(1) The role of injection molding system
The injection system is very widely used in industry and life. Simply speaking, it is to transport, melt and knead the plastic into a plasticized uniform melt, and then under a certain injection pressure, it is injected into the parison mold from the main pipe assembly through the nozzle , Made into a tubular parison with a bottom. The injection system is the most important part of the injection molding machine. It functions to uniformly plasticize the plastic material into a molten melt, and inject a certain amount of plastic melt into the mold cavity with a certain injection pressure and injection speed. There are three main types of plunger type, screw type and screw pre-plastic plunger injection type. At present, the most widely used is the screw type. Its function is that in a cycle of the plastic injection machine, after a certain amount of plastic is heated and plasticized within a specified time, under a certain pressure and speed, the molten plastic is injected into the mold cavity through the screw. Its role is that in a working cycle of the injection molding machine, after a certain amount of plastic is heated and plasticized within a specified time, under a certain pressure and speed, the molten plastic is injected into the mold cavity through the screw or plunger In the middle, after the injection is finished, the molten material injected into the mold cavity is pressure-preserved and shaped.
(2) Composition of injection molding system
There are many types of injection molding machines, but the basic composition is the same, mainly composed of four parts: mold clamping device, injection device, drive device and control system. See the injection system of the most widely used screw injection molding machine recently.
1. Types of plasticizers There are three types of plasticizers: plunger plasticizers, preplasticizer plasticizers, and reciprocating screw plasticizers.
(1) Plunger preplasticized injection molding machine:
An injection molding machine that plasticizes the molding material with a plunger-type plasticizing device and then fills the mold with a specially designed injection plunger. Compared with the ordinary plunger injection molding machine, it has the characteristics of uniform plasticization, stable injection volume and small pressure loss. The process is: the plastic particles pass through the metering device connected to the piston rod of the injection cylinder. When the injection piston moves forward, the plastic in the hopper enters the charging chamber, and the plastic is gradually melted and plasticized by the heating of the barrel heater. The plasticizing chamber injects the pre-plasticized melt into the mold cavity through a nozzle.
The plasticizing method of the plunger type plasticizing device is to use the heat conduction of external heating to melt and plasticize the plastic. Although this type of molding machine is relatively simple in structure, due to the existence of insufficient plasticization of the material, low plasticizing ability, and injection pressure It has the disadvantages of large loss, unstable injection speed, and difficulty in cleaning the barrel. It is difficult for molding larger products with high quality requirements or processing heat-sensitive plastics.
(2) Pre-plastic injection device of injection machine:
The pre-plastic injection device is composed of plasticized and injection parts. Its function is to evenly heat and plasticize the rubber material. The plasticized rubber material is injected into the mold cavity through the nozzle. After the injection is completed, the pressure is maintained to make the glue in the mold cavity. The material is kept under a set pressure for a certain period of time to prevent the backflow of the rubber material to cause loose tissue and uneven surface of the product. In view of the insurmountable defects of the plunger-type plasticizing device itself, in order to meet the requirements of large-scale injection, precision products and processing of some engineering plastics and heat-sensitive plastics, on the basis of this device, a pre-plasticizing device was developed.
The pre-plasticizing device is composed of a pre-plasticized heating cylinder and a plastic injection heating cylinder. Heating barrel for preplasticization is used to heat and melt molding material
Material, the heating cylinder for injection molding uses Song to inject the melted material. There are two types of pre-plasticizing devices: plunger pre-plasticizing and screw pre-plasticizing.
① Plunger pre-plasticized. The material is plasticized and melted in the heating cylinder, and then sent to the heating cylinder for injection molding. Because the force of the feeding material causes the injection rod to retreat, the injection amount can be determined (measured) by adjusting the amount of retreat. When the predetermined injection volume is reached, the injection piston advances and the molten material is injected into the mold.
② Screw pre-plasticized type. Using a screw extrusion device in the pre-plasticized heating cylinder, the material in the hopper is plasticized into a molten state while being sent to the heating cylinder by the rotation of the screw, and then sent to the injection heating cylinder, at this time, The amount of injection is also measured by controlling the amount of retraction of the injection rod. After measurement, the injection rod advances and the molten resin is injected into the mold.
In the above two pre-plasticizing devices, because the molten resin is likely to stagnate at the check valve at the junction of the two sets of heating cylinders, it is not possible to use materials that are prone to thermal decomposition. In addition, its structure allows the operation of changing the resin material and changing the color. It is very troublesome. Therefore, such pre-plasticizing devices are almost no longer used in Japan. However, compared with the plunger-type plasticizer, the advantage is that it can improve the plasticizing ability of the material. Therefore, for materials that are not easily thermally decomposed, such as polyethylene and polystyrene, especially when it is desired to shorten the molding cycle, it can effectively and fully This type of pre-plasticizing device is used.
(3) Reciprocating screw plasticizing device:
It is driven by an electric motor and decelerated by a gear reduction box, which drives the screw to rotate, and there is a stepless speed regulation driven by a hydraulic motor and a speed regulating motor. The types of electric motors and hydraulic motors that are used more are now used. This plasticizing device is mainly composed of a screw, a barrel, a screw head, a nozzle, a metering device, a heating device, etc. Its working process is: the screw rotates in cooperation with the transmission device, and the plastic is continuously forwarded from the hopper through the screw Under the heating of the heating device and the shearing and kneading of the screw, the plastic is gradually plasticized and melted uniformly. At the same time, the screw moves backward under the action of the melt pressure at the head of the screw and returns to a certain position. The device metering stops the screw rotation and prepares for injection molding. During injection molding, the end of the screw acts directly on the melt, and the melt is injected into the mold cavity at a certain pressure and high speed. In order to prevent the backflow of molten material during injection molding, a check ring is generally provided at the screw head to ensure the amount of molten material required for the product.
The reciprocating screw plasticizing device has good plasticizing quality, small injection pressure loss, stable and reliable metering, convenient screw assembly and disassembly and cleaning, and is suitable for large, medium and small injection molding machines.
The injection system of the screw injection molding machine is generally composed of a plasticizing device (barrel, screw and nozzle, etc.), a feeding component (feeding system hopper, etc.), a metering device, a screw drive device, a heating and cooling device, and an injection seat moving device. And travel limit device. As shown in the figure is the main component of a screw reciprocating injection system. The plasticized parts are the main parts of the screw injection machine, which are mainly composed of screw, barrel and nozzle. Unlike the plunger injection machine, the screw of the reciprocating screw injection machine has both axial movement and rotational movement in the barrel. When the screw rotates in the barrel, the plastic falling in the hopper is rolled into the barrel, and it is gradually pushed forward, compacted, vented and plasticized.
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