Polymer Processing Techniques PDF

Chapter II: Main Polymers Processing Techniques Introduction Plastic processing involves the methods used to transform polymers into finished or semi-finished products. Many contemporary transformation processes, such as single- screw and twin-screw extrusion, injection molding, blow molding of hol...

Chapter II: Main Polymers Processing Techniques Introduction Plastic processing involves the methods used to transform polymers into finished or semi-finished products. Many contemporary transformation processes, such as single- screw and twin-screw extrusion, injection molding, blow molding of hollow structures, film blowing extrusion, and calendering, were developed alongside the emergence of major commodity polymers. The choice of plastic processing techniques is influenced by the properties of the polymers and the intended applications of the final products. The principal industrial processes include: II.1. Extrusion Extrusion is a continuous transformation process. Similar to injection molding, pellets are fed into a heated tube equipped with a screw. The softened, homogenized material is pushed and compressed, then passes through a die to achieve the desired shape. This process resembles a "spaghetti machine," allowing for the production of semi-finished products in various forms. Using this technique, long products such as profiles for doors and windows, pipes, cables, optical fibers, tubes, meshes, and plastic sheets are manufactured. The tube or profile emerges continuously, is cooled, and then cut to the desired length. By layering multiple materials, products with combined properties can be created. The extruder is a machine that transports and processes solid or liquid materials using one or more screws rotating within a barrel, continuously forcing the material into a die. During the transfer within the barrel, the mixed and plasticized materials can be heated, occasionally cooled, and degassed; they may also pass through a filter before being extruded through a die or a die and punch. There are two types of extruders: 1. Single-screw extruder 2. Twin-screw extruder II.1.1. Single-Screw Extruder A single-screw extruder consists of a rotating screw contained within a heated barrel (cylinder). The basic schematic of a single-screw extruder is illustrated in Figure 1. Plasticization Pumping Die Polymer Loading Figure 1: Schematic of a Single-Screw Extruder The primary function of the extruder is to transport the polymer, melt it, and pressurize it to enable its passage through the die located at the end of the machine. From an industrial standpoint, the objective is to achieve a stable production rate characterized by a homogeneous material at a controlled temperature, while ensuring optimal production conditions (maximum flow, low energy consumption). A thorough understanding of the underlying mechanisms, along with their modeling, is essential. Observations of the polymer’s state within the machine reveal three main zones: - Solid transport Zone: This zone contains the polymer in its entirely solid state. - Melting Zone: In this area, both solid and melted polymer coexist. - Pumping Zone: Here, the polymer is fully melted. The energy required for melting and pressurizing the polymer is derived from two primary sources: - Mechanical Energy: Supplied by the rotation of the screw, which induces deformations in a highly viscous medium (shearing). - Thermal Energy: Provided by the regulation of the barrel through heating elements. II.1.2. Geometry of the Screw/Barrel System The geometry of the screw is designed to operate under optimal conditions depending on the used polymer. The essential geometric elements of the screw-barrel system are illustrated in Figure 2, where: 1. Barrel 2. Screw Element D1: Inner Diameter of the Barrel D2: Diameter of the Screw H: Channel (the space between the barrel and the screw) e: thread thickness B: screw pitch: This determines the angle the thread makes with a plane perpendicular to the screw axis. - The angle θ varies with the distance from the screw axis θ (r) is such that: 1- at bareel level, it is θ 1 (such that tg θ 1 = B / π D1) 2- at screw element level, it is θ 2 (such that tg θ 2 = B / π D2)  Figure 2: Geometry of the Screw/Barrel Note: In practice, a screw is characterized by the ratio L/D, where L is the length of the screw and D is its diameter. As the length of the screw increases, the degree of homogenization improves. L/D ↑ leads to a more homogeneous product. II.1.2. Twin-Screw Extruder Compared to single-screw extrusion, twin-screw extrusion is widely used for mixing, compounding, or reacting polymer materials. Due to the continuous production of highly homogeneous and finely structured products, twin-screw extruders are typically employed to produce a broader range of finished products. They enable more consistent material conversion and product quality management, as well as compensating for screw wear by adjusting the rotational speed. The extruder consists of two screws that rotate within separate openings of the barrel, with their axes parallel to the main axis of the barrel (Figure 3).  Figure 3: Twin-Screw Extruder The twin-screw extruder is recommended for: ✓ Producing large-profile products ✓ Processing stiffer materials ✓ Facilitating easier degassing ✓ Ensuring consistent feeding ✓ Achieving higher flow There are two types of twin-screw extruders: 1. Co-rotating Twin-Screw Extruders: The screws rotate in the same direction and have the same thread orientation. The open "8" configuration makes co-rotating twin-screw extruders well-suited for compounding, as they emphasize the mixing of materials rather than pressure buildup (Figure 4).  Figure 4: Co-rotating Twin Screws Extruder 2. Counter-rotating Twin-Screw Extruders: The two screws rotate in opposite directions. The material is contained within a "C"- shaped space, where it undergoes significant shearing and pressure buildup (Figure 5). Figure 5: Counter-rotating Twin Screws Extruder