The components related to plasticization in the injecti […]
The components related to plasticization in the injection part mainly include: screw, barrel, split shuttle, check ring, nozzle, flange, hopper, and the like. The following explains their roles and effects in the plasticization process.
The screw is an important part of the injection molding machine. Its role is to transport, compact, melt, agitate and apply pressure to plastics. All of this is done by the rotation of the screw in the barrel. When the screw rotates, the plastic will rub against each other on the inner wall of the barrel, the bottom surface of the screw groove, the screw advance surface, and between the plastic and the plastic. The forward advancement of the plastic is the result of this combination of motion, and the heat generated by the friction is also absorbed to increase the temperature of the plastic and melt the plastic. The structure of the screw will directly affect the extent of these effects.
Ordinary injection screw structure, also designed to improve the plasticizing quality into a separate type of screw, barrier type screw or split type screw.
The structure of the barrel is actually a round tube with a feed opening in the middle.
In the plasticization process of plastics, the power of advancement and mixing is derived from the relative rotation of the screw and the barrel. According to the different forms of plastic in the screw groove, the screw is generally divided into three sections: a solid conveying section (also called a feeding section), a melting section (also called a compression section), and a homogenization section (also called a metering section).
In the textbook on plastic plasticization, the plastic conveying section of the screw is regarded as a solid bed without plastic movement between the plastic particles, and then passes through the solid bed and the cylinder wall, the screw-edge advancing surface and the screw groove. The calculation of the ideal state of surface motion and friction to determine the speed at which the plastic is transported forward. This has a lot of gaps with the actual situation, and can not be used as a basis to analyze the feeding of plastic pellets of different shapes. If the particles of the plastic are not large, they will stratify and roll when they are pulled forward by the inner wall of the cylinder, and are gradually compacted to form a solid plug. When the diameter of the particles is similar to the depth of the groove, their trajectory is basically a linear motion along the radial direction of the groove plus a linear motion at an angle. Since the arrangement of the plastic in the groove is loose when the particles are large, the conveying speed is also slow. When the particles are large enough to enter the compression section and the diameter is greater than the depth of the groove, the plastic is caught between the screw and the barrel, and if the force of pulling forward is insufficient to overcome the force required to crush the plastic particles, Then the plastic will get stuck in the groove and not advance.
When the plastic is near the melting point temperature, the plastic in contact with the barrel begins to melt to form a molten 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 cylinder toward the root of the spiral ridge, thereby gradually collecting a swirling flow area on the advancing surface of the spiral ridge - Molten pool.
Due to the gradual shallowing of the depth of the molten section and the extrusion of the molten pool, the solid bed is squeezed toward the inner wall of the cylinder, thus accelerating the heat transfer process of the hot cylinder to the solid bed. At the same time, the rotation of the screw causes shearing of the melt film between the solid bed and the inner wall of the barrel, thereby melting the solid between the melt film and the solid bed interface. As the spiral of the solid bed moves forward, the volume of the solid bed gradually shrinks and the volume of the molten pool gradually increases. If the solid bed thickness is reduced at a speed lower than the shallower groove depth, the solid bed may partially or completely block the channel, causing plasticization to fluctuate or causing a sharp increase in frictional heat due to excessive local pressure. Local overheating occurs.
In the screw homogenization section, the solid bed has been broken due to too small a volume to form small solid particles dispersed in the molten pool. These solid particles are melted by friction and heat transfer with each of the melt around the coating. At this time, the function of the screw is mainly to mix and mix the plastic melt, and the speed of the melt is distributed from the highest speed close to the wall of the barrel to the lowest speed close to the bottom of the groove. If the depth of the groove is not large and the melt viscosity is high, 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 pyrolysis gas, and the coefficient of friction between plastic particles, it is generally common. When the screw processes some plastics with outstanding melt characteristics (such as Pc, PA, polymer ABS, PP-R, PVC, etc.), a certain degree of shear heat will appear. This phenomenon can generally be lowered.
The low screw speed is eliminated. But this will inevitably affect production efficiency. In order to achieve high-efficiency plasticization of these plastics, the company has developed special plasticizing screws and barrels for these plastics. The main problems that these special screws and barrels are designed for are the solid friction coefficient, melt viscosity, melting rate, etc. of the above plastics.
