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Injection molding machine maintenance
Browse: Release date:2018-05-23
Injection molding machines are capable of molding plastic products with complex shapes and precise dimensions or with metal inserts at a time. They are widely used in national defense, electromechanical, automotive, transportation, building materials, packaging, agriculture, culture, education, health and daily life. Life in all areas. The injection molding process has good adaptability to the processing of various plastics, has a high production capacity, and is easy to implement automation. In the rapid development of the plastics industry today, injection molding machines occupy an important position both in quantity and variety, and thus become one of the fastest growing and most productive machines in the plastics machinery industry.
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China's plastics processing companies are located in all parts of the country, and the technical level of equipment is uneven. Most of the processing companies' equipment needs technological transformation. Over the past few years, the technological progress of China's presses industry has been remarkable. In particular, the gap between the technological level of injection molding machines and foreign brand-name products has been greatly reduced. Significant changes have been made in the level of control, internal product quality and appearance. Selecting domestically produced equipment can also produce products with comparable quality to imported equipment, with less investment. These create conditions for the technological transformation of the company.
 
To have good products, we must have good equipment. The wear and corrosion of equipment is a natural law. With this rule, people can prevent or reduce equipment wear and corrosion, extend the life cycle of the equipment, and ensure the equipment's good rate.
 
In order to strengthen the use, maintenance, and management of plastics machinery, relevant departments in China have formulated related standards and implementation rules that require the equipment management and production companies to “scientifically manage, use, and lubricate the equipment properly.” Meticulous maintenance, regular maintenance, planned maintenance, improved equipment availability, and the equipment is always in good condition.
 
This article has written about the injection molding machine maintenance, maintenance of relevant knowledge and technical information for equipment management departments and production companies, management personnel and technical personnel reference.
 
Plastic injection molding technology is developed from the late 19th century to the early 20th century based on the die casting principle. It is one of the most commonly used methods in plastics processing. The law applies to all thermoplastics and partially thermosets (about 1/3 of total plastics).
 
1.1 Working principle of injection molding machine
 
The working principle of an injection molding machine is similar to that of a syringe used for injection. It uses the force of a screw (or plunger) to inject a plasticized molten state (ie, viscous flow state) into a closed mold cavity. After curing and setting, the process of the product is obtained.
 
Injection molding is a cyclic process, and each cycle mainly includes: quantitative feeding - melt plasticization - pressure injection - mold cooling - mold removal. Remove the plastic parts and then close the mold for the next cycle.
 
1.2 Structure of injection molding machine
 
Injection molding machine is divided into plunger injection molding machine and screw injection molding machine according to the plasticization mode; press the machine's transmission mode can be divided into hydraulic, mechanical and hydraulic-mechanical (linkage) type; according to the operation mode is divided into automatic, Semi-automatic, manual injection molding machine.
 
(1) Horizontal Injection Molding Machine: This is the most common type. The mold clamping part and the injection part are on the same horizontal center line, and the mold is opened in the horizontal direction. Its characteristics are: short body, easy operation and maintenance; low center of gravity of the machine, the installation is more stable; after the product ejected, it can be automatically dropped by gravity, and it is easy to realize fully automatic operation. At present, most injection molding machines on the market use this type.
 
(2) Vertical injection molding machine: The mold clamping part and the injection part are in the same vertical center line, and the mold is opened in the vertical direction. Therefore, the floor space is small, and it is easy to place the inserts, and it is more convenient to load and unload the molds, and the material falling from the hopper can be plasticized more uniformly. However, it is not easy for the product to fall automatically after it is ejected, and it must be removed by hand. It is not easy to realize automatic operation. Vertical injection molding machine should be used for small injection molding machine, generally more than 60 grams of injection molding machine used more, large and medium-sized machine should not be used.
 
(3) Angle injection molding machine: its injection direction and the mold interface are on the same plane. It is particularly suitable for the flat products that do not allow traces of gates in the machining center. It covers an area that is smaller than a horizontal injection molding machine, but inserts placed inside the mold are liable to tilt down. This type of injection molding machine should be used for small machines.
 
(4) Multi-mode rotating disc injection molding machine: It is a special injection molding machine with multi-station operation. Its feature is that the clamping device adopts a rotary disc type structure, and the mold rotates around the rotating shaft. This type of injection molding machine gives full play to the plasticizing ability of the injection device, which can shorten the production cycle and increase the production capacity of the machine. Therefore, this type of injection molding machine is particularly suitable for large batches of plastics that require a long cooling time or require more auxiliary time due to the placement of inserts. The production of products, but due to the large and complex clamping system, the clamping force of the clamping device is often small, so this injection molding machine is widely used in the production of plastic shoes and other products.
 
1.3 General injection molding machines include injection devices, mold clamping devices, hydraulic systems, and electrical control systems.
 
The basic requirements for injection molding are plasticization, injection and molding. Plasticization is the prerequisite for achieving and ensuring the quality of molded products. In order to meet the requirements of molding, injection must ensure sufficient pressure and speed. At the same time, due to the high injection pressure, a high pressure is generated in the mold cavity accordingly (the average pressure in the mold cavity is generally between 20 and 45 MPa, see Table 1), so a sufficient clamping force must be provided. It can be seen that the injection device and the mold clamping device are key components of the injection molding machine.
 
1.4 Injection Molding Machine Operation
 
1.4.1 Action Procedure of Injection Molding Machine
 
Nozzle forward→injection→maintain pressure→pre-molding→shrinkage→nozzle retreat→cooling→opening→extrusion→backoff→opening→closing→clamping→nozzle advancement.
 
1.4.2 Injection Molding Machine Operation Items: The injection molding machine operation items include control keyboard operation, electrical control cabinet operation and hydraulic system operation. The injection process operation, feeding operation, injection pressure, injection speed, ejection type selection, temperature and current and voltage monitoring of the barrel, adjustment of injection pressure and back pressure are performed.
 
1.4.2.1 Injection Process Action Selection:
 
General injection molding machines can be operated manually or semi-automatically and fully automatically.
 
Manual operation is in a production cycle, each action is achieved by the operator toggle the operating switch. It is usually used when the test machine adjusts the mold.
 
During semi-automatic operation, the machine can automatically complete a working cycle, but after each production cycle is completed, the operator must pull open the safety door, remove the work piece, close the safety door, and the machine can continue production in the next cycle.
 
In fully automatic operation, the injection molding machine can automatically enter the next working cycle after completing a work cycle. There is no need to stop and control and adjust during normal continuous operation. However, it should be noted that if automatic work is required, (1) do not open the safety door halfway, otherwise the automatic operation is interrupted; (2) feed in time; (3) if you use electric eye sensor, you should pay attention not to cover the electric eye.
 
In fact, in the fully automatic operation, it is usually also necessary to temporarily stop halfway, such as spraying mold release agent to the machine mold.
 
In 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 dialed accordingly.
 
The semi-automatic and fully automatic working procedure is determined by the line itself. The operator only needs to change the speed and pressure, the length of the time, the number of thimble, etc. on the surface of the electric cabinet. It will not be caused by the operator's mis-key. Make the work program appear confused.
 
When each action in one cycle is not adjusted properly, manual operation should be selected first, after confirming that each action is normal, then semi-automatic or fully automatic operation is selected.
 
1.4.2.2 Pre-plastic motion selection
 
According to whether or not the injection seat is retreated before and after pre-plastic feeding, that is, whether the nozzle leaves the mold, the injection molding machine generally has three options. (1) Fixed feeding: Before and after pre-molding, the nozzles are always attached to the mold, and the injection seat does not move. (2) Pre-feeding: The nozzles are pre-molded with the mold, the pre-molding is completed, the injection seat is back, and the nozzle leaves the mold. The purpose of this method is to use the mold injection hole to assist the nozzle during pre-molding to avoid the molten material flowing out from the nozzle when the back pressure is high. After the pre-molding, the nozzle and the mold can avoid heat transfer for a long time and affect the heat transfer. The respective temperature is relatively stable. (3) After feeding: After the injection is completed, the injection seat is retreated, the nozzle leaves the mold and is then pre-molded, and the pre-molding is completed and then the seat is advanced. This action is suitable for plastics with particularly narrow processing temperatures. Due to the short contact time between the nozzle and the mold, heat loss is avoided and solidification of the melt in the nozzle hole is avoided.
 
After the injection is completed and the cooling timer is timed, the pre-molding operation starts. The rotation of the screw melts the plastic and pushes it in front of the screw head. Due to the one-way valve functioned by the back-and-forth ring of the screw, the molten plastic accumulates on the front end of the barrel and forces the screw backwards. When the screw is retracted to a predetermined position (this position is determined by the travel switch, the distance to control the screw retreat, to achieve quantitative feeding), the pre-plastic stop, the screw stops rotating. This is followed by a retracting action. The retraction is a slight axial retraction of the screw. This action allows the pressure of the molten material collected at the nozzle to be relieved, and overcomes the problem of “retaining flaws” caused by the imbalance of the pressure inside and outside the barrel. phenomenon. If retraction is not required, the retraction stop switch should be adjusted to the proper position so that the retraction stop switch is pressed at the same time that the pre-plastic stop switch is pressed. When the screw is retracted to move back to the pressure stop switch, the retraction stops. Then the injection begins to recede. When the injection seat retreats to the pressure stop switch, the injection stop stops. If the fixed feeding mode is adopted, the position of the limit switch should be adjusted.
 
General production mostly uses fixed feeding methods to save injection seats and retreat operation time, speed up the production cycle.
 
1.4.2.3 Injection Pressure Selection
 
The injection pressure of the injection molding machine is adjusted by the pressure regulating valve. Under the condition of setting the pressure, the pressure of the high pressure and low pressure oil channels is switched on and off to control the injection pressure during the early and late periods.
 
Ordinary medium and above injection molding machines are equipped with three pressure options, namely high pressure, low pressure and low pressure after high pressure. High-pressure injection is achieved by injecting high pressure pressure oil into the injection cylinder. Due to the high pressure, the plastic enters the mold cavity at high pressure and high speed from the very beginning. Rapid injection of plastic into the mold during high pressure injection, injection cylinder pressure gauge readings rise quickly. Low-pressure injection is achieved by injecting low-pressure pressure oil into the injection cylinder. The reading of the pressure gauge during the injection rises slowly, and the plastic enters the mold cavity at low pressure and low speed. After the high pressure, the low pressure is achieved by controlling the pressure of the pressure oil introduced into the cylinder in time according to the type of the plastic and the actual requirements of the mold.
 
In order to meet different plastics requirements with different injection pressures, it is also possible to use different diameter screw or plunger replacement methods. This not only satisfies the injection pressure, but also gives full play to the machine's production capacity. In large-scale injection molding machines, there are often multiple stages of injection pressure and multi-stage injection speed control functions, which can better ensure the quality and accuracy of products.
 
1.4.2.4 Selection of Injection Speed
 
General injection molding machine control board has a fast-slow knob to meet the injection speed requirements. In the hydraulic system there is a large flow pump and a small flow pump running oil supply at the same time. When the oil circuit is connected to a large flow, the injection molding machine can realize fast 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 performed slowly.
 
1.4.2.5 Choice of ejector form
 
There are two types of ejector ejectors: mechanical ejector and hydraulic ejector, and some also have a pneumatic ejector system. The number of ejector ejectors is set to single and multiple times. The eject action can be manual or automatic.
 
The eject action is initiated by a die stop limit switch. The operator can achieve this by adjusting the push-out time button on the control cabinet as required. The speed and pressure of the ejector can also be controlled by the switch on the control cabinet surface. The distance between the front and the back of the ejector movement is determined by the travel switch.
 
1.4.2.6 Temperature Control
 
A thermocouple for temperature measurement is used as a temperature measuring element, and a millivolt meter for temperature measurement is used as a temperature control device to direct the on-off of the electric current of the barrel and die heating coil, and the temperature of each section of the barrel and the temperature of the die are selectively fixed. Table 5 lists some plastic molding processing temperature ranges for reference.
 
The barrel electric heating ring is generally divided into two sections, three sections or four sections of control. The ammeter on the electrical cabinet shows the size of each segment of the electric heating coil. The reading of the ammeter is relatively fixed. If the ammeter reading is found to be low for a long time during operation, there may be a failure of the electric heating ring, or poor contact of the wire, or oxidation of the heating wire, or burning of an electric heating ring. Both will increase the resistance of the circuit in parallel and cause the current to drop.
 
When the ammeter has a certain reading, it is also easy to use a plastic strip to wipe the outer wall of the heating coil one by one to see if the strip is melted or not to judge whether a certain electric circle is energized or burned.
 
1.4.2.7 Mold Control
 
Closing is the use of huge mechanical thrust to tighten the mold to withstand the high pressure injection and filling of the mold during the injection molding process.
 
The safety gate is closed and each trip switch gives a signal and the closing action starts immediately. The first is the slow start of the moving plate. After a short distance, the control bar that originally pressed the slow switch is disengaged, and the movable plate moves forward quickly. When it is approaching the end of closing the mold, the lever at the other end of the control lever presses the slow switch again, and at this time, the movable board turns slowly and advances at low pressure. In the process of low-pressure mold clamping, if there is no obstacle between the molds, it can be smoothly closed to press the high-voltage switch. The high-pressure switch is to straighten the machine hinge to complete the mold clamping action. This distance is very short, generally only 0.3 ~ 1.0mm, just turn the high pressure immediately hit the mold closure limit switch, then stop the action, the mold closing process.
 
The mold clamping structure of the injection molding machine is fully hydraulic and mechanical linkage. Regardless of the type of structure, the clamping force is finally exerted by the complete extension of the connecting rod. The straightening process of the connecting rod is a process in which the movable plate and the tail plate are stretched, and the process in which the four rods are pulled by force.
 
The size of the mold clamping force can be known from the highest value of the instantaneous oil pressure table of the tight mold. If the mold clamping force is large, the maximum value of the oil pressure meter will be high, and vice versa. Smaller injection molding machines do not have mold clamping oil pressure gauges. At this time, it is necessary to judge whether the molds are actually tight according to the straightening conditions of the connecting rods. If an injection molding machine molds the connecting rod to straighten easily when it is closed, or if the “difficult” fails to straighten, or if one pair of connecting rods is not fully straightened, the expansion mold will appear during the injection molding. Flashes or other problems occur.
 
1.4.2.8 Open mold control
 
When the molten plastic is injected into the mold cavity and cooling is completed, the product is removed as the mold opening action is performed. The mold opening process is also divided into three phases. The first stage slows the mold opening to prevent the part from tearing in the cavity. The second stage quickly opens the mold to shorten the mold opening time. The third stage slows the mold opening to reduce the impact and vibration caused by open mold inertia.
 
1.4.3 Control of injection molding process conditions
 
At present, various injection molding machine manufacturers have developed a variety of program control methods, generally include: injection speed control, injection pressure control, plastic injection into the mold cavity control, screw back pressure and speed and other plastic refining state control. The purpose of achieving process control is to