Created on:2021-06-23 15:50

Hydraulic fault diagnosis and maintenance of injection molding machine

Hydraulic fault diagnosis and maintenance of injection molding machine

A BOY15S injection molding machine is imported from Germany. It is a horizontal super small plastic injection molding machine. All the machines are driven by hydraulic pressure. The whole system uses only one variable vane pump. The control part adopts integrated block structure. It has manual, semi-automatic and automatic working cycle system. Main technical parameters: maximum injection volume 25g; The clamping force is 220kn; The diameter of screw is 22mm; The screw stroke is 80mm; The stroke volume is 30 cm3.

(1) Working principle of hydraulic transmission system

The working principle of the hydraulic system is shown in Figure a.

A.jpg

① The whole working cycle of clamping and locking starts from die closing. After the machine is started with the button, the die is closed quickly at first. When the moving die is close to the fixed die, S1 is powered off, S17 is powered on, and the speed is slowed down, so as to reduce the impact force of closing the die; After the mold is closed in place, press down the travel switches B13, S1 and S2 to be electrified, the mold locking booster cylinder IV starts to work, and the high-pressure oil enters the a cavity of the dynamic mold hydraulic cylinder I to lock the mold.

② When the injection position moves forward and the injection is locked, a pressure oil enters the e cavity of the hydraulic cylinder II, and the whole injection device moves forward to make the nozzle fit with the mold. Then S9 is charged. The pressure oil enters the j cavity of the injection cylinder III through the pressure reducing valve LG2 and the throttle valve drv1, and drives the screw to inject the molten material into the mold cavity at high pressure and high speed. At this time, the pressure of screw head acting on molten material is injection pressure, also known as primary pressure, which is regulated by direct acting relief valve db1.

③ Due to the cooling effect of the low-temperature mold, the molten material injected into the mold cavity shrinks. In order to make compact products, the molten material should be kept at a certain pressure for feeding: therefore, S18 is charged, the pressure in the left cavity of the injection cylinder III is controlled by the pressure valve db3, and the pressure exerted by the screw on the molten material is called the holding pressure, also known as the secondary pressure. During pressure holding, the screw moves forward slightly due to feeding.

④ Cooling and pre plasticizing of products when the molten material in the mold cavity loses the possibility of flowing out from the gate, the holding pressure is removed and the clamping pressure is also removed, so that the products are cooled and shaped in the mold. At this time, S13 is charged, the pressure oil enters the hydraulic motor through the check valve LG4 and the overflow throttle valve composed of lg5 and DR1, drives the screw to rotate, and transports the granular plastic from the hopper forward and plasticizes it; At the same time, the screw retreats again. When it retreats to the metering value, it stops rotating immediately.

⑤ The injection device retreats and the products are ejected by opening the mold to make S5 electrified. The pressure oil enters the right chamber of hydraulic cylinder II (chamber f) to drive the whole injection device back. Then S3 is electrified, the mold is opened, and S15 is electrified after the mold is opened in place. The pressure oil enters the working cavity of the ejection hydraulic cylinder, pushes the ejection rod, and ejects the product from the mold, which completes the whole working cycle.

(2) Fault diagnosis of hydraulic transmission system of injection molding machine

There are two kinds of faults in the injection molding machine: one is that the pressure of the mold lock is weak; The other is that the cavity is not filled with plastic.

① When this kind of failure occurs, the logic diagnosis method is used to diagnose. First, judge whether it is caused by electrical reasons. After inspection, the electrical part works normally, and eliminate the electrical reasons; Then analyze the possible faults in the hydraulic system.

a. Whether the sealing ring on the piston of clamping cylinder I is damaged and causes internal leakage, which makes the left and right cavities string oil and causes insufficient pressure.

b. Whether the piston of booster cylinder IV is stuck or whether the sealing ring on the piston is worn or not will cause serious internal leakage and lead to weak pressurization, which is the possible cause of weak pressurization. During the inspection, the oil path from the booster cylinder to the clamping cylinder was blocked, that is, the U point was blocked. The pressure gauge was observed and it was found that the pressure could reach 220kn, so the possibility of failure of the booster cylinder was ruled out.

c. Whether the above fault is caused by the change direction of the directional valve is not in place or the valve core is stuck. Because of the same type of spare directional valve, the two directional valves S1 and S2 were replaced successively, but the problem still existed, so the possibility of directional valve failure was ruled out.

d. Check whether the LG cone surface of the check valve is worn, not round or concentric, whether there is dirt on the cone surface causing leakage, and the oil pressure cannot be maintained. The check valve was disassembled, and the contact condition between the poppet valve and the valve seat joint surface was carefully checked. No leakage of the poppet surface was found, so the possibility of failure of the check valve LG was ruled out.

Finally, it can be judged that the fault occurred on the hydraulic cylinder I. when the hydraulic cylinder I was disassembled, it was found that the lock nut of the piston was loose, and the left end of the piston was seriously worn. The left end of the hydraulic cylinder was abraded up to 60mm, with a depth of 1.5-2mm. The sealing ring on the piston was also damaged, resulting in the internal leakage of the hydraulic cylinder. After repair, the machine was installed again, and the machine returned to normal.

② Generally speaking, this phenomenon is caused by the speed error of the hydraulic cylinder (about 5%) or insufficient pressure of the pressure valve. After careful observation, it was found that the speed of the injection cylinder was normal. When injection, the pointer of the pressure gauge will not rise after rising to a certain value, which can not meet the injection requirements, so the plastic will not fill the mold cavity. According to the analysis of hydraulic schematic diagram and working principle, the reasons for the pressure not to go up may be as follows: A. constant pressure variable pump failure; b. Injection cylinder failure; c. The directional valve S9 does not work normally; d. The direct acting relief valve db1 is faulty; e. Rvg3 hydraulic control check valve leakage.

Check each possibility one by one. First of all, rule out a, because the core insertion, mold closing and pre molding before injection are very normal, and these processes use the pump, but the position of the directional valve is different. Careful observation of these directional valves showed that they were working normally. Because of the complexity of the injection cylinder, the selection should be made according to the principle of easy first and then difficult, and the inspection should be carried out later, so that the analysis can be started. Check the directional valve with "component replacement method", find a spare directional valve of the same model, find that the problem still exists, eliminate the fault of directional valve S9. Then check the check valve rvg3, disassemble the check valve, and carefully check the joint between the poppet valve and the valve seat. There is no wear and non tightness. At this point, it is certain that the fault occurred in the direct acting relief valve and injection cylinder. When the overflow valve was removed, it was found that there was dirt sticking on the conical surface of the valve core, which stuck the valve core in the open state, and the spring was also broken, so the injection pressure was not high. After cleaning, replacing the damaged spring and re installing db1, the fault was eliminated.

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