Plastic Extruder Tips 7: Twin Screw Extruder Screw Design & Function

Twin Screw Extruder Screw Design & Function

Posted by Andy

The twin screw extruder screw adopts modular design, combined by shaft and screw element one piece by one piece. The screw element mainly consists of the transport unit and kneading unit.

Transport unit: large pitch like 64mm, medium pitch like 48mm, small pitch like 32mm.

Kneading unit: normally the greater the angle, the stronger the shear force. Like 30°, 45°, 60°, 90°.

1.Sealed section

As the first screw element of the screw, transport and prevent the material comes out from the screw.

2.Main feeder

The large pitch and force transport screw element make sure the material delivered to next melting section. Large pitch+medium pitch+small pitch to make sure the material fast transport and then slow down compressed the material and filled with the screw, preparation for the compounding.

3.Kneading section(melting & compounding)

Through the different combinations of kneading element to complete preliminary mixing.

  • Heat source: 10%-20%  come from barrel heating, 80%-90% come from the material shear and friction
  • Compounding: suitable for several different polymer resins or filler material mixing. It’s better to add filler material or liquid after the polymer melted.

4.Nature exhaust port

Screw design: Large pitch transport element. It must use one left-handed screw element before the exhaust port to form a backflow of material and form a high pressure for removing the volatile impurities and gases and vapors.

5.Side feeder feeding section

  • Purpose: adding suitable material to the melted polymer.
  • Screw design: Large pitch transport element for fasting deliver. And using small pitch element to compress the material before the side feeder.
  • Material: normally add glass fiber, carbon fiber or filler material etc.
  • Temperature: the temperature of barrel section for side feeder or the next section maybe low than the settled temperature. The heater needs continue working.

6.Kneading section(compounding)

7.Vacuum degassing port

  • Purpose: removing the volatile impurities and gases and vapors from the melting material by the pump.
  • Screw design: Large pitch transport element. It must use one left-handed screw element before the exhaust port to form a backflow of material and form a high pressure.
  • Device: through the pipe connected with the water-ring water pump.
  • Temperature control: the temperature may be lower than the settled, due to the gas absorption while absorbing heat.
  • Quality: if the vacuum is not enough, it will cause the pellets to foam or contain gas. And maybe also cause the material flowed back to the feeder and side feeder, form a stack.

8.Measurement pressure section

Pressurized the material and extrusion

PS: What’s the Left-handed Screw Element?

  • The shape of the reverse-threaded element is similar to that of the forward-threaded element except that the direction of the spiral of the groove is reversed. Because the reverse thread conveys the material in the opposite direction, the positive thread conveys the material in the extrusion direction, so the material establishes a high pressure in front of the entrance of the reverse thread section to overcome the resistance caused by the reverse flow in the reverse thread, so that the material passes the reverse thread Gap and forward delivery.
  • In the extruder, sometimes according to the needs of the extrusion process, different pressure zones (differential pressures sometimes as high as a few megapascals) may be formed at different positions in the screw axis or within a relatively short distance. Threaded components to achieve this purpose. For example, if a liquid additive or foaming agent is added at a certain position in the axial direction of the screw, the adding position must be in a low-pressure zone. At this time, a reverse-threaded element is provided before the joining zone to form a high-pressure seal to prevent the liquid additive or the blowing agent from flowing in the reverse direction. The formation of a low-pressure zone behind the reverse-threaded element facilitates the addition of additives or blowing agents. A counter-threaded element is placed in front of the exhaust port to create a high pressure in front of the exhaust zone and a sudden pressure reduction in the exhaust zone to facilitate venting.
  • The reason for the pressure drop in the reverse thread is that the reverse threading element itself has no forward conveying capability and the forward delivery of the material is at the expense of pressure loss. The size of the pressure drop in the reverse thread element is an important factor that must be considered when designing and selecting such elements. The reverse-threaded element is a resistance element and the pressure is increased. A positive-threaded conveying element should be provided in front of it to overcome the resistance and convey the material in the direction of the die. Adding a counter-threaded component will result in a pressure peak, adding two counter-threaded components will result in two pressure peaks; and so on, multiple reverse-threaded components can be set as needed.

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