Injection molding machines can mold plastic products with complex shapes, precise dimensions or dense textures with metal inserts in one go. They are widely used in various fields such as national defense, mechanical and electrical, automobile, transportation, building materials, packaging, agriculture, culture, education, health and people's daily life. Injection molding process has good adaptability to processing of various plastics, has high production capacity, and is easy to automate. Today, with rapid development of plastics industry, injection molding machines occupy an important position in both quantity and variety, making them one of the fastest growing and most produced types of plastic machinery.

 

Our country's plastic processing enterprises are spread all over the country, and technical level of their equipment is uneven. Most of processing enterprises' equipment needs technical transformation. In the past few years, technological progress of my country's plastic machine industry has been very significant. In particular, gap between technical level of injection molding machines and foreign famous brand products has been greatly narrowed, significant improvements have been made in terms of control level, internal product quality and appearance modeling. By choosing domestic equipment, with a smaller investment, you can also produce products of same quality as imported equipment. These create conditions for technological transformation of enterprises.
To have good products, you must have good equipment. Wear and corrosion of equipment is a natural law. If people master this law, they can prevent or reduce wear and corrosion of equipment, extend service life of equipment, and ensure integrity of equipment.
In order to strengthen use, maintenance and management of plastic machinery, relevant departments in China have formulated relevant standards and implementation details, requiring all equipment management departments and production enterprises to manage and use equipment to achieve "scientific management, correct use, reasonable lubrication, careful maintenance, regular maintenance, and planned maintenance, so as to improve equipment integrity rate and keep equipment in good condition."
This article has written relevant knowledge and technical information on maintenance and upkeep of injection molding machines, which can be used as a reference for managers and technicians in equipment management departments and production enterprises.
Plastic injection molding technology was developed from the late 19th century to the early 20th century based on principle of die-casting, and is currently one of the most commonly used methods in plastic processing. This law applies to all thermoplastics and some thermosetting plastics (accounting for about 1/3 of the total plastics).

Working principle of injection molding machine is similar to that of a syringe for injection. It uses thrust of screw (or plunger) to inject plasticized plastic into closed mold cavity. Process of obtaining finished products after curing and setting.
Injection molding is a cyclic process. Each cycle mainly includes: quantitative feeding-melting and plasticizing-pressure injection-mold filling and cooling-mold opening and picking. After taking out plastic part, mold is closed again and next cycle is carried out.

Injection molding machines are divided into plunger injection molding machines and screw injection molding machines according to plasticizing method; according to transmission mode of machine, they can be divided into hydraulic, mechanical and hydraulic-mechanical (connecting rod) types; according to operation mode, they are divided into automatic, semi-automatic and manual injection molding machines.

 

(1) Horizontal injection molding machine: This is the most common type. Mold clamping part and injection part are on same horizontal center line, and mold is opened in horizontal direction. Its characteristics are: short body, easy to operate and maintain; low center of gravity of machine, relatively stable installation; after product is ejected, it can automatically fall down by gravity, making it easy to realize fully automatic operation. At present, most injection molding machines on the market adopt this type.
(2) Vertical injection molding machine: Its mold closing part and injection part are on same vertical center line, and mold is opened in vertical direction. Therefore, it occupies a small area, is easy to place inserts, is more convenient to load and unload molds, and materials falling from hopper can be plasticized more evenly. However, it is not easy for product to fall automatically after being ejected. It must be removed by hand, and it is not easy to realize automatic operation. Vertical injection molding machines are suitable for small injection molding machines. Generally, injection molding machines under 60 grams are mostly used. Large and medium-sized machines are not suitable for use.
(3) Angle injection molding machine: Injection direction and mold interface are on same surface. It is especially suitable for flat products that do not allow gate marks to be left in processing center. It occupies a smaller area than a horizontal injection molding machine, but insert placed in mold is prone to tilt and fall. This type of injection molding machine is suitable for small machines.
(4) Multi-mode turntable injection molding machine: It is a special injection molding machine with multi-station operation. Its characteristic is that mold closing device adopts a turntable structure, and mold rotates around rotating shaft. This type of injection molding machine gives full play to plasticizing ability of injection device, can shorten production cycle and improve production capacity of machine. Therefore, it is especially suitable for production of large quantities of plastic products that require a long cooling and setting time or require more auxiliary time to place inserts. However, because mold clamping system is large and complex, clamping force of mold clamping device is often small, so this type of injection molding machine is widely used in production of plastic shoe soles and other products.
A general injection molding machine includes an injection device, a mold clamping device, a hydraulic system and an electrical control system.
Basic requirements for injection molding are plasticization, injection and molding. Plasticization is prerequisite for realizing and ensuring quality of molded products. In order to meet requirements of molding, injection must ensure sufficient pressure and speed. At the same time, due to high injection pressure, a correspondingly high pressure is generated in mold cavity (average pressure in mold cavity is generally between 20 and 45MPa, so there must be a large enough mold clamping force). It can be seen that injection device and mold clamping device are key components of injection molding machine.

 

Nozzle forward→Injection→Pressure maintaining→Pre-molding→Retraction→Nozzle backward→Cooling→Mold opening→Ejection→Needle withdrawal→Opening door→Close closing→Mold closing→Nozzle advancing.

Injection molding machine operation items include three aspects: control keyboard operation, electrical control cabinet operation and hydraulic system operation. Carry out selection of injection process actions, feeding actions, injection pressure, injection speed, ejection type, monitor temperature, current and voltage of each section of barrel, adjust injection pressure and back pressure, etc.

Generally, injection molding machines can be operated manually, semi-automatically and fully automatically.
Manual operation is a production cycle where every action is achieved by operator flipping operating switch. It is generally selected when testing machine and adjusting mold.
During semi-automatic operation, machine can automatically complete a work cycle, but after each production cycle, operator must open safety door, remove workpiece, then close safety door before machine can continue production in next cycle.
In fully automatic operation, injection molding machine can automatically enter next working cycle after completing one working cycle. There is no need to stop machine for control and adjustment during normal continuous working. However, it should be noted that if fully automatic operation is required. (1) Do not open safety door midway, otherwise fully automatic operation will be interrupted; (2) Add materials in time; (3) If electric eye induction is used, be careful not to cover electric eye.
In fact, in fully automatic operations, temporary shutdowns are usually required, such as spraying release agent on machine mold.
During normal production, semi-automatic or fully automatic operation is generally used. When operation starts, operation mode (manual, semi-automatic or fully automatic) should be selected according to production needs, and manual, semi-automatic or fully automatic switch should be moved accordingly.
Semi-automatic and fully automatic working procedures have been determined by circuit itself. Operator only needs to change speed and pressure, length of time, number of ejector pins, etc. on electrical cabinet. Work program will not be confused because operator adjusts wrong keys.
When each action in a cycle has not been adjusted properly, manual operation should be selected first. After confirming that each action is normal, semi-automatic or fully automatic operation should be selected.

Depending on whether injection seat retreats before and after pre-plastic feeding, that is, whether nozzle leaves mold, injection molding machine generally has three options. (1) Fixed feeding: nozzle is always attached to mold before and after pre-molding, and injection seat does not move. (2) Front feeding: Nozzle is pressed against mold for pre-molding and feeding. After pre-molding is completed, injection seat retreats and nozzle leaves mold. Purpose of choosing this method is to use injection hole of mold to resist nozzle during pre-molding to prevent molten material from flowing out of nozzle when back pressure is high. After pre-molding, it can avoid heat transfer caused by long-term contact between nozzle and mold, which affects relative stability of their respective temperatures. (3) Post-feeding: After injection is completed, injection seat retreats, nozzle leaves mold and then is pre-molded. After pre-molding is completed, injection seat advances. This action is suitable for processing plastics with a particularly narrow molding temperature. Due to short contact time between nozzle and mold, heat loss is avoided and solidification of molten material in nozzle hole is avoided.
After injection is completed and cooling timer is completed, pre-molding action begins. Screw rotates to melt plastic and squeeze it to the front of screw head. Because check ring at the front end of screw acts as a one-way valve, molten plastic accumulates at the front end of barrel, forcing screw backward. When screw retreats to a predetermined position (this position is determined by travel switch, which controls retreat distance of screw to achieve quantitative feeding), pre-molding stops and screw stops rotating. This is followed by a retraction action, which means that screw makes a slight axial retreat. This action can relieve pressure of melt accumulated at nozzle and overcome "saliva" phenomenon caused by imbalance of pressure inside and outside barrel. If retraction is not required, retraction stop switch should be adjusted to appropriate position so that retraction stop switch is also pressed at the same time that pre-molding stop switch is pressed. When screw retracts and stop switch is pressed, retraction stops. Then seats began to retreat. When injection seat retreats until stop switch is pressed, injection seat stops retreating. If a fixed feeding method is used, attention should be paid to adjusting position of travel switch.
Generally, fixed feeding methods are used in production to save time of advancing and retracting injection seat and speed up production cycle.

Injection pressure of injection molding machine is adjusted by pressure regulating valve. Under condition of setting pressure, injection pressure of early and late stages is controlled by switching on and off high-pressure and low-pressure oil circuits.
Ordinary medium-sized and above injection molding machines are equipped with three pressure options, namely high pressure, low pressure, high pressure first and then low pressure. High-pressure injection is achieved by introducing high-pressure oil into injection cylinder. Due to high pressure, plastic enters mold cavity at high pressure and high speed from beginning. During high-pressure injection, plastic enters mold quickly, and reading on injection cylinder pressure gauge rises quickly. Low-pressure injection is achieved by introducing low-pressure oil into injection cylinder. During injection process, pressure gauge reading rises slowly, plastic enters mold cavity at low pressure and speed. First high pressure and then low pressure is achieved by controlling pressure of pressure oil flowing into cylinder from a time perspective according to type of plastic and actual requirements of mold.
In order to meet different injection pressure requirements for different plastics, you can also replace screw or plunger with different diameters, which not only meets injection pressure, but also fully utilizes production capacity of machine. Large injection molding machines often have multi-stage injection pressure and multi-stage injection speed control functions, which can better ensure quality and accuracy of product.

 

Generally, there is a fast-slow knob on control panel of injection molding machine to meet injection speed requirements. In hydraulic system, a large-flow oil pump and a small-flow pump operate simultaneously to supply oil. When oil circuit is connected to a large flow rate, injection molding machine can quickly open and close mold, fast injection, etc. When hydraulic oil circuit only provides a small flow rate, various actions of injection molding machine will proceed slowly.

Injection molding machines have two types of ejection: mechanical ejection and hydraulic ejection. Some are also equipped with a pneumatic ejection system, and number of ejections is single or multiple. Ejection action can be manual or automatic.
Ejection action is started by mold opening stop limit switch. Operator can achieve this by adjusting ejection time button on control cabinet as needed. Speed and pressure of ejection can also be controlled by switch on control cabinet. Forward and backward distance of ejection pin movement is determined by travel switch.

A Temperature-measuring thermocouple is used as temperature-measuring element, a temperature-measuring millivolt meter is used as a temperature control device to direct current on and off of barrel and electric heating coil of mold, selectively fix temperature of each section of barrel and mold temperature. Table 5 lists molding processing temperature ranges of some plastics for reference.
Barrel electric heating coils are generally divided into two-stage, three-stage or four-stage control. Ammeter on electrical cabinet displays current of each section of electric heating coil. Ammeter reading is relatively fixed. If ammeter reading is found to be low for a long time during operation, heating coil may be faulty, or wire contact is poor, or heating wire is oxidized and thinned, or a certain heating coil is burned out. This will increase resistance of parallel resistor in circuit and decrease current.
When ammeter has a certain reading, you can also simply use plastic strips to scratch on outer wall of electric heating coil one by one to see whether strips are melted to determine whether a certain electric heating coil is energized or burned out.

Mold clamping uses a huge mechanical thrust to close mold tightly to withstand huge opening force of mold caused by high-pressure injection of molten plastic and filling of mold during injection molding process.
Close safety door, each travel switch will give a signal, and mold closing action will start immediately. First, movable mold plate is started at a slow speed. After advancing a short distance, control rod pressure block that originally pressed slow switch is detached, and movable plate rotates to advance quickly. When it advances to end of mold closing, the other end of control rod presses slow switch again. At this time, movable plate turns to move forward at slow speed and low pressure. During low-pressure mold closing process, if there are no obstacles between molds, they can be closed smoothly until high-pressure switch is pressed. Purpose of turning to high pressure is to straighten machine hinge to complete mold closing action. This distance is very short, generally only 0.3~1.0mm. As soon as high voltage is turned on, mold closing end limit switch is touched. At this time, action stops and mold closing process ends.
Mold clamping structure of injection molding machine includes fully hydraulic type and mechanical linkage type. Regardless of structural form, mold clamping force is finally implemented by connecting rod being fully extended. Straightening process of connecting rod is process of opening movable plate and tail plate, and it is also process of stretching four tie rods under force.

 

Size of mold clamping force can be known from maximum value of oil pressure gauge that rises at moment when mold is tightened. If mold clamping force is large, maximum value of oil pressure gauge will be high, and vice versa. Smaller injection molding machines do not have a mold closing oil pressure gauge. At this time, it is necessary to judge whether mold is really tightly closed based on straightening of connecting rod. If connecting rod straightens easily when injection molding machine closes mold, or fails to straighten "just a little bit", or one of several pairs of connecting rods does not straighten completely, mold expansion will occur during injection molding, and flash or other defects will appear on the part.

When molten plastic is injected into mold cavity and cooling is completed, mold is opened and product is taken out. Mold opening process is also divided into three stages. In the first stage, mold is opened slowly to prevent parts from tearing in mold cavity. In the second stage, mold is opened quickly to shorten mold opening time. In the third stage, mold is opened slowly to reduce impact and vibration caused by inertia of mold opening.

At present, various injection molding machine manufacturers have developed a variety of program control methods, which generally include: injection speed control, injection pressure control, control of plastic filling amount in injection mold cavity, control of plasticizing state such as screw back pressure and speed. Purpose of achieving process control is to improve product quality and maximize performance of machine.

Program control of injection speed is to divide injection stroke of screw into 3 to 4 stages, and use appropriate injection speed in each stage. For example: slow down injection speed when molten plastic first passes through gate, use high-speed injection during mold filling process, and slow down speed at the end of mold filling. Using this method can prevent overflow, eliminate flow marks and reduce residual stress on product.
When filling mold at low speed, flow rate is stable, size of product is relatively stable, and fluctuation is small, internal stress of product is low, stress in all directions inside and outside product tends to be consistent (for example, if a polycarbonate part is immersed in carbon tetrachloride, part molded by high-speed injection molding will tend to crack, but part molded by low-speed injection molding will not crack). Under relatively slow mold filling conditions, temperature difference of material flow, especially large temperature difference between material before and after gate, helps to avoid occurrence of shrinkage cavities and dents. However, due to long filling time, it is easy for parts to appear delamination and poor welding marks, which not only affects appearance, but also greatly reduces mechanical strength.
During high-speed injection, material flow speed is fast. When high-speed mold filling is smooth, melt fills mold cavity quickly, material temperature drops less, and viscosity drops less. Lower injection pressure can be used, which is a hot material filling situation. High-speed mold filling can improve gloss and smoothness of product, eliminate seam lines and delamination, reduce shrinkage and dents, make color uniform, and ensure fullness for larger parts of product. However, it is easy for product to become fat and blistered or yellow, or even burn and scorch, or cause difficulty in demoulding, or uneven mold filling. For high-viscosity plastics, it may cause melt rupture and cause cloud spots on the surface of part.
High-speed and high-pressure injection can be considered in following situations: (1) Plastics with high viscosity and fast cooling speed, long-process parts cannot completely fill all corners of cavity with low pressure and slow speed; (2) For parts with too thin wall thickness, molten material is easy to condense and stay when it reaches thin wall. A high-speed injection must be used to allow molten material to enter cavity immediately before a large amount of energy is consumed; (3) Glass fiber reinforced plastics, or plastics containing a large amount of filling materials, have poor fluidity , in order to obtain parts with smooth and uniform surfaces, high-speed and high-pressure injection must be used.
For high-end precision products, thick-walled parts, parts with large wall thickness changes, and parts with thick flanges and ribs, it is best to use multi-stage injection, such as level two, level three, level four or even level five.