JP4226498B2 - Plunger injection start position control method - Google Patents

Description

  The present invention relates to an injection molding method such as injection pressure molding, and more particularly, to a plunger injection start position control method for correcting and controlling the injection start position of a plunger in a cylinder after the molding material is metered before the next injection.

  As a main processing method in resin molding, injection molding has spread and developed together with hollow molding and extrusion molding. In this injection molding, the molding material (resin) is heated and softened (plasticized) in a cylinder, then injected into a mold closed at a high pressure, cooled and solidified in the mold, and finally the mold is molded. The mold is opened and the finished molded product is taken out.

  Various injection molding apparatuses have been developed for molding molded products. As this injection molding apparatus, for example, a pre-plastic method in which a screw for supplying a molding material while melting it is provided outside the cylinder, an in-line method in which a screw is provided integrally on the distal end side of the plunger inside the cylinder, etc. It is known (see, for example, Patent Document 1).

Here, a general pre-plastic injection molding apparatus will be described with reference to FIG.
The injection molding apparatus 100 includes a cylinder 101 having an injection port 101A at one end and slidably receiving a plunger 102 therein, and a driving means 103 such as a ball screw and a motor for moving the plunger 102 inside the cylinder 101; And a plasticizing device (barrel) 104 provided with a screw 105 for supplying a molding material supplied from a hopper 104B to the cylinder 101 with a supply port 104A communicating with the cylinder 101.

  The injection port 101A of the cylinder 101 communicates with the cavity 106A of the mold 106 through a gate and is provided with a load cell 102A at the rear end of the plunger 102. The plunger 102 is provided with a check valve 102A (not shown) that slides on the inner circumferential surface of the cylinder 101 so as to be movable within a predetermined range.

  In this injection molding apparatus, as shown in FIGS. 5 and 6A, on the mold 106 side, the molding material in the cavity 106A that has already been injected and injected is cooled and solidified (cooling step). The mold 106 is opened (mold opening process), the molded product in the cavity 106A is taken out (extraction process), and then the mold 106 is closed again (mold closing process).

  In this way, when the molded product is taken out from the mold 106 and the cavity 106A is emptied, the molding material that has already been metered into the cylinder 101 during the aforementioned cooling step is added by the forward movement of the plunger 102. The molding material is injected and injected into the cavity 106A communicated with the injection port 101A (injection process). Thereafter, the internal pressure of the cylinder 101 is controlled (pressure holding step), the plunger 102 is then retracted to generate a negative pressure, and the screw 105 is rotated to measure and supply the molding material again (metering step). In the following, a large number of molded products can be continuously produced by repeating the same steps.

By the way, in recent years, it has been performed to mold a thin disk or lens using a transparent plastic resin material, for example, a molding material such as PMMA. As a method of molding such a thin product with high accuracy, Injection compression molding is known.
In this injection compression molding, the plate thickness accuracy as a molded product and the quality as a product vary greatly depending on whether or not the molding material to be injected can be accurately measured. For this reason, the molding material to be used for injection is accurately determined in the metering process and the necessary amount to be injected is determined. Therefore, if there is an error in the volume at the time of injection (this is referred to as “injection amount”), for example, when a disk such as a DVD is formed, variation in residual stress occurs, which is birefringent in the disk. There are problems such as bringing about sex.

  As a cause of such variation in the injection amount of the molding material, for example, leakage of the molding material from a gap generated between the cylinder and the check valve of the plunger due to a differential pressure generated inside and outside the cylinder is known. Therefore, a control method for preventing this has been proposed.

That is, as shown in FIG.
(1) First, after completion of the weighing process (see (A) in the same figure) (when performing a suckback (see (D) in the same figure) described later), the resin is weighed. The plunger is temporarily retracted until the pressure in the supplied reservoir is nearly zero (for example, 1 MPa in the case of the present embodiment) (see FIG. 5B).
(2) Thereafter, the plunger is moved forward again, and the pressure in the storage portion that has been metered in is increased to a predetermined reference pressure (referred to as “relative position detection pressure P ₀ ”) ((C )reference). As a result, leakage of the molding material from the gap to the outside can be prevented. This process is called “relative position detection control process”, and this control is called “relative position detection control” (for example, patents). Reference 2).

Usually, when this relative position detection control is performed, the plunger 102 is once retracted until the internal pressure of the cylinder 101 approaches almost zero, as shown in FIG. 8A, for example (see FIG. 8A). As shown in FIG. 5B, the pressure is increased to the aforementioned predetermined reference pressure (this is referred to as “relative position detection pressure P ₀ ”). Then, with reference to the plunger position at this time (referred to as “relative position origin X ₀ ”), the injection operation is started again from there. Therefore, “servo lock control” is generally performed after the relative position detection process until the injection process starts until the operation of the plunger 102 is locked at the relative position origin.
The resin pressure is increased to a predetermined value (relative position detection pressure P ₀ ) by sealing the gap between the cylinder 101 and the check valve 102A, thereby preventing the occurrence of a gap that causes resin leakage. Because.
JP-A-10-58501 Japanese Patent No. 2615948

In the case of performing injection molding with a servo-lock control, while to the start of injection operation for injecting a molding material into the cavity of the mold, thereby locking the movement of the plunger in the relative position origin X _0. Therefore, following injection operation by the plunger, but it is made to start from this relative position origin X _0, the relative position origin X ₀ is the position at the inner pressure fluctuations of the cylinder is changed subtly each time.

  Therefore, the plunger waiting at the relative position origin changes the start stroke every time the injection operation is started (see FIG. 6B), thereby causing variations in the injection stroke. As a result, the injection time varies, and the injection amount of the molding material injected into the mold may also vary. For this reason, the injection quantity is not stable. In addition, since the mold is closed at the timing of this injection operation, the filling alignment is not successful due to the timing shift, and the thickness of the molded product may vary.

  By the way, in the above-described conventional injection molding machine, when the pressure (internal pressure) of the molding material supplied to the reservoir for molding the next molded product is normal as shown in FIG. If the pressure is too high compared to the internal pressure (see FIG. 10B), the molding material is pushed into the cavity from the tip of the injection port of the cylinder, resulting in a so-called “hanging” state, A flow mark (a drooping droop that hardens) is generated.

  On the other hand, if the internal pressure of the reservoir is reduced, drooping and flow marks will not occur. However, if the internal pressure is reduced too much compared to the normal internal pressure (see FIG. 10C), As the molding material used at the time of molding becomes uncertain, the "void (bubble)" is generated, or the bubble is ruptured and "jetting" (the void is ruptured, or the silver stream) is generated. This is a cause of product defects.

  Therefore, after the weighing operation (weighing process), the internal pressure is adjusted to be slightly higher than the time of weighing, but the internal pressure is increased to prevent air entrainment. There is also known a control method for performing so-called “suck back” in which the plunger is further moved backward from the position when pushed back backward. However, even when this suck back is performed, the start position of the plunger waiting at the relative position origin changes every time the injection operation is started, as shown in FIGS. .

  Therefore, after the measuring step or after the relative position detecting step, as shown in FIGS. 11 (A) to (D), the occurrence of these phenomena is controlled by controlling the pressure (internal pressure) of the storage portion of the cylinder to a low pressure. Methods to suppress it are also being studied. However, even in such a low-pressure control method, since the internal pressure value at the time when the low-pressure control is finished varies, the injection start position varies. Therefore, even in the case of this low pressure control, the plunger stroke and the injection time vary during injection.

Further, the relationship between the plunger position and the cylinder internal pressure will be described in detail in the section of [Best Mode for Carrying Out the Invention], but it is unique to a quadratic function or an exponential function as shown in FIG. It is known that there is a correlation indicating a correlation function P = F (X). According to the graph shown by this function, especially when the pressure (P) is low as in the above-described low-pressure control, the fluctuation range of the plunger position becomes considerably large even with a slight pressure fluctuation (variation per unit pressure).
Therefore, when the above-described low-pressure control is performed, the variation of the plunger position becomes considerably large, so that this stroke also varies more greatly and the injection time also varies more greatly. As a result, it becomes difficult to obtain a highly accurate molded product such as variations in the thickness of the molded product.

  In view of the circumstances described above, an object of the present invention is to provide a plunger injection start position control method that can hold the injection time constant each time, and thus can form a highly accurate molded product. To do.

In the plunger injection start position control method according to the present invention, the molding material is measured and supplied into the cylinder, and a predetermined relative position detection operation is performed in the cylinder to detect the plunger relative position, and then the molding material is injected. Prior to, a plunger injection start position control method for correcting and controlling the injection start position of the plunger in the cylinder, the relative pressure being a pressure in the cylinder when the plunger is advanced in the relative position detection operation From the relative position origin, which is the plunger position at the position detection pressure, the plunger position is moved by an offset value calculated in advance based on the correlation between the plunger position and the cylinder internal pressure, and the injection start position is adjusted. The molding material is injected from the injection start position.

In the plunger injection start position control method according to the present invention, the cylinder in which the offset value is the injection start position from the relative position origin in a unique correlation function that satisfies the relationship between the plunger position and the cylinder internal pressure. This is a configuration in which the plunger position is subtracted when the injection start position pressure is a constant value near zero.

  Further, in the injection start position control method of the plunger of the present invention, the injection start position pressure of the cylinder when the plunger is at the injection start position does not cause the molding material to droop from the cylinder, And it is the structure which exists in the specific pressure range which does not flow in air from the outside in the said cylinder.

  According to the present invention, since the injection time can be kept constant every time, the timing of the injection on the cylinder side and the filling / mold closing on the mold side can be achieved, and as a result, highly accurate molding It is possible to provide a method for controlling the injection start position of a plunger capable of molding an article.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Prior to describing the injection start position control method for the plunger of the present invention, first, an injection molding machine to which this method is applied will be described.
FIG. 1 shows a main part of a pre-plastic type injection molding machine to which the plunger start position detection control method of the present invention is applied. This injection molding machine 1 is roughly composed of a cylinder 2 and a barrel 3. And a mold 4 and a control unit 5.

  The cylinder 2 has a nozzle 2A serving as an injection port at the tip (lower end in FIG. 1), and accommodates a plunger 21 therein so as to be movable in the front-rear direction (vertical direction in FIG. 1). In addition, a cylinder head 2B is provided at the front end side of the cylinder 2, and a check valve 2C is provided in the cylinder 2 at a constricted (small diameter) portion between the cylinder head 2B and the main body. The outer peripheral surface of the plunger 21 is slidably fitted to the peripheral portion of the plunger 21 so as to be slidable. Note that the internal space on the tip side of the cylinder 2 is filled with the resin material supplied from the barrel 3 side through the check valve 2C, in other words, the storage portion 2D for temporarily storing the resin material. Is configured.

  The plunger 21 includes a load cell 22 that physically detects a pressure (internal pressure) in the storage portion 2D from a pressure acting on the plunger 21 at a rear end portion (upper end portion in FIG. 1), and a plunger 21 via a connecting member 23. , A slider 24, a ball screw 25 in which the female screw portion of the slider 24 is screwed, and a plunger 21 that slides forward and backward by rotationally driving the ball screw 25. Servo motor 26 to be operated.

  The slider 24 is inserted into a guide pin 20 </ b> A installed between the cylinder 2 and the fixed frame 20 and is prevented from rotating so as not to rotate.

  The control unit 5 inputs a signal from the encoder 261 connected to an output shaft (not shown) of the servo motor 26 and a signal from the load cell 22 that is output according to the pressure in the storage unit 2D (hereinafter referred to as internal pressure). On the other hand, a control signal is output to the servo motor 26 for control. That is, the control unit 5 can drive and control the servo motor 26 so that the internal pressure of the storage unit 2C detected by a signal from the load cell 22 becomes a predetermined value. In addition, the encoder 261 is configured to calculate the amount of movement of the plunger 21 and the (current) position in the cylinder 2 by the control unit 5 measuring the number of rotations based on a signal output from the encoder 261. .

On the other hand, the barrel 3 is provided with a supply port 3 </ b> A communicating with the cylinder 2 at the tip, and a hopper (not shown) and a screw 31 for supplying the resin material charged from the hopper to the cylinder 2.
The mold 4 is configured to take out a molded product from a cavity 4 </ b> A formed between the lower mold 41 and the upper mold 42 by moving the lower mold 41. The cavity 4A of the mold 4 communicates with an injection port at the tip of the nozzle 2A of the cylinder 2 through a gate (not shown).

Next, an injection compression molding method to which the injection start position control method of the plunger of the present invention is applied will be described in detail using the aforementioned pre-plastic injection molding machine 1 shown in FIG.
FIG. 2 is a time chart showing an injection compression molding method to which the injection start position control method of the plunger of the present invention is applied. In the injection compression molding method of this injection molding machine, an injection process, a pressure holding process, a cooling process (simultaneously The next measurement process, the relative position detection process, and the suck back process are performed in parallel, and the mold opening process, the removal process, and the mold closing process constitute one cycle. In FIG. 2, the description from the injection process to the pressure holding process is omitted.

The steps from the injection step to the cooling step in the present invention are the same as those in the prior art described in the section “Background Art”. In addition, the relative position detection process and the suck back process also perform the same operation as before. Further, pressure control and servo lock control are performed immediately after the suck back process and immediately before the next injection process.
That is, in order to maintain the inside of the cylinder 2 at a low pressure within a predetermined range from the latter half of the cooling process to the mold opening process and the unloading process from when the relative position detection process by speed control is completed. Next, the pressure inside the cylinder 2 is controlled. Specifically, sequence control is performed. Further, in this embodiment, the servo lock control for locking the plunger 21 without moving back and forth is performed only in the mold closing process (however, immediately before the injection process, the servo lock control is terminated and the plunger is injected. Start position control).

In the sequence control, based on the internal pressure of the cylinder 2 detected via the load cell 22, when the detected internal pressure of the cylinder 2 rises to a certain value, the internal pressure is reduced to a certain value.
That is, in FIG. 1, the ball screw 25 is rotated by the servo motor 26, and the plunger 21 is pulled up via the slider 24 screwed into the screw. For this reason, when a signal from the load cell 22 is input, the control unit 5 outputs a control signal to the servo motor 26 to control the servo motor 26. Here, the internal pressure of the cylinder 2 increases mainly due to the next expansion of the molding material in the storage part of the cylinder 2. Note that the pressure range during the low pressure control is set to a range in which no drooping, flow marks, voids, jetting, or the like is observed.

  In the present invention, “plunger start position control” is performed to correct the plunger position by a required offset amount (injection start position correction) from the final plunger position (that is, relative position origin) in the relative position detection step. It has become.

This plunger injection start position control is performed after the next molding material is measured while the molded product in the mold is cooled, and before the next injection (after detecting the relative position of the plunger and sucking back). The “injection start position” of the provided plunger 21 is corrected and controlled.
That is, the injection start position control of the plunger of the present embodiment is performed after the molding material is measured (metering process) and immediately before the next injection (injection process) is started,

  (1) First, an offset value (Xoff) is calculated in advance using the correlation function P = F (X) in FIG. 3 showing the correlation between the position of the plunger and the cylinder internal pressure (first step S1). ).

Note that this offset value is the relative position detection pressure value P ₀ (for example, 5 MPa) described later in the pressure value (P) of the reservoir 2D in the inherent correlation function P = F (X) shown in FIG. Plunger position (X ₂ : injection start position) when the pressure value (P) is a preset constant pressure value P ₂ (for example, 1 MPa: injection start position pressure) from the plunger position (X ₀ : relative position origin) at the time Minus the value, i.e.
X _off = X ₀ −X ₂ (2)
Defined in

The internal pressure value (injection start position pressure P ₂ ) at which the plunger 21 becomes the injection start position (X ₂ ) is such that the molding material does not droop in the reservoir 2D, and the inside of the cylinder 2 In this embodiment, the pressure is set to 1 MPa as described above, although it is preferable that the pressure is as high as possible. Further, this offset value (X _off ) is the correlation function described later if the cylinder 2, the plunger 21, the mold 4 and the like are the same and the molding material to be used has the same characteristics (for example, viscosity). Will be the same. Therefore, the offset value (X _off ) calculated first can be used as it is every time.

(2) Next, as shown in FIG. 4A, the internal pressure of the storage unit 2D at the end of the relative position detection step (the measured molding material is stored until the next injection start), that is, the relative position detection. detecting the relative position origin X ₀ is a plunger position corresponding to the pressure value P ₀ (second step S2). Here, the relative position detection pressure value (P ₀ ) of the present embodiment is 5 MPa (however, atmospheric pressure (1 atmosphere) corresponds to 0.1 MPa). The relative position detection pressure value (P ₀ ) is detected by the control unit 5 based on the signal from the load cell 22, and the relative position origin (X ₀ ) is determined based on the output signal of the encoder 261, and the control unit 5 determines the number of pulses. calculate.

(3) Since the relative position origin (X ₀ ) at this time is used as a part of the position correction processing data in the current injection start position control, the data about the relative position origin (X ₀ ) is controlled. The data is stored in a memory (not shown) of the unit 5 (third step S3).

  (4) A suck-back process is performed after the relative position detection process, and after this suck-back process, the low-pressure control is performed during the mold closing process as shown in FIG. 4B until the cooling process is completed. Also, in the next mold opening process, as shown in FIG.

(4) Next, just before or when the mold closing process ends, the current plunger position (X ₃ ) is calculated from the signal from the encoder, and this data is output to the control unit 5 (fourth step). S4).

(5) Then, the control unit 5 calculates in advance in the first step S1 from the relative position origin (X ₀ ) calculated in the second step S2 and stored in the memory in the third step S3. The plunger 21 is moved to the position where the offset value (X _off ) is subtracted, that is, the start position in the next injection process (where the cylinder internal pressure becomes zero (P = 0)) (fifth step S5).
That is, considering the current position (X ₃ ) when the mold closing process is completed,
ΔX = X ₃ − (X ₀ + X _off ) (3)
From this, the movement amount (ΔX) to be moved to the next injection start position (X ₂ ) and the movement direction thereof are calculated.

This moving direction is behind (the upper side in FIG. 1) from the relative position origin (X ₀ ), but differs depending on the situation at that time. In this embodiment, whether to move backward or forward is determined by the plunger position during servo lock control when the mold closing process is completed. For example, in the case of the expression (3), when the movement amount ΔX to be moved this time is positive, the movement is performed backward.

(5) Thereby, as shown in FIG. 4D, the plunger is pulled back in any direction (either up or down in the figure) by a certain offset value to correct the injection start position, and this injection start is started. Wait until the next injection operation at the position. Then, the next injection operation by the plunger is started from this position (X ₂ ) where the internal pressure of the reservoir 2C becomes P = 1 MPa.
In this way, a series of injection molding steps are repeated.

Next, a method for deriving a specific correlation function used in the present invention will be described.
This correlation function is determined by measuring the movement amount and the internal pressure based on the output signals from the load cell 22 and the encoder 261 by pushing the plunger 21 toward the tip, and using the correlation function used here. Find out.

For example, the plunger 21 is gradually pushed in from the internal pressure position of 0 (zero) MPa to pressurize it. And the data about the plunger position (movement amount) at this time and the internal pressure of the reservoir are monitored at intervals of 0.1 msec.
Therefore, since the data measurement of the position and the internal pressure is repeated several times by such a method and the average value of these is obtained, a unique correlation function P = F (X) as shown in FIG. 3 can be obtained. .
For example, when the average value of these data is calculated and the following data is obtained,
Position (mm) Pressure (MPa)
0.10
0.2 0
0.3 (= X ₂ ) 1 (= P ₂ )
0.4 2
0.5 (= X ₀ ) 5 (= P ₀ )
The relative position origin (X ₀ ) is 0.3 mm, and the relative position detection pressure value (P ₀ ) is 5 MPa. Accordingly, the offset value (X _off ) from the correlation function P = F (X) (data table) is 0.2 (= 0.5−0.3) mm.

  In addition, this invention is not limited to embodiment mentioned above at all, In the range which does not deviate from the summary, it can implement with a various form. That is, in this embodiment, pressure control and servo lock control are performed from immediately after the suck back process to immediately before the injection process, but without the suck back control and pressure control, the servo lock is performed from the relative position detection process to the mold closing process. Control may be performed.

  According to the injection start position control method of the plunger of the present invention, the injection operation can be started from a fixed position every time, and the injection time can be kept constant every time. Can be molded into a molded product with high accuracy, and is useful when an optical element such as a disk or lens is injection molded.

The block diagram which shows the principal part of the injection molding machine used for embodiment which concerns on this invention The time chart which shows the injection start position control method by the injection molding machine which concerns on embodiment of this invention The graph which shows the intrinsic | native correlation function used for the injection start position control method of this invention Process drawing which shows the movement operation | movement of the plunger by the injection start position control method of this invention Sectional drawing showing the configuration of a pre-plastic injection molding machine Time chart showing molding method by conventional injection molding machine Process diagram showing plunger movement in conventional injection molding method Process diagram showing the principle of relative position control Process diagram showing a conventional molding method using suckback control (A) and (B) are time charts showing changes in internal pressure when drooping and air entrainment occur in a pre-plastic injection molding machine, respectively. Process diagram showing conventional molding method using low pressure control

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Injection molding machine 2 Cylinder 2A Nozzle 2B Cylinder head 2C Check valve 2D Storage part 21 Plunger 22 Load cell 23 Connecting member 24 Slider 25 Ball screw 26 Servo motor 261 Encoder 27 Control part 3 Barrel 31 Screw 4 Mold 4A Cavity 41 Lower metal Mold 42 Upper mold 5 Control part P ₀ Relative position detection pressure P ₂ Injection start position pressure X ₀ Relative position origin X ₂ Injection start position Xoff Offset value

Claims (3)

After the molding material is measured and supplied into the cylinder and the relative position of the plunger is detected by performing a predetermined relative position detection operation in the cylinder, the injection of the plunger in the cylinder is performed prior to the injection of the molding material. A method for controlling an injection start position of a plunger for correcting and controlling a start position,
Based on the correlation between the plunger position and the cylinder internal pressure from the relative position origin, which is the plunger position at the relative position detection pressure that is the pressure in the cylinder when the plunger is moved forward in the relative position detection operation . An injection start position control method for a plunger that moves the plunger position by an offset value to adjust the injection start position and injects the molding material from the injection start position. The offset value is an inherent correlation function that satisfies the relationship between the plunger position and the cylinder internal pressure, and the injection start position where the pressure value of the cylinder that is the injection start position is a constant value near zero from the relative position origin. 2. The plunger injection start position control method according to claim 1, which is a value obtained by subtracting the plunger position at the time of pressure.   The injection start position pressure of the cylinder when the plunger is at the injection start position is such that the molding material does not droop from the cylinder and no air flows into the cylinder from the outside. The injection start position control method for the plunger according to claim 1 or 2, wherein the pressure is within the pressure range.

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