How does a twin screw extruder put energy into the material?

Anyone who runs a twin screw extruder regularly meets the same situations. Slightly raising the screw speed makes the melt temperature jump. Changing the screw configuration shifts the pressure profile. Increasing the throughput suddenly alters the mixing state. These events feel routine on the shop floor, but when you step back, a twin screw extruder is essentially a machine designed around one question: how is energy delivered to the material?

That energy does not come from barrel heaters alone. Mechanical energy from the rotating screws acts at the same time, driving melting, mixing, dispersion, and reaction. Total energy input during the extrusion process therefore needs to be considered separately as mechanical energy and thermal energy. The sections below describe each.

Mechanical energy input
Determined mainly by screw speed and transferred to the material as shear stress and shear strain.

Thermal energy input
Supplied from the outside through the barrel and die temperature settings.

In a twin screw extruder, the conveying zone after the feed point (feed to early melting region) is mainly governed by external thermal energy from the barrel, and initial melting begins there. In the kneading zone, mechanical energy input dominates, driven by shear stress generated between screw elements and between the screws and barrel, and this is where melting, dispersive and distributive mixing, and overall compounding progress. In co rotating, fully intermeshing twin screw extruders especially, the wiping action between the screws exposes material to the shear field at high frequency, so the transfer efficiency of mechanical energy to the material becomes very high.

Two design conditions that determine twin screw extrusion process behavior

The twin screw extrusion process is shaped by two kinds of conditions: those fixed by the machine itself, and those an operator can adjust during running. To organize process design variables, we will call the former the structural variables and the latter the operational variables. The terms may sound unfamiliar. Structural variables are hardware side conditions set in advance by the machine specification. Typical examples include screw diameter, L/D, and screw formation. Operational variables, on the other hand, can be changed during operation and represent the software side of the process. Examples include screw speed, throughput, and barrel temperature settings.

An actual twin screw extrusion process emerges from the combination of these two condition sets. At the same screw speed, a different screw configuration or a different L/D will impose a very different shear history and residence state on the material. Conversely, with the same machine configuration, changing only the screw speed or the feed rate produces large changes in melt temperature and pressure behavior.

Structural variables: mechanical design elements of a twin screw extruder

Screw diameter (D) and L/D ratio

Screw diameter is the structural variable that mainly governs the throughput capacity of the extruder, while the L/D ratio sets the effective process length of the extrusion line. At the same time, screw diameter directly affects the shear rate imposed on the material, so scale up requires careful attention to increased mechanical energy input and the resulting rise in material temperature.

Screw formation

Screw formation directly creates the shear field experienced by the material, and it is a key design factor that determines the performance of a twin screw extruder. By arranging functions in stages, including material conveyance with flight elements, melting promotion and kneading and pressure build with kneading elements, and flow splitting and recombination with mixing elements, the mixing intensity can be designed to match the material characteristics and the goal of the extrusion process.

Auxiliary equipment and overall machine configuration

The placement of feeders, side feeders, vent ports, gear pumps, and other auxiliary devices influences the location and timing of material addition, pressure build, and devolatilization behavior.

Operational variables: running conditions in the twin screw extrusion process

Screw speed (rpm)

Screw speed has a direct effect on the shear rate imposed on the material.

Feed rate (throughput) (kg/h)

Feed rate is the basic variable that determines the fill state inside the screw channel. Through changes in fill ratio, it affects shear stress and pressure gradient.

Barrel and die temperature setting (°C)

Barrel temperature can be viewed as the external thermal energy input that controls melting onset and melting progression. Alongside mechanical energy input from shear, it is one of the factors that drives material temperature rise and thermal history during the twin screw extrusion process.

Indicators evaluated as process outcomes

The combination of operational variables set as running conditions and structural variables fixed by mechanical design appears as the running behavior during extrusion. The following indicators capture and evaluate this behavior. They serve as important clues for judging whether the process design and operating conditions are appropriate.

Melt temperature at the discharge (°C)

Melt temperature, measured by a melt temperature and pressure sensor installed at the extruder tip, is a key indicator that reflects the thermal history the material has experienced during the extrusion process. It expresses changes in material properties, thermal degradation, and the progress of reactions through a single temperature value. Melt temperature is the result of the energy input state created by the operational and structural variables, and it is one of the most important indicators among the evaluation parameters.

Melt pressure at the discharge (MPa)

Melt pressure measured at the discharge is an important indicator for evaluating the stability of flow inside the die. Discharge pressure is also used to assess the pressure balance with downstream equipment such as filters, screen changers, and gear pumps, and pressure fluctuation can be viewed as a source of throughput instability. Melt pressure at the discharge is not determined by the screw configuration alone; the pressure distribution forms in combination with the die geometry and flow channel conditions located at the tip.

Specific energy (kWh/kg)

Energy per unit mass of resin is the indicator that represents the mechanical energy actually delivered to the material during the extrusion process. Specific energy is calculated from the motor output, after accounting for mechanical losses that occur as power passes through the reducer, coupling, and bearings to the screw. The effective power input to the screw shaft is divided by the throughput per unit time. This indicator indirectly reflects the mechanical energy received by the material. It is very useful as a common reference for comparing the energy input state across conditions with different materials, screw configurations, throughputs, or screw speeds. Alongside melt temperature, specific energy is one of the most important indicators among the evaluation parameters.

Residence time (min)

Residence time indicates how long the material takes from the feed point to the discharge. It is shaped by operational variables such as screw speed and feed rate, together with structural variables such as screw configuration and L/D ratio. In actual extrusion, material does not move at a uniform rate. Flow splitting, recombination, and the action of the kneading section create a residence time distribution. This distribution reflects the variation in thermal history that the material experiences.

Basic experiments with the ULTnano15
_{Effect of screw speed and throughput on melt temperature and pressure}

To observe how melt temperature and melt pressure respond to changes in operational variables such as screw speed and feed rate, we ran experiments on a small twin screw compounding extruder. The equipment was the ULTnano15 small recirculating twin screw compounding extruder. Because the goal was to evaluate the effect of operational variables on the extrusion process, we did not use the recirculation path. The unit was operated in single pass continuous mode.

Experimental setup

Running conditions / screw speed, throughput, Q/N

	①	②	③	④	⑤	⑥	⑦	⑧	⑨
Throughput g/hr	500	2500	5000	1500	1500	1500	300	1500	3000
Screw speed rpm	100	500	1000	100	500	1000	300	300	300
Q/N	5C	5C	5C	15C	3C	1.5C	1C	5C	10C

Comparison of ①/②/③: difference in fill ratio at three levels with the same Q/N.
Comparison of ④/⑤/⑥: difference in fill ratio at three levels with different Q/N (constant throughput).
Comparison of ⑦/⑧/⑨: difference in fill ratio at three levels with different Q/N (constant screw speed).

Running conditions / screw formation

Results / melt temperature and melt pressure

	①	②	③	④	⑤	⑥	⑦	⑧	⑨
Throughput g/hr	500	2500	5000	1500	1500	1500	300	1500	3000
Screw speed rpm	100	500	1000	100	500	1000	300	300	300
Q/N	5C	5C	5C	15C	3C	1.5C	1C	5C	10C
Pressure MPa	1.0	1.9	2.3	1.9	1.3	1.1	0.6	1.5	2.5
Temperature °C	165	167	169	165	167	169	166	166	167

Experimental results

When compared at the same Q/N, the data show that both melt temperature and melt pressure rise as screw speed and throughput increase. Even with Q/N held constant, the absolute values of screw speed and throughput grow, and the material temperature rises as a result.

When screw speed was raised at constant throughput, melt temperature increased while melt pressure dropped. The higher screw speed increased the shear rate imposed on the material, which promoted shear heating and raised the melt temperature. At the same time, the viscosity of the material fell, reducing the pressure loss at the die.

When throughput was raised at constant screw speed, both melt temperature and melt pressure went up. The higher feed rate raised the fill ratio in the screw channel, increasing the shear stress and pressure gradient acting on the material. The mechanical energy delivered to the material per unit time also grew, which contributed to the same trend.

From these results, melt temperature and melt pressure are not determined by a single operational variable. They are formed by the combination of screw speed and throughput, that is, by the interaction between the fill state and the energy input state.

A related page explains in detail how the balance between throughput and screw speed influences residence behavior and fill behavior of the material. The results there support the view that Q/N is a useful parameter for understanding the extrusion process.

*For details, please also see the page on “Fill ratio and residence time in twin screw extruders.”

Dominant factors for melt temperature rise and points to watch during scale up

The results indicate that the operational variable with the strongest effect on melt temperature in twin screw extrusion is screw speed. Higher screw speed directly raises the shear rate experienced by the material. Shear heating grows, and the melt temperature rises easily. In practice, it is common to see melt temperature change significantly with screw speed alone, even when throughput and barrel temperature are barely changed. In co rotating, fully intermeshing twin screw extruders especially, the wiping action between the screws gives high efficiency in transferring mechanical energy to the material, so the contribution of screw speed to temperature rise becomes more pronounced.

This effect also becomes easier to see during scale up. As screw diameter grows, the peripheral speed at the screw outer surface increases at the same rpm. In other words, running a larger machine at the same screw speed as a smaller one actually imposes a higher shear rate on the material. Conditions that worked on a lab unit can therefore lead to a melt temperature higher than expected, or to material burning, when moved directly to a larger machine.

*For details, please also see the page on “Scale up of twin screw extruders.”

Specific energy as an evaluation indicator for twin screw extrusion

Melt temperature and melt pressure are commonly used in the operational evaluation of twin screw extruders, mainly because they are easy to measure. These are important indicators, but both describe the final state reached. They do not directly show what mechanical action the material received inside the extruder.

Mixing in twin screw extrusion depends strongly on the history of shear and elongation imposed on the material by the rotating screws. This mechanical action does not always appear as a change in melt temperature or pressure. Once the material is fully melted, differences in mixing state often fail to show up as differences in temperature or pressure. An evaluation that relies only on temperature and pressure can therefore miss differences in the mixing state inside the process.

This is where the concept of specific energy becomes useful. Specific energy expresses the mechanical energy delivered to the material per unit mass, and it shows how much mechanical load the material received inside the extruder. The indicator is easy to compare across conditions with different screw speeds, throughputs, and screw configurations, and it organizes the process from the perspective of how much mechanical action was applied. Adding specific energy to the evaluation makes it easier to separate thermal effects from the mechanical effects of screw rotation, and to capture the twin screw extrusion process closer to its actual state.

Summary

The process behavior of a twin screw extruder reflects the energy input state formed by the combination of operational variables (software side conditions adjustable during running) and structural variables (hardware side conditions fixed by machine specification). The resulting condition set produces various process indicators such as melt temperature, melt pressure, specific energy, and residence time, which together allow a quantitative evaluation of melting and mixing.

Optimizing a twin screw extrusion process requires more than treating each condition in isolation. The relationship between operational and structural variables needs to be viewed from the question of what kind of energy is being delivered to the material, and in what form. Designing the conditions while tracing how that energy input state appears in temperature, pressure, specific energy, and residence behavior, and how these indicators relate to each other, is essential for stabilizing the twin screw extrusion process.

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ABOUT THE PUBLISHER

Technovel Corporation — Extrusion Machinery Specialists

Osaka based Technovel specializes in extrusion machinery. We built the world’s first horizontally multi screw extruder, and our Quad and Octa screw extruders now serve diverse industries. Our twin screw range runs from the world’s smallest 6 mm lab unit, through our best-selling 15 mm model, to large production machines. This column shares the know how behind them.

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