The Importance of Fill Factor (Q/N) in Twin Screw Extruders

What is Fill Factor?

The fill factor represents the degree to which materials occupy the screw channels in an extruder, typically expressed as a value between 0 and 1.

Q: Actual discharge rate, Qmax: Theoretical maximum discharge rate.

What is Q/N?

In twin screw extruders, the Q/N ratio (discharge rate/rotation speed) is a critical parameter for evaluating the material fill level and transport efficiency within the extruder. Here, Q refers to the discharge rate, and N represents the screw rotation speed.

Conversely, at a constant discharge rate, an increased screw rotation speed reduces the fill factor, whereas a lower rotation speed increases it.

At a constant rotation speed, a higher discharge rate results in a higher fill factor, while a lower discharge rate results in a lower fill factor.

C: Capacity coefficient of the barrel and screw (a constant that varies depending on screw size and element design).

Fill Levels in the Extrusion Process

The fill level during extrusion can be adjusted by selecting specific screw elements that perform different functions:

• Full-flight screws with high conveying capacity create low-fill regions as they make it difficult for materials to occupy the screw fully.
  
• Screw elements with no conveying function or those with reverse conveying roles can generate high-fill regions.

Optimizing the screw configuration is crucial to achieving the desired fill levels based on processing objectives.

Impact of Fill Factor

Achieving Uniform Mixing and Dispersion

A high fill factor increases the pressure inside the screw, enhancing shear action and promoting better mixing and dispersion of materials and additives. This improves mixing efficiency and product quality.A low fill factor, on the other hand, may lead to voids within the screw, making uniform mixing and dispersion challenging, which can result in inconsistent product properties. However, an excessively high fill factor can increase pressure within the screw, leading to overheating and potential thermal degradation of materials.

Product Homogeneity and Process Stability

Properly adjusted fill factors help achieve uniform density and physical properties across the product cross-section. In twin screw extruders, where processing conditions vary by section, maintaining a stable fill factor is essential for consistent process stability. An unstable fill factor can introduce defects such as air bubbles or surface irregularities, especially for high-viscosity resins or resins with fillers for flame retardancy or improved strength.

Fill Factor and Residence Time

Residence time and fill factor are closely related process parameters that directly influence material flow and quality in the extruder. A low fill factor results in shorter residence time, allowing materials to pass quickly through the screw. This can lead to insufficient mixing and dispersion, resulting in uneven product characteristics. Residence time tends to increase with a higher fill factor, leading to more effective mixing and improved product quality. However, excessive residence time can cause material degradation, particularly for heat-sensitive materials.

Effect of Filling Ratio on Residence Time

When the filling ratio is high, the residence time becomes longer. As a result, greater shear is applied to the material, promoting mixing and improving mixing quality. However, if the residence time becomes excessively long, especially for heat-sensitive materials, degradation or thermal decomposition may occur.

When the filling ratio is low, the residence time becomes shorter and the material passes through the screws more quickly. In such cases, mixing and dispersion may be insufficient, which can lead to non-uniform properties in the final product.

Influence on Chemical Reactions

In processes that require chemical reactions, such as reactive extrusion, optimizing the filling ratio allows the residence time to fall within a range suitable for the reaction, enabling efficient processing. Conversely, if the filling ratio is too low, the residence time becomes too short, increasing the likelihood that the reaction will not be completed.

Verification Experiment of Filling Ratio Using the ULTnano15

An experiment using a small twin screw compounding extruder was conducted to visually examine how changes in the Q/N index affect the filling ratio. A circulation-type small twin screw compounding extruder, the ULTnano15, was used for this study. However, in this experiment, priority was given to observing the filling condition in the plasticizing and melting section, which represents the initial stage of the extrusion process. Therefore, continuous extrusion was carried out in a singlepass mode without using the circulation path.

In addition, the Q/N index was adjusted by changing extrusion conditions such as screw rotation speed and throughput using the same equipment, and the resulting effects on the filling behavior were visually observed. By using a special vertically split barrel design, it was possible to visualize the filling condition of the screws during extrusion operation with high reproducibility, compared with conventional methods that rely on confirming the filling state by pulling out the screws after operation.

Experimental Overview

Operating Conditions / Output rate, Screw rotation speed and Q/N

	①	②	③	④	⑤	⑥	⑦	⑧	⑨
Output g/hr	500	2500	5000	1500	1500	1500	300	1500	3000
Screw 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:
Differences in filling ratio at three levels under the same Q/N

Comparison of ④/⑤/⑥:
Differences in filling ratio at three levels under different Q/N values (constant throughput)

Comparison of ⑦/⑧/⑨:
Differences in filling ratio at three levels under different Q/N values (constant screw speed)

Operating Conditions / Screw Configuration

Comparison of ①/②/③ ※ under same Q/N

Comparison of ④/⑤/⑥: ※ under same output rate

Comparison of ⑦/⑧/⑨: ※ under same screw rotation speed

Experimental Results

As a result, when comparing under the same Q/N value, only minor variations were observed, and no significant differences in the fill ratio were confirmed.
In contrast, when comparing different Q/N values, clear differences in fill ratio were observed. In particular, the data showed a distinct trend of increasing fill ratio within the twin screw extruder as the Q/N ratio increased.

This suggests that the residence state and fill behavior of the melt are highly sensitive to the balance between feed rate and screw speed, reinforcing the effectiveness of the Q/N parameter as a useful metric for understanding the extrusion process.

Furthermore, for both comparisons under the same Q/N conditions and across different Q/N conditions, data on melt pressure and melt temperature at the extruder discharge were successfully obtained.
These data provide valuable insights for a deeper understanding of fill behavior in the plasticizing and melting section and yielded highly interesting findings.
Experimental results related to melt temperature and pressure are available at the following link, where the effects of different operating conditions on process parameters such as temperature and pressure are discussed in detail.

Importance of Filling Ratio Design — Devolatilization as an Example

For example, in the devolatilization process of a twin screw extruder, the filling ratio in the screw section has a significant influence on devolatilization efficiency. When the filling ratio is high and the molten resin nearly fills the barrel, the amount of volatile components generated increases. However, because no free surface is formed, there is no effective path for gas release. This can lead to sudden pressure release at the vent section, causing resin ejection known as “vent-up,” which tends to reduce continuous operability and effective devolatilization efficiency.

In contrast, when the filling ratio is intentionally reduced in the devolatilization zone to secure a free surface of the resin, volatile components can be efficiently removed through a continuous mechanism involving diffusion, surface arrival, and vacuum extraction. In other words, by selecting screw patterns that combine pressurization and pressure release—features characteristic of twin-screw extruders—it is possible to achieve both high devolatilization performance and stable operation.

Importance of Filling Ratio Design — Other Influences

In twin screw extrusion, the filling ratio affects not only devolatilization but also a wide range of factors, including thermal behavior inside the machine, flow characteristics, mixing performance, reaction behavior, and ultimately the quality of the final product. Under high filling ratio and high-pressure conditions, the resin fills the barrel as a continuous phase, and viscous dissipation becomes dominant. As a result, internal heat generation may exceed heat dissipation through the barrel, leading to localized temperature rise. This can increase the risk of material degradation, such as thermal decomposition, molecular weight reduction, and gel formation.

In the screw mixing section, high shear stress is advantageous for dispersing fillers and pigments. However, in filler compounding processes involving materials such as glass fibers or carbon fibers, excessive shear may cause fiber breakage and a reduction in fiber length.

On the other hand, under low filling ratio and low-pressure conditions, internal heat generation is suppressed, and the influence of external heater control becomes relatively larger, improving mechanical temperature controllability. However, reduced shear stress leads to weaker mixing performance. Under extremely low filling conditions, insufficient melting, temperature non-uniformity, throughput fluctuations, and die pressure pulsation may occur, potentially resulting in poorer dispersive and distributive mixing as well as reduced dimensional stability.

Therefore, in twin screw extrusion processes, it is essential to understand the characteristics and limitations of both high and low filling conditions. By comprehensively considering material properties, mixing and reaction requirements, mechanical limitations, and product quality, optimal design and control of the filling ratio and pressure state become key technical factors for achieving both stable operation and high-quality production.

Summary

The relationship between residence time and fill factor affects all aspects of the extrusion process, including material flow, mixing, temperature distribution, and reaction efficiency. Higher fill factors lead to longer residence times, improving mixing and temperature uniformity but increasing the risk of material degradation. Lower fill factors result in shorter residence times, which may lead to insufficient mixing and reduced reaction completion .Optimizing the relationship between residence time and fill factor is key to achieving efficient, stable extrusion processes and minimizing product quality variations. By carefully controlling these parameters, extrusion can be conducted with enhanced efficiency and consistent product quality.

Twin Screw Extruders by Technovel
– A Dedicated Japanese Extruder Manufacturer

　

As a world-first innovation, we have developed a new type of horizontal multi screw extruder. Our Quad screw and Octa screw extruders are already being utilized across various industries. Even in the field of conventional twin screw extruders, we offer a wide lineup ranging from ultra-compact models—such as the world’s smallest 6 mm diameter screw and our best-selling 15 mm model—to large-scale production machines.

Japanese Extrusion Experts, Always Advancing