The huge market of plastic products has greatly promoted the development of plastic machinery industry, injection moulding machine has naturally become the key equipment for plastic moulding in modern industrial system. In recent years, injection moulding machinery has developed towards the trend of high speed and high precision, green and energy saving.
The key index to measure the productivity and control performance of injection moulding machine is the cycle time of opening and closing mould action. The cycle time, also known as the empty cycle time, refers to the time required for the closing mechanism to operate for one cycle without injection and pre-moulding action, specifically including the closing time, the opening time, and the changeover time between the closing and opening of the mould.
As shown in Fig. 1, the opening and closing actions are relatively independent in the whole production process, and reducing the time of this part of the action unrelated to the process can directly improve the production efficiency without affecting the quality of the products, and the study of the cycle time is of great significance to the improvement of the productivity and performance of the injection moulding machine.
In terms of improving the rapidity, many experts and scholars for the control valve spool structure, spring stiffness coefficient, the control cavity damping hole diameter, etc. to improve the response speed of the control valve in order to improve the dynamic characteristics of the system to improve the rapidity of the system.
Figure 1 injection moulding machine working production flow
Opening and closing mould action principle and system composition
This study focuses on the opening and closing system of the three-plate hydraulic injection moulding machine, as depicted in Figure 2. During the closing process, it generates sufficient clamping force to ensure that the mould cavity is tightly sealed, preventing the plastic melt from overflowing during injection. After injection, it provides space for the ejector cylinder to eject the moulded products smoothly.
Fig. 2 Mould opening and closing structure of injection moulding machine
The three-plate hydraulic injection moulding machine’s mould opening and closing system primarily consists of an electrical control system, a hydraulic transmission system, a mould opening and closing mechanism, and a lubrication system.
The hydraulic system under study belongs to the servo pump control system, and the control logic is the composite control of ‘servo pump control + hydraulic valve control’, i.e., the servo driver sends out a speed command to the motor, drives the oil pump to rotate, and then supplies the oil to the system, and the flow value of the output of the oil pump is only related to the speed of the servo motor.
By adjusting the valve control logic, switch to different working oil circuit, and then complete the opening and closing of the mould process of different sub-action.
Study on the influencing factors of mould opening and closing cycle time
The rapidity of the mould opening and closing system determines the cycle time performance. The electro-hydraulic servo system’s rapidity is influenced by the pressure building time, the dynamic response of the components, and the actuator’s working speed. We investigate the impact of various parameters on this rapidity from these three perspectives and derive corresponding countermeasures to reduce the cycle time.
1. Elastomer pressure building time
Fluid and pipeline are typical elastomers, with high-pressure clamping stage for analysis, when the pressure increases, the fluid, pipeline and cylinder will produce the corresponding elastic deformation, resulting in the original closed cavity within the dynamic cavity, replenishment of a certain volume of working fluid to fill the cavity, bringing a certain amount of time loss for the elastomer pressure building time.
High-pressure clamping stage, the oil is intensely compressed, the cylinder inlet cavity appears dynamic volume, then compensate for the part of the oil to be supplemented:
Formulas 1.
Where: Vs for the cylinder and high-pressure hose initial volume, m3;
Force rate of change, N/m; Δx for the cylinder displacement, m; A for the cylinder working chamber area, m2; βe for the volume of the fluid modulus of elasticity, MPa.
Compensate for the dynamic volume of the pipeline to be supplemented by the oil:
Formulas 2.
Where: r is the radius of the working surface of the pipeline, m; ΔFx is the deformation resistance, N; k‘ is the pipeline stiffness, N/m; Δx’ is the pipeline path directional deformation, m.
In summary, the additional pressure building time caused by elastomer deformation is:
Formulas 3.
Where: Q is the oil supply flow rate, m3/s.
Fig. 3 Simulink simulation model of high-pressure hose
After constructing the high-pressure hose simulation circuit as depicted in Figure 3, we substitute the key parameters into the model, set various pipe diameters and lengths, simulate different initial volumes, and present the simulation results in Figure 4.
The curve in Figure 4 indicates that as the initial volume increases, the pipe pressure oscillation effect diminishes. It takes longer for the system to build pressure and attain a stable state. The figure label L represents the initial volume.
Fig. 4 Pressure building time of elastomer under different initial volume.
2. Dynamic characteristics of hydraulic components
2.1 Dynamic Response of Motor Pump
As illustrated in Fig. 5, we model the motor pump using the MATLAB/Simulink platform. The response time of the motor pump is determined by the time interval t1 between the command signal received by the servo driver and the subsequent output speed of the servo motor.
Fig. 5 Simulink simulation model of servo drive unit
After iteratively debugging the Kp and Ki parameters in the rotational speed PI closed-loop control algorithm, we provide the rotational speed combinations for step-up and then step-down scenarios. Initially, the rotational speed increases from 0 r/min to 500 r/min, 1000 r/min, and 1500 r/min, respectively, and then decreases back to 0 r/min in each case.
We process the obtained data and present the simulation results for the speed step-up and step-down settings in Table 1 and Table 2, respectively.
Table 1: Simulation results for step-up speed setting
Table 2 Simulation results for speed step down condition
The simulation results reveal that the time interval t1, which spans from the receipt of the command signal by the servo driver to the output speed of the servo motor, is between 45 and 70 milliseconds. During this period t1, there is no output speed from the motor, representing an invalid time for the opening and closing actions.
2.2 Control valve response
The response characteristics of the control valve group directly affect the opening speed of the element.
The oil needs to pass through the switching valve and safety valve, which are two control valves, to enter the rodless cavity. The response of the valve group directly influences the speed at which oil enters the rodless cavity, thus the response time of the valve group is analyzed.
2.2.1 Changing the spring stiffness of the main control valve control chamber
The spring stiffness of the control cavity of the main control valve has a big influence on the switching speed of the left and right positions of the spool, so take three sets of stiffness values, respectively 11.6 N/mm, 21.6 N/mm and 31.6 N/mm.
Observe the spool displacement change of the main control valve, as shown in Fig. 6. The simulation curves indicate that, during the spool’s transition from the right position to the center position, a greater spring stiffness results in a faster spool switching rate and a higher spool oscillation frequency.
2.2.2 Changing the control oil flow rate
We set three constant flow sources with values of 80 L/min, 100 L/min, and 120 L/min as inputs to the system, and present the simulation results in Figure 7-1.
The outlet flow response of the control valve group becomes faster with the increase of the control oil flow value. The original circuit is an internal control method, so the overall dynamic response can be improved by setting the external control oil port and increasing the control oil flow.
2.2.3 Changing the safety valve control chamber area
The size of the control pressure on the spool is influenced by the area of the safety valve’s control chamber. We set the control chamber diameters to 30 mm, 34 mm, and 38 mm, respectively, and the simulation results are shown in Figure 7-2.
The smaller the area of the control chamber, the smaller the hydraulic control force, but the control chamber volume is also small, so the time required for the oil to fill the control chamber is shorter, the flow response of the valve group is faster, and the oil enters the cylinder more quickly.
Fig. 6 Response of valve group under different spring stiffness of main control valve
Figure 7 Safety valve manifold flow
Conclusion
Taking the opening and closing mould system of the injection moulding machine as the research object, the influence of the pressure building time and the dynamic response of the components on the rapidity of the system is studied with regard to the cycle time of the opening and closing mould system action.
Reducing the initial volume of the cylinder and its connected high-pressure hose helps to reduce the elastomer’s pressure building time, but the time required for the system to build up pressure and reach a pressure steady state is getting longer and longer.
The response characteristics of the motor pump and control valve group and the parameters of the control valve group model all influence the rapidity of the system.
Improve the injection moulding machine opening and closing system rapidity can take the following measures:
(1) According to the actual demand, try to reduce the initial volume in order to improve the system rapidity;
(2) Reduce the response time of the motor pump can improve the rapidity;
(3) Improve the spring stiffness of the main control valve, increase the control oil flow, reduce the area of the safety valve control chamber, and change the parameter value of the control valve group to improve the speed.
