Tips and precautions for designing hydraulic system of injection molding machine

Hydraulic technology has been introduced into the industrial field for more than a hundred years. With the rapid development of industry, hydraulic technology is changing with each passing day. With the development of mathematics, control theory, computers, electronic devices and hydraulic fluidics, hydraulic servo systems have emerged and have matured as an applied science, forming their own systems and a set of effective analysis and design methods. The author takes the application of hydraulic systems in injection molding machines as an example, focusing on the methods of hydraulic system design and issues that should be paid attention to. The basis for designing the hydraulic transmission system of injection molding machine
(1) The overall layout and process requirements of the ceramic micro injection molding machine, including the types of movements of the injection molding machine accomplished by hydraulic transmission, the types and models of possible hydraulic actuators proposed during mechanical design, the location of the actuators and the size range of the space, The degree of automation required, etc.
(2) The working cycle of the injection molding machine, the movement mode of the actuator (moving, rotating or swinging), and the scope of the completed work.
(3) The movement speed, speed regulation range, working stroke, load properties and change range of the hydraulic actuator.
(4) The action sequence and interlocking requirements of each component of the injection molding machine, as well as the working environment and floor space of each component, etc.
(5) The working performance of the hydraulic system, such as working stability, reliability, reversing accuracy, dwell time and punch-out volume, etc.
(6) Other requirements, such as pollution, corrosion, flammability, and the quality, dimensions and economy of the hydraulic device. Steps to design hydraulic transmission system
1. Clarifying the working requirements for the hydraulic transmission system is the basis for designing the hydraulic transmission system, and is proposed by the user department in the form of a technical mission statement.
2. Draw up hydraulic transmission system diagram
(1) According to the movement form of the working parts, rationally select the hydraulic actuator;

(2) According to the performance requirements and action sequence of the working Electronic parts, list the various basic circuits that may be realized. At this time, attention should be paid to selecting the appropriate speed regulation scheme and speed switching scheme, and determining safety measures and unloading measures to ensure the completion of the automatic working cycle and the sequence and reliability of the operation.

After the hydraulic transmission scheme is drawn up, a formal schematic diagram should be drawn according to the graphic symbols specified by national standards. The model and specifications of each hydraulic component should be marked in the diagram, as well as the action cycle diagram of the actuator and the action cycle table of the electrical components. At the same time, a list of standard (or universal) components and auxiliary components should be listed.
3. Calculate the main parameters of the hydraulic system and select hydraulic components
(1) Calculate the main parameters of the hydraulic cylinder;

(2) Calculate the required flow rate of the hydraulic cylinder and select a hydraulic pump;

(3) Select oil pipes;

(4) Select component specifications;

(5) Calculate the actual working pressure of the system;

(6) Calculate the power and select the motor;

(7) Calculate the heat generation and fuel tank volume;

4. Carry out necessary hydraulic system verification.
5. Structural design of hydraulic device.
6. Draw hydraulic system working diagrams and prepare technical documents. Issues that should be paid attention to when designing the hydraulic transmission system
1. When combining basic circuits, attention should be paid to preventing mutual interference between circuits to ensure a normal working cycle.
2. Improve system efficiency and prevent system overheating. For example, if the power is small, a throttling speed control system can be used; if the power is large, it is best to use a volumetric speed control system; frequent parking brakes should enable the pump to unload in time; systems with greatly different fuel consumption rates in each working cycle , high-efficiency circuits such as accumulators or pressure-compensated variable pumps should be considered.
3. Prevent hydraulic shock. For high-pressure and large-flow systems, you should consider using a hydraulic reversing valve instead of an electromagnetic reversing valve to slow down the reversing speed; use an accumulator or add a buffer circuit to eliminate hydraulic shock.
4. The system should strive to be simple on the premise of meeting the working cycle and productivity. The more complex the system, the more chances of failure. The system must be safe and reliable. The actuators that perform vertical motion to lift heavy objects should be equipped with a balance loop; the actuators that have strict sequential action requirements should use a sequential action loop with stroke control. In addition, there should also be interlocking devices and some safety measures.
5. Try to achieve standardized and serialized design and reduce the design of special steel parts. Issues to note when using hydraulic systems

(1) Users should understand the working principle of the hydraulic system and be familiar with various operations and the position and rotation direction of the adjustment handle.

(2) Before driving, check whether the adjustment handles and handwheels on the system have been touched by unrelated persons, whether the positions of the electrical switches and travel switches are normal, whether the tools on the main engine are installed correctly and firmly, etc., and then check whether the guide rails and piston rods are properly installed. Wipe the exposed parts before driving.

(3) When driving, first start the hydraulic pump for the control oil circuit. If there is no dedicated hydraulic pump for the control oil circuit, the main hydraulic pump can be started directly.

(4) Hydraulic oil should be checked and replaced regularly. For newly put into use hydraulic equipment, the oil tank should be cleaned and replaced with new oil after about 3 months of use. From now on, clean and oil change every six months to one year.

(5) Pay attention to the oil at all times during work. During normal operation, the temperature of the oil in the tank should not exceed 60°C. If the oil temperature is too high, try to cool it down and use hydraulic oil with a higher viscosity. When the temperature is too low, preheating should be performed, or intermittent operation should be performed before operation, so that the oil temperature gradually increases before entering the formal operating state. (6) Check the oil level to ensure there is sufficient oil in the system.

(7) Systems with exhaust devices should be exhausted, and systems without exhaust devices should be run back and forth multiple times to allow them to naturally discharge gas.

(8) The fuel tank should be covered and sealed, and an air filter should be installed at the ventilation hole above the fuel tank to prevent the intrusion of dirt and moisture. When refueling, filtering should be performed to keep the oil clean. (9) The system should be equipped with coarse and fine filters as needed, and the filters should be inspected, cleaned and replaced frequently.

(10) To adjust the pressure control components, generally first adjust the system pressure control valve – the relief valve, open it when the pressure is zero, and gradually increase the pressure until it reaches the specified pressure value; then adjust the pressure of each circuit in turn. Pressure control valve. The adjustment pressure of the safety relief valve of the main oil circuit hydraulic pump is generally 10%–25% greater than the required working pressure of the actuator. The adjustment pressure of the pressure valve of a fast-moving hydraulic pump is generally 10%–20% greater than the required pressure. If unloading pressure is used to supply the control oil circuit and lubricating oil circuit, the pressure should be maintained in the range of 0.3–0.6MPa. The adjustment pressure of the pressure relay should generally be 0.3–0.5MPa lower than the oil supply pressure.

(11) The flow control valve should be adjusted from small flow rate to large flow rate, and should be adjusted gradually. The flow control valves of synchronous motion actuators should be adjusted at the same time to ensure smooth movement:

① the system produces noise and vibration;

② the moving parts crawl;

③ the pressure in the system is insufficient;

④ the speed of the moving parts is abnormal;

⑤ the oil temperature is too high;

⑥ Abnormal reversing or starting;

Causes and troubleshooting methods for crawling of working parts

(1) Because the compressibility of air is large, when the liquid containing bubbles reaches the high-pressure area and is severely compressed, the volume of the oil will become smaller, causing the Working parts crawl. ​ ​ ​

Take measures: Set up an exhaust device at a high position in the system loop to remove the air.

(2) Because the friction resistance between relatively moving parts is too large or the friction resistance changes, the working parts will crawl during movement. Measures to be taken: There are certain requirements for the geometric tolerance and surface roughness of parts such as hydraulic cylinders, pistons and piston rods; and the cleanliness of the hydraulic system and hydraulic oil should be ensured to prevent dirt from being caught between the surfaces of relatively moving parts, thus Increase friction resistance.

(3) Poor lubrication between the surfaces of moving 3d printing sports car parts results in dry friction or semi-friction, which can also easily lead to crawling. ​ ​​Measures to be taken: Frequently check the lubrication between surfaces of relatively moving parts to keep them in good condition.

(4) If the sealing and centering of the piston and piston rod of the hydraulic cylinder are poor, crawling will also occur. ​ ​Measures to be taken: The load should be removed, the hydraulic cylinder should be operated alone, and the centering should be corrected after measuring the friction resistance.

(5) Crawling occurs due to severe hydraulic cylinder leakage.
Measures to be taken: reduce leakage losses, or increase hydraulic pump capacity.