Injection molding is a cycle process, each cycle mainly includes: quantitative feeding – melting and plasticization – pressure injection – mold filling and cooling – mold opening and taking parts. After taking out the plastic part, close the mold again for the next cycle[ Edit this paragraph] structural injection molding machines are divided into plunger injection molding machines and screw injection molding machines according to the plasticization mode. According to the transmission mode of the machine, they can be divided into hydraulic, mechanical and hydraulic mechanical (connecting rod) types. According to the operation mode, they can be divided into automatic, semi-automatic and manual injection molding machines.
(1) Horizontal injection molding machine: This is the most common type. The clamping part and the injection part are on the same horizontal centerline, and the mold is opened along the horizontal direction. Its features are: low fuselage, easy to operate and maintain; The center of gravity of the machine is low and the installation is stable; After the products are ejected, they can fall automatically by gravity, which is easy to realize full-automatic operation. At present, most injection molding machines on the market adopt this type.
(2) Vertical injection molding machine: its clamping part and injection part are on the same vertical centerline, and the mold is opened along the vertical direction. Therefore, the utility model has the advantages of small floor area, easy placement of inserts, convenient loading and unloading of molds, and uniform plasticization of materials falling from the hopper. However, the products are not easy to fall automatically after ejection, and must be removed by hand, which is not easy to realize automatic operation. Vertical injection molding machines are suitable for small injection molding machines. Generally, injection molding machines below 60g are more used, and large and medium-sized machines are not suitable.
(3) Angle injection molding machine: its injection direction is on the same surface as the die interface. It is especially suitable for plane products that are not allowed to leave gate traces in the machining center. The floor area is smaller than that of the horizontal injection molding machine, but the inserts placed in the mold are easy to fall obliquely. This type of injection molding machine is suitable for small machines.
(4) Multi mode rotary table injection molding machine: it is a special injection molding machine with multi station operation. Its feature is that the clamping device adopts rotary table structure, and the mold rotates around the rotating shaft. This type of injection molding machine gives full play to the plasticizing capacity of the injection device, which can shorten the production cycle and improve the production capacity of the machine. Therefore, it is especially suitable for the production of mass products with long cooling and setting time or requiring more auxiliary time due to the placement of inserts. However, because the clamping system is huge and complex, and the clamping force of the clamping device is often small, this kind of injection molding machine is widely used in the production of plastic soles and other products.
General injection molding machine includes injection device, clamping device, hydraulic system and electrical control system.
The basic requirements of injection molding are plasticization, injection and molding. Plasticization is the premise to realize and ensure 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 (the average pressure in the mold cavity is generally between 20 ~ 45MPa), so there must be a large enough clamping force. It can be seen that the injection device and clamping device are the key components of the injection molding machine[ Edit this paragraph] operation 3.1 action program of injection molding machine
Mold closing → pre molding → backward shrinkage → nozzle advancing → injection → pressure maintaining → nozzle retreating → cooling → mold opening → ejection → door opening → workpiece taking → door closing → mold closing.
3.2 operation items of injection molding machine
The operation items of injection molding machine include control keyboard operation, electrical control cabinet operation and hydraulic system operation. Select the injection process action, feeding action, injection pressure, injection speed and ejection form, monitor the temperature, current and voltage of each section of the barrel, and adjust the injection pressure and back pressure.
3.2.1 action selection during injection
General injection molding machines can be operated manually, semi-automatic and fully automatic.
Manual operation is in a production cycle, each action is realized by the operator turning the operation switch. Generally, it is only selected when commissioning and mold adjustment.
During semi-automatic operation, the machine can automatically complete the action of one working cycle, but after each production cycle, the operator must open the safety door, remove the workpiece, and then close the safety door before the machine can continue the production of the next cycle.
During fully automatic operation, the injection molding machine can automatically enter the next working cycle after completing the action of one working cycle. In the normal continuous working process, there is no need to stop for control and adjustment. However, it should be noted that if full-automatic operation is required, (1) do not open the safety door halfway, otherwise the full-automatic operation will be interrupted（ 2) Feed in time（ 3) If electric eye induction is selected, pay attention not to cover the electric eye.
In fact, it is usually necessary to stop temporarily in the middle of full-automatic operation, such as spraying release agent on the machine mold.
During normal production, semi-automatic or full-automatic operation is generally selected. At the beginning of operation, the operation mode (manual, semi-automatic or full-automatic) shall be selected according to the production needs, and the manual, semi-automatic or full-automatic switches shall be toggled accordingly.
The semi-automatic and full-automatic working procedures have been determined by the line itself. The operator only needs to change the speed and pressure, the length of time, the number of thimbles, etc. on the electric cabinet, so as not to confuse the working procedures due to the operator’s wrong key adjustment.
Before each action in a cycle is not adjusted properly, select manual operation first, and then select semi-automatic or full-automatic operation after confirming that each action is normal.
3.2.2 selection of pre molding action
According to whether the injection seat retreats before and after pre plastic feeding, that is, whether the nozzle leaves the mold, the injection molding machine is generally equipped with three options.
(1) Fixed feeding: the nozzle is always stuck into the mold before and after pre molding, and the injection seat does not move.
(2) Front feeding: the nozzle carries out pre molding feeding against the die. After the pre molding is completed, the injection seat retreats and the nozzle leaves the die. The purpose of this method is to use the injection hole of the mold to hold the nozzle during pre molding to prevent the molten material from flowing out of the nozzle when the back pressure is high. After pre molding, the heat transfer caused by long-time contact between the nozzle and the mold can be avoided, affecting the relative stability of their respective temperatures.
(3) Post feeding: after the injection, the injection seat retreats, the nozzle leaves the mold, and then the pre molding, and the injection seat advances after the pre molding. This action is suitable for processing plastics with particularly narrow molding temperature. Due to the short contact time between the nozzle and the mold, the loss of heat and the solidification of molten material in the nozzle hole are avoided.
After the injection is completed and the cooling timer is timed, the pre molding action starts. The screw rotates to squeeze the plastic melt to the front of the screw head. Due to the action of the check valve at the front end of the screw, molten plastic accumulates at the front end of the barrel, forcing the screw back. When the screw retreats to a predetermined position (this position is determined by the travel switch to control the backward distance of the screw and realize quantitative feeding), the pre molding stops and the screw stops rotating. Followed by the backward contraction action, which means that the screw makes a small amount of axial retreat. This action can relieve the pressure of molten material gathered at the nozzle and overcome the “salivation” phenomenon caused by the imbalance of internal and external pressure of the barrel. If it is not necessary to shrink back, the shrink back stop switch shall be adjusted to the appropriate position to stop the pre molding. At the same time when the switch is pressed, the retraction stop switch is also pressed. When the screw moves backward to press the stop switch, the backward retraction stops. Then the injection seat began to retreat. When the injection seat retreats to press the stop switch, the injection seat stops retreating. If fixed feeding mode is adopted, pay attention to adjust the position of travel switch.
Generally, fixed feeding mode is adopted in production to save the operation time of injection seat advance and retreat and speed up the production cycle.
Post time: Sep-03-2021