Injection molding machines can form plastic products with complex shapes, precise dimensions or dense textures with metal inserts in one go. They are widely used in defense, electromechanics, automobiles, transportation, building materials, packaging, agriculture, culture, education, health and people's daily life.

 

Injection molding process has good adaptability to processing of various plastics, high production capacity, and is easy to automate. In today's rapidly developing plastics industry, injection molding machines occupy an important position in terms of quantity and variety, thus becoming one of the fastest growing and most produced models of plastic machinery.
China's plastic processing enterprises are scattered all over country, and technical level of equipment is uneven. Most processing enterprises' equipment needs technical transformation. In recent years, technological progress of China's plastic machinery industry has been very significant, especially gap between technical level of injection molding machines and foreign famous brands has been greatly narrowed, and significant improvements have been made in control level, product internal quality and appearance. Choosing domestic equipment can also produce products of comparable quality to imported equipment with a smaller investment. These have created 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 rules, 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, planned maintenance, improve equipment integrity, and keep equipment in good condition.
This article writes relevant knowledge and technical information on maintenance of injection molding machines for reference by managers and technicians of 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. It is currently the most important part of plastic processing. One of the most commonly used methods. This method is applicable to all thermoplastics and some thermosetting plastics (accounting for about 1/3 of the total amount of plastics).

Working principle of injection molding machine is similar to that of syringe for injection. It uses the thrust of screw (or plunger) to inject plasticized molten state (i.e. viscous flow state) plastic into closed mold cavity, obtains product after solidification and shaping.
Injection molding is a cyclic process, and each cycle mainly includes: quantitative feeding - melt plasticization - pressure injection - mold filling cooling - mold opening and taking out. After taking out plastic part, mold is closed again for next cycle.

Injection molding machines are divided into plunger type injection molding machines and screw type injection molding machines according to plasticizing method; they can be divided into hydraulic type, mechanical type and hydraulic-mechanical (connecting rod) type according to transmission method of machine; they can be divided into automatic, semi-automatic and manual injection molding machines according to operation method.
(1) Horizontal injection molding machine: This is the most common type. Its 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 by gravity, and it is easy to achieve full automatic operation. At present, most injection molding machines on the market adopt this type.
(2) Vertical injection molding machine: Its mold clamping 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 convenient to load and unload molds, and materials dropped from hopper can be plasticized more evenly. However, product is not easy to fall automatically after being ejected, and must be removed by hand, which is not easy to achieve automatic operation. Vertical injection molding machines are suitable for small injection molding machines, generally more for injection molding machines below 60 grams, are not suitable for large and medium-sized machines.

 

(3) Angle injection molding machine: Its injection direction and mold interface are on same plane. It is particularly suitable for flat products where gate marks are not allowed in machining center. It occupies a smaller area than horizontal injection molding machine, but inserts placed in mold are easy 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, characterized by a clamping device Turntable structure is adopted, and mold rotates around rotating shaft. This type of injection molding machine fully utilizes plasticizing ability of injection device, which can shorten production cycle and improve production capacity of machine. Therefore, it is particularly suitable for production of large quantities of plastic products that require long cooling and shaping time or more auxiliary time due to the placement of inserts. However, due to large and complex clamping system, clamping force of clamping device is often small, so this type of injection molding machine is widely used in production of products such as plastic soles.
A general injection molding machine includes an injection device, a clamping device, a hydraulic system, and an electrical control system.
Basic requirements of injection molding are plasticizing, injection, and molding. Plasticizing is prerequisite for achieving and ensuring quality of molded products, and in order to meet requirements of molding, injection must ensure sufficient pressure and speed. At the same time, due to high injection pressure , correspondingly, a very high pressure is generated in mold cavity (average pressure in mold cavity is generally between 20 and 45 MPa), so there must be a sufficiently large clamping force. It can be seen that injection device and clamping device are the key components of injection molding machine.

 

1. Action procedure of injection molding machine
Nozzle forward → injection → pressure holding → pre-plasticization → retraction → nozzle retreat → cooling → mold opening → ejection → needle withdrawal → door opening → door closing → mold clamping → nozzle forward.
2. Injection molding machine operation items: Injection molding machine operation items include three aspects: control keyboard operation, electrical control cabinet operation and hydraulic system operation. Injection process action, feeding action, injection pressure, injection speed, ejection type selection, monitoring of temperature, current and voltage of each section of barrel, adjustment of injection pressure and back pressure are performed respectively.
3. Injection process action selection:
Generally, injection molding machines can be operated manually, semi-automatically, or fully automatically.
Manual operation is in a production cycle, and each action is achieved by operator turning operating switch. It is usually selected when testing machine and adjusting mold.
In 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 next cycle of production.
In fully automatic operation, injection molding machine can automatically enter next work cycle after completing a work cycle. There is no need to stop machine for control and adjustment during normal continuous operation. However, it should be noted that if fully automatic operation is required, then
(1) Do not open machine in the middle Open safety door, otherwise fully automatic operation will be interrupted;
(2) Add materials in time;
(3) If electric eye induction is selected, be careful not to cover electric eye.
In fact, temporary shutdown is usually required in fully automatic operation, such as spraying mold release agent on machine mold.
In normal production, semi-automatic or fully automatic operation is generally used. At the beginning of operation, 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 turned accordingly.
Semi-automatic and fully automatic working procedures have been determined by circuit itself. Operator only needs to change speed and pressure, time, number of ejection pins, etc. on electrical cabinet surface, and working procedure will not be confused due to operator's wrong adjustment of button.
Before all actions in a cycle are properly adjusted, manual operation should be selected first. After confirming that each action is normal, semi-automatic or fully automatic operation can be selected.

Depending on whether injection seat retreats before and after pre-molding, 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 presses against mold for pre-molding. After pre-molding, injection seat retreats and nozzle leaves mold. Purpose of choosing this method is to use mold injection hole to assist nozzle during pre-molding to prevent molten material from flowing out of nozzle when back pressure is high. After pre-molding, it can avoid nozzle and mold from contacting for a long time, generating heat transfer, which affects relative stability of their respective temperatures.
(3 ) Post-feeding: After injection is completed, injection seat retreats, nozzle leaves mold and then pre-molding, and injection seat moves forward after pre-molding. This action is suitable for plastics with a particularly narrow molding temperature. Since contact time between nozzle and mold is short, heat loss is avoided and solidification of molten material in nozzle hole is avoided.
After injection is completed and cooling timer is finished, pre-molding action begins. Screw rotates to melt plastic and squeeze it to the front of screw head. Due to role of one-way valve played by stop ring at the front end of screw, molten plastic accumulates at the front end of barrel, forcing screw to retreat backwards.
When screw retreats to predetermined position (this position is determined by travel switch, which controls distance screw retreats to achieve quantitative feeding), pre-molding stops and screw stops rotating. This is followed by retraction action, which is a slight axial retraction of screw , this action can release pressure of molten material gathered at nozzle, overcome "drooling" phenomenon caused by imbalance of pressure inside and outside barrel.
If retraction is not required, retraction stop switch should be adjusted to an appropriate position so that retraction stop switch is pressed at the same time as pre-plasticization stop switch is pressed. When screw retracts and retreats to stop switch, retraction stops. Then injection seat begins to retreat. When injection seat retreats to stop switch, injection seat stops retreating. If a fixed feeding method is used, attention should be paid to adjusting position of stroke switch.
In general, fixed feeding methods are often used in production to save injection seat advance, retreat operation time and speed up production cycle.

 

Injection pressure of injection molding machine is adjusted by pressure regulating valve. Under condition of set pressure, injection pressure of injection molding machine is adjusted by pressure regulating valve. By opening and closing high-pressure and low-pressure oil circuits, injection pressure at the front and back ends is controlled.
Ordinary medium-sized and larger injection molding machines are equipped with three pressure options, namely high pressure, low pressure, high pressure first and low pressure later. High-pressure injection is achieved by passing 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 pressure gauge reading of injection cylinder rises quickly.
Low-pressure injection is achieved by passing low-pressure oil into injection cylinder. Pressure gauge reading rises slowly during injection process, plastic enters mold cavity at low pressure and low speed. High pressure first and low pressure later is achieved by controlling pressure of pressure oil entering cylinder in time according to actual requirements of type of plastic and mold.
In order to meet requirements of different plastics, there are different injection pressures , or you can use method of replacing screws or plungers of different diameters, which not only meets injection pressure, but also gives full play to production capacity of machine. Large injection molding machines often have multi-stage injection pressure and multi-level injection speed control functions, which can better ensure quality and precision of products.

Generally, there are fast-slow knobs on control panel of injection molding machine to meet requirements of injection speed. In hydraulic system, a large-flow oil pump and a small-flow pump are set to run and supply oil at the same time. When oil circuit is connected to a large flow, injection molding machine can realize fast opening and closing of mold, fast injection, etc. When hydraulic oil circuit only provides a small flow, various actions of injection molding machine are carried out slowly.

Injection molding machine ejection forms include mechanical ejection and hydraulic ejection. Some are also equipped with pneumatic ejection systems. Number of ejections is set to single and multiple. Ejection action can be manual or automatic.
Ejection action is started by mold opening stop limit switch. Operator can adjust ejection time button on control cabinet as needed. Ejection speed and pressure can also be controlled by switch on control cabinet, front and rear distance of ejector movement is determined by travel switch.

Temperature measuring thermocouple is used as temperature measuring element, temperature measuring millivoltmeter is used as a temperature control device to command on and off of current of barrel and mold electric heating coil, selectively fix temperature of each section of barrel and mold temperature. Table 5 lists molding processing temperature range of some plastics for reference.
Barrel electric heating coil is generally divided into two, three or four sections for control. Ammeter on electrical cabinet displays temperature of each section respectively. Current of electric heating coil. Reading of ammeter is relatively fixed. If reading of ammeter is found to be low for a long time during operation, electric heating coil may have a fault, or wire contact is poor, or heating wire is oxidized and thinned, or a certain electric heating coil is burned. These will increase resistance of circuit in parallel and reduce current.
When ammeter has a certain reading, you can also simply use a plastic strip to scratch outer wall of electric heating coil one by one to see whether material strip is melted to determine whether a certain electric heating coil is energized or burned.

Mold closing is to close mold with a huge mechanical thrust to resist high-pressure injection of molten plastic during injection molding process and huge opening force of mold caused by filling mold.
After safety door is closed, all travel switches give signals, and mold closing action starts immediately. First, movable mold plate starts at a slow speed. After moving forward a short distance, control rod pressure block that originally pressed slow switch is disengaged, and movable plate turns to move forward quickly. When it moves to the end of mold closing, the other end of control rod presses slow switch again, movable plate turns to move forward slowly and at low pressure.
During low-pressure mold closing process, if there is no obstacle between molds, they can be closed smoothly until high-pressure switch is pressed. High pressure is turned to straighten machine hinge to complete mold closing action. This distance is extremely short, generally only 0.3~1.0mm. As soon as high pressure is turned, mold closing end limit switch is touched. At this time, action stops and mold closing process ends.
Mold closing structure of injection molding machine is fully hydraulic and mechanical connecting rod type. Regardless of structural form, mold closing is finally implemented by fully straightening connecting rod. Straightening process of connecting rod is process of movable plate and tail plate being stretched, and it is also process of four tie rods being stretched by force.
Size of mold clamping force can be known from maximum value of oil pressure gauge at the moment of mold clamping. The larger mold clamping force, the higher maximum value of oil pressure gauge, and vice versa. Smaller injection molding machines do not have mold clamping oil pressure gauges. At this time, it is necessary to judge whether mold is really closed based on straightening of connecting rod. If connecting rod of an injection molding machine is easily straightened when mold is clamped, or it is "almost" unable to straighten, or one of several connecting rods is not fully straightened, mold will bulge during injection molding, and product will have flash or other defects.

When molten plastic is injected into mold cavity and cooled, mold is opened to take out product. Mold opening process is also divided into three stages. The first stage is to open mold slowly to prevent product from tearing in mold cavity. The second stage is to open mold quickly to shorten mold opening time. The third stage is to open mold 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 are roughly: injection speed control, injection pressure control, control of amount of plastic filling injected into mold cavity, control of plasticizing state such as back pressure and speed of screw. Purpose of achieving process control is to improve quality of product and maximize efficiency of machine.

 

Program control of injection speed is to divide injection stroke of screw into 3~4 stages, and use appropriate injection speed in each stage. For example: slow down injection speed when molten plastic just starts to pass through gate, use high-speed injection during filling process, and slow down speed at the end of filling. This method can prevent overflow, eliminate flow marks and reduce residual stress of product.
When filling at a low speed, flow rate is stable, product size is relatively stable, fluctuation is small, internal stress of product is low, stress in all directions inside and outside product tends to be consistent (for example, a polycarbonate part is immersed in carbon tetrachloride, part molded by high-speed injection has a tendency to crack, while part molded at a low speed does not crack).
Under relatively slow filling conditions, temperature difference of material flow, especially temperature difference between material before and after gate, is large, which helps to avoid shrinkage and dents. However, due to long duration of filling time, it is easy to cause part to have stratification and poorly bonded weld marks, which not only affects appearance, but also greatly reduces mechanical strength.
When high-speed injection is used, material flow speed is fast. When high-speed filling is smooth, molten material quickly fills cavity, material temperature drops less, and viscosity drops less. A 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 line phenomenon and delamination phenomenon, reduce shrinkage depression, uniform color, and ensure fullness for most of product.
However, it is easy to cause product to become fat and blister or product to turn yellow, or even burn and scorch, or cause demolding difficulties, or uneven mold filling. For high-viscosity plastics, it may cause melt rupture and produce cloud spots on the surface of product.

Control of injection pressure is usually divided into one injection pressure, two injection pressure (holding pressure) or more than three injection pressure controls. Whether timing of pressure switching is appropriate is very important to prevent excessive pressure in mold, overflow or lack of material.
Specific volume of molded product depends on melt pressure and temperature when gate is closed during holding stage. If pressure and temperature are same each time from holding pressure to product cooling stage, specific volume of product will not change. At a constant molding temperature, the most important parameter that determines product size is holding pressure, the most important variables that affect product size tolerance are holding pressure and temperature.
For example: after mold filling is completed, holding pressure is immediately reduced. When surface layer forms a certain thickness, holding pressure rises again. In this way, large thick-walled products can be molded with low clamping force to eliminate pits and flash.
Holding pressure and speed are usually 50%~65% of maximum pressure and speed when plastic fills mold cavity, that is, holding pressure is about 0.6~0.8MPa lower than injection pressure. Since holding pressure is lower than injection pressure, oil pump load is low during considerable holding time, service life of solid oil pump is extended, and power consumption of oil pump motor is also reduced.
Pressure injection can not only make parts fill mold smoothly, but also avoid weld lines, dents, flash and warping deformation. It is beneficial for molding of thin-walled parts, multi-head small parts, long-process large parts, even molding of parts with unbalanced cavity configuration and untight clamping.

A certain amount of metering is pre-adjusted so that a small amount of melt (buffer amount) remains at the end of screw near the end of injection stroke. According to filling situation in mold, further injection pressure (secondary or tertiary injection pressure) is applied to add a small amount of melt. In this way, product can be prevented from sinking or shrinkage rate of product can be adjusted.

High back pressure can make melt obtain strong shear, low speed will also make plasticization time of plastic in barrel longer. Therefore, program design control of back pressure and speed is currently used more.
For example: in full stroke of screw metering, high speed and low back pressure are first used, then switched to lower speed and higher back pressure, switched to high back pressure and low speed, finally plasticized at low back pressure and low speed. In this way, pressure of melt in the front of screw is mostly released, reducing rotational inertia of screw, thereby improving accuracy of screw metering.
Too high back pressure often causes colorant to change color more; mechanical wear of pre-molding mechanism barrel and screw increases; pre-molding cycle is extended, production efficiency decreases; nozzle is prone to drooling, and amount of recycled materials increases; even if a self-locking nozzle is used, if back pressure is higher than designed spring locking pressure, it will also cause fatigue damage. Therefore, back pressure must be adjusted appropriately.
With advancement of technology, it has become possible to incorporate small computers into control system of injection molding machines and use computers to control injection molding process. Nippon Steel's N-PACS (microcomputer control system) can achieve four feedback controls (pressure holding adjustment, mold pressure adjustment, automatic metering adjustment, resin temperature adjustment) and four process controls (injection speed program control, pressure holding test, screw speed program control, back pressure program control).

 

Preparations before molding may include a lot of content. Such as: inspection of material processing performance (determination of plastic fluidity, moisture content, etc.); dyeing and particle selection before raw material processing; preheating and drying of pellets; cleaning and preheating of inserts; cleaning of trial molds and barrels, etc.

According to characteristics of plastics and feeding conditions, appearance and process performance of raw materials should generally be tested before molding.
If plastic used is in powder form, such as polyvinyl chloride, it should also be batched and dry mixed; if product has coloring requirements, an appropriate amount of colorant or masterbatch can be added; supplied pellets often contain varying degrees of moisture, flux and other volatile low molecular weight substances, especially some plastics with hygroscopic tendencies, moisture content always exceeds limit allowed by processing.
Therefore, drying treatment must be carried out before processing, and moisture content must be measured. Moisture content of polycarbonate, which is sensitive to water at high temperatures, is required to be below 0.2%, or even 0.03%~0.05%, so it is often dried in a vacuum drying oven. Dried plastic must be properly sealed and stored to prevent plastic from absorbing moisture from air and losing drying effect. For this reason, use of a drying chamber hopper can continuously provide dry hot materials for injection molding machine, which is beneficial to simplifying operations, keeping clean, improving quality, and increasing injection rate. Loading amount of drying hopper is generally 2.5 times hourly material consumption of injection molding machine.

Injection molded products need to embed metal inserts in products for assembly and strength requirements. During injection molding, when cold metal insert placed in mold cavity cools together with hot plastic melt, due to significant difference in shrinkage rate of metal and plastic, a large internal stress is often generated around insert (especially for rigid chain polymers such as polystyrene).
Existence of this internal stress causes cracks around insert, resulting in a significant reduction in performance of product. This can be achieved by selecting metals with large thermal expansion coefficients (aluminum, steel, etc.) as inserts, and preheating inserts (especially large metal inserts). At the same time, when designing products, measures such as arranging larger thick walls around inserts are taken.

Before the first use of a newly purchased injection molding machine, or when it is necessary to change product, change raw materials, change color, or find decomposition in plastic during production, barrel of injection molding machine needs to be cleaned or disassembled.
Barrel cleaning method is generally used for heating barrel cleaning. Cleaning material is generally plastic raw materials (or plastic recycled materials). For heat-sensitive plastics, such as polyvinyl chloride stock, low-density polyethylene, polystyrene, etc. can be used for transitional material cleaning, then transitional cleaning material can be replaced with processed new material.

Release agent is a substance that can make plastic products easy to demold. Zinc stearate is suitable for general plastics except polyamide; liquid paraffin is better for polyamide plastics; silicone oil is expensive and troublesome to use, so it is rarely used.
Use of release agent should be controlled in moderation, and it should be used as little as possible or not at all. Excessive spraying will affect appearance of product and will also have an adverse effect on color decoration of product.
High-speed and high-pressure injection can be considered in following situations:
(1) Plastic has high viscosity and cools quickly. Long-process parts cannot be completely filled in every corner of cavity by low pressure and slow speed;
(2) For parts with too thin walls, molten material is easy to condense and stay at thin wall. It is necessary to use a high-speed injection to allow molten material to enter cavity immediately before energy is consumed;
(3) Plastics reinforced with glass fiber 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-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 two-stage, three-stage, four-stage or even five-stage.

Developing good injection molding machine operation habits is very beneficial to improving machine life and production safety.

(1) Check whether there is water or oil in electrical control box. If electrical appliance is damp, do not start machine. Maintenance personnel should blow dry electrical parts before starting machine.

 

(2) Check whether power supply voltage meets requirements, which should generally not exceed ±15%.
(3) Check whether emergency stop switch, front and rear safety door switches are normal. Verify whether rotation direction of motor and oil pump is consistent.
(4) Check whether cooling pipes are unobstructed, pass cooling water into cooling water jackets at oil cooler and the end of barrel.
(5) Check whether there is lubricating oil (grease) in each moving part, and add enough lubricating oil.
(6) Turn on electric heating and heat each section of barrel. When temperature of each section reaches requirement, keep it warm for a period of time to stabilize machine temperature. Insulation time varies according to requirements of different equipment and plastic raw materials.
(7) Add enough plastic in hopper. According to requirements of different plastics for injection molding, some raw materials are best dried first.
(8) Cover heat shield on barrel, which can save electricity, extend life of electric heating coil and current contactor.

(1) Do not cancel function of safety door for sake of convenience.
(2) Pay attention to temperature of pressure oil and make sure it does not exceed specified range. Ideal operating temperature of hydraulic oil should be maintained between 45 and 50℃, and is generally more suitable within range of 35 to 60℃.
(3) Pay attention to adjusting travel limit switches to avoid collisions when machine is in motion.

(1) Before stopping machine, clean plastic in barrel to prevent residual material from oxidizing or decomposing due to long-term heat.
(2) Mold should be opened to keep toggle mechanism in a locked state for a long time.
(3) Workshop must be equipped with lifting equipment. Be very careful when installing and removing heavy parts such as molds to ensure production safety.