Plunger injection molding machine and screw injection molding machine
(1) Plunger injection molding machine The plunger injection molding machine is plasticized by the plunger. The plastic raw material is melted in the barrel by the heat transferred from the barrel wall and the diverter shuttle, and then pushed by the plunger. Plasticized and injected into the cavity through the nozzle. Its plasticization and injection mechanism is relatively simple, and it is composed of nozzle, diverter shuttle, electric heating ring, barrel, feeding device, hopper, metering chamber, injection plunger transmission arm, injection plunger, injection seat moving cylinder and other parts. The plasticizing mechanism itself has the disadvantages of uneven plasticization of materials, low plasticizing ability, large injection pressure loss (friction between particulate materials), unstable injection speed, and difficulty in cleaning materials. The air contained in the material, bubbles or silver bars often appear on the injection molded products. At the same time, it is difficult to mold larger products, products with high quality requirements, or process heat-sensitive plastics. For example, when processing nylon, the filled glass fibers cannot be fully dispersed and the physical properties are not uniform, making it difficult to ensure the appearance quality of the product. At present, plunger injection molding machines are mostly used for molding small plastic products.
The main disadvantages of the plunger injection device are uneven plasticization, large injection pressure loss, and difficulty in providing stable process conditions. In addition, the cleaning of the cartridge is difficult. However, the plunger-type injection device has a simple structure and low equipment manufacturing cost, and is suitable for molding small and medium-sized plastic parts.
(2) Screw-type injection molding machine In injection molding, a screw is used to replace the plunger, and a screw is used to provide injection pressure. This injection molding machine can overcome the defects of the plunger injection molding machine and meet the continuously improving quality requirements of users. The working principle is that the screw rotates, and the plastic moves forward through the screw groove of the screw. Due to shear heat generation and external heating of the barrel, the plastic in the barrel is plasticized, and the plasticized melt cannot be pressed out in front, so the screw is pushed backward to move backward and be measured. When the melt in the part has accumulated to the required amount, the screw will stop rotating. Due to the use of a screw plasticizing device, and through the outside of the barrel heating method? Granular plastic can be fully plasticized, easy to control temperature and metering. Because the hydraulic motor is easy to achieve low and medium speed stepless speed regulation and effective overload protection, the screw drive is usually implemented by a hydraulic motor.
As the injection volume increases, the pre-plastic screw drive mode also changes. The motor is driven by constant power, and the hydraulic motor is driven by constant torque. During plasticizing, the torque received by the screw is basically constant torque. Therefore, the frequent start of the hydraulic motor will consume more energy than the motor 130%. However, the hydraulic motor can achieve stepless speed regulation and effectively implement overload protection. Therefore, for a large injection molding machine that changes materials and molds less frequently, it is economical to use a motor-driven screw structure; and for a small injection molding machine that changes materials and molds frequently, a hydraulic motor drive method should be used. With the advent of DC and AC servo motors, stepless speed regulation has been achieved. In order to save energy, a variable displacement motor (secondary knot) pre-plastic system has also appeared.
At present, most of the factories use reciprocating screw injection molding machines, but there are also a considerable number of plunger injection molding machines. The plunger injection molding machine is generally used to produce small products under 60c-3; the reciprocating screw injection molding machine is used to form heat-sensitive plastics, poorly fluid plastic products, and medium and large plastic products. The reciprocating screw injection molding machine can be used to process thermosetting plastics in addition to thermoplastics. The quality of processed products is stable, the performance and dimensional accuracy of the products are improved, the molding cycle is shortened, and the working conditions are also appropriately improved. An oil pressure camera rod is installed behind the screw of the extruder to rotate and plasticize the material and advance it. At the same time, due to the back pressure of the material, the screw retreats and the material is accumulated in front of the screw. After a certain amount of plasticization is obtained, the screw stops rotating, and under the action of the hydraulic cylinder, the material is pushed forward into the injection mold. The advantage of this method is that the entire cycle can be continuous. And there is no need to maintain a very large clamping force.
Compared with the plunger injection molding machine, the screw injection molding machine has the following advantages: 1. The quality of the screw pre-plasticized raw material is better than that of the plunger plasticized raw material. 2. The mixing performance of the screw type is better than that of the plunger type. The structure and working method of the screw injection molding machine are the same as the plunger injection molding machine, except that the plunger in the first barrel is changed to a screw. The screw first plasticizes the raw material, and then pushes the molten material into the forming mold through the plunger to cool and form.
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