Analysis on the principle and technical conditions of plastic injection molding equipment
The machine has a dense plastic product that can be molded in a complex shape, accurate in size or with metal inserts. It is widely used in national defense, electromechanical, automobile, transportation, building materials, packaging, agriculture, culture and education, and people's daily life. each field. The injection molding process has good adaptability to the processing of various plastics, high productivity, and easy automation. In the rapid development of the plastics industry, injection molding machines occupy an important position in terms of quantity and variety, and thus become one of the fastest growing and most productive models of plastic machinery.
China's plastics plastic injection molding processing enterprise Star Luoqi, all over the country, the technical level of equipment is uneven, most processing enterprises need equipment for technological transformation. In recent years, the technological progress of China's plastic machine industry has been remarkable. In particular, the gap between the technical level of injection molding machines and foreign famous brand products has been greatly reduced, and significant improvements have been made in terms of control level, internal quality and appearance of products. The choice of domestically produced equipment, with a small investment, can also produce products of comparable quality to imported equipment. These have created conditions for the technological transformation of enterprises.
To have good products, you must have good equipment. The wear and corrosion of equipment is a natural law. When people master this law, they can prevent or reduce the wear and corrosion of equipment, extend the life cycle of equipment, and ensure the integrity of equipment.
In order to strengthen the use, maintenance and management of plastic machinery, relevant departments of China have formulated relevant standards and implementation rules, requiring all equipment management departments and production enterprises to “scientific management, correct use, reasonable lubrication, and management of equipment. Careful maintenance, regular maintenance, planned maintenance, improve equipment integrity, so that equipment is always in good condition.
Plastic injection molding technology was developed from the end of the 19th century to the beginning of the 20th century according to the principle of die casting. It is one of the most commonly used methods in plastic processing. This method applies to all thermoplastics and some thermoset plastics (about 1/3 of the total amount of plastics).
1.1 Working principle of injection molding machine
The working principle of the injection molding machine is similar to that of the syringe for injection. It is to inject the plasticized molten state (ie, the viscous flow state) into the closed cavity by the thrust of the screw (or plunger). The process of obtaining the product after curing and setting.
Injection molding is a cyclic process, and each cycle mainly includes: quantitative feeding - melt plasticization - pressure injection - filling mold cooling - mold extraction. After removing the plastic part, the mold is closed again and the next cycle is performed.
1.2 Structure of injection molding machine
According to the plasticizing method, the injection molding machine is divided into a plunger type injection molding machine and a screw type injection molding machine; according to the transmission mode of the machine, it can be divided into hydraulic type, mechanical type and hydraulic type-mechanical (linkage) type; Semi-automatic, manual injection molding machine.
(1) Horizontal injection molding machine: This is the most common type. The mold clamping portion and the injection portion are on the same horizontal center line, and the mold is opened in the horizontal direction. Its characteristics are: short body, easy to operate and maintain; low center of gravity, smooth installation; after the product is ejected, it can be automatically dropped by gravity, easy to achieve fully automatic operation. At present, injection molding machines on the market mostly adopt this type.
(2) Vertical injection molding machine: the clamping part and the injection part are on the same vertical center line, and the mold is opened in the vertical direction. Therefore, the floor space is small, the insert is easy to be placed, the mold is convenient to be loaded and unloaded, and the material falling from the hopper can be plasticized more uniformly. However, it is not easy to automatically fall after the product is ejected, and must be removed by hand, which is not easy to achieve automatic operation. The vertical injection molding machine should be used for small injection molding machines. Generally, it is used in injection molding machines below 60 grams, and large and medium-sized machines should not be used.
(3) Angle injection molding machine: the injection direction and the mold interface are on the same surface, and it is especially suitable for the planar product in which the machining center portion is not allowed to leave the gate mark. It has a smaller footprint than a horizontal injection molding machine, but the inserts placed in the mold are easily tilted down. This type of injection molding machine is suitable for small machines.
(4) Multi-mode rotary injection molding machine: It is a special injection molding machine with multi-station operation. Its characteristic is that the clamping device adopts a rotary structure, and the mold rotates around the rotating shaft. This type of injection molding machine fully utilizes the plasticizing ability of the injection device, can shorten the production cycle and increase the production capacity of the machine, and is therefore particularly suitable for large-volume molding which requires a long time for cooling or requires more auxiliary time for inserting the insert. The production of products, but because of the large and complex clamping system, the clamping force of the clamping device is often small, so the injection molding machine is widely used in the production of plastic soles and other products.
Typical injection molding machines include injection devices, clamping devices, hydraulic systems, and electrical control systems.
The basic requirements for injection molding are plasticization, injection and molding. Plasticization is the premise to achieve and ensure the quality of molded products, and in order to meet the molding requirements, injection must ensure sufficient pressure and speed. At the same time, due to the high injection pressure, a correspondingly high pressure is generated in the cavity (the average pressure in the cavity is generally between 20 and 45 MPa, see Table 1), so a sufficient clamping force must be obtained. It can be seen that the injection device and the clamping device are key components of the injection molding machine.
1.4 Injection Molding Machine Operation
1.4.1 Action procedure of the injection molding machine
Nozzle advance → injection → pressure holding → pre-plastic → retraction → nozzle retreat → cooling → mold opening → ejection → retraction → opening the door → closing the door → closing the mold → nozzle advancement.
1.4.2 Injection molding machine operation items: The injection molding machine operation items include three aspects: control keyboard operation, electrical control cabinet operation and hydraulic system operation. The injection process action, the feeding action, the injection pressure, the injection speed, the selection of the ejection type, the monitoring of the temperature and current, the voltage of each section of the barrel, the adjustment of the injection pressure and the back pressure are respectively performed.
1.4.2.1 Injection process action selection:
Generally, the injection molding machine can be operated manually or semi-automatically and fully automatically.
Manual operation is achieved in a production cycle, each action is triggered by the operator toggle the operation switch. Generally used when the test machine is adjusted.
In semi-automatic operation, the machine can automatically complete one cycle of work, but after each production cycle, the operator must open the safety door, remove the workpiece, and close the safety door, the machine can continue production in the next cycle.
When fully automatic operation, the injection molding machine can automatically enter the next working cycle after completing a work cycle. No downtime is required for control and adjustment during normal continuous operation. However, it should be noted that if full-automatic work is required, (1) do not open the safety door in the middle, otherwise the automatic operation will be interrupted; (2) it should be fed in time; (3) if electric eye induction is used, care should be taken not to block the electric eye.
In fact, in fully automatic operation, it is usually necessary to temporarily stop the machine midway, such as spraying the mold release agent to the machine mold.
For normal production, semi-automatic or fully automatic operation is generally used. When the operation starts, the operation mode (manual, semi-automatic or fully automatic) should be selected according to the production needs, and the manual, semi-automatic or fully automatic switch should be triggered accordingly.
The semi-automatic and fully automatic working procedure has been determined by the line itself. The operator only needs to change the speed and pressure on the panel surface, the length of time, the number of thimbles, etc., without the operator adjusting the button. Make the work program confusing.
Before each action in a cycle is not adjusted properly, you should first select manual operation to confirm that each action is normal, then select semi-automatic or fully automatic operation.
1.4.2.2 Pre-plastic action selection
According to whether the nozzle is retracted before and after the pre-plastic feeding, that is, whether the nozzle leaves the mold, the injection molding machine generally has three options. (1) Fixed feeding: the nozzles are always stuck into the mold before and after pre-molding, and the nozzle does not move. (2) Pre-feeding: The nozzle is pre-plasticized with the mold, the pre-molding is completed, the nozzle is retracted, and the nozzle leaves the mold. The purpose of this method is to use the mold injection hole to assist the nozzle during the pre-molding to avoid the melt flowing out of the nozzle when the back pressure is high. After the pre-molding, the nozzle and the mold can be prevented from contacting for a long time to generate heat transfer, affecting them. The respective temperatures are relatively stable. (3) After feeding: After the injection is completed, the injection seat is retreated, the nozzle leaves the mold and then pre-plasticized, and the pre-molding is completed. This action is suitable for processing plastics with a particularly narrow molding temperature. Since the contact time between the nozzle and the mold is short, the loss of heat is avoided, and the solidification of the melt in the nozzle hole is also avoided.
After the injection is completed and the cooling timer is timed, the pre-forming action starts. The screw rotates to melt the plastic and squeeze it into front of the screw head. Due to the action of the check valve from the retaining ring at the front end of the screw, molten plastic accumulates at the front end of the barrel, and the screw is forced back. When the screw retreats to the predetermined position (this position is determined by the stroke switch, the distance of the screw is controlled to retreat to achieve the quantitative feeding), the pre-plastic stops, and the screw stops rotating. Followed by the retracting action, the retraction means that the screw makes a slight axial retreat. This action can relieve the pressure of the melt collected at the nozzle and overcome the "stubble" caused by the imbalance of the pressure inside and outside the barrel. phenomenon. If it is not necessary to retract, the retraction stop switch should be adjusted to the appropriate position, and the retraction stop switch is also pressed at the same time when the pre-plastic stop switch is pressed. When the screw is retracted and then retracted to the upper stop switch, the retraction stops. Then the seat begins to retreat. When the seat is retracted to the upper stop switch, the seat stops and retreats. If the fixed feeding method is adopted, care should be taken to adjust the position of the travel switch.
In general, the fixed production method is adopted to save the operation time of the advance and retreat of the injection and accelerate the production cycle.
1.4.2.3 Injection pressure selection
The injection pressure of the injection molding machine is regulated by the pressure regulating valve. Under the condition of setting the pressure, the injection pressure of the high pressure and low pressure oil passages is controlled to control the pressure of the injection pressure before and after.
There are three pressure options for the injection molding machine above the medium and medium size, namely high pressure, low pressure and high pressure and low pressure. High pressure injection is achieved by the injection of high pressure oil into the injection cylinder. Due to the high pressure, the plastic enters the cavity at high pressure and high speed from the beginning. When the high pressure injection, the plastic is quickly inserted into the mold, and the reading of the injection cylinder pressure gauge rises quickly. Low-pressure injection is achieved by the injection cylinder introducing low-pressure pressure oil. The pressure gauge reading rises slowly during the injection process, and the plastic enters the mold cavity at low pressure and low speed. The high pressure and low pressure are firstly controlled according to the type of plastic and the actual requirements of the mold to control the pressure of the pressure oil flowing into the cylinder from time to time.
In order to meet the different injection requirements of different plastics, it is also possible to replace the screw or plunger of different diameters, which not only satisfies the injection pressure, but also fully exerts the production capacity of the machine. In large injection molding machines, there are often multiple injection pressures and multi-stage injection speed control functions, which can ensure the quality and precision of the products.
1.4.2.4 Selection of injection speed
There are fast-slow knobs on the control panel of the injection molding machine to meet the injection speed requirements. In the hydraulic system, a large flow oil pump and a small flow pump are simultaneously operated to supply oil. When the oil circuit is connected to a large flow rate, the injection molding machine realizes rapid opening and closing of the mold, rapid injection, etc. When the hydraulic oil circuit only provides a small flow rate, various actions of the injection molding machine are slowly performed.
1.4.2.5 Selection of the ejector form
The injection molding machine has two types of mechanical ejection and hydraulic ejection, and some are equipped with a pneumatic ejector system. The number of ejections is single and multiple. The ejection action can be manual or automatic.
The ejection action is initiated by the mold opening stop limit switch. The operator can do this by adjusting the eject time button on the control cabinet as needed. The speed and pressure of the ejection
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