METHOD OF UNCLOGGING NOZZLE OF ELECTRIC INJECTION MOLDING MACHINE

Abstract: This invention provides a method of unclogging a nozzle of an electric injection molding machine which can dislodge a resin from a nozzle without applying continuously an excessive­ ly large load to a servo motor for driving a screw. When the res­ in clogs the nozzle, pulse distribution to a servo circuit is started (S1) up to the foremost screw position. When the difference be­ tween a moving distance instructed to the servo motor and an actual moving distance of the servo motor exceeds a predeter­ mined value and if a feed stop state is not cancelled before time­ out of a timer which has been started at the point of time when a feed stop state for stopping the pulse distribution is attained (S3-­ S6), forward movement of a screw is stopped by alarm process­ ing (S7).

Description: The present invention relates to a method for removing resin clogged in a nozzle of a motor-driven injection molding machine by moving forward a screw of the machine and, more particularly, to a method of this kind adapted to protect a servomotor for driving the screw from being subjected to an overload. While an injection molding machine is operated, there are occasions when molten resin is cooled and solidified in a nozzle and finally clogs the nozzle, thus preventing continuation of injection molding cycles. Particularly in the case where retracting of the nozzle, or a so-called sprue break, is not performed in each injection molding cycle to set the nozzle apart from the mold, the nozzle is readily cooled by the mold contacting the nozzle, resulting in solidification of molten resin at the distal end of the nozzle and hence clogging of the same. Measures conventionally taken to remove the solidified resin when the nozzle becomes clogged are to disassemble the nozzle, or move the screw forward through manual operation of an injection mechanism to discharge the solidified resin with a large pressure applied thereto. If the latter-mentioned method for removing nozzle clogging is applied to a hydraulic injection molding machine, no particular problems will arise. However, if this method is used with a conventional injection molding machine according to the preamble of the attached claim 1 using a motor, e.g., a servomotor, as the driving power source of the injection mechanism, an excessive load may act on the motor when the resin clogged in the nozzle is discharged On such occasions, a large current flow occurs in the motor for a long period of time, and thus the motor and the components of an electronic circuit for driving the motor, e.g., power transistors, will be shortened in life or be destroyed. The object of the present invention is to provide a method for removing clogging of a nozzle in a motor-driven injection molding machine, which is capable of discharging resin clogged in the nozzle without an excessive load being continuously applied to a servomotor for driving a screw. Of course, methods for judging whether a drive device is in an overload state are known in the art, and thus see for example JP-A-60-64817, JP-A-61-262802 and JP-A-61-84218, but none of these is applied to the specific case of removing clogged resin in a nozzle of a motor-driven injection molding machine. According to the present invention there is provided a method for removing clogging of the nozzle in an injection molding machine comprising an injection screw and a servomotor for driving the injection screw axially, the method being characterised by the steps of: (a) moving the screw forwardly by the servomotor towards a predetermined axial position at which resin clogged in the nozzle is ensured to be discharged; (b) judging whether the servomotor is in an overload state, during forward movement of the screw in said step (a), by detecting when a deviation of an actual movement amount of the servomotor from a commanded value thereof rises above a predetermined value; (c) stopping the forward feed of the screw in said step (a), by interrupting the commanded movement amount to the servomotor, when the servomotor is judged to be in an overload state by said step (b); (d) terminating the stoppage of the screw forward feed of said step (c) when said actual/commanded movement amount deviation falls below said predetermined value, indicating a release from the overload state, before passage of a predetermined period of time (T) after the beginning of said stoppage; and (e) de-actuating the servomotor when said deviation does not fall below said predetermined value before passage of said predetermined period of time (T) after the beginning of said stoppage, thereby definitely stopping the forward movement of the screw when the overload state is not released. Thus, according to the present invention, the forward movement of the screw is stopped when the servomotor continuously undergoes an overload state for the predetermined time while being actuated to move the screw forward for discharging resin clogged in the nozzle. Clogging of the nozzle can therefore be removed without an excessive load being continuously applied to the servomotor, and the servomotor and the components of an electronic circuit for actuating the servomotor can be protected and thus prolonged in their service life. Brief Description of the Drawings Fig. 1 is a flowchart of a control program for executing a nozzle clogging removal method according to one embodiment of the invention; and Fig. 2 is a schematic diagram of a principal part of a motor-driven injection molding machine to which is applied the nozzle clogging removal method according to the embodiment of Fig. 1. Referring first to Fig. 2, a motor-driven injection molding machine comprises a screw 1 disposed in a heat cylinder 4 having a nozzle 5 at the distal end thereof. The screw 1 is coupled to a servomotor 2 for injection by means of a transmission device (not shown), in such a manner that it is axially movable with rotation of the motor 2. A position sensor, e.g., a pulse coder 3, is mounted to the motor shaft of the servomotor 2, to detect the rotational position of the motor and, accordingly, the axial position of the screw 1. The injection molding machine is provided with a control system, for example, a numerical control system (hereinafter referred to as "NC system") 10 which includes a microprocessor (hereinafter referred to as "CPU") 11 for NC, and a CPU 12 for a programmable machine controller (hereinafter referred to as "PMC"). To the PMCCPU 12 are connected a ROM 15 for storing sequence programs such as a program for a clogging removal process described hereinafter, and a RAM 22 used for temporary storage of data and the like. Connected to the NCCPU 11 are a ROM 14 for storing a monitor program for global control of the injection molding machine, and a RAM 21 used for temporary storage of data and the like. Servo-circuits for controlling the operation of individual servomotors for various axes, such as an injection axis, a mold clamping axis, a screw rotation axis, an ejector axis, an injection unit axis, are also connected to the NCCPU 11 through a servo-interface 16. Among these, Fig. 2 illustrates only the servomotor 2 for injection and the associated servo-circuit 17. A nonvolatile shared RAM 18, comprising a bubble memory or CMOS memory, serves to store an NC program for controlling various operations of the injection molding machine, various setting values, parameters and the like. Connected to a bus arbiter controller (hereinafter referred to as "BAC") 13 are respective buses of the NCCPU 11, PMCCPU 12, shared RAM 18, input circuit 19, and output circuit 20. The BAC 13 is adapted to control buses to be used. Also connected to the BAC 13 through an operator panel controller 23 is a manual data input device with a CRT display (hereinafter referred to as "CRT/MDI") 24. In the following, a nozzle clogging removal operation of the injection molding machine constructed as above will be explained. When the nozzle 5 becomes clogged with resin, an operator inputs an injection unit retraction command through the CRT/MDI 24, to detach the nozzle 5 from a mold (not shown). The operator then inputs a nozzle clogging removal command through the CRT/MDI 24, and in response to this command, a process for removing clogging of the nozzle, shown in Fig. 1, is started. On receiving the above command, the PMCCPU 12 delivers movement command data to the NCCPU 11 through the BAC 13 and the shared RAM 18, to move the screw 1 to a predetermined axial position, preferably the foremost position thereof, at which resin clogged in the nozzle 5 is ensured to be discharged. The NCCPU 11 reads this movement command data, and then starts delivery of a command pulse train (command movement amount), i.e., distribution of pulses, to move the screw to the foremost position (Step S1). Each command pulse from the CPU 11 is supplied to an error register in the servo-circuit 17 through the servo-interface 16, and, in response to each pulse, the value stored in the register is updated in an incremental direction. The servo-circuit 17 drives the servomotor 2 for rotation with a driving current corresponding to the register value, thereby to move the screw 1 forward. The output pulses from the pulse coder 3, generated with rotation of the motor, are supplied to the error register, and the register value is updated in a decremental direction each time the pulse is applied to the register. Therefore, the register value represents the deviation of the actual movement amount of the servomotor 2 from the command movement amount supplied thereto up to the current point of time. If the forward movement of the screw 1 is delayed or hindered by resin clogged in the nozzle 5, the distribution pulses are accumulated in the error register and thus the register value is increased. As a result, the output torque of the servomotor 2 increases, so that the screw 1 pushes the resin with an increased force. Following Step S1, the CPUs 11 and 12 execute the below-mentioned steps repeatedly. The NCCPU 11 judges whether or not the screw 1 has reached the foremost position (Step 2). More specifically, when the pulse distribution up to the foremost screw position has been completed and at the same time the value of the error register is within a predetermined in-position range, the CPU 11 judges that the foremost position has been reached, and sends a signal representing this to the PMCCPU 12 through the BAC 13 and the shared RAM 18. On receiving the signal representing the arrival at the foremost position, the CPU 12 judges that the resin clogged in the nozzle 5 is already discharged, because the screw 1 has reached the foremost position, and accordingly ends the clogging removal process. If it is judged in Step S2 that the screw 1 has not reached the foremost position, a decision is made as to whether a signal requested a feed stop has been delivered (Step S3). As mentioned above, when the forward movement of the screw 1 is delayed or hindered by the resin clogged in the nozzle 5, the force of pushing the resin is increased. If the resin still continues to delay or hinder the forward movement of the screw, the value of the error register increases above a predetermined value, which is set previously and represents an overload state of the servomotor 2. The injection molding machine of this embodiment has the feed stop function of interrupting the pulse distribution on such occasions. During execution of the feed stop function, the NCCPU 11 sends a feed stop indication signal to the PMCCPU 12 through the BAC 13 and the shared RAM 18. On receiving the feed stop indication signal, the PMCCPU 12 starts a timer T (Step S4), and repeats a decision as to whether the feed stop indication signal is still being supplied from the NCCPU 11 (Step S5), and a decision as to whether the time set by the timer T is up (Step S6). While in the feed stop state, the value of the error register is large and thus large current flows in the servometer 2. Accordingly, the motor actuates the screw 1 with the maximum output torque, so that the screw 1 pushes the resin clogged in the nozzle 5 with a large force. As a result, the value of the error register may be decreased below the aforementioned predetermined value to terminate the feed stop function, before passage of the time set by the timer T, i.e., before passage of a maximum allowable period in which the motor and the components of the servo-circuit 17 etc. are not endangered even if an overload state permitting a large current flow in the servomotor 2 continues. In such case, the PMCCPU 12 resets the timer T (Step S8), and repeats the process of Step S2 and the subsequent steps. If the feed stop is again requested before the screw 1 reaches the foremost position, the PMCCPU 12 starts the timer T and repeats execution of Steps S5 and S6 as mentioned above. If the feed stop state is ended before the time set by the timer T passes (Step 5), the timer T is reset (Step S8) and the program returns to Step S2. If the screw 1 reaches the foremost position during execution of these steps, then it is judged that clogging of the nozzle 5 is removed, and accordingly the nozzle clogging removal process is ended. If it is judged in Steps S5 and S6 that the feed stop state is continued until the time of the timer T passes, an alarm process is performed wherein an alarm is displayed on the CRT screen of the CRT/MDI 24 and the actuation of the servomotor 2 is stopped (Step S7). As mentioned above, the time within which large current is allowed to continuously flow in the servomotor 2 is restricted to the time set by the timer T, and this restricted time is set to such a safe time period that the servomotor 2 and electronic components for driving the servomotor 2, such as power transistors and the like, are never endangered even if the large current flow is continued. Therefore, the servomotor and the electronic components can be protected from being damaged and shortened in their life. Meanwhile, when the feed stop occurs once but is ended afterwards, that is, when the resin clogged in the nozzle 5 is moved, no further greater force is usually required thereafter to remove the clogged resin. Therefore, the feed stop state is scarcely repeated. In the foregoing embodiment, an NC system is used as the control system for controlling the injection molding machine, and the feed stop function of the NC system is utilized. A similar process to the above-described one can be also applied to an injection molding machine which is controlled by a microcomputer or the like as the control system, to remove resin clogged in the nozzle.

Claims: A method for removing clogging of the nozzle (5) in an injection molding machine comprising an injection screw (1) driven axially by a servomotor (2), the method being characterised by the steps of: (a) moving the screw (1) forwardly by the servomotor (2) towards a predetermined axial position at which resin clogged in the nozzle is ensured to be discharged; (b) judging whether the servomotor (2) is in an overload state, during forward movement of the screw in said step (a), by detecting when a deviation of an actual movement amount of the servomotor (2) from a commanded value thereof rises above a predetermined value; (c) stopping the forward feed of the screw in said step (a), by interrupting the commanded movement amount to the servomotor (2), when the servomotor is judged to be in an overload state by said step (b); (d) terminating the stoppage of the screw forward feed of said step (c) when said actual/commanded movement amount deviation falls below said predetermined value, indicating a release from the overload state, before passage of a predetermined period of time (T) after the beginning of said stoppage; and (e) de-actuating the servomotor when said deviation does not fall below said predetermined value before passage of said predetermined period of time (T) after the beginning of said stoppage, thereby definitely stopping the forward movement of the screw when the overload state is not released. A method according to claim 1, wherein said predetermined axial position of the screw (1) is a foremost position thereof. A method according to claim 1 or 2, wherein said step (c) comprises starting a timer for setting a time corresponding to said predetermined period of time (T), simultaneously with the stoppage of the forward feed of the screw (1). A method according to claim 1, 2 or 3, wherein said step (e) comprises performing an alarm indication simultaneously with the definite stoppage of the forward movement of the screw (1).

Inventors: KAMIGUCHI M ROOM - FANUC MANSH, KAMIGUCHI, M. ROOM 6-207 FANUC MANSHONHARIMOMI
Applicants: FANUC LTD, FANUC LTD.
Application year: 1988
Main classification: B29C45
IPC codes: B29C45, G05B19