Abstract: This paper, through careful study of the control system of a small, old, vertical pneumatic injection molding machine from Japan, selected a Mitsubishi PLC-FX1S-30MR and a GOT930-BWD-C touchscreen, and successfully designed a safe and practical control program for the pneumatic injection molding machine using the SFC (Sequential Function Chart) programming method. It also elaborates on the methods of industrial control system analysis, programming techniques, and key points of wiring, connection, and debugging in the actual technical renovation of old equipment. Keywords: Pneumatic injection molding machine technical renovation, PLC, Touchscreen, STL, Energy saving Introduction: Equipment introduced to many factories in Guangdong Province in the 1980s and 1990s has been in use for nearly 20 years. Due to the severe aging of electrical control components, the machine suffers from poor operating stability, high failure frequency, heavy maintenance workload, numerous safety hazards, and high energy consumption. This results in increased product quality defects, low production efficiency, frequent workplace accidents, high maintenance costs, and rising production costs, placing a heavy economic burden on factories! However, it is a pity to discard these old equipment. Given the age of these devices and the lack of relevant information, technical upgrades are quite difficult. How can we give these old devices a new look? This is the sacred duty that the times have bestowed upon us, the vast number of equipment maintenance technicians! While continuously improving our professional skills, we hope that this article can provide some reference and inspiration for technicians in the technical upgrade of old equipment! Paper content: Our factory is a Japanese-owned enterprise, specializing in the production of wires and cables and connecting terminals with wire plugs. In August 1991, 25 small vertical pneumatic injection molding machines (manufactured in October 1987, and used in Japan for nearly four years) were imported from Daito Special Electric Wire Co., Ltd. of Japan. These machines are centrally supplied with air by an air compressor network, resulting in low noise, energy efficiency, and environmental friendliness. The required working air pressure is 0.45-0.6 MPa, single-mold injection volume is 2-8g, and single-mold product net weight is 2-6.5g. They operate on single-phase 200-220V power (for heating and solenoid valves) and can produce various small plastic products in a single molding process. I. Problems: Due to their age, these 25 vertical pneumatic injection molding machines exhibit the following conditions: ① The paint on the equipment is peeling, and most machines are heavily rusted. ② The wiring design is complex and outdated. The main components include: 19 small intermediate relays (frequently prone to poor contact or broken coils), 6 time controllers, 4 old-style pneumatic solenoid valves, and 1 temperature controller. ③. High failure frequency and low production efficiency increase overall machine maintenance costs. ④. Aging electrical control components pose potential safety hazards. In May 2006, an employee's finger was crushed during mold closing while the machine was running. ⑤. Sometimes the control precision is inaccurate, which somewhat affects product quality. (See attached Figure 12-1 for the actual control box of the old machine.) II. Analysis of the actual working principle of the old pneumatic injection molding machine: Since there was no circuit diagram, pneumatic circuit diagram, or operating manual for this machine, I was keenly aware of the arduousness and significance of the task. To fulfill my mission, in June 2006, I spent nearly a week in the production workshop fully understanding the actual control principles of the machine's circuit and pneumatic systems. The machine's operation process is as follows: Turn on the main power switch of the control box, and turn on the power switch on the control box panel (Note: When drawing the control principle system flowchart, it was discovered that the previous control system had safety hazards. Previously, the machine did not have a start-up button; this was added during my technical modification using a start/stop interlock button. Pressing the start-up button prevents the actuators from malfunctioning in manual mode, causing accidental personal injury or machine damage). Manual or automatic operation modes can be selected. 1. When production is required and automatic operation is selected: With the automatic switch closed, press the start button. The sliding pneumatic solenoid valve operates, and the sliding/retracting cylinder drives the pull rod under air pressure, causing the moving template to slide forward. When it encounters the sliding proximity switch, the sliding stops. Then, the mold-closing pneumatic solenoid valve is energized, and the mold-closing/opening cylinder drives the guide rod under air pressure, causing the moving template to quickly close upwards with the fixed template. (Note: During the mold sliding and closing stages, in case of an emergency, the safety light curtain sensor will activate or the foot pedal will be pressed to stop the emergency stop switch. The mold closing solenoid valve will then de-energize and reset. The cylinder, under air pressure, will switch to mold opening action. When the mold opening position reaches the mold opening limit proximity switch, the sliding solenoid valve will be energized. The sliding/retracting cylinder, under air pressure, will cause the moving platen to slide backward, completing the human-machine protection process.) When the mold closing time is up, the injection/retracting pneumatic solenoid valve and the feeding pneumatic solenoid valve will be energized simultaneously. When the feeding cylinder drives the guide rod to the feeding stroke, the feeding pneumatic solenoid valve will de-energize, ending the feeding process. When the injection time reaches the set value, the injection inlet pneumatic solenoid valve will de-energize and reset. The cylinder, under air pressure, will switch to the injection rod return action (i.e., injection retraction). Next, when the plastic product in the mold cavity has cooled and solidified, the product starts counting. When the counting signal ends, the mold closing solenoid valve is de-energized and resets itself until the sliding solenoid valve is energized, automatically completing one production cycle. 2. When machine adjustment is required and manual operation is selected: turn on each conversion switch to energize or de-energize each pneumatic solenoid valve, so that the cylinder drives the guide rod under air pressure to perform feeding, sliding/sliding, mold closing/opening, and injection/injection actions, completing the machine adjustment work. (Note: The arrows in the box indicate the direction of movement of each actuator; the arrow 1 with a dotted line was added during the technical modification for the machine's safety!) Flowchart of the electrical control principle system of the old pneumatic injection molding machine [align=center] Diagram of the air circuit control system of the old pneumatic injection molding machine[/align] According to the air circuit control system of this machine, it can be analyzed that: when the air pressure switch is turned on, the compressed air passes through the water filter dryer, and the compressed air passes through the pressure reducing valves marked "1" and "2" for two parts of the work. ①. Part 1: Marked "3": When the 2-position 5-way pneumatic single-control valve is energized, point A is vented through the pressure regulating valve, causing the mold opening (closing) cylinder guide rod to move upwards for mold closing; when de-energized, point B is vented, causing it to move downwards for mold opening. Marked "6": When the 2-position 5-way pneumatic single-control valve is energized, point A is vented through the pressure regulating valve, causing the feeding cylinder guide rod to move forward for feeding; when de-energized, point B is vented, causing it to move backwards to stop feeding. Part 2: Marked "4": When the 2-position 5-way pneumatic single-control valve is energized, point A is vented through the pressure regulating valve, causing the injection (retraction) cylinder guide rod to move downwards for injection; when de-energized, point B is vented, causing it to move downwards for injection retraction. Marked "5": When point B of the 2-position 5-way pneumatic solenoid dual control valve is energized, air is supplied through the pressure regulating valve, causing the guide rod of the sliding (retracting) cylinder to slide forward; when point A is energized, air is supplied at point A, causing it to slide backward. III. Compiling a safe and practical control program for an old pneumatic injection molding machine: 1. Determine the model configuration of I/O points, PLC, and touch screen: ⑴. After systematically understanding the working process and control principle of the entire machine, a practical theoretical basis was provided for writing the control program for the PLC of this machine. (2) From the above (Table 4-1), it can be concluded that: 12 input points and 6 output points are required, totaling 18 I/O points. Since the Mitsubishi FXN series has a higher cost performance, while the FXS series has a lower cost performance, referring to the "FX1S Series Micro Programmable Controller User Manual", the functions of the FX1S can meet the programming of this machine. Purchasing 20 PLC input points is just enough. Since the FX1S-20MR PLC only has 13 input points, considering future maintenance or the addition of other functions, the Mitsubishi FX1S-30MR PLC is selected. After connecting all the input points of this machine, there are still 4 points left. These 4 points can be reserved for future use. (3) From the electrical control principle system flow chart of this machine (see Figure 2-1), it can be seen that there are 3 time settings and 1 count display that need to be frequently adjusted according to the production situation. Considering that all buttons and switches must be controlled on the control panel of the control box, a Mitsubishi GOT930-BWD-C touch screen with a screen of "240×80 dots" and an effective display size of 117×42 mm (3″ type) is selected as the human-machine interface to facilitate the adjustment and operation of the machine by workshop workers. 2. Program design of PLC control system: ⑴. Based on the reasonable configuration of PLC input and output points, further arrangements are made for writing the program. The I/O system configuration of this machine's PLC is as follows (it should be noted that: ①. The start button and the automatic button are connected in series to point X2. ②. The safety light curtain and the emergency stop foot switch are connected in parallel to point X5. ③. Y10 is the output point of the PLC to display when the battery in the DU is low, which is convenient for maintenance personnel to inspect later). [align=center] PLC of old pneumatic injection molding machine FX1S-30MR I/O System Configuration Diagram[/align] ⑵. According to the results, the working process of this machine is as follows: under the sequential action of each input signal and each time control, each actuator works automatically and orderly in sequence according to its own working condition. If the control circuit of the intermediate relay of this machine is used to compile the PLC control ladder diagram, it will be cumbersome and complicated, and will also bring certain difficulties to the PLC program compilation. Therefore, the SFC (Sequential Function Chart) programming method is selected for the PLC control system of this machine, which can easily compile the program and achieve the purpose of reasonable and safe control. When consulting the "Graphical Operation Terminal GOT-F900 Operation Manual", it was found that: because T and C can select the current value and the set value, while the data register can only select the current value. Therefore, when compiling the PLC program, D, V, and Z were not selected as the indirect data registers of T and C. The description of the state transition of the PLC sequential control system diagram of this machine is as follows: When the run preparation button X0 is pressed after power-on, and X0 is "1", S0 is self-locked (i.e., SET S0). ①. When the signal is transferred to S0 (i.e. STL) When S0), the relevant signals that need to be processed are: First, the running indicator Y5 is "1" and the light is on. Select manual X1 or automatic X2. A). When manual mode is selected, X1 is "1"; X2 is disconnected and is "0". When Y3 is "0" and power is off: a). If manual sliding switch X3 is "1" and Y0 is "1", the template slides forward after being powered on. a). If manual mold closing switch is selected, the signal is transferred to S22; at the same time, once X5 is "1", the signal is transferred to ⑥S24. ④. When the signal is transferred to S22 (i.e. STL S22), the relevant signals that need to be processed are: Y2 is "1" and is powered on and self-locked, Y4 is "1", and T1 starts timing. When the feeding stroke X12 is reached, Y4 is de-powered; when the soft contact T1 is "1", the signal is transferred to S23. ⑤. When the signal is transferred to S23 (i.e., STL S23), the relevant signals to be processed are: Y2 resets and de-energizes; T2 starts timing; when the soft contact T2 is "1", C18 starts counting once; simultaneously, the signal is transferred to S24. ⑥. When the signal is transferred to S24 (i.e., STL S24), the relevant signals to be processed are: Y1 resets and de-energizes; when X13 and X4 are both "1", the signal is transferred to S25. ⑦. When the signal is transferred to S25 (i.e., STL S25), the relevant signals to be processed are: T3 starts timing; when the soft contact T3 is "1", the signal is transferred to S26. ⑧. When the signal is transferred to S26 (i.e., STL S26), the relevant signals to be processed are: Y3 is "1", waiting for X2 to start; when X2 is "1", the signal is transferred to S0, the sequential control program is completed, and the program returns (RET) to the main program, completing one work cycle. 3. Ladder diagram of PLC control system program: (See the PLC "Pneumatic Injection Molding" program package for the program). Ladder diagram of PLC sequential control for old pneumatic injection molding machine [align=center][/align] 4. Touch screen screen and program design: Considering that operators may sometimes make mistakes, for human-machine safety, not all switches are replaced by touch keys. Therefore, the touch screen only displays 5 screens. (See the relevant program design in the "Pneumatic Injection Molding Machine Touch Screen DU Program.dup package") Screen "0" has: The current display screen has three times: mold closing time T0, injection time T1, and product cooling and molding time T2; There is a product counting screen. In the FX-PCS-DU-WIN-C touch screen design software, the values ​​of the three times are set to the current value for display. The upper limit of the current value for mold closing time T0 and injection time T1 is set to 9.9 seconds, and the lower limit is set to 0.0 seconds. The upper limit of the current value for cooling time T2 is set to 99.9 seconds. The lower limit is set to 0.0 seconds, and the upper limit for displaying the current product count is set to 9999. Setting the lower limit to 0 will reset the count to zero when it reaches 9999. Touching the "Settings" or "Convert" button for this item will open the corresponding settings screen. Screen "1" shows the mold closing time T0 setting, with an upper limit of 9.9 seconds and a lower limit of 0.0 seconds. Pressing the "Return" button returns to screen "0". Screen "2" shows the injection time T1 setting, with an upper limit of 9.9 seconds and a lower limit of 0.0 seconds. Pressing the "Return" button returns to screen "0". Screen "3" shows the cooling time T2 setting, with an upper limit of 99.9 seconds and a lower limit of 0.0 seconds. Pressing the "Return" button returns to screen "0". Screen "4" shows the product count reset screen, displaying the current product count with an upper limit of 9999. The lower limit is set to 0. Pressing the "Product Clear" key will clear the product count; pressing the "Return" key will return to screen "0". 5. Wiring, connection, and debugging of PLC and touch screen: 1) After the PLC and touch screen programs are designed, connect the entire machine according to the PLC I/O system configuration (see Figure 5-1) and the external wiring diagram of the machine (see Figure 10-1). 2) After checking that the wiring is correct, first turn on the power, connect the programming data signal line to the PLC's data interface, and input the written program from the PC to the PLC; then connect the RS232C data signal line to the DU's data interface, and input the written program from the PC to the DU. External wiring diagram 3) Connect the PLC and touch screen with the RS232C data signal line. After debugging and checking, the current screen display is normal. The display showed all settings were normal, and the PLC program was running normally. The only issue was that the backlight on the GOT930-BWD-C touchscreen remained on for an extended period. Adjusting the off-light time on the DU (Distributor Unit) had no effect. Consulting the "GOT-F900 Operation Manual" revealed that auxiliary relays M+0 to M+7 had special uses. M+2, when ON, turned off the backlight after a specified time; while M+6, ON, indicated a low battery in the DU. To improve the design, two lines of code were added after the PLC sequential control ladder diagram. To prevent operators from accidentally deleting programs on the DU by opening the main menu, four main menu call keys were set diagonally, while the other two diagonally opposite keys were disabled. This effectively protected operators from easily accessing the DU's main menu and safeguarded their work. After debugging, the machine underwent a three-day trial run. It operated stably, with precise control and good product quality, and no abnormalities were found. This marked a good start to the technical upgrade and received praise from the supervisor. IV. Conclusion: The only drawback of this machine is the lack of an ejector pin device and the inability to install and use slightly thicker molds. Adding an ejector pin device would reduce the workload for workers; further modifications to the height of the fixed mold plate would undoubtedly expand the machine's usability, allowing even slightly thicker molds to be installed and used. During the writing of this article, I searched online for domestic injection molding machine manufacturers, and found that they all produced hydraulic vertical injection molding machines; even the smallest models used 380V hydraulic motors. 5.5KW hydraulic injection molding machines share a common characteristic: during production, the hydraulic motor must run continuously once the machine is turned on, while no kinetic energy is needed for product cooling or handling, resulting in energy waste. The research and production of energy-saving pneumatic small injection molding machines is still largely unexplored in China. Especially today, as my country strives to build a harmonious society and promotes resource conservation and emission reduction, it is even more worthwhile to vigorously promote this type of small injection molding machine—with centralized air supply via pipelines, low noise, and both energy saving and environmental protection—in the small plastic products industry.