The injection molding machine (injection molding machine, injection machine) is the main molding equipment which uses the thermoplastic or thermosetting materials to make all kinds of plastic products with different shapes. It can be divided into vertical type, horizontal type and all electric type. Today, Huahong screw and you know each part of the injection molding machine. The mechanical structure of injection molding machine is mainly divided into injection part and mold locking part. The function of the injection part is to melt the plastic and inject it into the mold cavity. The function of the mold locking part is to control the opening and closing of the mold, ejecting the products and other actions.
1. Injection part
There are two main injection methods: piston type and reciprocating screw type. Usually piston injection molding machine has been very rare, here is no introduction.
The reciprocating screw injection molding machine coagulates and mixes the solid plastic particles (or powder) through the rotation of the screw in the heating barrel, and then extrudes them into the cavity at the front end of the barrel, and then the screw moves forward along the axial direction, injecting the plastic melt in the cavity into the mold cavity. During plasticization, the plastic is compacted in the screw groove under the promotion of the screw edge, and receives the heat transferred from the cylinder wall. In addition, the heat generated by friction between the plastic and the plastic, plastic and the cylinder and the screw surface gradually rises to the melting temperature. The condensed plastic is further mixed by the screw, and enters the front of the barrel along the screw slot and pushes the screw backward.
Injection part and plasticization related parts mainly include: screw, barrel, split shuttle, check ring, nozzle, flange, feeding hopper, etc. The function and influence in the plasticizing process are described below.
Screw is an important part of injection molding machine. Its function is to stop the delivery, compaction, condensation, mixing and pressure of plastic. All these are completed by the rotation of the screw in the barrel. When the screw rotates, the friction and mutual movement between the plastic and the inner wall of the barrel, the bottom surface of the screw groove, the advancing surface of the screw edge and between the plastic and the plastic will occur. The forward movement of plastic is the result of this combination of movements, and the heat generated by friction is also absorbed to improve the temperature and condensation of plastic. The screw structure will directly affect the degree of these effects.
Common injection screw structure, but also in order to improve the quality of plastic design components of screw, barrier screw or split screw.
The structure of the barrel is actually a round pipe with a blanking opening in the middle.
In the process of plastic plasticization, the driving force of its moving and mixing is from the relative rotation of screw and barrel. According to the different shapes of plastics in screw grooves, the screw is generally divided into three sections: solid conveying section (also known as feeding section), melting section (also known as contraction section), homogenization section (also known as metering section). (injection molding machine screw)
In the teaching materials of plastic plasticization, the solid conveying section of plastic in screw is regarded as a solid bed without mutual movement between plastic particles. Then, the ideal state of mutual movement and friction between the solid bed and the cylinder wall, the advancing surface of screw edge and the surface of screw groove is calculated to confirm the forward conveying speed of plastic. There are many gaps between this and the theoretical situation, and it can not be used as a basis to analyze the feeding situation of different shapes of plastic particles. Assuming that the plastic particles are small, they will show stratification and rolling when they are pulled forward by the inner wall of the barrel, and will be gradually compacted to form a solid plug. When the diameter of the desired particles is about the same as the depth of the screw groove, their trajectories are basically a straight-line movement along the radial direction of the screw groove plus a straight-line movement at an angle. Due to the loose arrangement of plastics in the screw groove when the particles are large, the conveying speed is also slow. When the particle size reaches a certain degree, when it enters the contraction section and its diameter is greater than the depth of the screw groove, the plastic will be stuck between the screw and the barrel. Assuming that the force of pulling forward is insufficient to suppress the force required to flatten the plastic particle, the plastic will be stuck in the screw groove and will not advance.
When the plastic is close to the melting point temperature, the plastic in contact with the cylinder has begun to melt to form a melting film. When the thickness of the melt film exceeds the gap between the screw and the barrel, the top of the spiral edge scrapes the melt film radially from the inner wall of the barrel to the root of the spiral edge, and then gradually integrates the whirlpool, which is a whirlpool active area, on the advancing surface of the spiral edge.
As the depth of the spiral groove in the melting section becomes shallower and the molten pool is extruded, the solid bed is squeezed to the inner wall of the barrel, thus accelerating the heat transfer process from the heat engine barrel to the solid bed. At the same time, the rotation of the screw makes the film between the solid bed and the inner wall of the barrel shear, so that the solid between the film and the solid bed interface condenses. As the spiral shape of the solid bed moves forward, the volume of the solid bed decreases and the volume of the molten pool increases. If the reduction speed of the solid bed thickness is lower than that of the depth of the spiral groove, the solid bed may partially or completely block the spiral groove, which may shake the plasticization, or generate a sharp increase of friction heat due to the excessive local pressure, resulting in local overheating.
In the screw homogenization section, the solid bed was once divided into small solid particles dispersed in the molten pool due to its small volume. These solid particles are melted by friction and heat transfer with the surrounding melt. At this time, the function of the screw is mainly to mix the plastic melt evenly by stirring, and the speed distribution of the melt is from the highest speed close to the barrel wall to the lowest speed close to the bottom of the screw groove. If the depth of the spiral groove is not large and the viscosity of the melt is high, then the friction between the melt molecules will be very strong.
Because of the melting speed, melt viscosity, melting temperature range, sensitivity of viscosity to temperature and shear rate, corrosion of high-temperature synthetic gas