Extrusion is a widely used molding method for processing materials such as plastics, rubbers, and chemical fibers. It involves heating and plasticizing the material through an extruder, which continuously pushes it through a die of a specific shape to produce products or semi-finished products with a constant cross-sectional shape.
1. Definition and basic principles of extrusion process
In plastic processing, this process is also known as extrusion molding or extrusion; in rubber processing, it is commonly referred to as extrusion. The core principle is that the material is heated and plasticized between the barrel and screw of the extruder, and simultaneously pushed forward by the pressure generated by the rotating screw, ultimately passing through the die head continuously for molding. This process has high production efficiency and can achieve continuous production.
2. Main equipment composition
The core equipment of the extrusion process consists of an extruder and a head (die), supplemented by devices such as feeding, sizing, cooling, traction, cutting, and winding, which work in coordination.
00001. Extruder: It is mainly divided into single-screw extruder and twin-screw extruder according to the number of screws.
Single screw extruder: Its structure is relatively simple, with key parameters including screw diameter, aspect ratio (the ratio of screw length to diameter), and speed range. The design of the screw (such as its geometric shape and aspect ratio) is crucial for the plasticizing effect of the material, production capacity, and product quality.
Twin-screw extruder: Two screws rotate within an ∞-shaped barrel, which can be classified into meshing and non-meshing types. The co-rotating meshing type is commonly used for plastic blending, filling, and reinforcement modification; while the counter-rotating meshing type is mostly used for processing products such as rigid polyvinyl chloride.
00002. Die head and die: Located at the front end of the extruder, they serve as the guiding and shaping components for the molten material.
· Head: It converts the rotational motion of the screw into linear motion, and generates the necessary extrusion pressure.
· Die: It determines the cross-sectional shape and dimensional specifications of the final product. By changing the die, the same extruder can produce products of different shapes. The die design needs to ensure smooth laminar flow of the material and minimize pressure drop.
3.Typical Process Flow and Product Types
The specific process of extrusion technology requires the selection of appropriate equipment and auxiliary devices based on the type of polymer being processed and the shape of the target product. Additionally, key process parameters such as screw speed, head pressure, material temperature, shaping conditions, and traction speed must be determined. Extruded products usually undergo plasticization, but the shaping methods vary. For example, plastic products often require cooling and solidification, while rubber semi-finished products need further vulcanization. By adjusting the equipment and process, a variety of products can be produced:
00001. Pellets: The mixed material is melted and mixed by an extruder, formed into strips through a multi-orifice die, and then hot-cut or cold-cut into pellets.
00002. Tubing: The material forms a tubular shape through the annular gap formed by the die and the mandrel, and then undergoes sizing and cooling to obtain a precisely sized tube. The commonly used method is the outer diameter sizing method, which utilizes pressure difference to make the tube billet closely adhere to the inner wall of the sizing sleeve. For example, the extrusion of high-performance polymethyl pentene (PMP) tubing requires precise control of temperature, screw speed, die design, and cooling and sizing processes to ensure uniform wall thickness and stable performance.
00003. Sheeting and Film: Flat products with a thickness greater than 0.25mm are classified as sheeting, while those with a thickness less than 0.25mm are classified as film. Flat films can be produced through extrusion casting (with cooling and drum shaping), or high-strength biaxially oriented films can be produced through longitudinal and transverse stretching. Low-foam boards can be produced by adding foaming agents and using special screw die molds.
00004. Blow-molding film: The extruded tubular material is inflated into a film bubble using compressed air, cooled by air, and then wound up. It is widely used in the manufacture of packaging bags, agricultural films, and so on.
00005. Coated wire: The molten plastic is used to coat the bare metal wire through a T-shaped die, and after cooling, it is made into wire and cable.
00006. Composite products: Utilizing multiple extruders to supply different plastics, and through extrusion with a shared head, forming an integrated composite film, sheet, plate, profile, or pipe.
00007. Rubber product processing: In the rubber industry, the extrusion process can be used for rubber compound filtration (removal of impurities), tire tread (integral extrusion or layered extrusion), and the manufacture of inner tube carcass.
00008. Melt spinning: In the chemical fiber industry, extruders can be used to melt high-viscosity resins, providing feedstock for melt spinning.
5. Control of key process parameters (taking high-performance pipes as an example)
Taking the extrusion of high-performance pipe materials like TPX DX810 as an example, the control of key process parameters directly affects the quality of the final product
Temperature control: It is necessary to precisely set the temperature of the extruder barrel in different sections (such as the feeding section, compression section, and homogenization section) to prevent material degradation caused by excessive temperature.
Screw speed and die design: The screw speed needs to be controlled within a certain range (such as 20-40rpm) to avoid excessive shear heat generated by high speed, which may affect uniformity; the die adopts a gradual flow channel design to facilitate smooth flow of the melt and control wall thickness deviation.
Cooling and solidification: A solidification water tank is used for cooling, with temperature and cooling length (such as 5-8 times the pipe diameter) controlled to ensure rapid crystallization and solidification of the material, reducing internal stress.
Post-processing: After extrusion, post-processing such as annealing is often required to eliminate internal stress, and attention should be paid to storage conditions.
In summary, extrusion technology is an efficient and flexible continuous molding technique that can produce a wide range of products, from simple granules to complex composite products, through precise control of equipment selection and process parameters. It has broad applications in industrial fields such as plastics, rubber, and chemical fibers.
