS is added to or subtracted from the respective filling pressure and/or speeds and the switching-over position setting values of the holding pressure changeover position setter 6, to thereby correct the set values. Control signal is fed through the sequencer 21 and through the amplifier 9' to the pressure control valve 11 and/or the flow regulating valve 12, to thereby carry out filling speed and/or prepressure changeover control. The screw position immediately of the injection of the last shot stored in the memory 16 is transferred to the memory 16' in accordance with the holding pressure completion signal 20. Fig. 7 shows the positional relationship according to the system shown in Fig. 6. The corrected changeover posi­ tions for the filling pressure and/or speed and the holding pressure and designated by characters Si' and Sh', respectively. The following relationships are established, Si' = Si + ΔS, and Sh' = Sh + ΔS where Si and Sh are the set values for the changeover positions for the filling pressure and/or the prepressure. Since the control signal outputted from the adder/subtracter 8 has been corrected with the deviation ΔS, the relationship, Sm(2) - Sh' = Sm(1) - Sh, is established, to thereby keep constant the screw stroke in every shot, where Sm(1) and Sm(2) represent the actual screw positions of the former shot and the present shot immediately before the injection start, respectively.

Claims: 1. A method for controlling an injection molding machine characterised by the steps of: detecting, by a screw position detector (4) a position of a screw (2) of said injection molding machine immediately before start of injection; comparing said detecting signal with a predetermined value for a plasticising/measurement position set in a measurement position setter to calculate a deviation therebetween; and adding/subtracting said deviation to or from predetermined values representative of switching-over positions set in switching-over position setters (6) thereby keep constant an actual screw stroke from said plasticising/measurement position to the switching-over position in every shot of said injection molding machine. 2. A method for controlling an injection molding machine characterised by the steps of: detecting a position of a screw by a screw position detector; setting injection speed and/or pressure change-over positions in advance; setting a rearmost position of said screw; detecting a deviation between a value representative of said rearmost position and a value representative of said position, immediately before an injection molding operation, by said screw position detector; adding or subtracting the deviation to respective values representative of injection speed and/or pressure change-over positions; storing results of calculation in the step of adding or subtracting; and controlling the injection molding operation in accordance with the results. 3. A method for controlling an injection molding machine characterised by the steps of: detecting, by a screw position detector, a position of a screw of said injection molding machine immediately before a start of injection; calculating a deviation between said position detected by said screw position detector and a value outputted from a memory that stores a screw position of a former shot immediately before a start of injection,; and adding/subtracting said deviation to or from predetermined values representative of switching-over positions set in switching-over position setters, thereby performing a switching over operation. 4. An injection molding machine comprising: a screw (2); and a screw position detector (4) for detecting a position of said screw (2); characterised by injection speed and/or pressure change-over position setting means for setting injection speed and/or pressure change-over positions in advance; means (5') for setting a rearmost position of said screw; means (7') for detecting a deviation between a value representative of said rearmost position set by said rearmost position setting means and a value representative of a position, immediately before an injection molding operation, detected by said screw position detector; an adder or subtractor (8) for adding or subtracting said deviation to respective values representative of injection speed and/or pressure change-over position set in said injection speed change-over position setting means; and storing means (20) for storing results of calculation of said adder or subtractor, thereby controlling the injection molding operation with signals stored in said storing means. 5. An injection molding machine comprising: a screw (2); a screw position detector (7') for detecting a position of said screw immediately before a start of injection; means for setting a plasticising/measurement position; means (7) for comparing said detecting value from said screw position detector with the plasticising/measurement position set in said measurement position setting means; a switching-over position setter (9', 10) for setting switching over positions of said screw; and adder/subtractor (8) for adding/subtracting said deviation to or from predetermined values representative of switching-over positions set in the switching-over position setter, thereby keep constant an actual screw stroke from the plasticising/measurement position to the switching position in every shot of said injection molding machine. 6. An injection molding machine comprising: a screw (2); a screw position detector (7') for detecting a position of said screw immediately before a start of injection; a memory (20) for storing a screw position of a former shot immediately before a start of injection; calculating means (7) for calculating a deviation between said position detected by said screw position detector and a value outputted from said memory; a switching-over position setter (9', 10) for setting a plurality of switching-over positions for said screw (2); and, adder/subtractor (8) for adding/subtracting said deviation to or from predetermined values representative of switching-over positions set in said switching-over position setter, thereby performing a switching-over operation.

Inventors: EGI, TADAO, KATO, MASAHI, YOKOTA, AKIRA
Applicants: THE JAPAN STEEL WORKS, LTD.
Application year: 1986
Main classification: B29C45
IPC codes: B29C45