JP2005254532A - Pressure control method of injection molding machine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure control method of an injection molding machine capable of preventing the occurrence of dripping or a flow mark and not causing voids (bubbles) or jetting (or silver streams). <P>SOLUTION: In the control method of the injection molding machine for injecting a plasticized molding material in a mold 4 by a plunger 21, low pressure control for holding the pressure in a storage part 2C for allowing a next molding material to stand by to a preset pressure value or below is performed in a process for allowing the next molding material after metering to stand by during a period from the cooling of the molded product in the mold 4 to the taking-out of the molded product after mold opening. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a pressure control method for an injection molding machine.

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

  Various injection molding apparatuses have been proposed and developed for molding molded products. As this injection molding apparatus, a pre-plastic system in which a screw for melting a molding material is provided outside a cylinder, an in-line system in which a screw is integrally provided on the tip side of a plunger inside the cylinder, and the like are known (for example, Patent Document 1).

  Here, a pre-plastic injection molding apparatus will be described with reference to FIG. This injection molding apparatus 100 has a cylinder 101 having an injection port 101A at one end and slidably receiving a plunger 102 therein, a driving means 103 such as a motor for moving the plunger 102 inside the cylinder 101, and a cylinder 101. A plasticizer (barrel) 104 having a screw 105 for supplying a molding material supplied from a hopper 104B to a cylinder 101 is provided at a leading end (lower end in the figure) with a supply port 104A communicating with the cylinder. The injection port 101A of the cylinder 101 communicates with the cavity 106A of the mold 106 through a gate, and a pressure sensor 101B is provided on the inner wall surface of the cylinder 101 in the vicinity of the injection port 101A.

In this injection molding apparatus, as shown in FIG. 6, pressure control in the cylinder 101 is performed by the pressure sensor 101B 107 (pressure holding process), and a required amount of molding material is supplied to the storage portion 101C in the cylinder 101. Then, the plunger 102 is retracted to a predetermined position inside the cylinder 101 (relative position detection step). In this injection molding apparatus, as shown in FIG. 5, a relative position detection step is provided after the weighing step (for details of the relative position detection step, see, for example, Patent Document 2). .)
On the other hand, in the mold 106, the molding material in the cavity 106A is solidified by cooling (cooling process), then the mold 106 is opened (mold opening process), and the molded product inside the mold 106 is taken out (extraction process). Then, the mold 106 is closed again (mold closing process).

  In this way, when the cavity 106A of the mold 106 is emptied, the next molding material supplied into the cylinder 101 is pressurized by the forward movement of the plunger 102 and communicated with the injection port 101A. The molding material is injected and injected into the cavity 106A (injection process). Thereafter, the pressure inside the cylinder 101 is controlled, the plunger 102 is moved backward, and the molding material is supplied again from the screw 105 side by the negative pressure inside the cylinder 101 (pressure holding step). Thereafter, a large number of molded products can be continuously produced by repeating the same steps.

However, the conventional injection molding machine described above has the following problems.
For example, in a pre-plastic type injection molding apparatus, if the pressure (or back pressure) of the molding material on the plunger head inside the cylinder prepared for molding the next molded product is too high, or the molding material in the cylinder When the mold expands, when the mold is opened and the finished molded product is taken out, the molding material is pushed into the cavity from the lower end of the injection port, so that it becomes a so-called “hanging” state, A flow mark (a drooping droop that hardens) is generated.

On the other hand, if the back pressure is reduced in order to prevent the above-described drooping and flow marks, the molding material used in the next molding becomes uncertain. Therefore, the back pressure is adjusted to be slightly higher so as not to cause drooping or a flow mark, and when the plunger is pushed back by the back pressure, the plunger is returned slightly backward from the position. A method of performing so-called “suckback” (suckback step; see FIG. 6) is also known.
JP-A-10-58501 Japanese Patent No. 2615948

  However, since the inside of the cylinder is in a reduced pressure state during suck back, air is drawn into the cylinder from the outside of the cylinder on the rear end side of the plunger, and this air is mixed into the molded product, resulting in “void (bubble) ", Or bubbles are ruptured and" jetting "(both ruptured voids or silver stream) is generated, leading to product defects.

  The present invention has been made in view of the above circumstances, and it is possible to prevent the occurrence of drooping and flow marks, and the pressure control method for an injection molding machine that does not cause voids (bubbles) or jetting (or silver stream). The purpose is to provide.

The pressure control method of an injection molding machine of the present invention is a molding method of an injection molding machine that injects a plasticized molding material into a mold by a screw or a plunger,
In the period from the cooling of the molded product in the mold until the mold is opened and taken out, and in the period for waiting for the molding material after the next measurement, the pressure in the storage unit for waiting the next molding material is set in advance. The configuration is such that low pressure control is performed to keep the pressure value below the set pressure value.

  In the pressure control method for an injection molding machine according to the present invention, the low pressure control sets a maximum value and a minimum value in advance for the pressure in the storage section, and the pressure in the storage section reaches the maximum value. Then, the screw or plunger is pulled back in the direction away from the supply direction, and the screw or plunger sequence control is performed to stop the screw or plunger pull-back operation when the screw or plunger is returned to the minimum value.

  Further, according to the pressure control method of the injection molding machine of the present invention, the shape of the nozzle at the tip of the cylinder communicating with the cavity of the mold is narrower when facing the storage part and wider when facing the injection port. It has a configuration exhibiting an expanded shape.

  According to the present invention, it is possible to provide a pressure control method for an injection molding machine that can prevent drooping and generation of flow marks and does not cause voids (bubbles) or jetting (or silver stream).

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Prior to explaining the pressure control method of the injection molding machine of the present invention, first, an injection molding machine to which this method is applied will be explained.
FIG. 1 shows the main part of a pre-plastic injection molding machine to which the pressure control method of the present invention is applied. The injection molding machine 1 is roughly composed of a cylinder 2, a barrel 3, a mold, and the like. 4 etc.

  The cylinder 2 has a nozzle 2 </ b> A serving as an injection port at the tip (lower end in FIG. 1, hereinafter referred to as the lower end), and accommodates a plunger 21 so as to be movable in the front-rear direction. In addition, the inside of the lower end side of the cylinder 2 constitutes a storage portion 2C in which the resin material supplied from the barrel 3 side is temporarily stored.

The nozzle 2A is provided with a front end portion (hereinafter referred to as a lower end portion) of an injection port 2B communicating with the cavity 4A of the mold 4 through a gate, and as shown in FIG. A narrow injection path 2D is formed between the injection port 2B. In addition, the injection path 2D has an opening (taper) shape in which the direction facing the storage portion 2C is narrowed and the direction facing the injection port 2B is widened. In this embodiment, the expansion angle of the injection port 2B is 22 degrees or more.
Thus, by forming the tapered structure in which a part of the injection path 2D is constricted, even if the internal pressure of the storage part 2C inside the cylinder 2 is too high, the molding material is formed in the constricted part in the injection path 2D. Is easy to harden and can be divided at this hardened part. This constricted portion is likely to be hardened because the amount of the molding material per unit time passing through this portion is suppressed to a small amount, so that the molding material comes into contact with the inner peripheral surface of the nozzle 2A (cylinder 2) and is cooled. This is because the cooling efficiency is increased.

  The plunger 21 includes a load cell 22 that detects a force acting on the plunger 21 at a rear end portion (upper end portion in FIG. 1), and a female screw portion that is integrated with the plunger 21 via a connecting member 23 and has a female screw cut. A slider 24, a ball screw 25 in which the female screw portion of the slider 24 is screwed, a servo motor 26 that slides the plunger 21 in the front-rear direction by rotationally driving the ball screw 25, and a control unit 27. have.

The slider 24 is configured to be inserted into a guide pin 20A installed between the cylinder 2 and the fixed frame 20 so as not to rotate.
The control unit 27 inputs a position signal from an encoder 261 connected to a rotating shaft (not shown) of the servo motor 26 and a load signal from the load cell 22 and outputs a control signal to the servo motor 26. . That is, the control unit 27 drives and controls the servo motor 26 so that the output torque of the servo motor 26 itself, the resin pressure in the storage unit 2C detected by the load signal from the load cell 22 and the like become a predetermined value.

On the other hand, the barrel 3 includes a supply port 3 </ b> A communicating with the cylinder 2 at one end (right end in FIG. 1), 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 injection port of the cylinder 2 communicates with the cavity 4A of the mold 4 through a gate.

Next, the pressure control method of the injection molding machine of the present invention will be described in detail using the above-described pre-plastic injection molding machine shown in FIG.
FIG. 3 shows an injection molding method to which the pressure control method of an injection molding machine according to the present invention is applied. In the injection molding method of this injection molding machine, an injection process, a pressure holding process, a cooling process (simultaneously the next measurement) Process), mold opening process, removal process, and mold closing process constitute one cycle, and the conventional suck back process (the plunger is pulled back (upward in FIG. 1) to perform position control and is located at the bottom of the cylinder) It does not include that the molding material moves backward (downward in FIG. 1).

  Specifically, from the latter half of the cooling process, specifically, from the completion of the relative position detection process by speed control to the mold opening process and the removal process, the conventional servo lock control (completion of the suck back process) Instead of performing the process until the completion of the mold closing process), pressure control in the cylinder 2 is performed, and sequence control is performed to maintain the inside of the cylinder 2 at a low pressure (particularly, an ultra-low pressure). In the present invention, the servo lock control for locking the plunger 21 without moving in the front-rear direction (vertical direction in FIG. 1) is performed only in the mold closing process.

  As shown in FIG. 4, this sequence control is based on the internal pressure of the cylinder 2 detected via the load cell 22, and when the internal pressure of the cylinder 2 rises to a constant value (maximum value Pmax; P1), the internal pressure is increased to a constant value (minimum value). The value is reduced to the value Pmin; P2). In other words, the ball screw 25 is rotated by the servo motor 26, and the plunger 21 is pulled up via the slider 24 screwed to the servo motor 26. For this reason, when the load signal from the load cell 22 is input, the control unit 27 outputs the control signal to the servo motor 26.

Here, the maximum value P1 corresponds to the maximum allowable value of the pressure increase in the cylinder 2 generated by the expansion of the molding material in the cylinder 2, and when the pressure exceeds this value, Causes drooling and flow marks. In other words, this value corresponds to the minimum pressure value that causes drooping or flow marks, and in this embodiment, P1 = 2.0 MPa.
On the other hand, the minimum value P2 is an allowable minimum pressure value that is set in order to prevent the occurrence of voids and jetting, and is set to P2 = 0.1 MPa in the present embodiment. That is, when the internal pressure in the storage portion 2C of the cylinder 2 falls below the minimum value P2, the inside of the cylinder 2 is depressurized, causing an air drawing phenomenon from the outside of the cylinder 2 on the rear end side of the plunger 21 to the inside of the cylinder 2. There is a risk that the above-mentioned voids and jetting may occur. The atmospheric pressure (1 atm) corresponds to this 0.1 MPa.

  Therefore, since such sequence control is performed from the completion of the relative position detection process by the speed control described above to the mold opening process and the unloading process, drooping and flow marks that have conventionally occurred are performed. Can be prevented, and voids (bubbles) and jetting (or silver stream) can be avoided.

That is, as described in the section [Prior Art], conventionally, in these processes, the pressure (or back pressure) of the molding material in the reservoir inside the cylinder is too high from the beginning, or over time. By gradually expanding the molding material in the storage part inside the cylinder, when the mold is opened and the finished molded product is taken out, the molding material is pushed out from the tip of the injection port (lower end in FIG. 1) into the cavity, Troubles such as so-called “sagging” and “flow marks (things with drooling drastically hardened)” have occurred.
On the other hand, in the present invention, such sequence control is performed at an ultra-low pressure. That is, since the pressure (back pressure) of the molding material on the plunger head side inside the cylinder is not too high, it is possible to prevent drooping and flow marks due to this high back pressure.

  Further, conventionally, after the molding material is weighed and taken into the cylinder for the next molding, the plunger is pulled backward so that the molding material is not pushed out in the gate direction, and the suck back is sometimes performed. However, when this suck-back is performed, as described in the “Prior Art” column, the inside of the cylinder is decompressed, and air is drawn into the cylinder from the outside of the cylinder on the rear end side of the plunger. For this reason, this air is mixed in the molded product to generate “voids (bubbles)”, or the bubbles are ruptured to generate jetting (“voids ruptured or silver stream”). However, in the present invention, since the suckback is not performed, such inconvenience can be avoided.

  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, the pressure control method of the present invention is applied to a pre-plastic type injection molding machine, but in addition to this, even in various types of injection molding machines such as an inline screw method and a plunger method, The pressure control method of the present invention can be applied. In an inline screw type injection molding machine, it is only necessary to monitor the pressure only on the tip side of the screw.

  The pressure control method of the injection molding machine according to the present invention can prevent drooping and flow marks, and can also avoid the occurrence of voids (bubbles) and jetting (or silver stream), thereby reducing material loss. In addition, particularly in the case of performing DVD molding with a stamper, the life of the stamper is increased, which is useful.

The block diagram which shows the principal part of the injection molding machine used for embodiment of this invention (A) is principal part sectional drawing which shows the nozzle part of the injection molding machine used for embodiment of this invention, (B) is the A section expanded sectional view. A time chart showing a molding method by an injection molding machine according to an embodiment of the present invention The time chart which shows the pressure control method of the injection molding machine which concerns on embodiment 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

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Injection molding machine 2 Cylinder 2A Nozzle 2B Injection port 2C Storage part 2D Injection path 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 mold 42 Upper mold P1 Maximum value Pmax
P2 Minimum value Pmin

Claims (3)

A pressure control method for an injection molding machine for injecting a plasticized molding material into a mold by a screw or a plunger,
In the period from the cooling of the molded product in the mold until the mold is opened and taken out, and in the period for waiting for the molding material after the next measurement, the pressure in the storage unit for waiting the next molding material is set in advance. A pressure control method for an injection molding machine that performs low-pressure control to maintain a state equal to or lower than a set pressure value.   In the low pressure control, a maximum value and a minimum value are set in advance for the pressure of the storage portion that temporarily stores the molding material in the cylinder that movably accommodates the plunger, and the pressure in the storage portion is 2. The pressure control of an injection molding machine according to claim 1, wherein when the maximum value is reached, the plunger is pulled back in a direction away from the supply direction, and when the minimum value is returned, the plunger sequence control is performed to stop the pull-back operation of the plunger. Method.   3. The injection molding according to claim 2, wherein a nozzle shape of the tip portion of the cylinder communicating with the cavity of the mold exhibits an expanded shape in which a direction facing the storage portion is narrowed and a direction facing the injection port is widened. Machine pressure control method.

Images