Understanding the Impact of Key Parameters on the Screw Extrusion Process and Performance

The main parameters of the screw are diameter, length-diameter ratio, compression ratio, pitch, groove width, groove depth, helix angle, gap between the screw and the barrel, etc., which have a great impact on the extrusion process and performance.

Extruder Screw diameter

The screw diameter is the outer diameter of the thread, and the production capacity of the extruder is approximately proportional to the square of the screw diameter, when other conditions are the same, the screw diameter increases slightly, which will cause a significant increase in the extrusion volume, and its impact is even greater than the increase in the number of screw revolutions on the extrusion volume. Therefore, the screw diameter is often used to characterize the technical parameters of the size of the extruder.

Extruder Screw length-diameter ratio

The ratio of the length of the working part of the screw to the diameter of the screw is called the length-diameter ratio, and when other conditions are certain (such as the diameter of the screw), increasing the length-diameter ratio means increasing the length of the screw. The large aspect ratio and reasonable temperature distribution are conducive to the mixing and plasticization of plastics, and the heating time of the plastic in the barrel is also longer, and the plasticization of the plastic will be more adequate and uniform, so as to improve the plasticization quality.

Under the premise that the plasticizing quality requirements remain unchanged, the rotation speed of the screw can be increased after the length-diameter ratio is increased, thereby increasing the extrusion amount of plastic. However, if you choose an excessively large aspect ratio, the power consumed by the screw will increase accordingly, and the processing and assembly difficulty of the screw and barrel will increase, and the possibility of screw bending will also increase, which will cause the scraping of the inner wall of the screw and the barrel and reduce the service life.

In addition, for heat-sensitive plastics, excessive aspect ratios can be thermally decomposed due to long residence time, which affects the plasticization and extrusion quality of plastics. Therefore, the selection of the aspect ratio should be determined according to the physical properties of the processed plastics and the extrusion quality requirements of the product.

Compression ratio

Compression, also known as the geometric compression ratio of the screw, is the ratio of the volume of the first screw groove in the screw feeding section to the volume of the last screw groove in the homogenization section. It is determined by the physical compression ratio of the plastic – that is, the ratio of the density of the part to the performance density of the incoming material. The large compression ratio allows the granular plastic to be fully plasticized and compacted. When the type of plastic being processed is different, the choice of compression ratio should also be different.

Spiral rising angle

The spiral angle is the angle between the thread and the screw cross-section. If the helix angle is too large, the plasticizing time can not be guaranteed, and the plasticizing quality of the screw will be reduced, and if it is too small, the volume of the screw groove will be reduced, which will affect the extrusion amount. For the feeding section, the 30° helix angle is most suitable for the powder; The 15° helix angle is suitable for square grains; The helix angle of about 17° is suitable for spherical or columnar grains.

Pitch and groove width

The pitch is the axial distance of the thread, and the groove width is the groove width perpendicular to the ridge. When other conditions are equal, the change of pitch and groove width not only determines the helix angle of the screw, but also affects the volume of the groove, thus affecting the extrusion amount and plasticization degree of the plastic. The increase of the width of the screw groove means that the width of the screw edge decreases, the volume of the screw groove increases accordingly, and the extrusion volume increases. At the same time, the width of the screw edge is reduced, and the frictional resistance of the screw rotation is reduced, so the power consumption is low.

Groove depth

The depth of the groove is the difference between the outer radius of the thread and the radius of the root. According to the requirements of the compression ratio, the groove depth of the feeding section is greater than that of the molten section, and the groove depth of the molten section is greater than that of the homogenization section.

The deep screw groove of the feeding section is large, which is conducive to improving its conveying capacity; However, if the groove depth is too deep, the strength of the screw will decrease, resulting in the screw shearing under the action of greater torque; Second, it is too deep to make the plastic unevenly mixed and stirred unevenly between the tanks, which affects the heat conduction and thermal balance, and leads to the reduction of the plasticizing capacity of the screw.

The screw groove in the molten section and the homogenization section is gradually shallow, and the screw produces a higher shear rate on the material, which is conducive to the heat transfer from the cylinder wall to the material and the mixing and plasticization of the material. However, if it is too shallow, the volume of the screw groove decreases, which directly affects the extrusion volume.

The gap between the screw and the barrel

The gap between the screw and the barrel is half of the difference between the inner diameter of the barrel and the outer diameter of the screw. The size of the gap between the screw and the barrel has a great impact on the quality and output of extrusion, especially for plasticization.

When the gap between the screw and the barrel is too large, especially when the gap in the homogenization section increases, the counterflow and leakage phenomenon of the plastic increases, which not only causes the fluctuation of extrusion pressure and affects the extrusion amount, but also causes the plastic to overheat due to the increase of these refluxes, which is the result of the aggravation of friction, this overheating, especially in the environment of poor heat dissipation, often leads to the decomposition of the plastic, resulting in poor plasticization and difficult molding. Therefore, the gap between the screw and the barrel is generally controlled at 0.1~0.6mm.

Screw head structure

The shape and geometric size of the screw head determine whether the material can enter the machine head from the screw in a balanced manner, and whether it can avoid stagnation, so as not to avoid the thermal decomposition of local materials due to too long heating time.

Different shapes of the screw head will cause the plastic to flow differently from the screw into the machine head. The head of the screw is often shaped with a cone with a small cone angle, and in order to increase the stirring effect, a continuous thread with the screw homogenization section can be made on the cone shape.

The number of heads of the screw thread

When other conditions are the same, compared with the single-head thread, the multi-head thread has a greater positive thrust on the material, the ability to transport the material is stronger, and the backflow phenomenon of the plastic melt can be reduced. However, when all the threads are multi-head threads, it will easily cause fluctuations in extrusion volume and pressure fluctuations due to the melting and homogenization of each screw groove or the inconsistency of the melt conveying capacity, which is not conducive to the extrusion quality. Therefore, sometimes just to improve the ability of the feeding section to convey materials, double-ended threads are set in the feeding section to improve the conveying capacity of plastic particles.

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