The function and working process of extruder screws

The extruder screw is the most important component in plastic extruder equipment, which directly affects the plasticizing effect and output of the plastic machine. The rotation of the extruder screw in the material barrel is carried out under high temperature, high pressure, and high torsion resistance. Due to its strong push of the material forward during rotation, as well as its ability to withstand strong frictional forces and corrosive gases from plastic decomposition, the material of the extruder screw must have high mechanical strength, withstand huge torsional moments, and remain deformation free under high temperature and high pressure conditions.

During the rotation process of the extruder screw, the plastic is mainly sheared and plasticized by the screw edges, which push the plastic forward. Therefore, the screw edges bear huge shear stress and friction force. Due to long-term operation under harsh conditions, the screw edges wear out, become smaller, and the gap between the same barrel increases, resulting in a decrease in plastic extrusion volume. In severe cases, plastic reflux may occur, and the plasticizing effect may decrease, leading to a serious decrease in grain size and production capacity.

The process of melt extrusion is to feed the pre mixed material from the feeding port into the extruder barrel. The first section of the barrel is the feeding section, where the material will not melt. It is driven by the extruder screw and brought into the second section, which is the compression section. This section is the heating stage, where the material begins to melt and the friction between the materials increases, forming a high viscosity body. It continues to be driven by the extruder screw into the high shear third section, which is the homogenization section, effectively separating pigment aggregates and achieving full dispersion. At present, the extruder equipment used in powder coatings includes double extruder screw extruders, single extruder screw extruders, and star extruder screw extruders. Although the types and internal structures of extruders are different, the design purpose is the same, which is to maximize the uniform dispersion of materials. Therefore, Qingdao Guohao Plastic Machinery believes that the quality of extruders directly determines the degree of material dispersion.
The main unit of a plastic extruder is the extruder, which consists of an extrusion system, a transmission system, and a heating and cooling system.

1. Extrusion system The extrusion system includes a screw, barrel, hopper, head, and mold. Plastic is plasticized into a uniform melt through the extrusion system, and under the pressure established during this process, it is continuously extruded by the screw head.

(1) Screw: It is the most important component of the extruder, which directly affects the application range and productivity of the extruder. It is made of high-strength and corrosion-resistant alloy steel.

(2) Barrel: It is a metal cylinder, generally made of alloy steel or composite steel pipes lined with alloy steel with high heat resistance, pressure resistance, durability, wear resistance, and corrosion resistance. The combination of the barrel and screw achieves the crushing, softening, melting, plasticizing, venting, and compaction of plastics, and continuously and uniformly delivers the rubber material to the molding system. The length of the general barrel is 15-30 times its diameter, in order to ensure that the plastic is fully heated and plasticized.

(3) Hopper: The bottom of the hopper is equipped with a cutoff device for adjusting and cutting off the material flow, and the side of the hopper is equipped with a sight hole and a calibration measuring device.

(4) Head and mold: The head is composed of an alloy steel inner sleeve and a carbon steel outer sleeve, and the head is equipped with a forming mold. The function of the machine head is to convert the rotating plastic melt into parallel linear motion, uniformly and smoothly introduce it into the mold sleeve, and give the plastic the necessary molding pressure. Plastic is plasticized and compacted inside the barrel, and flows through a porous filter plate along a certain channel through the neck of the machine head into the forming mold. The mold core and sleeve are appropriately matched to form a circular gap with a continuously decreasing cross-section, allowing the plastic melt to form a continuous and dense tubular coating layer around the core wire. To ensure a reasonable plastic flow channel inside the machine head and eliminate dead corners where plastic accumulates, diversion sleeves are often installed. To eliminate pressure fluctuations during plastic extrusion, pressure equalization rings are also installed. The machine head is also equipped with a device for mold calibration and adjustment, which facilitates the adjustment and correction of the concentricity of the mold core and sleeve.

The extruder is divided into angled heads (with an angle of 120 degrees) and right angled heads according to the direction of the material flow at the head and the centerline of the screw. The outer shell of the machine head is fixed to the body with bolts, and the mold inside the machine head has a core seat, which is fixed to the wire inlet port of the machine head with a nut. The front of the core seat is equipped with a core, and the center of the core and the core seat has a hole for passing the core wire; A pressure equalization ring is installed at the front of the machine head to balance the pressure; The extrusion molding part consists of a mold sleeve seat and a mold sleeve. The position of the mold sleeve can be adjusted by bolts through support to adjust the relative position of the mold sleeve to the mold core, facilitating the uniformity of the extrusion layer thickness. The outside of the machine head is equipped with heating and temperature measuring devices.

2. The function of the transmission system is to drive the screw and supply the torque and speed required by the screw during the extrusion process. It is usually composed of an electric motor, a reducer, and bearings.

3. Heating and cooling devices are necessary conditions for the plastic extrusion process to proceed.

(1) Nowadays, extruders usually use electric heating, which is divided into resistance heating and induction heating. The heating element is installed in the body, neck, and head of the machine. The heating device heats the plastic inside the external heating cylinder to achieve the temperature required for process operation.

(2) The cooling device is set up to ensure that the plastic is within the temperature range required by the process. Specifically, it is to eliminate the excess heat generated by the shear friction of the screw rotation, in order to avoid excessive temperature that may cause plastic decomposition, burning, or difficulty in shaping. Barrel cooling can be divided into two types: water-cooled and air-cooled. Generally, air-cooled is more suitable for small and medium-sized extruders, while water-cooled or a combination of both forms of cooling are more commonly used for large extruders; Screw cooling mainly adopts central water cooling, aiming to increase the solid material conveying rate, stabilize the adhesive output, and improve product quality; But the cooling at the hopper is to strengthen the conveying effect of solid materials, prevent plastic particles from sticking and blocking the material mouth due to temperature rise, and ensure the normal operation of the transmission part.