Unveiling the Key Parameters of Twin-Screw Extruders

Extruder machines are now an essential component of the industry. Viscous materials are easily transformed into well-structured products by the extrusion process. By precisely regulating the processing conditions, all of this is accomplished.

A twin screw extruder, which is frequently used in the plastic extrusion process, is probably something you have used or seen before. This procedure involves the mixing or compounding of two or more substances.

The two co-rotating and intermeshing screws in this machine are housed in a closed barrel with splined shafts. A double screw extruder is very helpful in producing stiff PVC and wood fiber mixtures.

twin-screw extruder

Screw Configuration

The screw configuration is an important component that influences product transformation, residence time distribution, degree of fill, and energy input into the material. The screw is the main component of a extruder.

Two parallel screws that revolve in opposing directions are contained inside a barrel and make up the twin screw extrusion machine. Via a hopper, the raw material is inserted into the barrel and moved toward the end of the barrel via the screws.

When the longer reversed screw element is used or the screw is put closer to the die, the material breakdown rate rises. Furthermore, when the mixing components are relocated farther from the die, longer, and spaced with greater spacing between the elements, the material breakdown, mechanical energy input, and water solubility index all rise. The use of a reversed screw element, kneading element, and mixing elements improves the specific mechanical energy, expansion ratio, and water solubility index. The severe screw configuration provided a more enlarged product with lower bulk density than the medium screw configuration.

Screw Speed

A 2.8″ (71 mm) twin screw extruder can have a feed rate Q of 900 pph and a screw speed N of 300 rpm. The Q/N value would be 3.0 pph/rpm. greater screw speeds (up to 1000 rpm) would result in correspondingly greater rates while maintaining the same 3.0 pph/rpm.

Barrel Temperature

The feeding section of a dual screw extruder typically ranges between 50 and 90℃, depending on its design. The temperature at the entrance is 50-90℃ and should be equal to the melting point or viscosity-temperature at the end. The temperature should rise linearly in this portion.

Throughput

The throughput of a twin screw extruder can be determined using this model by subtracting the entire quantity of leakage from the total volume of the C-shaped chambers that become free per unit time. This has been tested with model extruders fully loaded with Newtonian liquids, with absolutely satisfying results.

Residence Time

The residence time distribution is a crucial parameter for understanding axial conveying and mixing in a twin-screw extruder. A model for the residence time distribution (RTD) is validated by analyzing data from polyethylene extrusion on a 30-mm corotating twin-screw extruder.

The transformation of the RTD to obtain both the residence-volume distribution (RVD) and the residence-revolution distribution (RRD) provides fresh physical insights into the extrusion process. It is discovered that operating settings with equal specific throughput provide equivalent RVD and RRD, and that for a particular screw arrangement, the axial mixing of extrusion material as measured by a tracer is roughly the same under all working conditions. This allows the experimental RVD curves to be layered, resulting in a single master curve for a specific screw shape.

The creation of a basic residence model that can distinguish between screw configurations and operating situations and characterize partially and fully filled screw sections is motivated by these new tools. The RTD model’s parameters were determined using the least-square error-fit approach, which showed that the model function adequately describes the RTD experimental data.

Torque

Usually computed using motor power. For a single-screw extruder, the output torque is T=P*9550/n/1.05, where P is the motor power, 9550 is the coefficient, n is the maximum screw speed, and 1.05 represents the energy loss during transmission through the gearbox. % loss

In the case of a twin screw extruder machine, the output torque T=P*9550/n/2/1.05, and the other two are because the output torque is distributed between the two screws, and the torque of the single screw is computed.

The value obtained by this technique is typically slightly greater than the real value, because, for example, the planned single-axis torque is 200N.m, and the required motor power is computed using the above calculation.

The closest gear is chosen to provide sufficient output torque. The computed motor power is not precisely there because the standard motor power is omitted.

Twin screw extruders have the following advantages: positive displacement capability, particularly for counter-rotating screws, which facilitates pumping of hard-to-feed materials, reduces the sensitivity of flow rate to pressure, and gives a more uniform residence time; better mixing without excessive local heating; low shear rate and better temperature control to allow processing of heat sensitive materials; and versatility through the use of modular sections for specialized functions.

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