Injection Molding Machine is mainly divided into the injection part and the clamping part in the mechanical structure. The function of the injection part is to melt the plastic and inject it into the mold cavity, and the function of the clamping part is to control various actions such as opening and closing the mold and ejecting the product. In the below details of Injection Molding Machine structure has described.
1. Injection part of Injection Molding Machine
There are two main types of injection parts: piston type and reciprocating screw type. Piston-type injection molding machines are rare now and will not be introduced here.
The reciprocating screw injection molding machine melts and mixes solid plastic particles (or powder) through the rotation of the screw in the heating barrel, and extrudes it into the cavity at the front end of the barrel, and then the screw moves forward in the axial direction to remove the plastic in the cavity. The melt is injected into the mold cavity. During plasticization, the plastic is compacted in the screw groove under the impetus of the screw thread, and receives the heat transferred from the barrel wall. In addition, the friction between the plastic and the plastic, the plastic and the barrel and the surface of the screw generates heat, and the temperature gradually rises. up to the melting temperature. The melted plastic is further mixed by the screw, and enters the front of the barrel along the screw groove and pushes the screw back.
The plasticizing-related components of the injection part mainly include: screw, barrel, shunt shuttle, check ring, nozzle, flange, feeding hopper, etc. The functions and influences in the plasticization process are explained below.
The screw is an important part of the injection molding machine . Its function is to convey, compact, melt, stir and press the plastic. All of this is done by the rotation of the screw within the barrel. When the screw rotates, the plastic will produce friction and mutual movement between the inner wall of the barrel, the bottom surface of the screw groove, the screw flight advancing surface, and the plastic and the plastic. The forward advancement of the plastic is the result of this combination of motions, and the heat generated by friction is also absorbed to raise the temperature of the plastic and melt it. The structure of the screw will directly affect the extent of these effects. Ordinary injection
screw The structure of ordinary injection screw is also designed to separate screw, barrier screw or split screw in order to improve the plasticization quality.
The structure of the barrel is actually a round tube with a feeding opening in the middle.
In the plasticizing process of plastic, the power of its advancement and mixing comes from the relative rotation of the screw and the barrel. According to the different shapes of the plastic in the screw groove, the screw is generally divided into three sections: the solid conveying section (also called the feeding section), the melting section (also called the compression section), and the homogenization section (also called the metering section).
In the teaching materials about plastic plasticization, the plastic in the solid conveying section of the screw is regarded as a solid bed that does not move between plastic particles, and then passes through the solid bed and the barrel wall, the screw propulsion surface and the screw groove. Calculation of the ideal state of motion and friction of the surfaces against each other to determine the speed at which the plastic is transported forward. This is quite different from the actual situation, and it cannot be used as a basis to analyze the feeding situation of plastic particles of different shapes. If the plastic particles are not large, they will delaminate and tumble as they are pulled forward by the inner wall of the barrel, and gradually compacted to form a solid plug. When the diameter of the desired material particles is similar to the depth of the screw groove, their motion trajectory is basically a linear motion along the radial direction of the screw groove plus a linear motion at an angle. Since the arrangement of the plastic in the screw groove is very loose when the particles are large, the conveying speed is also slow. When the particles are large enough to enter the compression section and their diameter is greater than the depth of the screw groove, the plastic will be stuck between the screw and the barrel. If the force of pulling forward is not enough to overcome the force required to flatten the plastic particles, Then the plastic will get stuck in the screw groove and not advance forward.
When the plastic is close to the melting point temperature, the plastic in contact with the barrel has begun to melt to form a molten film. When the thickness of the molten film exceeds the gap between the screw and the barrel, the top of the spiral rib scrapes the molten film radially from the inner wall of the barrel to the root of the helical rib, thereby gradually converging into a vortex-shaped flow area on the propulsion surface of the helical rib—— molten pool.
Due to the gradually shallow depth of the screw groove in the melting section and the extrusion of the molten pool, the solid bed is squeezed towards the inner wall of the barrel, which accelerates the heat transfer process from the heat barrel to the solid bed. At the same time, the rotation of the screw causes shearing of the molten film between the solid bed and the inner wall of the barrel, so that the solid between the molten film and the interface of the solid bed is melted. As the spiral shape of the solid bed progresses forward, the volume of the solid bed gradually decreases, while the volume of the molten pool gradually increases. If the rate at which the thickness of the solid bed decreases is lower than the rate at which the groove depth becomes shallow, the solid bed may partially or completely block the groove, causing fluctuations in plasticization, or a sharp increase in frictional heat generation due to excessive local pressure. Local overheating occurs.
In the screw homogenization section, the solid bed has been broken up due to its small volume to form small solid particles dispersed in the molten pool. These solid particles are melted by friction and heat transfer with the melt around the coating, respectively. At this time, the function of the screw is mainly to mix the plastic melt evenly by stirring it, and the speed distribution of the melt ranges from the highest speed close to the barrel wall to the lowest speed close to the bottom of the groove. If the groove depth is not large and the melt viscosity is high, then the friction between the melt molecules will be severe.
Due to the melting speed, melt viscosity, melting temperature range of various plastics, the sensitivity of viscosity to temperature and shear rate, the corrosiveness of high-temperature decomposition gas, and the coefficient of friction between plastic particles are very different. When the screw processes some plastics with outstanding melt characteristics (such as Pc, PA, polymer ABS, PP-R, PVC, etc.), there will be a phenomenon of excessive shear heat in a certain section. This phenomenon can generally be reduced by reducing Screw speed is eliminated. But this is bound to affect production efficiency. In order to achieve efficient plasticization of these plastics, many companies have successively developed special plasticizing screws and barrels for these plastics. The main problems in the design of these special screws and barrels are the solid friction coefficient, melt viscosity, melting speed, etc. of the above plastics.
2. Shunt Shuttle (Gluing Head)
The shunt shuttle is a part mounted on the front end of the screw shaped like a torpedo body. The function of the shunt shuttle in plastic plasticization is mainly to shunt and mix the plastic melt, so that the melt is further mixed evenly. At the same time, the shunt shuttle also has the function of limiting the position of the check ring during plasticization.
In order to further strengthen the mixing effect, it is recommended to use a barrier type mixing structure on the injection molding machine with a clamping force of more than 250 tons. shunting shuttle. It can not only improve the uniformity of the color of the product, but also make the mechanical strength of the product higher.
3. Anti-return ring (over the apron)
As the name suggests, the function of the anti-return ring is to stop the backflow. It is a part that prevents the plastic melt from leaking back during injection. When working, the non-return ring and the non-return gasket (over-glued gasket) contact to form a closed structure to prevent the leakage of plastic melt.
The precision of the weight of the injection molded products of an injection molding machine has a great relationship with the speed of the anti-return action of the non-return ring. The speed of a non-return ring’s action response is determined by its non-reverse action stroke, sealing and pressing time, and time to leave the shunt shuttle. We have tried a variety of non-return ring structures and parts parameters, and finally determined the optimal non-return surface parameters, the parameters of the non-return ring and the shunt shuttle fit, the non-return ring and the barrel clearance parameters through experiments. High-precision injection volume control can be achieved.
4. Nozzle of Injection Molding Machine
The nozzle is the transition part connecting the barrel and the mold. During injection, the molten material in the barrel is driven by the screw to flow through the nozzle at high pressure and fast into the mold. Therefore, the structure of the nozzle, the size of the nozzle hole and the manufacturing precision will affect the pressure and temperature loss of the melt, the distance of the shooting range, the pros and cons of the feeding effect, and whether the phenomenon of “drooling” occurs. There are many types of nozzles currently in use, and they all have their scope of application. Only the most used three are discussed here.
(1) Straight-through nozzle This nozzle is in the shape of a short tube. When the molten material flows through this nozzle, the pressure and heat loss are very small, and it is not easy to produce stagnation and decomposition, so the external heating device is generally not attached. However, due to the short nozzle body, the length extending into the fixed plate hole is limited, so the main flow channel of the mold used is long. In order to make up for this defect, the length of the nozzle is increased, and it has become an improved type of straight-through nozzle, also known as an extended nozzle. This nozzle must have a heat setting added. In order to filter out the solid impurities in the melt, a filter can also be installed in the nozzle. The above two nozzles are suitable for processing high-viscosity plastics. When processing low-viscosity plastics, salivation will occur.
(2) Self-locking nozzle During the injection process, in order to prevent the molten material from drooling or retracting, it is necessary to temporarily block the nozzle channel and use a self-locking nozzle. Among the self-locking nozzles, the spring type and the needle valve type are the most widely used. This type of nozzle is self-locking by means of the spring pressing the valve core in the nozzle body. During injection, the valve core is pushed open by the high pressure of the molten material, and the molten shape is then injected into the mold. When the glue is melted, the valve core is reset and self-locking under the action of the spring. Its advantages are that it can effectively prevent the “salivation” phenomenon when injecting low-viscosity plastics, is convenient to use, and has a remarkable self-locking effect. However, the structure is relatively complex, the injection pressure loss is large, the range is short, the feeding effect is small, and the requirements for the spring are high.
The lever needle valve nozzle is the same as the self-locking nozzle. It is also a kind of temporary opening and closing of the nozzle channel during the injection process. It uses an external hydraulic system to control the opening and closing valve of the linkage mechanism through a lever. core. When in use, the operated hydraulic system can open the valve core accurately and timely according to the needs, which has the advantages of convenient use, reliable self-locking, small pressure loss and accurate measurement. In addition, it does not use springs, so there is no need to replace the springs. The main disadvantage is that the structure is more complicated and the cost is higher.
During injection, the plastic melt flows through the nozzle at a very high shear rate and enters the mold cavity under the push of the screw. Under this high-speed shearing action, the melt temperature rises rapidly. Especially for high viscosity PVC, PP-R, PMMA, PC, high impact ABS, etc., too small nozzle hole diameter will cause high temperature decomposition of plastic. For thin-walled precision products that are difficult to fill, it is better to use a nozzle with a longer range, and for thick-walled products, a nozzle with good plasticity is required. In addition, for some plastics with very low melt viscosity (such as PA, etc.), it is necessary to use a self-locking nozzle with anti-salivation function.
On many machines, in addition to general-purpose nozzles for general viscosity, there are also special nozzles such as self-locking nozzles, PVC nozzles, and PMMA nozzles. Special nozzle for products.
- Flange The flange is the part that connects the nozzle and the barrel? It only acts as a channel in the plastic injection and injection process of plastic. If there is a large gap or groove between the flange and the nozzle or the flange and the barrel, black spots will appear on the product because the plastic stays in the gap or the groove for too long to decompose.
- Feeding hopper
Feeding hopper is a component for storing plastic raw materials, and some add heating and blowing devices to the hopper to make a drying hopper. The shape of the feeding hopper is generally a lower conical shape and an upper cylindrical shape. The conical taper slope has different optimal values for plastic parts with different particle sizes, different particle shapes, and different friction coefficients and cohesion coefficients between particles. Otherwise, it will either waste the storage capacity of the hopper or cause poor feeding. The reason for the phenomenon of “bridging” or “funneling into a tube” without cutting material at all
is that a clear bridge is formed between the plastic particles at the small conical mouth that can support the material above it. Large and irregularly shaped recycled materials are more likely to occur. “Funneling into a tube” occurs because the particles going down are not enough to pull their neighbors to flow together, which tends to happen when the plastic is smaller in size. The general solution is to install a vibrating device on the hopper or reduce the taper slope. If the heat on the barrel is transferred to the feeding hopper, the temperature of the feeding hopper is too high, and the surface of the plastic pellets softens or sticks into a block, which is more likely to form “bridging” or blockage.
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