The operating principle of the extruder

June 6, 2019

The operating principle of the extruder

 

With the wide application and development of plastic extruder molding processes, the types of plastic extruders are increasing and the classification methods are also inconsistent. According to the spatial position of the screw, it can be divided into horizontal and vertical extruders; according to the rotation speed of the screw, it can be divided into ordinary extruder, high speed and overspeed extruder; according to the number of screws, it is divided into screwless extrusion. Machine, single screw extruder, twin screw extruder and multi-screw extruder, etc., the following describes the working principle of single screw plastic extruder

When the extrusion system is heated to a given process temperature for a certain period of time, the motor is started and the torque and speed required by the screw are supplied to the screw. The plastic in the hopper enters the groove by the feeding port under the action of its own weight or the thrust of the feeder.

 

As the screw rotates, the plastic is transported forward under the action of the friction of the barrel screw.

 

After the plastic self-feeding port enters the screw, it is conveyed forward by the friction of the inner wall of the cylinder and the surface of the screw under the action of the rotating screw. In the molding process, after the material is added from the hopper to the plastic extruder in the form of powder or granules, the process of conveying, compacting, compressing, melt-plasticizing, and homogenizing into a uniform melt is completed. In the screw feeding section, the loose solid pellets (or the end of the material) are filled with the screw groove, and as the material is continuously conveyed, the material begins to be compacted.

 

After the material enters the compression section, the plastic gradually forms a high pressure due to the shallower depth of the screw groove and the resistance of the head, and is further compacted at the same time, heating outside the barrel and the inner surface of the screw and the barrel Under the action of the internal friction and shear heat generated by the strong agitation, mixing and shearing friction, the temperature of the plastic is continuously increased, and at a certain point in contact with the barrel, the temperature of the plastic reaches the melting point and begins to melt. As the material is transported, heating continues, and the amount of molten material gradually increases, while the amount of unmelted material decreases accordingly. At the end of the compression section, all materials are transformed into a viscous flow state. However, the temperature at each point is not uniform, and the homogenization of the homogenization section is relatively uniform. Finally, the screw quantifies, constants, and determines the temperature of the molten material. Squeeze into the nose. The die in the head of the plastic extruder is a molded part through which the material obtains the geometry and size of a certain section, and after cooling setting and other processes, the formed product can be obtained.

    

 

The operating principle of single screw extruder
       
    The function of the extruder is to convert the solid plastic into a uniform melt by heating, pressurizing and shearing, and to send the melt to the next process. The production of the melt involves processes such as mixing color masterbatches, blending resins, and re-grinding. The finished melt must be uniform in concentration and temperature. The pressurization must be large enough to squeeze the viscous polymer.

 

The extruder performs all of the above processes through a barrel with a screw and a spiral knife. The plastic pellets enter the barrel through the hopper at one end of the barrel and are then passed through the screw to the other end of the barrel. In order to have sufficient pressure, the depth of the threads on the screw decreases as the hopper increases. External heating and internal heat generated by the plastic and screw due to friction soften and melt the plastic.

 

The extruder usually consists of at least three stages. The first section, near the hopper, is the feeding section. Its function allows the material to enter the extruder at a relatively smooth rate. In general, this part will maintain a relatively low temperature in order to avoid blockage of the feed channel. The second part is the compression section, where the melt is formed and the pressure is increased. The transition from the feed section to the compression section can be either abrupt or gradual (smooth).

 

The last part of the metering section is next to the extruder exit. The main function is that the material flowing out of the extruder is uniform. In this part, to ensure the uniformity of composition and temperature, the material should have sufficient residence time. At the end of the barrel, the plastic melt exits the extruder through a handpiece that is designed to the desired shape through which the extruded melt stream passes.

 

Another important part is the drive mechanism of the extruder. It controls the rotational speed of the screw, which determines the output of the extruder. The power required is determined by the viscosity (flow resistance) of the polymer. The viscosity of the polymer depends on the temperature and flow rate and decreases with increasing temperature and shear. The extruder has a strainer that blocks impurities on the screen. In order to avoid downtime, the filter should be able to be replaced automatically. This is especially important when processing resins with impurities, such as recycled materials.

 

 

Introduction of the principle of twin-screw plastic extruder

 

The first twin-screw extruder has the principle of an extruder for a single-screw extruder: solids transport, melting, pressurization and pumping, mixing, stripping and devolatilization, but not simply. The research on the twin-screw extrusion theory started late, coupled with its many types, the screw geometry is complex, and the extrusion process is complicated, which brings many difficulties to the research.

 

On the whole, the research on twin-screw extrusion theory is still in the initial stage. This is what is called "technology more than science". From its extrusion process research, it is divided into three parts:

 

1. The change state of the physical state of the polymer during the extrusion process, the conveying principle, the solid, the melt transport, the exhaust truth and the law, establish a mathematical physical model to guide the design and extrusion of the twin-screw extruder Optimization of the process.

 

2. To understand the true state of the physical state of the two or more polymers and materials during the extrusion process, the mixed morphology, the process of structural changes, and the relationship between the final mixture and the properties.

3. As a twin-screw extruder, the internal relationship between the reaction process, speed, performance and screw configuration and operating conditions during extrusion reaction molding is established to guide the reaction molding extrusion.
 

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