JP2005334977A - Method for controlling drive of locally pressing pin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To automatically determine the setting of the control quantity in a short time by a simple process in order to control the drive so that the stroke of a pressing pin is a target value. <P>SOLUTION: When the feed flow rate to a pressing cylinder is employed as the control quantity of the pressing pin, the initial set value Q1 is set to be 50% in terms of the opening of a flow rate control value, and then, set to the flow rate Q2 corresponding to the opening of 75% between the maximum opening of 100% and 50%. Q3, Q4 and Q5 are successively set to the set value immediately before, and the intermediate value of the previous set values, and trial is performed before the stroke value is within a range of the predetermined stroke. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a pressure casting method in a die casting machine, and more particularly to a pressure pin drive control method for pressure casting.

  Normally, in pressure casting such as die casting machines, when the molten metal fills the mold cavity and solidifies, the volume shrinks, and at this time, shrinkage cavities are formed inside the cast product, and the strength and airtightness of the cast product etc. Adversely affect. In particular, die casting machines have a large temperature gradient and remarkable shrinkage. Various methods have been proposed for die casting machines in order to prevent the formation of shrinkage cavities. Among them, the molten metal is filled into the cavity and then squeezed using a pressure pin that protrudes into the cavity. Thus, the method of homogenizing the structure has become the mainstream. An example of a method using such a pressure pin will be described below with reference to FIGS.

  FIG. 4 shows a main configuration of a die casting machine to which the pressure pin control method is applied. In the figure, reference numeral 1 denotes a mold, and a molten metal 4 is poured into a cavity 2 of the mold 1 from an injection sleeve 3.

  The mold 1 is composed of a movable mold 5 and a fixed mold 6. The movable mold 5 is attached to a movable plate 51 of a die casting machine, while the fixed die 6 is attached to a fixed plate 61, and the movable plate 51 is moved. Thus, the mold clamping and mold opening are performed.

  The injection sleeve 3 is attached to a fixed plate 61, and the opening at the tip thereof communicates with the inside of the cavity 2 of the mold 1 through the gate portion 7, and the molten metal inside is injected by the injection plunger 8 inserted into the injection sleeve 3. 4 is injected. The injection plunger 8 is driven and controlled by an injection cylinder 9 disposed coaxially with the injection sleeve 3. The injection cylinder 9 is provided with an injection pressure detection sensor 10.

  The mold 1 includes a pressure pin 11 and a pressure pin cylinder 12 for driving the pressure pin 11 between the movable mold 5 and the movable platen 51.

  The pressure pin 11 is inserted into a pin hole 13 formed in the movable die 5, and its tip can protrude into the cavity 2. The pressure pin cylinder 12 and the pressure pin 11 are coaxial. Has been placed.

  Furthermore, a stroke detection sensor 14 for detecting the stroke of the pressure pin 11 is provided. The stroke detection sensor 14 is an absolute displacement detector. In this embodiment, a differential transformer is used. Since the absolute stroke detection sensor 14 determines the origin position by the sensor itself, the output value in a certain stroke is always constant. In this embodiment, the stroke detection sensor 14 is accommodated in the pressure pin cylinder 12.

  That is, as shown in FIG. 6, the coil portion 16 is fixed to the cylinder head 15 of the pressure pin cylinder 12, and a sleeve-like core 18 is built in the piston 17 so as to cover the coil portion 16. When the piston 17 moves, the built-in sleeve core 18 also moves together, so that the positional relationship between the coil portion 16 and the sleeve core 18 is shifted, and a voltage corresponding to the position of the piston 17 is output. Since the piston 17 and the pressure pin 11 are integrally connected, the stroke of the pressure pin 11 can be detected by detecting the position of the piston 17.

  The stroke detection sensor 14 is not limited to the absolute type, but an incremental type may be used. However, the incremental type requires an origin position signal, has a space and environmental problem, and has an absolute type. Is preferred.

  The stroke detection sensor 14 and the injection pressure detection sensor 10 are connected to an input device 21 of a controller 20 which is a control means, and each output signal is input. The controller 20 includes a central processing unit (CPU) 22, a storage device (memory) 23, and an output device 24 in addition to the input device 21.

  The output device 24 is directly connected to a solenoid valve 26 that drives and controls the pressure pin cylinder 12 through an amplifier 25.

  Reference numeral 27 denotes a control panel for performing various controls of the die casting machine, and is connected to the controller 20 for inputting various information such as initial setting of the optimum stroke So of the pressure pin 11.

  As shown in FIGS. 8 (a) and 8 (b), the pressurizing pin 11 is controlled when a start timer 201, which will be described later, is set when the injection pressure P reaches the set pressure Po, or when the pressure increase is switched in this diagram. Operate. When a predetermined start timing time To elapses and the time is up, a start signal is output, the solenoid valve 26 is switched, the pressurizing pin cylinder 12 is operated to start the pressurizing pin 11, and a squeeze is performed. It is.

  FIG. 7 shows a control block diagram of the controller 20.

  That is, the stroke detection sensor 14 detects the actual stroke Sa of the pressure pin 11 and feeds back the detected stroke Sa to be within the allowable range So ± α of the optimum stroke So stored in the storage device 23. Based on the comparison result, the start timing time correction means 200 adds and subtracts the minute time unit ΔT stored in the storage device 23 to the set value To of the start timing time and corrects it for the next step. Move.

  The corrected start timing time is set as the time-up time of the start timer 201. When the injection pressure P reaches a predetermined injection pressure Po stored in the storage device 23, the start timer 201 is set and the corrected start timing is set. A start signal is transmitted to the solenoid valve 26 after the time To elapses. Further, the pressure pin cylinder 12 is driven to detect the stroke of the pressure pin 11, feed back the detected value, and the injection stroke Sa of the pressure pin 11 is the allowable range So of the optimal injection stroke So by repeating the injection process sequentially. It automatically controls to be within ± α.

  Hereinafter, drive control of the pressure pin will be described based on the flowchart of FIG.

  First, the injection pressure Po, the optimal stroke So, the allowable range ± α of the optimal stroke So, the code No. A start timing time To which is a time-up time of the start timer 201, a minute time unit ΔT which is a correction unit of the start timing time To, and the number n of shots to be corrected data are preset (step 1), and an injection operation is started. (Step 2).

  Then, the injection pressure P detected by the injection pressure sensor 10 is input from the input device 21 of the controller 20, and when the injection pressure P reaches a predetermined pressure Po preset in the storage device 23, the start timer 201 is set ( Steps 3, 4).

  Next, it is determined whether or not the start timer 201 has timed up (step 4). When the time is up, a start signal for starting the pressurizing pin 11 is sent to the solenoid valve 26 through the output device 24. As a result, the solenoid valve 26 is switched to drive the pressure pin cylinder 12, and the pressure pin 11 is started to start squeezing the molten metal 4 in the mold 1 (step 5).

  The squeezed molten metal 4 is in the middle of solidification, and as the solidification proceeds, the pressure pin 11 does not advance any further and stops. The pressure pin stroke Sa at the time of the stop is detected by the stroke detection sensor 14 and compared with the optimum stroke So stored in the storage device 23 in the central processing unit 22, and the detected stroke Sa is allowed for the optimum stroke So. It is determined whether it is within the range So ± α (step 7).

  If the detected stroke Sa is within the allowable range, the initially set start timing time To is stored in the storage device 23 (step 10), and the next injection process is started.

  If the detected stroke Sa is not within the allowable range, the start timing time To is corrected and the next injection process is started. The start timing time is corrected by adding or subtracting a preset minute time unit ΔT to To (step 9).

  That is, when the stroke Sa of the pressurizing pin 11 is smaller than the allowable range So-α of the set value So, the solidification is completed before the pressurizing pin 11 reaches the set stroke So. ΔT is subtracted from the start timing time To so that the timing of ejecting the pin 11 is advanced.

  On the other hand, when the stroke Sa of the pressure pin 11 is larger than the allowable range So + α of the set value, the squeeze is applied before the portion to be squeezed is solidified, so that the start timing time is delayed. ΔT is added to To.

  In this way, the set stroke gives an allowable range ± α in the preset value So, and the controller 20 constantly applies feedback so that the detected actual stroke Sa of the pressure pin 11 falls within that range. By providing the display means 202, the detection stroke Sa can be displayed by digital display or the like. If the detection stroke is displayed in this manner, comparison evaluation with an actual product can be easily performed.

  Here, the correction process is not limited to one shot, but may be performed every several shots. In this example, the number of shots n as data is input, and the stroke determination process in step 7 and the start in step 9 are performed. A discrimination process for discriminating the number of shots is provided between the timing time correction process (step 8). For example, when n is set to 1, the start timing time To of the start timer 201 is corrected for each shot, and when n is set to a plurality of, for example, 3, for example, based on the data of the previous two shots every three shots Thus, the start timing time To is corrected.

  In addition, a certain criterion is provided, and the start timing time of the start timer 201 at that time is stored in advance in the storage device 23, and the value of the timer is stored for each mold or for each casting condition in the same mold. Therefore, the value is used as the initial value at the next casting.

  In the above example, the start timing time of the pressurizing pin 11 is determined based on the pressure of the injection cylinder 9, but other oil pressure, injection speed, injection stroke, strain amount of the mold and the extrusion pin, etc. are started. It may be a reference for timing determination.

  In the above-described example, the stroke detection sensor 14 is provided in the pressure pin cylinder 12, but a precise detector must be provided inside the mold. Japanese Patent Application Laid-Open No. 8-132211 discloses an unnecessary method of a stroke detector using another device. In this method, as shown in FIG. 4, a flow rate counter 100 is provided for counting the flow rate of the pressure oil supplied to the pressure pin cylinder 12 in a predetermined unit, and the counter value of the flow rate counter 100 is measured. Thus, the stroke amount of the pressure pin 11 is indirectly measured.

  Japanese Patent Laid-Open No. 8-132211 discloses that when the difference between the target stroke of the pressure pin and the measured value exceeds a predetermined amount, the timing for driving the pressure pin is changed, or the pressure is Methods such as changing the pressure P of the oil or changing the flow rate Q of the pressure oil are disclosed.

  FIG. 9 shows a control block when the pressure pin cylinder is controlled using the physical quantities T, P, and Q described above, and particularly, a portion related to correction amounts of ΔP, ΔQ, and ΔT is indicated by a chain line portion Z. FIG. 10 is a time chart for explaining the operation of the pressurizing pin at that time, and FIG. 11 is a diagram when the injection pressure P reaches the set pressure P1 to show the control state of the pressurizing pin that approximately follows the coagulation contraction. The stroke waveform S from when the pressure pin starts to move after a lapse of time T until the desired stroke end is reached is shown. In the stroke control of the pressure pin described above, if there is a difference between the stroke amount and the target value, in other words, the correction when the stroke position does not fall within the predetermined range is performed in the Z part of FIG. As shown, unit correction amounts ΔT, ΔP, and ΔQ are set in advance, and these values are used as unit amounts so as to gradually approach the target stroke, and the deviation amount is simply calculated by one shot measurement. Even if the deviation amount was given all the next time, it did not mean that it converged to the target value. This is because the solidification process of the molten metal filled in the cavity is complicated, and the temperature lowering process and volume shrinkage process are not fully understood. It is inevitable from trying to control.

  Therefore, as shown in FIG. 9, it is practical to provide ΔT, ΔP, and ΔQ so as to approach asymptotically. However, this requires a large number of shots to reach the target stroke value from the initial setting as shown in FIG. (The figure shows an example in which the asymptotic operation is performed 10 times or more to reach the target range.

JP-A-8-132211

  An object of the present invention is to solve the problem that a large number of shots are required in controlling the result of each shot for trial molding to reach an allowable target value or an allowable range by the asymptotic method described above. Is.

In order to solve the above-described problem, a pressure pin drive control method according to the present invention includes:
After a molten metal is injected and filled into a pair of mold cavities formed corresponding to a desired casting, a pressure pin provided so as to be able to advance and retreat into the cavity is pressed in accordance with the cooling and solidification process of the molten metal in the cavity. In the method of driving control to push in by a pin cylinder,
A first step of setting a target range of the pressure pin stroke amount when a desired casting is formed by the molten metal previously filled in the cavity;
Any of waiting time T from when at least the injection pressure reaches a predetermined set value until driving of the pressure pin is started, supply flow rate Q to the pressure pin cylinder, supply pressure P to the pressure pin cylinder A second step of specifying a value of a control amount related to driving of the pressure pin corresponding to the one;
A third step of performing trial molding of the cast product one or more times based on the control amount value specified in the second step and measuring an actual stroke value of the pressure pin for each molding;
When the actual stroke value of the pressure pin given by the measurement in the third stage is within the target range, the value of the control amount specified in the second stage is controlled for production molding for the cast product. A fourth stage to hold as a quantity value;
When the actual stroke value of the pressure pin given in the third stage belongs to one region outside the target range, the actual stroke value in the third stage based on the next new control amount value is at least the previous time. The control value specified in the target range and the second stage is set to the next new control amount value so as to be the other region outside the target range different from the region outside the target range to which the actual stroke value belongs. A fifth step of identifying based on the value of the quantity, and the value of the maximum allowable control amount or the value of the minimum allowable control amount;
When the actual stroke value obtained as a result of executing the third stage using the next new control amount value specified in the fifth stage becomes the other region outside the target range. Is a sixth step of executing the third step by specifying an intermediate value obtained from the previous control amount value and the previous control amount value as the next new control amount value;
When the actual stroke value obtained as a result of the sixth stage is within the target range, the value of the specified control amount is held as the value of the control amount for production molding for the cast product, and is also obtained. When the actual stroke value is outside the target range, a new control amount is specified by selecting one of the fifth and sixth steps corresponding to the region outside the target range to which the actual stroke value belongs. 7 stages.

  In that case, the identification of the intermediate value as the next new control amount in the sixth stage is performed by calculating the difference between each actual stroke value and the target value based on the previous control amount and the previous control amount. It may be configured to have a step of calculating by multiplying each value of the previous control amount and the previous control amount by the calculated ratio of the difference.

  According to the present invention, at least one of the control amounts T, Q, and P is gradually narrowed so as to gradually converge to the target value so that the target value or target range of the pressure pin stroke is gradually reached. Significantly reduce the number of trial molding operations when setting the pressure pin stroke for new castings and ensure that the control amount corresponding to the target stroke amount or range is reached without requiring special skills can do.

  An embodiment of the present invention will be described below with reference to FIGS.

  FIG. 1 is substantially the same as FIG. 6 in describing each component, and only the differences will be described below. That is, in FIG. 6, the stroke sensor 14 is attached to the cylinder 12 for the pressurizing pin 11, but in FIG. 1, there is no stroke sensor 14, and therefore the input device 21 has a flow rate counter instead. A count value of 100 is input for each shot.

  FIG. 2 is a flowchart for explaining the process of the present embodiment, and FIG. 3 is a graph for explaining the transition of the actual stroke amount of the pressure pin for each shot.

  FIG. 2A shows a case where the flow rate Q is used among the control amounts T, P, and Q. In step ST1, a target value or an allowable range of the stroke amount of the pressure pin is set. Next, in step ST2, an allowable minimum value and a maximum value of each control amount T, P, Q are set. Further, a predetermined injection pressure value, that is, an injection pressure attainment value as a condition for starting driving of the pressure pin is set in the same manner. The target value or the allowable range means a target range in the present invention.

  Next, in step ST3, it is determined whether or not Q is selected as the control amount. If NO in step ST3, it is further asked whether the control amount T is selected in step ST4. As a result, when the determination is NO, the control amount processing routine of step ST5 is entered. If YES, the program enters the T routine at step ST6. Here, since the control amount Q is selected, the details of the P and T routines are omitted. Hereinafter, a portion corresponding to the Q routine surrounded by a one-dot chain line will be described.

  In step 7, an initial value Q1 is set as the control amount Q. The initial value Q1 is now set to 50% of the flow rate corresponding to the control valve specification, that is, the maximum flow rate. Next, in step ST8, the value of index i is set to i = 1. Next, in step ST9, it is checked whether or not there is a trial molding start command. If there is no start command, a predetermined time is waited in ST10. When there is a start command, a shot operation for actual trial molding is performed in step ST11 (here, one shot is executed). Next, in step ST12, the pressure pin stroke amount L (i) is calculated from the count value of the flow meter during the shot operation. In step ST13, it is checked whether or not the stroke amount L (i) is within the target range. In step ST14, the control amount Q1 corresponding to the target and range is stored in the memory.

  Here, in the case of NO in step ST13, it is asked whether or not it is the first shot in step 15. Here, at the first shot, the index i is changed to i = i + 1 in step ST16, and a new value of the control amount Q is calculated in step ST18 corresponding to whether the target value is larger or smaller.

  In step ST15, when the second shot or later, the index i is set to i = i + 1 in step ST17, and then in step ST19, the control amount Q (i + 1) is newly calculated from the control amount commanded last time.

  The new control amount Q (i + 1) calculated in steps ST18 and ST19 executes the shot operation in step ST11 via the junction B. By repeating such a loop from the junction points A to B and from the junction point B to A several times, the stroke L (i) reaches the target range. In the flowchart of FIG. 2A described above, step 19 is executed after the second time in step ST15 for initial determination.

  In other words, the case of approaching asymptotically to the target range most effectively by doing in this way was illustrated. In this example, as shown in FIG. 3, even when the actual stroke amount of the pressure pin is outside the target range, each adjacent stroke amount may be alternately in a region opposite to the target range. It should be noted that this is a premise.

  Because of this premise, it is meaningful to use the previous and previous control amounts in step ST19.

  On the other hand, the actual stroke amount of the pressurizing pin includes the value of the initial control amount, other physical quantities related to the molten metal solidification process, for example, the start timing time T, the injection pressure P related to the start of the same time T, the molten metal It is closely related to temperature, viscosity, cavity volume, etc., and it is not always guaranteed that the actual stroke amount is alternately in the region opposite to the target range after the second time.

  Therefore, FIG. 2B shows a corresponding process in the case where the actual stroke amount is in the region on one side outside the target range both continuously, for example.

  In FIG. 2B, step ST17A for determination is provided after step ST17 in FIG. 2A, and the actual stroke amount outside the target range for the second and subsequent times continuously belongs to a region outside the same target range. Whether the region is on the same side, i.e., NO, indicates that the control amount is generated in step ST18 only when the region is different, i.e., YES. In this case, the number of asymptotic trials increases as compared with the case of FIG. 2A, but the involvement of know-how and the like can be further reduced in specifying the control amount. More specifically, in FIG. 2B, for example, the initial actual stroke value corresponding to the control value of the initial value is in the lower region of the target range. A new control amount is specified based on the amount and the control value of the initial value, and the value of the second actual stroke based on the specified control amount value is also in the same lower region of the target range. In this case, NO is determined in step ST17A, and a new control amount value is generated again in step ST18. In that case, the allowable maximum control amount is used as in the previous case. The value of the other control variable can be the value of the control variable specified next to the initial value.

  FIG. 3 shows a specific example. In the figure, the initial value Q1 is 50% as described above. As a result of the first shot, it is assumed that the stroke L (1) of the pressure pin has not reached the target stroke range. From the result of the first shot, it is necessary to increase the flow rate in the second shot. As the value, 75%, which is intermediate between the maximum flow rate of 100% and the previous value of 50%, is given as the next command value control amount Q2. If the stroke amount L (2) of the second shot at this time exceeds the target range as shown in the figure, the next shot is 62.5%, which is an intermediate value between the previous 75% and the previous 50%. Command value control amount Q3. Even with this command value Q3, the stroke amount L (3) has not yet reached the allowable range. Next, 68.75%, which is an intermediate value between the previous command value 62.5% and the previous command value 62.5%, is commanded. Since the stroke amount L (4) at that time exceeds the target range, the command value control amount Q5 of 65.625% is given as the next shot command value. . In the figure, the stroke amount of the pressurizing pin at the command value control amount Q5 is within the target range and is in a normal stroke state.

  Although one embodiment of the present invention has been described with reference to FIGS. 1 to 3, the present invention is not limited to this embodiment, and the following modifications should be included in the spirit of the present invention. That is, in the above example, the flow rate Q to the pressure pin cylinder is taken as an example of the control amount, but the waiting time T and the supply pressure P to the pressure pin cylinder may be targeted.

  In addition, in the example shown in FIG. 3, the new control amount value is simply an average of the previous and previous values as an intermediate value calculation, but each actual amount based on the previous control amount and the previous control amount value is used. It is also possible to calculate the difference value between the stroke value and the target value and weight the previous control amount and the previous control amount using the ratio of the difference.

  Specifically, when the previous control amount X and the previous control amount Y are set, the corresponding actual stroke amounts are 60 mm and 90 mm, respectively, and when the target value is 65 mm, the previous actual stroke amount and the target value. The difference is 5 mm, and the difference from the last time is 25 mm.

The ratio of the difference is 1: 5, and the ratio is 1/6, 5/6. Therefore, the new control amount Z is
Z = 5/6 ・ X + 1/6 ・ Y = (5X + Y) / 6
And In this case, the control amount corresponding to the control amount X, Y that is closer to the target value is X, and the weight is 5/6 for X and 1/6 for Y. In this way, the asymptotic number of trials can be more effectively reduced. By the way, when calculating a new control amount Z simply as an average of the previous and previous values,
Z = (X + Y) / 2
It is.

It is a principal part block diagram of the apparatus for performing the method of this invention. It is a flowchart explaining the method of this invention, Comprising: (a) shows the example which the actual stroke amount of the pressurization pin which made the frequency | count of trial molding comparatively small converges in a target range, (b) shows actual stroke amount. It is a partial flowchart which shows a process when becomes continuously the same area | region outside a target range. It is a figure which shows a mode that the stroke amount of a pressurization pin converges in the tolerance | permissible_range of a target value when the method of this invention shown to Fig.2 (a) is performed. It is a block diagram of the stroke control apparatus of the conventional pressurization pin. It is a flowchart explaining the control process in FIG. FIG. 5 is a detailed view of a position sensor unit of the system of FIG. 4. FIG. 5 is a control block diagram of the system of FIG. 4. It is a wave form diagram explaining operation | movement of the system of FIG. 4, Comprising: (a) is a wave form diagram of injection pressure, (b) is a wave form diagram of a pressurization piston stroke, respectively. It is a control block diagram of one Example of the conventional system which gives the fixed value increment (DELTA) P, (DELTA) Q, and (DELTA) T of control amount P, Q, and T. FIG. FIG. 10 is a waveform diagram of a pressing pin stroke with respect to time after filling in one shot in the conventional method shown in FIG. 9. It is a figure which shows the injection pressure of the whole shot, injection speed, and pressurization pin stroke in the conventional system shown in FIG. It is a figure which shows the arrival status to the target stroke of the pressurization pin by the conventional system shown in FIG.

Explanation of symbols

1 Mold 2 Cavity 3 Sleeve 4 Molten Metal 10 Injection Pressure Sensor 11 Pressure Pin 12 Pressure Cylinder 100 Flow Rate Counter

Claims (2)

After a molten metal is injected and filled into a pair of mold cavities formed corresponding to a desired casting, a pressure pin provided so as to be able to advance and retreat into the cavity is pressed in accordance with the cooling and solidification process of the molten metal in the cavity. In the method of driving control to push in by a pin cylinder,
A first step of setting a target range of the pressure pin stroke amount when a desired casting is formed by the molten metal previously filled in the cavity;
Any of waiting time T from when at least the injection pressure reaches a predetermined set value until driving of the pressure pin is started, supply flow rate Q to the pressure pin cylinder, supply pressure P to the pressure pin cylinder A second step of specifying a value of a control amount related to driving of the pressure pin corresponding to the one;
A third step of performing trial molding of the cast product one or more times based on the control amount value specified in the second step and measuring an actual stroke value of the pressure pin for each molding;
When the actual stroke value of the pressure pin given by the measurement in the third stage is within the target range, the value of the control amount specified in the second stage is controlled for production molding for the cast product. A fourth stage to hold as a quantity value;
When the actual stroke value of the pressure pin given in the third stage belongs to one region outside the target range, the actual stroke value in the third stage based on the next new control amount value is at least the previous time. The control value specified in the target range and the second stage is set to the next new control amount value so as to be the other region outside the target range different from the region outside the target range to which the actual stroke value belongs. A fifth step of identifying based on the value of the quantity, and the value of the maximum allowable control amount or the value of the minimum allowable control amount;
When the actual stroke value obtained as a result of executing the third stage using the next new control amount value specified in the fifth stage becomes the other region outside the target range. Is a sixth step of executing the third step by specifying an intermediate value obtained from the previous control amount value and the previous control amount value as the next new control amount value;
When the actual stroke value obtained as a result of the sixth stage is within the target range, the value of the specified control amount is held as the value of the control amount for production molding for the cast product, and is also obtained. When the actual stroke value is outside the target range, a new control amount is specified by selecting one of the fifth and sixth steps corresponding to the region outside the target range to which the actual stroke value belongs. And a step of controlling the driving of the pressure pin.   The intermediate value as the next new control amount in the sixth stage is determined by calculating the difference between each actual stroke value and the target value based on the previous control amount and the previous control amount value. 2. The pressure pin drive control method according to claim 1, further comprising the step of calculating the weight of the previous control amount and the previous control amount based on the difference ratio.