MODELING AND SIMULATION OF INTERNAL CIRCULATION TWO-PLATEN INJECTION MOLDING MACHINE BASED ON AMESIM

Transcription

1 MODELING AND SIMULATION OF INTERNAL CIRCULATION TWO-PLATEN INJECTION MOLDING MACHINE BASED ON AMESIM Lu Yang, Jiong Peng, Dongjie Chen and Jian Wang* Beijing Institute of Technology, Beijing , China Abstract The internal circulation direct hydraulic two-platen clamping system opened a new era of the development of the injection molding machine. This paper established the hydraulic system models for the internal circulation clamping system by AMESim. Displacement of the moving platen, pressure in the mold-clamping and flow in the internal circulation valves were calculated. The simulation results showed that the system design was reasonable and reflected the real dynamic characteristics of hydraulic system. The modeling and simulation for the internal circulation two-platen injection molding machine laid the foundation for further studies. Introduction Precision, cleanliness, saving and benefit are the main themes of the injection molding machine (IMM) in the world. The injection molding machine can make products with complicated structure in one-timely molded. So the machine has reliable quality, higher production, fewer post processing and lower waster rate. Although the weight and repeatability of the plastics are widely accepted as the measure of injection molding machine, the main factors of influence on IMM s performance are machine s design, parts processing and control precision. Clamping system is one of the important structures among the components of IMM. The functions of the clamping system are 1) opening and closing the mold, 2) pushing out the products, 3) providing mold clamping forces. The clamping forces counteracted the pressure of the melt plastics coming into the mold cavity. Therefore, due to the importance of the clamping system, IMMs are commonly classified according to the type they employ. There are two main types of clamping system in common use: toggle clamping and direct hydraulic clamping [1]. With energy conservation, efficient and high speed, the toggle clamping has obvious advantages in the field of standard machines. Compared with the toggle clamping, the direct hydraulic clamping has excellent performances: 1) uniform force for the template, 2) steady speeds for opening and closing mold, 3) excellent repetition of clamping force. In the early 90s, a new direct hydraulic injection molding machine was invented in Europe, and the direct hydraulic two-platen IMM was a representative [2]. The direct hydraulic twoplaten clamping system is mainly divided into three types: non-circulation, external circulation and internal circulation. The performance comparison of these three direct hydraulic clamping systems can be seen in Table 1. Table 1. Performance comparison of direct hydraulic clamping systems Circulation Non External Internal Mold-moving two the same Mold-clamping four Cylinder Moving small small big diameter Clamping big big Moving speed slow fast fast Movement of mold-moving and moldclamping move together Pump power high high low Energy consumption high high less Specialty supplement oil for moldmoving moldclamping interconnect Clamping stability bad general good Disadvantages inconformity between clamping force and speed greater resistance complicated structure, high precision, low strength [3] As shown in the above table, the internal circulation clamping system has all the advantages of toggle clamping and direct hydraulic clamping. The internal circulation clamping system is also excellent in stability. Operating principle of the internal circulation two-platen IMM The injection molding machine is an advanced piece of manufacturing equipment which concentrates machinery, hydraulic pressure, electric, data collection and control. The injection molding is a cyclical process, as shown in Figure 1.

2 This paper took the internal circulation two-platen injection molding machine as the object of study. The machine was manufactured by Bloomachine Company. The mold-clamping system was composed of three internal circulation and a cylinder connected to air. In the sketch mode, the model of internal circulation two-platen hydraulic system can be established by using hydraulic library, mechanical library and signal library. The AMESim has standard hydraulic model libraries. But there are lots of different forms of hydraulic components. Therefore, AMESim provides a library of hydraulic components design (HCD) for non-standard components. The internal circulation hydraulic had to be established by HCD. This model consisted of two cylinder blocks and a 2 position 2 port hydraulic control valve. The two cylinder blocks can realize circulation by using the 2 position 2 port hydraulic control valve, as shown in Figure 3. Figure 1. Cyclical process of the injection molding An IMM consists of mold-clamping system, moldmoving system, injection system, hydraulic system and control system [5]. The operating principle of the IMM can be seen in Figure 2. Firstly, the operating speed and working pressure of the machine were set. They were in accordance with the processing conditions. Secondly, the set of signals were compared with the feedbacks of the sensors. Then the differences were sent into controller. The feedbacks were the speed of the moving platen and the pressure of the pump. Lastly, after processing the differences, the output directive of controller was used to control the speed of servo motor. Then servo motor drove the pump to deliver hydraulic oil. The oil went into hydraulic through the servo valves. After that, the hydraulic completed a series of actions. Figure 2. Operating principle of the mold-clamping system This paper established the internal circulation hydraulic system models based on AMESim. The AMESim is a modeling, simulation and dynamic analysis software for hydraulic/mechanical systems based on bond graphs. It was released by French IMAGINE Company in Users can build complex multi-disciplinary systems on a single platform. Then, simulation and analysis are performed. The graphical interface let users release from a tedious mathematical model and focus on the physical system itself. This can improve the design efficiency greatly [4]. Establishment of AMESim model Piston Two cylinder blocks Piston rod 2 position 2 port hydraulic control valve Figure 3. Internal circulation cylinder model The pipeline characteristic was set to direct connection. The diameter of the cylinder was set to 190 mm. The diameter of the piston rod was set to 90 mm. The flow rate at maximum opening of the valve was set to L/min. The equivalent cross sectional area at maximum opening of the valve was set to 5153 mm 2. In a real situation, the internal circulation valve was installed in the piston of clamping cylinder. When the internal circulation valve was fully opened, the valve was equivalent to an orifice. The diameter of the orifice was set to 81 mm. Other parameters were set the same as the former. This model with an orifice can be regarded as the real model under the condition of the valve was fully open. The model can be seen in Figure 4. The piston rods of the two models were exerted a force of 100 N, respectively. There were no pressure source. The other parameters remained the default. The 2 position 2 port hydraulic control valve was open. The simulation ran 15 s. The displacement of the piston and the flow rate through the valve were carried out, as shown in Figure 5 and Figure 6. The displacement curves of the two

3 models are almost identical. And the flow rate curves are also similar. So the model of the internal circulation cylinder can be equivalent to the real. Orifice Figure 4. The equivalent model of the real model Figure 7. The clamping system of internal circulation twoplaten IMM Parameter Settings Figure 5. The displacement curves of the internal circulation cylinder model and the equivalent model Figure 6. The flow curves of the internal circulation cylinder model and the equivalent model The hydraulic system model of the internal circulation two-platen IMM was established, as shown in Figure 7. The mold-moving system was composed of two hydraulic which were connected differentially. This connection can improve the speed of the mold-moving and reduce cycle time. The mold-clamping system was composed of three internal circulation and a cylinder connected to air. This model accomplished a series of actions through position servo control system and hydraulic pressure servo control system. After clicking the button of premier sub-model, each element was assigned the sub-model. In the parameter mode, the parameters of main elements can be seen in Table 2. All other parameters are kept as defaults. Table 2. Parameters of main elements Name Moldmoving Moldclamping Power system Control system Cylinders Valve Cylinders Valve Booster cylinder Internal circulation valve Pump Servo motor Decompressi on valve Piecewise linear signal source Parameter piston diameter 60 mm rod diameter 40 mm length of stroke 0.65 m Import and export flow 128 L/min piston diameter 190 mm rod diameter 90 mm length of stroke 0.65 m Import and export flow 128 L/min piston diameter 110 mm rod diameter 75 mm length of stroke m characteristic flow rate at maximum opening L/min equivalent cross sectional area at maximum opening 5153 mm 2 pump displacement 64 ml/r maximum flow rated speed rated power pressure relief output at start of stage 1 output at end of stage 1 duration of stage 1 output at start of stage L/min 2000 r/min 25 kw 160 bar 0 null 4.26 s

4 Moving platen Mass with ideal end stops output at end of stage 2 duration of stage 2 output at start of stage 3 output at end of stage 3 duration of stage 3 mass higher displacement limit s 0 null 1.8 s 1000 kg m The hydraulic system of IMM was a nonlinear complex system. For the convenience of controller design, the system was simplified as a combination of proportional link, first-order inertia link and delaying link. The moldmoving system adopted position closed-loop control. The mold-clamping system adopted pressure closed-loop control. When the pressure of mold-clamping reached 100 bar, the booster cylinder began to work. When the pressure reached the set value, the servo motor stopped turning and the system continue to the next procedure. (2) The pressure of the mold-clamping can be seen in Figure 9. The extreme and slope of the curve show the pressure of the mold-clamping can reach the set value. The mold-clamping pressure curve has a step. That s because the clamping pressure is too high. The clamping have to reduce pressure step by step. This way is helpful to reduce the impact on the hydraulic system and improve the stability of the hydraulic system. Mold-clamping Results of the simulation and analysis According to the actual process of IMM, the simulation ran 18 s. The following is the simulation results. (1) The displacement of the moving platen can be seen in Figure 8. Figure 9. The pressure of the mold clamping (3) The flow curve of the 2 position 2 port hydraulic control valve can be seen in Figure 10. The dashed line is the flow of the oil returning to the hole of mold-clamping. The flow is very small from 0 to 4.26 s, that s because the back of the clamping connects directly with the fuel tank. When the mold closes, the oil directly returns to the fuel tank. So the flow through the internal circulation valve is very small. After comparing the two curves, the variation tendency of the flow is almost the same. This suggests that the mold-clamping achieve the internal circulation in the process of opening and closing mold. Figure 8. The displacement of the moving platen As can be seen from the Figure 8, the simulative displacement is accorded with the actual movement. The speed of mold opening is faster than that of mold closing. The fast mold opening can shorten product cycle and enhance work efficiency. The slow mold closing can reduce the impact on mold. According to the simulation displacement curve of the moving platen, the machine can run steadily. When compared with the simulation displacement and the actual displacement, there are some delays in simulation results. It is evident from Figure 8 that the delay of mold opening is higher than that of mold closing. That s because the force on the left of mold-moving piston is bigger than the force on the right side. So the left chambers of the mold-moving need more hydraulic oil. The delay of mold opening is higher. Figure 10. The flow curve of the 2 position 2 port hydraulic control valve Conclusions This paper established the hydraulic system models by AMESim. The internal circulation were

5 established by HCD. Through verification, the model of the internal circulation was reasonable. Then, the model parameters were set and a simulation was carried out. Displacement curves of the moving platen, pressure curve of the mold-clamping and flow curves of the internal circulation valve were carried out. The results show that: (1) The simulation results can reflect the real dynamic characteristics of hydraulic system. The hydraulic system model is reasonable. The modeling and simulation for internal circulation two-platen injection molding machine provide basis for the further manufacture. (2) The simulation results are ideal. There is no difference in the forces between the three internal circulation clamping and the clamping cylinder connected to air. There are certain differences between actual situation and parameter settings. (3) The injection molding machine has multiple hydraulic systems and is also a complex and hysteretic system. Only using the closed-loop control has some certain shortcomings. In the future, the joint simulation method of AMESim and ADAMS will provide a new way for the dynamic simulation of the injection molding machine. ADAMS was developed by MDI(Mechanical Dynamics Inc) in USA. ADAMS is the first computer software oriented simulating working characteristic of integer mechanical system. A more reasonable and effective control system will be design by adopting the AMESim and MATLAB/Simulink joint simulation method. Thus, the joint simulation will design a more accurate model to help the optimization of the injection molding machine. References 1. Z.W. Jiao, P.C. Xie, Y. An, X.T. Wang and W.M. Yang, Materials Processing Technology, 211, 1076 (2011). 2. Z.W. Jiao, P.C. Xie, Y.M. Ding and W.M. Yang, the 5th CAE Engineering Analysis and Technology Conference of China (2009). 3. W. Wohlrab and Weissenburg, U.S. Patent 5,336,462 (1994). 4. X.H. Li and R.L. Zhu, Chemical Equipment Technology, 30, 60 (2009). 5. C. Sasikumar, S. Srikanth and S.K. Das, Eng. Fail. Anal, 13, 1246 (2006).