The function of an extruder is to convert solid plastic into a uniform, consistent melt through heating, pressurization, and shear, and then deliver the melt to the next processing step. Melt production involves processes such as blending color masterbatches and other additives, compounding resins, and re‑grinding. The finished melt is homogeneous in both concentration and temperature. The pressure applied is sufficient to force the viscous polymer through the die. An extruder accomplishes these operations within a barrel equipped with a screw and helical flighting. Plastic pellets enter the barrel via a hopper at one end and are conveyed by the screw toward the opposite end. To generate adequate pressure, the depth of the screw’s threads decreases as the distance from the hopper increases. External heating, combined with internal heat generated by friction between the plastic and the screw, softens and melts the material.

Different polymers and diverse applications often impose distinct design requirements on extruders. Many options involve the die configuration, multiple feed ports, mixing elements along the screw, melt cooling and heating, or the absence of an external heat source (heat‑free extrusion), as well as the relative magnitude of clearance variations between the screw and barrel, and the number of screws. For example, compared with single‑screw extruders, twin‑screw extruders can achieve more thorough melt blending. In tandem extrusion, the melt produced by the first extruder serves as feedstock for the second extruder; this process is commonly used to manufacture extruded polyethylene foam.

At the end of the barrel, the molten plastic exits the extruder through a die, which is designed with an optimal geometry to guide the flow of the extrudate. Another critical component is the extruder’s drive system, which controls the screw’s rotational speed; this speed directly determines the extruder’s throughput. The required power depends on the polymer’s viscosity—its resistance to flow—which, in turn, is influenced by temperature and shear rate, decreasing as both increase. Extruders are equipped with screens that trap contaminants; to minimize downtime, these screens should be capable of automatic replacement. This feature is especially important when processing resins containing impurities, such as recycled materials.

Basic parameters of the screw rod

Typically, a screw is divided into three sections: the feeding section, the compression section, and the metering section.

Feed section—having a smaller bottom diameter, its primary function is to convey the material to the downstream section; thus, the key issue is conveying capacity. The relevant parameters are L1 and h1, where h1 = (0.12–0.14)D.

Compression section—characterized by a change in the barrel diameter—primarily serves to compact and melt the material, thereby building up pressure. Key parameters include the compression ratio ε = h1/h3 and the length L2; more accurately, the degree of taper A is defined as A = (h1 − h3)/L2.

Homogenization section (metering section) — delivers the molten material from the compression section to the front end of the screw at a constant rate and temperature; parameters: L3, h3, where h3 = (0.05–0.07)D.

For the entire screw, the parameter L/D—length-to-diameter ratio—

Advantages and disadvantages of L/D: The length-to-diameter ratio (L/D) and the screw speed (n) are critical factors affecting the screw’s plasticizing capacity and efficiency. A larger L/D increases the residence time of the material in the barrel, which promotes better plasticization while reducing pressure flow and leakage, thereby enhancing plasticizing performance. It is particularly beneficial for materials with stringent temperature‑distribution requirements. However, increasing L/D also has adverse effects on manufacturing, assembly, and operation. Typically, L/D ranges from 18 to 20, though there is currently a trend toward further enlargement.

For other pitches S, the helix angle φ is given by φ = πD tan φ; typically, when D = S, φ = 17°40′.

Phi has an impact on plasticizing capability; generally, a larger phi corresponds to a faster conveying speed. Consequently, phi varies depending on the material’s shape. For powdered materials, a phi of approximately 25° is appropriate; for cylindrical materials, around 17°; and for block‑shaped materials, about 15°. However, differing phi values can also pose challenges during processing, so a standard value of 17°40′ is typically adopted.

The clearance width e: for materials with low viscosity, e should be as large as possible; if too small, leakage is likely, but if too large, power consumption increases and overheating may occur. Typically, e = (0.08–0.12)D.

In summary, under the current circumstances, due to the lack of adequate experimental methods, there is no comprehensive design methodology for screw extruders. Most designs rely on empirical parameterization tailored to the specific properties of the material at hand, with each manufacturer generally following a similar approach.

10. What extrusion techniques are used for sealant strips?

What are the extrusion techniques for sealing strips? Here’s an analysis: PVC sealing strips, also known as polyvinyl chloride sealing strips, are primarily made from polyvinyl chloride (PVC). They are commonly used in applications such as standard automotive seals, window and door seals, wooden door seals, and aluminum alloy seals.

The sealant strip machine boasts low energy consumption, high efficiency, simple operation, and a high degree of automation. During operation, molds can be easily swapped to produce aluminum alloy door and window seals, automotive seals, plastic‑steel door and window seals, and other products tailored to customer requirements. Seal strips have been widely adopted across diverse industries, including automotive, doors and windows, cabinets, refrigerators, and shipping containers, making this equipment an ideal choice for investors seeking to start a profitable business. Backed by advanced manufacturing processes, robust technical expertise, superior product quality, and comprehensive after-sales service—including on-site installation and hands-on training in formulation and technology—we are committed to supporting your success.

Our extruder for producing sealing strips features an L/D ratio of 20:1, a 7.5 kW motor, and a 10 kW heating system, with a production capacity of 50–150 kg per hour. The machine is designed to work in conjunction with an electrical control unit, an extruder, a water tank, an automatic calendering speed regulator, and a winding device. It offers advantages such as low energy consumption, high efficiency, simple operation, and a high degree of automation. During operation, molds can be easily interchanged to produce sealing strips of various shapes, including those for aluminum alloy doors and windows, automotive applications, and plastic‑steel doors and windows.