The plastic extruder converts solid plastic into a uniform melt by applying pressure and shear, and then sends the melt to the next process. The production of the melt involves mixing additives such as masterbatch, blending resins, and re-grinding. The finished melt must be uniform in concentration and temperature. The pressure must be large enough to extrude the viscous polymer.
Plastic extruders complete all of the above processes through a barrel with a screw and a spiral path. Plastic pellets enter the barrel through a hopper at one end of the barrel and are then conveyed to the other end of the barrel by the screw. In order to have enough pressure, the depth of the screw threads decreases as the distance from the hopper increases. External heating and internal heat generated by friction between the plastic and the screw soften and melt the plastic. Different polymers and different applications often require different design requirements for plastic extruders. Many options involve discharge ports, multiple feed ports, special mixing devices along the screw, cooling and heating of the melt, or no external heat source (insulated plastic extruders), the relative size of the gap between the screw and the barrel, and the number of screws. For example, twin-screw plastic extruders can mix the melt more thoroughly than single-screw plastic extruders. Tandem extrusion is the use of the melt extruded by the first plastic extruder as a feedstock to the second plastic extruder, which is usually used to produce extruded polyethylene foam.
The characteristic dimensions of a plastic extruder are the diameter (D) of the screw and the ratio (L) of the length (L) of the screw to the diameter (D). Plastic extruders usually consist of at least three sections. The first section, near the L/D) hopper, is the feeding section. Its function is to allow the material to enter the plastic extruder at a relatively steady rate. Generally, this section will be kept at 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 sudden or gradual (smooth). The last section, the metering section, is close to the plastic extruder outlet. Its main function is to ensure that the material flowing out of the plastic extruder is uniform. In this section, to ensure the uniformity of composition and temperature, the material should have sufficient residence time.
At the end of the barrel, the plastic melt leaves the plastic extruder through a die, which is designed to be an ideal shape, and the extruded melt stream passes through here.
Another important part is the drive mechanism of the plastic extruder. It controls the rotation speed of the screw, which determines the output of the plastic extruder. The required power is determined by the viscosity (flow resistance) of the polymer. The viscosity of the polymer depends on the temperature and flow rate, and decreases with the increase of temperature and shear force. Plastic extruders are equipped with filters that can block impurities on the filter. To avoid downtime, the filter should be able to be replaced automatically. This is particularly important when processing resins with impurities, such as recycled materials. The screw of the extruder is divided into a feeding section, a plasticizing section, and a melting section. The temperature is based on the process parameters of the plastic particles, and the model is divided into 20, 36, 52, 65, 75, 95, 120, and 135 according to the screw diameter. After the plastic particles are heated, the movement of the screw changes the original state. There are many types, depending on the specific application. The capacity of the frequency converter is proportional to the diameter of the screw, and is adjusted according to the different raw materials.