Extrusion molding is a high-efficiency, continuous, low-cost, and wide-ranging molding processing method. It is an early technology in the processing of polymer materials. After more than 100 years of development, extrusion molding has become a molding processing method which deals with the most varieties of products, meets the most changes, and takes up the largest proportion of output in the polymer processing field. Also, it is with high productivity, strong adaptability and has wide range of applications.
Extrusion molding is one of the most important processing forms of plastic extrusion materials. It is suitable for most plastic materials except for some thermosetting plastics. About 50% of thermoplastic products are completed by extrusion molding. At the same time, it is also widely used in the molding of chemical fibers, thermoplastic elastomers, and rubber products. Extrusion molding is used to produce continuous products in various forms such as pipes, bars, sheets, profiles, wire and cable sheaths, monofilaments, etc.. It can also be used for mixing, plasticizing, granulating, coloring, and high blending modification of molecular materials, etc.. in using extrusion line machine. In addition, based on extrusion molding, extrusion-blow molding technology and extrusion-tentering technology are important methods for manufacturing films and hollow products, which are combined with inflation and stretching methods.
Extrusion molding mainly refers to the extruding action of the screw or plunger to force the polymer material to melt by heating through the die of the machine head to be molded under pressure. The result is a continuous profile with a constant cross-section. The extrusion molding process mainly includes the processes of feeding, melting and plasticizing, extrusion molding, shaping, and cooling.
The extrusion process can be divided into two stages. The first stage is to plasticize the solid plastic (that is, into a viscous fluid) and pass it through a special-shaped die under pressure to become a continuum with a cross-section similar to the shape of the die. The second stage is to use an appropriate method to make the extruded continuum lose its plastic state and become solid to obtain the desired product.
According to different plasticization methods, the extrusion process can be divided into two types, namely, dry extrusion and wet extrusion.
Dry plasticization relies on heating to turn plastic into a melt, and plasticization and pressure can be carried out in the same equipment. The shaping treatment is only simple cooling and wet plasticization, and the plastic is fully softened with a solvent. Therefore, plasticization and pressurization must be divided into two independent processes, and the shaping treatment must be removed with a more troublesome solvent, and solvent recovery must be considered.
Although wet extrusion has advantages in uniform plasticization and avoiding excessive heating of plastics, based on the above shortcomings, its scope of application is limited to the extrusion of nitrocellulose and a few cellulose acetate plastics.