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              EN CN
              Structural analysis and working principle of injection molding machine

              2020-10-15

              1. Working principle of injection molding machine
               
              Injection molding machine is abbreviated as injection molding machine.
               
              Injection molding uses the thermal physical properties of plastics to add materials from a hopper to a barrel. The outside of the barrel is heated by a heating ring to melt the material. The barrel is equipped with a screw driven by an external power motor. The role of the material on the screw Under the dual action of external heating and screw shear, the material is gradually plasticized, melted and homogenized. When the screw rotates, the friction and shear force of the material in the screw groove Under the action, the molten material is pushed to the head of the screw. At the same time, the screw retreats under the reaction of the material, so that the head of the screw forms a storage space to complete the plasticization process. Then, the screw is thrust by the piston of the injection cylinder Under the action of high speed and high pressure, the molten material in the storage chamber is injected into the mold cavity through a nozzle. After the melt in the cavity is pressure-maintained, cooled, solidified and shaped, the mold is under the action of the clamping mechanism , Open the mold, and eject the shaped product from the mold through the ejector device.
               
              The operating cycle flow of the injection molding machine is shown in Figure 1.
               
              Second, the classification of injection molding machines
               
              There are three types of configuration according to the clamping part and the injection part: horizontal, vertical and angular
               
              (1) Horizontal injection molding machine: Horizontal injection molding machine is the most commonly used type. Its characteristic is that the center line of the injection assembly is concentric or consistent with the center line of the clamping assembly, and is parallel to the installation ground. It has the advantages of low center of gravity, stable work, convenient mold installation, operation and maintenance, large mold opening and small space occupation; but it covers a large area and is widely used in large, medium and small machines.
               
              (2) Vertical injection molding machine: Its characteristic is that the clamping device and the axis of the injection device are arranged in a line and perpendicular to the ground. It has the advantages of small footprint, convenient mold assembly and disassembly, easy installation of inserts, uniform plasticization of materials falling from the hopper, easy automation and automatic line management of multiple machines. The disadvantage is that the ejected product is not easy to fall off automatically, and often requires manual or other methods to take out, and it is not easy to achieve fully automated operation and large product injection; the body is high, and the feeding and maintenance are inconvenient.
               
              (3) Angle injection molding machine: the axes of the injection device and the clamping device are arranged perpendicular to each other. According to the relative position of the center line of the injection assembly and the installation base, there are horizontal and vertical, vertical and horizontal, horizontal and horizontal: ①Horizontal and vertical, the injection assembly line is parallel to the base, and the center line of the clamping assembly is The base is vertical; ②Vertical and horizontal, the center line of the injection assembly is perpendicular to the base, and the center line of the clamping assembly is parallel to the base. The advantage of the angle injection machine is that it has the advantages of both horizontal and vertical injection machines. It is especially suitable for opening molds of asymmetric geometrical products with side gates.
               
              Three, the composition structure analysis of the injection molding machine
               
              According to the requirements of the injection molding process, the injection molding machine is a model with strong mechatronics, mainly composed of injection parts, clamping parts, fuselage, hydraulic system, heating system, control system, feeding device, etc. as shown in picture 2.
               
              (1) Typical structure of injection molded parts
               
              1. Composition of injection parts
               
              At present, the common injection molding devices have single-cylinder and double-cylinder forms. The injection molding machines of our factory are all double-cylinder forms, and they are all directly driven by hydraulic motors to inject the screws. Because the composition of machines of different manufacturers and models is not completely the same, the following is a specific analysis of the machines used in our factory.
               
              The composition diagrams of the vertical machine and horizontal machine injection device are shown in Figure 3 and Figure 4, respectively.
               
              The working principle is: When pre-plasticizing, the screw in the plasticizing part is driven by the hydraulic motor to rotate the main shaft. One end of the main shaft is connected with the screw key, and the other end is connected with the hydraulic motor key. When the screw rotates, the material is plasticized and plasticized. The molten material is pushed into the storage chamber at the front end of the barrel. At the same time, the screw retreats under the reaction of the material, and the thrust seat is retreated through the thrust bearing, and the piston rod is pulled back straight through the nut to complete the measurement. When injecting, the injection cylinder The oil in the rod cavity of the piston pushes the piston rod through the bearing to complete the action, and the rod cavity of the piston pushes the piston rod and the screw to complete the injection action.
               
              (A) is a top view; (b) is a plan view between the injection seat and the guide rod support
               
              1- hydraulic motor; 2, 6-guide rod support; 3- guide rod; 4- injection cylinder; 5- feeding port;
               
              7-Thrust seat; 8-Injection seat; 9-Plasticized parts; 10-Seat shifting cylinder
               
              1- hydraulic motor; 2- thrust seat; 3- injection cylinder; 4- injection seat; 5- feeding port; 6-seat shifting cylinder; 7- plasticized parts; 8-upper template
               
              2. Plasticized parts
               
              There are two types of plasticized parts: plunger type and screw type. The screw type is introduced below.
               
              The screw-type plasticizing part is shown in Figure 5, which is mainly composed of screw, barrel, nozzle, etc. During the continuous advancement of the rotating screw, the plastic realizes the change of physical state, and finally it is injected into the mold cavity in a molten state. Therefore, the plasticized component is the core component for uniform plasticization and quantitative injection.
               
              1- nozzle; 2- screw head; 3- check ring; 4- barrel; 5- screw; 6- heating ring; 7- cooling water ring
               
              The working principle of the screw-type plasticizing component: During pre-plasticizing, the screw rotates to continuously advance the material that falls into the screw groove from the material port. The heating ring transfers heat to the material in the screw groove through the barrel wall. Under the dual action of external heating and screw rotation and shearing, and through the thermal history of each functional section of the screw, the material is plasticized and melted. The melt pushes off the anti-reverse ring, flows into the front end of the screw through the surrounding passage of the screw head, and produces back Press and push the screw to move backward to complete the measurement of the melt. During injection, the screw acts as a plunger. Under the action of the cylinder, it moves forward quickly to inject the melt in the storage chamber into the mold through the nozzle.
               
              Screw-type plasticized parts generally have the following characteristics:
               
              ① The screw has two functions: plasticization and injection;
               
              ②The screw is only used for pre-plasticizing when plasticizing;
               
              ③In the plasticization process, the thermal history of plastics is longer than that of extrusion;
               
              ④The screw must undergo axial displacement during plasticization and injection, and at the same time the screw is in an intermittent working state where the screw is turned and stopped. Therefore, the instability of the screw plasticization process is formed.
               
              (1) Screw
               
              The screw is a key component of the plasticized part, and it is in direct contact with the plastic. The effective length of the plastic passing through the screw groove has to go through a long thermal history, and it has to go through three states (glass state, viscoelastic state, viscous fluid state). The length, geometry, and geometric parameters of each functional segment will directly affect the delivery efficiency and plasticization quality of the plastic, and will ultimately affect the injection molding cycle and product quality.
               
              Compared with extrusion screw, injection screw has the following characteristics:
               
              ① The length to diameter ratio and compression ratio of the injection screw are relatively small;
               
              ② The screw groove in the homogenization section of the injection screw is relatively deep;
               
              ③ The feeding section of the injection screw is longer, while the homogenization section is shorter;
               
              ④ The head structure of the injection screw has a special form.
               
              ⑤ When the injection screw is working, the plasticizing capacity and melt temperature will change with the axial displacement of the screw.
               
              (Ⅰ) Classification of screw
               
              According to its adaptability to plastics, injection screws can be divided into general-purpose screws and special screws. General-purpose screws are also called conventional screws, which can process most thermoplastics with low and medium viscosity, crystalline and non-crystalline civilian plastics and engineering Plastic is the most basic form of screw. Corresponding to it, there are special screws, which are used to process plastics that are difficult to process with ordinary screws. According to the screw structure and geometric characteristics, it can be divided into conventional screws and new types of screws. It is called a three-section screw, which is the basic form of the screw. There are many new types of screw, such as a separate screw, a split screw, a corrugated screw, and a screw without metering section.
               
              The effective thread length of conventional screw is usually divided into feeding section (conveying section), compression section (plasticization section), metering section (homogenization section). According to different plastic properties, it can be divided into gradual, abrupt and general screw.
               
              ① Gradual screw: The compression section is longer and the energy conversion is gentle during plasticization. It is mostly used for PVC and other plastics with poor thermal stability.
               
              ② Abrupt screw: The compression section is short, and the energy conversion is more intense during plasticization. It is mostly used for polyolefin, PA and other crystalline plastics.
               
              ③General-purpose screw: A general-purpose screw with relatively strong adaptability, which can adapt to the processing of a variety of plastics, avoids frequent screw replacement, and is beneficial to improve production efficiency.
               
              The length of the conventional screw name section is as follows:
               
              Screw type Feeding section (L1) Compression section (L2) Homogenization section (L3)
               
              Gradient type 25~30% 50% 15~20%
               
              Mutant 65~70% 15~5% 20~25%
               
              General type 45~50% 20~30% 20~30%
               
              (Ⅱ) Basic parameters of screw
               
              The basic structure of the screw is shown in Figure 6, which is mainly composed of the effective thread length L and the connecting part of the tail.
               
              ds — The outer diameter of the screw. The screw diameter directly affects the plasticizing capacity, which directly affects the theoretical injection volume. Therefore, an injection molding machine with a large theoretical injection volume has a large screw diameter.
               
              L/ds-screw length to diameter ratio. L is the effective length of the screw thread part. The larger the screw length to diameter ratio, the longer the thread length, which directly affects the thermal history of the material in the screw and the ability to absorb energy. The energy source has two parts: one part is the barrel The other part of the external heating ring is the friction heat and shear heat generated when the screw rotates, which is converted by external mechanical energy. Therefore, L/ds directly affects the melting effect and melt quality of the material, but if L/ds If it is too large, the transmission torque will increase and energy consumption will increase.
               
              L1—The length of the feeding section. The feeding section is also called the conveying section or the feeding section. In order to improve the conveying capacity, the surface of the screw groove must be smooth and clean. The length of L1 should ensure that the material has enough conveying length, because too short L1 will cause the material to melt prematurely, making it difficult It is difficult to ensure the plasticizing quality and plasticizing ability of the subsequent sections of the screw if the conveying conditions of stable pressure are ensured. The plastic slides into the screw groove from the hopper under its own gravity. When the screw rotates, the material is compressed into a dense solid plug nut under the action of the friction force of each thrust surface composed of the barrel and the screw groove, along the thread direction Do relative movement. In this section, the plastic is in a solid state, that is, a glass state.
               
              h1—The depth of the screw groove of the feeding section. If h1 is deep, it will hold more materials, which improves the feeding volume and plasticizing capacity, but it will affect the plasticizing effect of materials and the shear strength of the screw root, generally h1≈(0.12~0.16)ds.
               
              L3 — The length of the melting section. The melting section is also called the homogenization section or the metering section. The melt is further homogenized in the spiral groove of the L3 section, the temperature is uniform, the composition is uniform, and a good melt quality is formed. The length of L3 helps the melt in the spiral groove. The fluctuation in the groove has the effect of stabilizing the pressure, so that the material is extruded from the screw head in a uniform amount, so it is also called the metering section. L3 is short, it helps to improve the plasticizing ability of the screw, generally L3=(4~5)ds.
               
              h3 — the depth of the screw groove in the melting section, h3 is small and the screw groove is shallow, which improves the plasticizing effect of the plastic melt and is beneficial to the homogenization of the melt. However, if h3 is too small, it will lead to excessive shear rate and excessive shear heating If h3 is too large, it will cause the degradation of the molecular chain and affect the quality of the melt. On the contrary, if h3 is too large, the back pressure of the screw will increase the back pressure during pre-plasticization, which will reduce the plasticizing ability.
               
              L2 — The thread length of the plasticizing section (compression section). The material is continuously compressed, sheared and mixed in this conical space. The material starts from the entry point of section L2, and the molten pool is continuously enlarged. At the exit point, the molten pool has occupied the full spiral groove. The glass state undergoes a transition from a viscoelastic state to a viscous fluid state, that is, at this stage, the plastic is in a state of coexistence of particles and melt. The length of L2 will affect the conversion process of the material from the glass state to the viscous flow state. If it is too short, it will be too late to transform. The solid material will block at the end of the L2 section to form a high pressure, torque or axial force; too long will increase the screw Torque and unnecessary consumption, generally L2=(6~8)ds. For crystalline plastics, the melting point of the material is obvious, and the melting range is narrow. L2 can be shorter, generally (3 ~ 4) ds. For heat-sensitive plastics, this section can be longer.
               
              S-the pitch, its size affects the helix angle, thereby affecting the conveying efficiency of the screw groove, generally S≈ds.
               
              ε —compression ratio. ε=h1/h3, that is, the ratio of the groove depth h1 in the feeding section to the groove depth h3 in the melting section. A large ε will enhance the shearing effect but weaken the plasticizing ability. Generally speaking, a smaller ε is better to improve the plasticizing ability and increase the adaptability to materials. For crystalline plastics, the compression ratio is general Take 2.6~3.0. For low-viscosity, heat-stable plastics, a high compression ratio can be selected; for high-viscosity, heat-sensitive plastics, a low compression ratio should be used.
               
              (2) Screw head
               
              In the injection screw, the function of the screw head is: when pre-plasticizing, it can discharge the plasticized melt into the storage chamber, and during high-pressure injection, it can effectively seal the melt in the front of the screw head to prevent Backflow.
               
              Screw heads are divided into two categories, those with check rings and those without check rings. For those with check rings, the melt in the homogenization section of the screw pushes the check rings away during pre-plasticizing, and passes through the screw head. The formed gap flows into the storage chamber. During injection, the melt pressure on the screw head forms a thrust to block the non-return ring backflow channel to prevent backflow.
               
              Table 1 Injection screw head form and use
               
              For some high-viscosity materials such as PMMA, PC, AC or PVC with poor thermal stability, in order to reduce the shearing effect and the residence time of the material, the non-return ring is not necessary, but such injection will produce backflow and prolong the pressure holding time.
               
              Requirements for screw head:
               
              ① The screw head should be flexible and smooth;
               
              ② The clearance between the check ring and the barrel should be appropriate, that is, to prevent the melt from flowing back, but also to be flexible;
               
              ③ There is not only a sufficient flow cross section, but also a return force on the end face of the check ring, so that it can be quickly closed during injection;
               
              ④ The structure should be easy to disassemble and assemble, and easy to clean;
               
              ⑤ The thread of the screw head is opposite to that of the screw to prevent the screw head from loosening during pre-molding.
               
              (3) Barrel
               
              (Ⅰ) The structure of the barrel
               
              The barrel is an important part of the plasticization component. The screw is installed inside and the heating ring outside is installed to bear the action of compound stress and thermal stress. The structure is shown in Figure 7:
               
              1- Front barrel; 2- Electric heating ring; 3- Screw hole; 4- Feeding port
               
              The screw hole 3 is equipped with a thermocouple, which should be in close contact with the thermocouple to prevent false floating, otherwise it will affect the temperature measurement accuracy.
               
              (Ⅱ) Feeding port
               
              The structure of the feeding port directly affects the feeding effect and the feeding capacity of the plasticized parts. Most of the injection molding machines rely on the weight of the material in the hopper to feed. The commonly used feed port section is shown in Figure 8: the symmetrical feed port is shown in Figure 8. 8(a), the manufacturing is simple, but the feed is unfavorable; now asymmetrical forms are often used, as shown in Figures 8(b) and 8(c), this type of feed port has a large contact area due to the large contact angle between the material and the screw , It is beneficial to improve the feeding efficiency, and it is not easy to open a bridging cavity in the hopper.
               
              (Ⅲ) Wall thickness of the barrel
               
              The wall thickness of the barrel requires sufficient strength and rigidity, because the barrel has to withstand melt and gas pressure, and the ratio of length to diameter of the barrel is large, and the barrel requires sufficient heat capacity, so the barrel wall must have a certain thickness , Otherwise it is difficult to ensure the temperature stability; but if it is too thick, the barrel will be bulky, waste materials, large thermal inertia, slow temperature rise, and large hysteresis in temperature adjustment.
               
              (Ⅳ) Barrel gap
               
              The barrel gap refers to the one-sided gap between the inner wall of the barrel and the outer diameter of the screw. If the gap is too large, the plasticizing ability will be reduced, the injection leakage will increase, and the injection time will be prolonged, which will cause partial degradation of the material in the process; if it is too small, thermal expansion The effect makes the friction between the screw and the barrel increase, energy consumption increases, and even jams, this gap Δ=(0.002~0.005)ds.
               
              (Ⅴ) Heating and cooling of the barrel
               
              The heating method of the injection molding machine barrel is
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