HK1060713B - A control system for an injection molding machine and a method for controlling the injection molding machine - Google Patents

Description

Injection molding machine control system and injection molding machine control method

Technical Field

The present invention relates to a control system and a control method for an injection molding machine, and more particularly to a control system and a control method for an injection molding machine that executes a molding cycle including a filling step and a subsequent pressure maintaining step.

Background

First, a molding cycle of an injection molding machine, including a plasticizing process, a metering process, a filling process, and a pressure holding process, will be described by taking an electric injection molding machine as an example.

(1) Plasticising/metering process

Rotating the screw rod by a servo motor for rotating the screw rod, wherein the screw rod is arranged in the heating cylinder; supplying resin to the rear portion of a screw rod in a heating cylinder through a hopper, and feeding a predetermined amount of resin to the front end portion of the heating cylinder by rotating the screw rod while melting the resin supplied by rotating the screw rod; during this time, the screw rod is gradually subjected to the pressure (back pressure) of the molten resin gradually accumulated in the distal end portion of the heating cylinder and retreats.

The rear end of the screw rod is directly connected with the injection shaft. The injection shaft is freely rotatably supported on the pressure plate by means of bearings. The injection shaft is driven in the axial direction by an injection servomotor supported on a pressure plate. The pressure plate is moved forward and backward along the guide rod by a ball screw driven by an injection servo motor. As will be described later, the above-described back pressure of the molten resin may be detected by a load cell, and the detected value of the load cell is controlled using a pressure feedback control loop.

(2) Filling process

In the next filling step, the pressure plate is moved forward by the injection servomotor, and the molten resin is filled into the mold with the tip of the screw as a plunger. The resin pressure at the tip of the screw at this time can be detected as an injection pressure.

And at the end of the filling process, the molten resin is filled in the inner cavity of the mold. At the moment, the forward movement of the screw rod is switched from speed control to pressure control; the switching from speed control to pressure control is called V/P switching, which affects the quality of the molded product.

(3) Pressure maintaining process

After the V/P switching, the resin in the inner cavity of the mold is cooled under a preset pressure. This is called a pressure holding step (or a blocking step). In the pressure maintaining step, the resin pressure is controlled by a feedback control loop as in the back pressure control described above.

In an injection device of an injection molding machine, a post-control system of a pressure maintaining process (3) returns to the process (1) and enters the next molding cycle. In the mold clamping device, the mold is opened and the cooled and solidified molded product is pushed out by the pushing-out mechanism while the step (1) is performed, and then the mold is closed to proceed to the step (2).

In injection molding, as a factor for determining the dimensional accuracy of a molded product, the influence of a resin pressure variation curve (profile) in an injection step and a pressure holding step is large. For example, in the case of thin-walled molded products, molding defects such as less shot, voids, flash, and warpage are often caused by a difference in resin pressure particularly after the V/P switching.

In order to avoid the poor molding, the V/P switching position is set in terms of molding conditions. Injection speed setting, dwell pressure setting, and the like, which are referred to as molding condition designations. However, the molding condition designation is complicated due to the mutual influence between the above-described various settings. The molding conditions in the existing holding pressure process include only pressure setting and time setting. The pressure setting is generally carried out in different stages.

For example, the resin pressure is high at the time of V/P switching. When the holding pressure setting value is set to a holding pressure at which the resin pressure is low, the screw is retracted, but the retraction speed is not controlled at this time. As will be described later, this causes a problem of bringing about a negative influence on the formed article.

Fig. 1 illustrates a filling process and a pressure holding process of a molding cycle when a conventional control method is implemented.

Specifically, fig. 1 (a) illustrates a variation curve of injection speed in a filling process and a pressure holding process of a molding cycle when a conventional control method is implemented; FIG. 1(b) is a view illustrating a resin pressure variation curve in a filling process and a pressure holding process of a molding cycle in the case of implementing a conventional control method; fig. 1 (c) shows an example of a variation curve of the position of the screw (distance between the front of the screw and the nozzle-side end of the heating cylinder) in the filling step and the pressure holding step of the molding cycle in the case of implementing the conventional control method.

In fig. 1 (a), the positive injection speed indicates that the screw advances, and the negative injection speed indicates that the screw retreats. Here, for convenience of description, only the case where the pressure is set at the first stage in the pressure holding process is shown. The pressure holding process control is performed by setting the pressure holding pressure (Ph1) and time (t 1).

As the injection starts, the screw advances as shown in fig. 1 (a), and the distance between the front of the screw and the nozzle-side end of the heating cylinder is shortened, so that the resin is filled into the cavity of the mold.

There may be a case where: when the molten resin compressed at the gate of the leading end portion of the screw is injected into the cavity, the volume of the resin in the cavity increases. In this case, even if the pressure of the head of the screw is changed (by changing the holding pressure in the holding step), the resin in the cavity is not affected to some extent or an excessive reaction is caused.

When the resin pressure is high at the time of the V/P switching and the first pressure-holding set value is set so that the resin pressure is reduced, the screw is retracted, that is, the screw retraction speed is generated. In particular, when the difference between the resin pressure and the first pressure holding set value Ph1 is large at the time of V/P switching, the screw-retracting speed becomes high as shown by the dotted circle portion in fig. 1 (a). If the screw rod retreating speed becomes high, negative pressure is given to the molten resin. As a result, the molten resin is adversely affected, for example, by the formation of voids in the resin.

Disclosure of Invention

It is a general object of the present invention to provide a novel and useful control system and control method for an injection molding machine that avoids one or more of the above-mentioned disadvantages.

Specifically, the present invention aims to provide an injection molding machine control system and an injection molding machine control method that contribute to improvement in dimensional accuracy of molded articles by controlling high-speed retraction of a lead screw after V/P switching in a pressure maintaining process.

The purpose of the invention is realized as follows: an injection molding machine control system that executes a molding cycle including a filling process of controlling a screw of the injection molding machine to advance and a pressure maintaining process of controlling a pressure of molten resin after the filling process, comprising: a pressure detecting means for detecting a pressure holding pressure at which the resin is melted in an axial direction of the screw rod in the pressure holding step; a speed detection unit for detecting the retreating speed of the screw rod; and a retraction speed limiting means for limiting the screw retraction speed to a predetermined retraction speed threshold value after the pressure control is started in the pressure holding step based on the pressure holding pressure detected by the pressure detecting means.

The reverse speed limiting unit includes: a 1 st arithmetic unit for calculating a 1 st operation amount based on a 1 st difference between the retreat speed detected by the speed detecting unit and the preset retreat speed threshold, a 2 nd arithmetic unit for calculating a 2 nd operation amount based on a 2 nd difference between the holding pressure detected by the pressure detecting unit and the holding pressure preset value, and a control unit for performing the screw retreat speed control based on the 1 st operation amount when the 1 st operation amount is larger than the first operation amount and performing the holding pressure control based on the 2 nd operation amount when the 1 st operation amount is smaller than or equal to the 2 nd operation amount.

The control system of the injection molding machine comprises a screw rod which is pushed by a driving part to fill the molten resin into the inner cavity of the mold, wherein after the mold is filled with the molten resin, the screw rod is retracted according to the difference between the detection pressure of the molten resin and the preset pressure of the molten resin; the system includes a controller for limiting the backward speed of the screw until the detected pressure of the molten resin reaches the predetermined pressure.

The object of the invention is also achieved in that: an injection molding machine which performs a molding cycle including a filling process of controlling a screw of the injection molding machine to advance and a pressure maintaining process of controlling a pressure of molten resin after the filling process, comprising the steps of: a step of detecting a pressure holding pressure, which is a pressure of the molten resin in the axial direction of the screw rod in the pressure holding step; detecting the retreating speed of the screw rod; and limiting the screw retraction speed to a preset retraction speed threshold after the pressure control is started in the pressure holding process based on the detected holding pressure.

The control method of the injection molding machine comprises the following steps:

(a) advancing a screw of an injection molding machine to fill molten resin into an inner cavity of a mold;

(b) withdrawing the screw according to a difference between a detected pressure of the molten resin and a preset pressure of the molten resin;

(c) and limiting the retreating speed of the screw rod until the detection pressure of the molten resin reaches the preset pressure.

As described above, according to the present invention, the screw retraction speed limit is added as the molding condition in the pressure holding step, and the control in the pressure holding step can be automatically switched between the pressure holding control and the screw retraction speed control.

Other objects, features and advantages of the present invention will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings.

Drawings

Fig. 1 is an explanatory diagram of a filling process and a pressure holding process according to a conventional control method.

Fig. 2 is a configuration diagram of an electric injection molding machine including a servomotor-driven injection device to which a control system and a control method according to an embodiment of the present invention are applied.

Fig. 3 is a structural view of a hydraulic injection molding machine to which the control system and the control method according to the embodiment of the invention are applied.

FIG. 4 is a diagram illustrating a filling process and a pressure maintaining process when the control method according to the embodiment of the present invention is implemented.

Fig. 5 is a block diagram of a backward speed limiting unit in a pressure maintaining process of the electric injection molding machine realized by the control system controller according to an embodiment of the present invention.

Fig. 6 is a block diagram of a reverse speed limiting unit in a hydraulic injection molding machine pressure maintaining process implemented by a control system controller according to an embodiment of the present invention.

Detailed Description

Embodiments of the present invention will be described with reference to fig. 2 to 6.

The structure of the injection molding machine applicable to the present invention is described first.

Fig. 2 illustrates a configuration of an electric injection molding machine including an injection servo motor to which the control system and the control method according to the embodiment of the present invention are applied.

According to fig. 2, the rotation of the injection servomotor 11 is transmitted to the bead screw 12. The nut 13, which moves forward and backward by the rotation of the ball screw 17, is fixed to the pressure plate 14. The pressure plate 14 is movable along guide rods 15, 16 fixed to a chassis (not shown), and the back and forth movement of the pressure plate 14 is transmitted to a screw 20 through a bearing 17 and a load cell 18 and an injection shaft 19.

The screw 20 is mounted in a heating cylinder 21 both rotatably and axially movably. A hopper 22 for supplying resin is provided at a rear portion of the heating cylinder 21 facing the screw 20. The rotation of the screw rotation servomotor 24 is carried out by a belt. A coupling member 23 such as a pulley is transmitted to the injection shaft 19. Therefore, the injection shaft 19 is driven to rotate by the screw rotation servomotor 24, and the screw 20 is rotated.

In the plasticizing/metering step, the screw 20 is retracted while rotating in the heating cylinder 21 by the rotational movement of the servomotor 24, and thereby the molten resin is accumulated in front of the screw 20, i.e., on the nozzle 21-1 side of the heating cylinder 21.

In the filling step, the screw 20 is advanced by the rotational movement of the servomotor 11, and the accumulated molten resin is filled in the mold and pressurized, thereby realizing molding. At this time, the force pressing the resin can be detected by the load cell 18 as a reaction force, that is, the resin pressure at the leading end portion of the screw is detected, and the detection value output from the load cell 18 is amplified by the load cell amplifier 25 and then output to the controller 26.

A position detector 27 for detecting the amount of movement of the screw 20 is attached to the pressure plate 14, and a detection value of the position detector 27 is amplified by an amplifier 28 and then input to the controller 26.

The controller 26 outputs current (torque) commands corresponding to the respective steps to the servo amplifiers 29, 30 in accordance with set values set in advance by an operator via the input device 35.

The servo amplifier 29 controls the drive current of the servo motor 11 to control the output torque of the servo motor 11. The servo amplifier 30 controls the servo motor 24 drive current to control the servo motor 24 torque.

The servo motors 11, 24 have encoders 31, 32 for detecting their rotations, respectively. The numbers of rotations detected by the encoders 31, 32 are both input to the controller 26.

Fig. 3 shows a structure of a hydraulic injection molding machine to which the control system and the control method of the embodiment of the invention are applied.

Referring to fig. 3, a hydraulic injection molding machine according to the present invention is adapted to drive an injection shaft 42 with a screw driving cylinder 41. That is, the injection shaft 42 is driven in the axial direction by controlling the inflow and outflow of the working oil to and from the screw driving cylinder 41.

The cylinder driving servo valve 43 is a valve for switching an oil path between the screw driving cylinder 41, the hydraulic pressure source 44, and the hydraulic oil tank 45. Injection pressure detectors 46 and 47 are provided in the hydraulic piping to detect the injection pressure as the resin pressure. A position detector 48 serving as a lead screw position detection is provided on the injection shaft 42.

The injection pressure detectors 46, 47 correspond to the load cell 18 shown in fig. 2. The controller 50 receives detection signals from the injection pressure detectors 46 and 47 and the position detector 48, and the controller 50 controls the cylinder driving servo valve 43, thereby realizing speed control in the filling step and pressure control in the holding pressure step.

The following describes embodiments of the control system and control method of the present invention.

In the embodiment of the control system and the control method, after the pressure control in the pressure maintaining process after the V/P switching is started, the screw rod retreating speed is controlled to be a preset threshold value, namely the screw rod retreating speed threshold value.

That is, the screw-withdrawal-speed threshold is provided as one of the molding conditions in the holding pressure process, so that the holding pressure process control can be automatically switched to the holding pressure control or the screw-withdrawal-speed control.

Fig. 4 illustrates a filling process and a pressure holding process in a forming cycle when the control method according to the embodiment of the present invention is implemented.

Specifically, (a) in fig. 4 illustrates the injection speed variation curve in the filling process and the holding pressure process of the molding cycle when the control method according to the embodiment of the present invention is implemented; FIG. 4 (b) is a graph showing the variation of resin pressure in the filling step and the holding step of the molding cycle when the control method according to the embodiment of the present invention is carried out; fig. 4 (c) shows a variation curve of the position of the screw (distance between the front of the screw and the nozzle-side end of the heating cylinder) in the filling step and the pressure holding step of the molding cycle in the case of implementing the control method according to the embodiment of the present invention.

In fig. 4 (a), the positive injection speed indicates that the screw advances, and the negative injection speed indicates that the screw retreats.

As shown in fig. 4 (b), for convenience of description, only the case of the first stage of pressure setting in the pressure holding process is shown. The pressure holding process control is performed by setting the pressure holding pressure (Ph1) and time (t 1). However, the present invention is not limited to the above examples, and the present invention can be applied to a case where the holding pressure is set in multiple stages.

As the injection is started, the screw advances as shown in fig. 4 (a), the distance between the front of the screw and the nozzle-side end of the heating cylinder is shortened, and the resin is filled into the cavity of the mold.

That is, in the after-V/P-switching pressure maintaining process, the 1 st operation amount is obtained from the 1 st difference between the detected screw rod retreating speed after the start of the pressure control and the preset screw rod retreating speed threshold, and the 2 nd operation amount is also obtained from the 2 nd difference between the detected pressure maintaining pressure after the start of the pressure control and the preset pressure maintaining pressure in the after-V/P-switching pressure maintaining process; the larger of the 1 st operation amount and the 2 nd operation amount is selected for control.

Specifically, after the pressure control is started in the pressure maintaining step after the V/P switching, if the 1 st operation amount is equal to or less than the 2 nd operation amount, the same control as before is executed; and if the 1 st operation amount becomes larger than the 2 nd operation amount, controlling the screw rod retreating speed to be a screw rod retreating speed threshold value. The screw retreat speed control is actually realized by the screw position control.

Fig. 5 shows a block diagram of a reverse speed limiting unit in the pressure holding process implemented by the control system controller according to an embodiment of the present invention.

First, an example in which the present control system and control method are applied to the electric injection molding machine shown in fig. 2 will be described.

The holding pressure is detected as the resin pressure by a load cell 18 having the function of a pressure detecting unit shown in fig. 2. The detection value of the position detector 27 shown in fig. 2 is differentiated by the speed detection means to realize the detection of the screw speed. The controller 26 shown in fig. 2 functions as a reverse speed limiting unit.

Referring to fig. 5, after the pressure control in the V/P switching holding pressure process is started, a 1 st subtractor 1 calculates a 1 st difference between a screw rod retreating speed detection value (negative value) and a preset screw rod retreating speed threshold value (negative value). Here, as described above, the detected value of the screw-back speed is obtained by differentiating the detected value of the position detector 27 shown in fig. 2. The 2 nd difference value between the holding pressure detected by the load cell 18 and the preset holding pressure value is obtained by the 2 nd subtracter 2. The 1 st compensator 3 outputs a 1 st operation amount according to the 1 st difference, and the 2 nd compensator 4 outputs a 2 nd operation amount according to the 2 nd difference. These operation amounts belong to the same category, of course. In particular, when the above-described operation amount is converted into dimensional accuracy, the related operation is easily achieved.

The controller 26 shown in fig. 2 compares the 1 st operation amount with the 2 nd operation amount, and selects the larger one of the 1 st operation amount and the 2 nd operation amount to supply to the injection servomotor 11. Accordingly, if the 1 st operation amount is equal to or less than the 2 nd operation amount, the same control as before is executed; if the 1 st operation amount becomes larger than the 2 nd operation amount, the screw rod retreating speed control is performed, that is, the screw rod retreating speed is maintained at the screw rod retreating speed threshold.

Thus, since the screw retreating speed in the pressure maintaining process after the V/P switching is limited to the threshold value, the high-speed retreat of the screw in the pressure maintaining process of the electric injection molding machine can be prevented. Therefore, adverse effects on the molded product can be avoided.

Next, an example of applying the control system and the control method to the hydraulic injection molding machine shown in fig. 3 will be described.

The holding pressure is detected as a resin pressure by injection pressure detectors 46, 47 having a function of a pressure detecting means shown in fig. 3. The detection value of the position detector 48 shown in fig. 3 is differentiated by the speed detection means to realize the detection of the screw speed. The controller 50 shown in fig. 3 functions as a reverse speed limiting unit.

Fig. 6 illustrates a retreat speed limiting unit in a hydraulic injection molding machine pressure maintaining process implemented by a control system controller according to an embodiment of the present invention.

Referring to fig. 6, after the pressure control in the pressure maintaining process after the V/P switching is started, a 1 st difference value between a detected value (negative value) of the screw rod retreating speed and a preset threshold value (negative value) of the screw rod retreating speed is obtained by a 1 st subtracter 1. Here, as described above, the detected value of the screw-back speed is obtained by differentiating the detected value of the position detector 48 shown in fig. 3.

A2 nd difference value between the holding pressure detected by the injection pressure detector 46 and a preset holding pressure value is obtained by a 2 nd subtracter 2. The 1 st compensator 3 outputs a 1 st operation amount according to the 1 st difference, and the 2 nd compensator 4 outputs a 2 nd operation amount according to the 2 nd difference. These operation amounts belong to the same category, of course.

The controller 50 shown in fig. 3 compares the 1 st operation amount with the 2 nd operation amount, and selects the larger one of the 1 st operation amount and the 2 nd operation amount to supply to the cylinder driving servomotor 43. Accordingly, if the 1 st operation amount is equal to or less than the 2 nd operation amount, the same pressure control as before is performed; if the 1 st operation amount becomes larger than the 2 nd operation amount, the screw rod retreating speed control is performed, that is, the screw rod retreating speed is maintained at the screw rod retreating speed threshold.

In this way, since the screw retreating speed in the pressure maintaining step after the V/P switching is limited to the threshold value, the high-speed retreat of the screw in the pressure maintaining step of the hydraulic injection molding machine can be prevented. Therefore, adverse effects on the molded product can be avoided.

Since the threshold value is set for the screw retreating speed in the pressure holding step, it is possible to prevent adverse effects on the molded product due to the high-speed retreat of the screw in the pressure holding step. Therefore, the dimensional accuracy of the molded article in injection molding can be improved, and the specification of molding conditions can be simplified.

Further, when the resin is filled in the cavity of the mold before the detected resin pressure reaches the peak value, the V/P switching is sufficiently performed (so that the filling process time is shortened), and the screw-retracting speed is controlled to be the screw-retracting speed threshold after the pressure control is started in the pressure maintaining process under the V/P switching, for example, in the case of a thin-walled molded product, it is possible to effectively prevent the occurrence of molding defects such as a small amount of shot, blowholes, burrs, and warping, and thus it is possible to obtain a high-quality molded product.

The present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the general inventive concept.

For example, in the above embodiment, the control method of the present invention is applied to an application where: the preset holding pressure value is set in the first stage of the holding pressure process under the conditions of the preset holding pressure value Ph1 and the time t 1.

However, the present invention is not limited to the above-described embodiment, and the present invention can be applied to an optional holding pressure in a multi-stage setting of the holding pressure.

The present application is based on the prior application with application No. 2002-141423 filed in japan on 5, 16/2002, the entire contents of which are incorporated herein by reference.

Claims (16)

1. An injection molding machine control system for performing a molding cycle including a filling step of controlling a screw of the injection molding machine to advance and a pressure maintaining step of controlling a pressure of a molten resin after the filling step, the injection molding machine comprising: a pressure detecting means for detecting a pressure holding pressure at which the resin is melted in an axial direction of the screw rod in the pressure holding step; a speed detection unit for detecting the retreating speed of the screw rod; and a retreat speed limiting unit for limiting the retreat speed of the screw to a preset retreat speed threshold value after the pressure control is started in the pressure maintaining process based on the pressure detected by the pressure detecting unit.

2. The injection molding machine control system according to claim 1, wherein the retraction speed limiting unit includes: a 1 st arithmetic unit for calculating a 1 st operation amount based on a 1 st difference between the retreat speed detected by the speed detecting unit and the preset retreat speed threshold value, a 2 nd arithmetic unit for calculating a 2 nd operation amount based on a 2 nd difference between the holding pressure detected by the pressure detecting unit and a holding pressure preset value, and a control unit for performing screw retreat speed control based on the 1 st operation amount when the 1 st operation amount is larger than the 2 nd operation amount and performing holding pressure control based on the 2 nd operation amount when the 1 st operation amount is smaller than or equal to the 2 nd operation amount.

3. The control system of an injection molding machine according to claim 1, wherein the screw is advanced by a driving member so that a mold cavity is filled with a molten resin, and after the mold is filled with the molten resin, the screw is retracted based on a difference between a detected pressure of the molten resin and a preset pressure of the molten resin; the retraction speed limiting unit limits the retraction speed of the screw until the detected pressure of the molten resin reaches the preset pressure.

4. The control system of claim 2, wherein a plurality of preset dwell pressure values are set in the dwell process; the 2 nd operation unit obtains the 2 nd operation amount based on a difference between the holding pressure detected by the pressure detection unit and a preset value selected from the plurality of holding pressure preset values.

5. The control system of claim 2, wherein the 2 nd arithmetic unit obtains the 2 nd operation amount based on a 2 nd difference between the holding pressure detected by the pressure detecting unit and the holding pressure preset value immediately after the start of the holding pressure process.

6. An injection molding machine control system according to claim 2, wherein the injection molding machine includes an electric injection molding machine including a servo motor for injection; the pressure detection unit comprises a force sensor for detecting the axial resin pressure of the screw rod; the speed detection unit comprises a position detector for detecting the position of the screw rod; detecting a backward speed of the screw by differentiating a detection value obtained by the position detector; the screw retraction speed control is performed by controlling the injection servomotor in accordance with the 1 st operation amount.

7. An injection molding machine control system according to claim 2, wherein the injection molding machine includes a hydraulic injection molding machine having an injection cylinder and a servo valve for controlling inflow and outflow of working oil to and from the injection cylinder; the pressure detection unit comprises an injection pressure detector for detecting the hydraulic pressure of the injection cylinder; the speed detection unit comprises a position detector for detecting the position of the screw rod; detecting a backward speed of the screw by differentiating a detection value obtained by the position detector; the screw retraction speed control is performed by controlling the servo valve in accordance with the 1 st operation amount.

8. An injection molding machine control system according to claim 3, further comprising a 1 st computing element for generating a 1 st operation amount based on a 1 st difference between the screw retracting speed and the screw preset retracting speed threshold, and a second computing element for generating a 2 nd operation amount based on a 2 nd difference between the molten resin detection pressure and the molten resin preset pressure.

9. An injection molding machine control system as claimed in claim 8, wherein said lead screw is controlled in accordance with said 1 st operation amount when said 1 st operation amount is larger than said 2 nd operation amount; and when the 1 st operation amount is less than or equal to the 2 nd operation amount, the screw rod is controlled according to the 2 nd operation amount.

10. A control method of an injection molding machine which executes a molding cycle including a filling process of controlling a screw of the injection molding machine to advance and a pressure maintaining process of controlling a pressure of molten resin after the filling process, characterized by comprising the steps of: a step of detecting a pressure holding pressure, which is a pressure of the molten resin in the axial direction of the screw rod in the pressure holding step; detecting the retreating speed of the screw rod; and limiting the screw retraction speed to a preset retraction speed threshold after the pressure control is started in the pressure holding process based on the detected holding pressure.

11. An injection molding machine control method according to claim 10, wherein the 1 st operation amount is calculated based on a 1 st difference which is a difference between the detected retreat speed and the retreat speed threshold; calculating the 2 nd operation amount according to the 2 nd difference value which is the difference value between the detected holding pressure and the holding pressure preset value; performing the screw rod retraction speed control according to the 1 st operation amount when the 1 st operation amount is larger than the 2 nd operation amount; and performing the pressure control according to the 2 nd operation amount when the 1 st operation amount is not more than the 2 nd operation amount.

12. An injection molding machine control method as claimed in claim 10, comprising the steps of:

(a) advancing a screw of an injection molding machine to fill molten resin into an inner cavity of a mold;

(b) withdrawing the screw according to a difference between a detected pressure of the molten resin and a preset pressure of the molten resin;

(c) and limiting the retreating speed of the screw rod until the detection pressure of the molten resin reaches the preset pressure.

13. The control method of an injection molding machine according to claim 11, wherein a plurality of preset holding pressure values are set in the holding pressure process; and calculating the 2 nd operation amount according to the 2 nd difference value between the holding pressure detected by the pressure detection unit and the selected preset value in the plurality of holding pressure preset values.

14. An injection molding machine control method according to claim 12, wherein the pressure of the molten resin is detected by a pressure detecting unit; and the speed detecting unit detects the backward speed.

15. An injection molding machine control method as claimed in claim 12, wherein a 1 st operation amount is generated based on a 1 st difference between said retreat speed and a preset retreat speed threshold; the 2 nd operation amount is generated according to the 2 nd difference value between the pressure of the molten resin detected by the pressure detection unit and the preset pressure value.

16. An injection molding machine control method as claimed in claim 15, wherein said screw is operated in accordance with said 1 st operation amount when said 1 st operation amount is larger than said 2 nd operation amount; and when the 1 st operation amount is less than or equal to the 2 nd operation amount, the screw rod is operated according to the 2 nd operation amount.