High-pressure foaming machine is one of the key equipment in refrigerator and freezer production line. According to the author's long-term experience in repairing high-pressure foaming machine, the common mechanical failures that may occur in important parts of high-pressure foaming machine are analyzed one by one, and specific solutions are proposed. This article has certain guiding significance for maintenance personnel who are new to high-pressure foaming machine and for operators and maintainers of high-pressure foaming machine.
Foaming is a key process in the manufacturing process of household refrigerators and freezers. The quality of foaming directly affects the refrigeration effect and strength of refrigerators, and high-pressure foaming machine is an important basis for providing excellent foaming. High-pressure foaming machine is mainly composed of raw material tank, variable pump, high and low pressure reversing valve, mixing head, hydraulic station, metering system and electrical control system.
Common mechanical failures are concentrated in variable pump, mixing head, high and low pressure reversing valve, hydraulic station and other components. This article analyzes and describes them one by one.
1. Variable pump
The variable pump is the heart of the high-pressure foaming machine. Most of the currently used ones are open axial piston variable pumps, and some closed pumps are also in use. The plunger group of this pump is movable and can swing left and right. Its discharge volume is proportional to the angle of the pump group to the axis. The larger the angle, the greater the discharge volume. The discharge volume and the swing angle basically change linearly. Closed and open variable pumps can be interchanged, as long as the raw material pipeline is slightly changed. Common problems with variable pumps include unstable flow, no flow output, loud pump noise, pump head leakage and other faults.
1.1 Unstable flow of variable pump
The main reasons are: insufficient feed at the pump input or too large gap between the pump plunger and the inner wall. Insufficient suction of variable pump may be caused by dirty filter or too low pre-pressure of raw material tank. Clean the filter and increase the air pressure. Generally, according to process requirements, the air pressure of the material tank should be controlled at 2.5-3.0BAR. If the flow fluctuation during low-pressure cycle and high-pressure cycle is too large, this is because the gap between the pump plunger and the inner wall is too large, and a new pump needs to be replaced. In addition, there are several sealing rings at the discharge port of the pump. If there is a leak, it will also cause unstable flow. Whether the rotating motor speed of the variable pump is balanced also affects the stability of the flow rate.
1.2 Variable pump has no flow output
When the output of the variable pump suddenly becomes zero, it is very likely that the disc-shaped spring piece pressed between the oil distribution plate and the piston head in the pump has lost its elasticity or the retaining ring has fallen off. Just replace the disc-shaped gasket and reinstall the retaining ring. When a shallow groove on the oil distribution plate is completely worn out, the oil distribution plate and the piston head that matches it will be scrapped. There is a channel between the end cover and the piston jacket for the high-pressure raw material to flow. There is a sealing ring between these two surfaces. If it is broken, the variable pump output will also be lost.
1.3 Noise of variable pump
It is also a common fault during the use of the variable pump. The main reason is that the material temperature is too low, resulting in high viscosity of the raw material and too much gas in the raw material (especially combined polyether). If the material temperature is too low, the raw material can be heated by turning on the heater, or the pressure of the mixing head can be reduced, and the raw material temperature can be increased through continuous high-pressure circulation. In order to avoid excessive gas in the raw material, the raw material should not be stirred too much when not working, and the raw material in the material tank should not be too little, and it should be controlled at 1/2 of the material tank. In addition, if the bearing in the bearing group at the variable pump port is broken, it is also easy to produce noise. At this time, just replace the bearing. When the material tank is empty, if the pump is still started, it will produce a harsh sound, which is very easy to bite the pump and scrap it. Be careful not to run the pump empty. The electrical protection circuit is also set to prevent the high-pressure pump from running empty.
1.4 Leakage of variable pump
The shaft seal where the variable pump main shaft extends out of the pump body often leaks. The shaft seal and sleeve need to be replaced regularly. When replacing the shaft seal, pay attention to the direction and gentle movement. Since the sleeve and the shaft are interference fit, special tools must be used to remove them. Be careful not to put stolen goods into the pump. When installing the sleeve, heat the sleeve. Use an industrial hair dryer. Never use acetylene to heat it. When replacing the shaft seal, we must fill the two shaft seals with DOP to prevent air intrusion and lubricate the outer ring.
2. Mixing head
The mixing head is a component for mixing and pouring two raw materials. The L-shaped gun head with two pistons is used now. Compared with the first-generation single-piston gun head, it has the advantages of good mixing effect and small splashing. The main faults of the mixing head include large piston deadlock, oil leakage, pressure fluctuation, etc.
2.1 Mixing head deadlock
The main reasons are that the gap between the large piston rod and the inner wall is too large, the surface of the large piston rod is rough, the foaming liquid is not cleaned up, and the lubrication is not good. The solution is to keep the mixing head clean and add DOP to the piston rod at regular intervals and quantitatively to lubricate it. The time and number of self-cleaning of the mixing head at the end of the pouring should also be adjusted according to the changes in the proportion of cyclopentane, the changes in ambient temperature, and the changes in the composition of pure white materials.
2.2 Oil leakage of the mixing head
The solution to the oil leakage problem is to replace the seal. Pay special attention to the shaft seal of the small piston (because its leakage oil hole is often blocked by the foaming liquid, even if the shaft seal is broken, it is not easy to find, causing the hydraulic oil to enter the foaming liquid when the small cylinder is opened (that is, when the mixing head is injected), causing serious quality accidents, so the leakage oil hole of the small cylinder should be kept unobstructed and the shaft seal should be checked and replaced regularly. If the service life is too long, the gap between the small piston rod and its matching inner wall is too large, causing the black and white materials to penetrate each other from the return groove and cause the circuit to be blocked. At this time, the mixing head can only be scrapped.
2.3 Unstable pressure of mixing head
Unstable pressure during injection is a common problem of mixing head, mainly caused by dirty nozzles and improper installation. It should be cleaned regularly. In addition, if the filter screen has too low filtration accuracy or is damaged, the dirt will enter the nozzle and cause pressure fluctuations, so the filter element should also be carefully checked for damage and cleaned regularly. When installing the nozzle, clean the nozzle base and ensure that the sealing ring between high and low pressure is intact. Due to the different viscosities of black material and white material, a pressure difference is generated between the two return grooves of the small piston during high-pressure circulation, causing the white material with higher pressure to penetrate into the black material with lower pressure, resulting in dirty and blocked black material circuit and unstable pressure, so it should be cleaned regularly.
3. High and low pressure reversing valve: It generates material pressure together with the mixing head nozzle.
3.1 High and low pressure reversing valve is not closed
If the high and low pressure reversing valve is not closed, it will cause pressure between the mixing heads; if the main high and low pressure reversing valve is not closed or is not closed tightly, it will cause the mixing head to have no pressure or the pressure will be reduced, resulting in failure of raw material infusion. The sealing ring between the valve core pressure rod of the high and low pressure reversing valve should be replaced frequently, otherwise the leaked black material will soon be brought into the reversing valve through the air path, causing the reversing valve to be blocked.
3.2 High and low pressure reversing valve is opened Not open
When such a fault occurs, you should generally check whether there is a problem with the air valve first. Sometimes, although the air valve is already working with power from the electrical signal, the air source pressure supplied to the air valve is too small, which can also cause the reversing valve to be unable to open. Since black material is easy to crystallize, the valve core of the rotary valve is often stuck and cannot move. It should be cleaned regularly and lubricated with DOP.
4. Hydraulic station
The hydraulic station provides power for the mixing head filling action. Since the filling amount of the mixing head is closely related to the opening and closing time of the cylinder, the oil pressure it provides must be high enough and stable. The common problem of the hydraulic station is that it cannot output pressure and the pressure drops during output. Since the hydraulic oil continues to flow in the high-pressure pipeline, the temperature will rise, and its viscosity will decrease with the increase in temperature, affecting the normal operation of the hydraulic system, so the hydraulic station must have a heat exchanger and a chiller or use a circulating The water circuit of the ring water and the control of the oil temperature can ensure the long-term operation of the hydraulic components and the non-deterioration of the hydraulic oil. .
4.1 The pressure of the hydraulic station does not rise high
If the filter screen at the suction port of the hydraulic pump is too dirty, the pump cannot absorb the oil. Not only will the oil pressure not rise high, but it will also accelerate the wear of the pump, so the filter element should be cleaned frequently. The low pressure rise is also related to the wear of the pump and the overflow valve. The gear pump currently used is prone to wear and needs to be replaced frequently due to its own structure. The overflow valve is prone to fatigue due to long-term use of its spring, and should be adjusted regularly to prevent pressure leakage.
4.2 Pressure fluctuation of the hydraulic station
We also often encounter excessive pressure fluctuations, which are mostly caused by the bladder in the accumulator being broken or the nitrogen pressure in it being too low. We can replace the bladder and refill the nitrogen. Note that the pressure of the nitrogen should not be too high or too low, and the pressure can be filled to 100MPa. In addition, the internal leakage of the reversing valve core also causes hydraulic pressure fluctuations.