An article explaining the main structure of a plastic extruder

Plastic extruder is one of the types in the plastic machinery industry, accounting for 31% of the total output value of plastic machinery. Plastic machinery extrusion equipment, with its unique characteristics, has made decisive contributions to the special talents of large processes. In recent years, Chinese plastic machinery enterprises have expanded their market competitiveness by adding extruders to the new market. The market competitiveness will be further enhanced, and the share of imports and exports to foreign countries will increase. Now, the orders of plastic extruder enterprises in China have significantly rebounded, and the increase in imports and exports is steadily advancing. In plastic extruder forming equipment, the plastic extruder is generally referred to as the main machine, and the subsequent equipment that is matched with it, the plastic extruder forming machine, is called the auxiliary machine. After more than 100 years of development, plastic extruders have evolved from single screw to various models such as twin-screw, multi screw, and even no screw. The plastic extruder (host) can be matched with various plastic molding auxiliary machines such as pipes, films, rods, monofilaments, flat wires, packing belts, extruded nets, plates (sheets), profiles, granulation, cable wrapping, etc., to form various plastic extrusion molding production lines and produce various plastic products. Therefore, plastic extrusion molding machinery is one of the commonly used models in the plastic processing industry, both now and in the future.

The main structures of plastic extruders are:

1. Screw

The productivity, plasticizing quality, dispersion of fillers, melt temperature, power consumption, etc. of an extruder are mainly determined by the function of the screw.

Ordinary screw refers to a three-stage structure screw with full thread from the feeding section to the homogenization section, whose main mission is to complete plastic plasticization and transportation.

Ordinary full thread three-stage screws are divided into two types: gradient screws and abrupt change screws.

Gradient screws are mostly used for the processing of amorphous plastics. They provide good thermal conductivity to most plastics, have a small shear effect on materials, and can be controlled. They are suitable for thermosensitive plastics and can also be used for crystalline plastics.

The compression section of the sudden change screw is relatively short, which can cause significant shear effects on the material, making it suitable for crystalline plastics with low viscosity and sudden change melting point. High viscosity plastics are prone to local overheating and should not be used.

2. Material cylinder

The second most important component after the screw, it works together with the screw to complete the mission of material transportation, melting, and quantitative constant pressure extrusion. The structural form of the material cylinder is related to the stability and uniformity of heat transfer.

Other components of extruder

1. Heating and cooling system

To ensure that plastic can always melt within the required range of its processing technology, a heating and cooling system must be installed on the extruder. There are two methods for obtaining heat during the extrusion process of plastic: one is the heat supplied by the external heater of the barrel; The second is conflict cutting heat.

Most of the heat in the heating section comes from external heaters; Most of the heat in the homogenization section comes from conflict shear heat. Therefore, in order to control the temperature of each section, it is necessary to heat and cool it in sections.

(1) Heating system

The commonly used methods include liquid heating, steam heating, electric heating, and far-infrared heating.

a, Liquid heating

First, heat the liquid (water, oil, useful solvents), and then heat the material cylinder with the liquid. Temperature control is achieved by adjusting the flow rate or modifying the liquid temperature. Its characteristic is stable heating. But the heating system is complex and not widely used.

b, Electric heating resistor heating:

Using current to generate a lot of heat through a resistance wire to heat the barrel and machine head. The diagram shows a cast aluminum heater, with the resistance wire insulated with magnesium oxide powder and the shell made of cast aluminum.

Due to the fact that resistance heating is transmitted to the plastic through the barrel, a temperature gradient is formed in the thickness direction of the barrel, resulting in a long heat transfer time.

Induction heating:

Through electromagnetic induction, eddy currents are formed inside the material cylinder, causing it to heat up and heat up the plastic inside.

When current passes through a coil, magnetic field lines occur. Changing the direction of the current and the direction of the magnetic field lines is equivalent to cutting the magnetic field lines in the barrel, which will induce an electromotive force in the barrel, causing secondary voltage and induced current, also known as eddy current. Eddy currents treat the material cylinder as a resistor and generate heat when passing through it. Therefore, heat is generated throughout the entire thickness of the barrel, resulting in a small gradient and a slow heating rate.

(2) Cooling system

The purpose is to cool excessive heat and allow the plastic to form at normal temperatures. Regularly cool the barrel and screw.