Solid plastics are transformed into a homogeneous melt through pressure and shearing, and then fed to the next process. Melt production involves mixing additives such as masterbatches, blending resins, and re-grinding. The finished melt must be homogeneous in concentration and temperature. Sufficient pressure must be applied to extrude the viscous polymer.
A plastic extruder accomplishes all of the above processes through a barrel with a screw and a feed path. Plastic granules enter the barrel through a hopper at one end and are then conveyed to the other end by the screw. To maintain sufficient pressure, the depth of the screw thread decreases with increasing distance from the hopper. External heating and internal heat generated by friction between the plastic and the screw soften and melt the plastic. Different polymers and applications often require different design specifications for plastic extruders. Many options involve the discharge port, multiple feed ports, special mixing devices along the screw, melt cooling and heating, or no external heat source (insulated plastic extruder), the relative size of the gap between the screw and barrel, and the number of screws. For example, a twin-screw plastic extruder allows for more thorough mixing of the melt compared to a single-screw plastic extruder. Tandem extrusion uses the melt extruded from the first plastic extruder as feedstock for the second plastic extruder, typically used to produce extruded polyethylene foam.
The characteristic dimensions of a DL plastic extruder are the screw diameter (D) and screw length (L) relative to the diameter DL/D (D), and the ratio (L/D). Plastic extruders typically consist of at least three sections. The first section, near the L/D hopper, is the feeding section. Its function is to allow material to enter the plastic extruder at a relatively steady rate. Generally, this section will maintain a relatively low temperature to avoid clogging of the feeding channel. The second section is the compression section, where the melt is formed and the pressure increases. The transition from the feeding section to the compression section can be abrupt or gradual (smooth). The final section, the metering section, is located immediately after the extruder outlet and its primary function is to ensure a uniform flow of material exiting the extruder. Sufficient residence time is required in this section to guarantee the uniformity of composition and temperature.
At the tail end of the barrel, the molten plastic exits the extruder through a die head designed with an ideal shape, through which the extruded melt flow passes.
Another crucial component is the extruder's drive mechanism. It controls the screw's rotational speed, which determines the extruder's output. The required power is determined by the polymer's viscosity (flow resistance). Polymer viscosity depends on temperature and flow rate, decreasing with increasing temperature and shear force. Plastic extruders are equipped with filters to trap impurities. To avoid downtime, the filters should be automatically replaceable. This is particularly important when processing resins containing impurities, such as recycled materials. Extruder screws are divided into feeding, plasticizing, and melting sections. Temperatures vary depending on the process parameters of the plastic particles. Extruder models are available in screw diameters of 20, 36, 52, 65, 75, 95, 120, and 135°C. The plastic particles are heated, and the screw's movement changes their state. There are many types of extruders, depending on the specific application. The capacity of the frequency converter is directly proportional to the screw diameter and further adjusted according to the raw material.
