Maintenance methods and experience summary of injection molding machines
1.1 Working principle of injection molding machine
The working principle of an injection molding machine is similar to that of an injector used for injection molding. It relies on the thrust of a screw (or plunger) to inject plastic that has already been plasticized in a molten state (i.e. viscous flow state) into a closed mold cavity, and after solidification and shaping, the process of obtaining the product is carried out.
Injection molding is a cyclic process, with each cycle primarily consisting of: quantitative feeding, melting and plasticization, pressure injection, mold filling and cooling, and mold opening and picking. After removing the plastic part, close the mold again and proceed to the next cycle.
1.2 Structure of injection molding machine
Injection molding machines are divided into plunger injection molding machines and screw injection molding machines based on the plasticization method; According to the transmission method of the machine, it can be divided into hydraulic, mechanical, and hydraulic mechanical (connecting rod) types; According to the operation method, it is divided into active, semi-active, and manual injection molding machines.
(1) Horizontal injection molding machine: This is a more common type. The clamping part and the injection part are on the same horizontal centerline, and the mold is flipped horizontally. Its characteristics are: low body, easy to operate and repair; The center of gravity of the machine is low, and the equipment is relatively stable; After the product is ejected, it can be automatically lowered using the gravity effect, making it easy to complete full active operation. At present, this type of injection molding machine is commonly used in the market.
(2) Vertical injection molding machine: Its clamping part and injection part are on the same straight centerline, and the mold is flipped in a straight direction. Therefore, it has a smaller footprint, simple placement of inserts, convenient loading and unloading of molds, and the material falling into the hopper can be plasticized more evenly. But it is not easy for the product to fall off voluntarily after being pushed out, so it is necessary to remove it by hand, which is not easy to complete the active operation. Vertical injection molding machines are suitable for small injection molding machines. Generally, injection molding machines weighing less than 60 grams are more commonly used, and large and medium-sized machines are not suitable.
(3) Angle injection molding machine: Its injection direction and mold interface are on the same surface, making it particularly suitable for flat products with no gate marks allowed in the machining center. It occupies a smaller area than a horizontal injection molding machine, but the embedded parts placed in the mold are slightly skewed and fall off. 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 operated at multiple workstations, characterized by the use of a turntable structure for mold closing equipment, and the mold rolls around the rotary shaft. This type of injection molding machine fully utilizes the plasticizing ability of injection equipment, can shorten the production cycle, improve the production ability of the machine, and is particularly suitable for the production of large quantities of plastic products with long cooling and shaping time or requiring more auxiliary time due to the placement of inserts. However, due to the large and chaotic clamping system, the clamping force of the clamping equipment is often small, so this type of injection molding machine is more commonly used in the production of plastic shoe soles and other products.
A general injection molding machine includes injection equipment, mold closing equipment, hydraulic system, and electrical control system.
The basic requirements for injection molding are plasticization, injection, and molding. Plasticization is a condition for completing and ensuring the quality of formed products, and in order to meet the requirements of satisfactory molding, it is necessary to ensure that there is satisfactory pressure and speed for injection. Together, due to the high injection pressure, a corresponding high pressure occurs in the mold cavity (the average pressure in the mold cavity is generally between 20~45MPa, so it is necessary to have a satisfactory clamping force. Therefore, it can be seen that the injection equipment and clamping equipment are crucial components of the injection molding machine.
1.4 Operation of injection molding machine
1.4.1 Action program of injection molding machine
Nozzles moving → Injecting → Maintaining pressure → Pre molding → Inverting → Nozzles retracting → Cooling → Mold opening → Ejection → Needle retraction → Door opening → Door closing → Mold closing → Nozzles moving.
1.4.2 Injection molding machine operation items: The injection molding machine operation items include three aspects: keyboard operation, electrical control cabinet operation, and hydraulic system operation. Separation involves selecting the injection process, feeding action, injection pressure, injection speed, and ejection type, monitoring the temperature, current, and voltage of each section of the material barrel, and adjusting the injection pressure and back pressure.
1.4.2.1 Selection of injection process actions:
Generally, injection molding machines can be operated manually, as well as semi-active and fully active operations.
Manual operation is completed by the operator turning the operation switch during a production cycle. It is usually selected during machine testing and mold adjustment.
During semi-active operation, the machine is able to actively complete an operation cycle, but after each production cycle is completed, the operator needs to swing open the safety door, remove the workpiece, and then close the safety door, so that the machine can continue to produce for the next cycle.
During fully active operation, the injection molding machine can actively enter the next operation cycle after completing one operation cycle. There is no need to stop the machine for control and adjustment during the normal continuous operation process. However, it should be noted that if full active operation is required, (1) do not open the safety door halfway, otherwise full active operation will be terminated; (2) Timely addition of materials; (3) If using electric eye sensing, be careful not to cover the electric eye.
In practice, in fully active operation, it is generally necessary to temporarily stop the machine halfway, such as spraying release agent on the machine mold.
During normal production, semi-active or fully active operation is generally chosen. At the beginning of the operation, the operation method (manual, semi-active, or fully active) should be selected based on the production demand, and the manual, semi-active, or fully active switches should be turned accordingly.
The semi-active and fully active operation procedures have been determined by the circuit itself. The operator only needs to change the size of speed and pressure, the duration of the time, the number of pins, and so on on on the electrical cabinet surface, without causing the operation procedure to appear chaotic due to the operator adjusting the wrong key button.
Before adjusting each action in a cycle, manual operation should be selected first. After acknowledging that each action is normal, semi-active or fully active operation should be selected.
1.4.2.2 Selection of Pre molding Actions
According to whether the injection seat before and after pre molding feeding is withdrawn, that is, whether the nozzle is detached from the mold, injection molding machines generally have three options to choose from. (1) Fixed feeding: The nozzle is always attached to the mold before and after pre molding, and the injection seat does not move. (2) Pre feeding: The nozzle is pressed against the mold for pre molding feeding. After the pre molding is completed, the injection seat is withdrawn and the nozzle is detached from the mold. The intention of selecting this method is to use a mold needle hole to support the nozzle during pre molding, to prevent molten material from flowing out of the nozzle when the back pressure is high. Pre molding can prevent heat transfer between the nozzle and the mold due to prolonged contact, which affects their relative temperature stability. (3) Post feeding: After the injection is completed, the injection seat is withdrawn, the nozzle is detached from the mold, and then pre molded. After pre molding, the injection seat moves forward. This action is suitable for processing plastics with particularly narrow forming temperatures, as the contact time between the nozzle and the mold is short, preventing heat loss and solidification of the molten material in the nozzle hole.
After the injection is completed and the cooling timer is timed, the pre molding action begins. The rotation of the screw melts the plastic and squeezes it to the front of the screw head. Due to the one-way valve effect of the stop ring at the front end of the screw, molten plastic accumulates at the front end of the barrel, forcing the screw backwards. When the screw retreats to the predetermined position (determined by the travel switch, control the interval for screw retreat, and complete quantitative feeding), the pre molding stops and the screw stops rolling. Next is the retraction action, which involves a slight axial retreat of the screw. This action can relieve the pressure of the molten material gathered at the nozzle, overcoming the phenomenon of "drooling" caused by the imbalance of pressure in the barrel gauge. If reverse retraction is not required, the reverse retraction stop switch should be adjusted to the appropriate position, so that at the same time as the pre molded stop switch is pressed, the reverse retraction stop switch is also pressed. When the screw performs a retraction action and retreats to the upper stop switch, the retraction stops. Then the seat begins to retreat. When the injection seat retreats to the stop switch, the injection seat stops retreating. If using a fixed feeding method, attention should be paid to adjusting the position of the travel switch.
Generally, fixed feeding methods are used in production to save the time of injection seat forward and backward operations and accelerate the production cycle.
1.4.2.3 Selection of injection pressure
The injection pressure of the injection molding machine is regulated by a pressure regulating valve. Under the condition of adjusting the pressure, the injection pressure is controlled by the opening and closing of the high-pressure and low-pressure oil circuits, and the concavity and convexity of the injection pressure in the early and later stages are controlled.
There are generally three types of pressure selection for injection molding machines of medium and above size, namely high pressure, low pressure, and high pressure first and then low pressure. High pressure injection is completed by injecting high-pressure oil into the injection cylinder. Due to high pressure, plastic enters the mold cavity at high pressure and speed from the beginning. During high-pressure injection, the plastic enters the mold quickly, and the pressure gauge reading of the injection cylinder rises rapidly. Low pressure injection is completed by injecting low-pressure pressure oil into the injection cylinder. During the injection process, the pressure gauge reading rises slowly, and plastic enters the mold cavity at low pressure and low speed. Starting with high pressure and ending with low pressure is based on the practical requirements of the plastic variety and mold, and the pressure of the pressure oil entering the oil cylinder is controlled from time to time to complete the process.
In order to meet the different plastic requirements with different injection pressures, it is also possible to choose the method of replacing screws or plungers with different diameters, which not only satisfies the injection pressure but also fully utilizes the production ability of the machine. In large injection molding machines, there is often a function of controlling multi-stage injection pressure and multi-stage injection speed, which can better ensure the quality and accuracy of the product.
1.4.2.4 Selection of injection speed
Generally, there are fast slow knobs on the control panel of injection molding machines to meet the requirements of injection speed. There is a high flow oil pump and a low flow pump operating together in the hydraulic system to supply oil. When the oil circuit is connected to a large flow rate, the injection molding machine completes rapid mold opening and closing, rapid injection, etc. When the hydraulic oil circuit only provides a small flow rate, various actions of the injection molding machine will proceed slowly.
1.4.2.5 Selection of Ejection Methods
There are two types of ejection methods for injection molding machines: mechanical ejection and hydraulic ejection. Some are also equipped with pneumatic ejection systems, and the ejection times can be single or multiple. The ejection action can be manual or active.
The ejection action is initiated by the mold opening stop limit switch. The operator can reach it by adjusting the ejection time button on the control cabinet according to their needs. The speed and pressure of the ejection can also be controlled by the switch on the control cabinet, and the interval between the forward and backward movements of the ejector pin is determined by the travel switch.
1.4.2.6 Temperature control
Using temperature measuring thermocouples as temperature measuring elements, coupled with a temperature measuring millivolt meter as temperature control equipment, it commands the on/off of the current in the material barrel and mold heating coil, and selectively fixes the temperature of each section of the material barrel and mold. Table 5 lists the processing temperature scales for some plastics for reference.
The electric heating coil of the material cylinder is generally divided into two, three, or four sections for operation. The ammeter on the electrical cabinet displays the magnitude of the current in each section of the electric heating coil separately. The reading of the ammeter is relatively fixed. If the reading of the ammeter is found to be low for a long time during operation, it may be due to problems with the electric heating coil, poor wire contact, oxidation and thinning of the electric heating wire, or burning of a certain electric heating coil. These will increase the resistance value of the circuit in parallel and cause the current to decrease.
When the ammeter has a certain reading, a plastic strip can also be used to scratch the outer wall of the electric heating coil one by one, to determine whether a certain electric heating coil is energized or burned by observing whether the strip is melted.
When injecting at high speed, the material flow speed is fast. When the high-speed mold filling is smooth, the molten material quickly fills the mold cavity, causing less decrease in material temperature and viscosity. It is possible to choose a lower injection pressure, which is a hot material filling situation. High speed mold filling can improve the glossiness and smoothness of the parts, eliminate the phenomenon of seam lines and layering, shorten the depression, achieve uniform color, and ensure fullness for most parts. However, there may be slight issues such as product weight gain, blistering, yellowing, or even burns or charring, or difficulty in demolding or uneven mold filling. For high viscosity plastics, it is possible to cause melt cracking, resulting in cloudy spots on the surface of the parts.
High speed and high pressure injection can be considered in the following situations: (1) Plastic has high viscosity, fast cooling speed, and long process parts should be selected with low pressure and slow speed, which cannot completely fill all corners of the mold cavity; (2) For parts with thin wall thickness, the molten material is prone to condensation and retention when it reaches the thin-walled area. It is necessary to choose a high-speed injection to consume a lot of energy from the molten material, which can enter the mold cavity immediately before it is consumed; (3) It is necessary to use high-speed and high-pressure injection molding to obtain a smooth and uniform surface of the parts reinforced with fiberglass or plastics containing a lot of filling materials due to poor fluidity.
For high-end fine products, thick walled parts, parts with significant changes in wall thickness, and parts with thicker protrusions and ribs, it is better to choose multi-stage injection, such as second, third, fourth, and even fifth level.
Published on 2021-10-09
CONTACT US
