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Calculation and Application of Residence Time of Polymer Materials in Screw Extruders

Biobased biodegradable materials 2025-09-09 14:22:23

In polymer material processing, especially in the extrusion molding of degradable materials, the residence time of the material in the screw extruder is a critical parameter.Also known as residence time(HUT) refers to the time plastic pellets take from entering the extruder to flowing out of the die, which is directly related to whether the material will undergo thermal degradation due to prolonged heating. Today, we will discuss how to scientifically calculate this time and its importance.

Calculation formula of residence time

In actual production, the commonly used residence time (T) calculation formula is based on the volumetric flow rate of the material and the filled volume of the material inside the extruder, as follows:

T =V/Q

Among them:

T represents the dwell time (unit: minutes).

V is the effective filling volume of the material inside the extruder (unit: liters), which can typically be estimated based on the screw diameter, effective length, and filling rate (for simplified calculations, it can be approximated as the effective volume of the screw, i.e., V = Πd).²L/4000, where d is the screw diameter (cm) and L is the effective length of the screw (cm).

Q is the volumetric flow rate of the material (unit: L/min), which can be obtained by dividing the mass flow rate by the density of the material in its molten state (Q = G/ρ, where G is the mass flow rate (kg/min), and ρ is the density of the molten material (kg/L)).

It should be noted that:

In the formula, use "V = Πd".²L/4000" calculates the total volume of the barrel (i.e., the volume of the cylindrical space inside the barrel). However, in reality, the screw itself occupies a portion of the space inside the barrel, and the material can only fill the threads between the screw and the barrel. Therefore, this formula is essentially a simplified estimation for the following reason:

Screw structure is complex: different screws (such as gradient type, step type, barrier type) vary greatly in parameters like groove depth, thread width, and pitch. The actual volume of the groove requires detailed drawings for accurate calculation and is difficult to express with a unified formula.

Industry Practical Practice: In actual production, the effective volume of material is usually approximated by "barrel total volume × filling rate." The filling rate is generally taken based on experience (mostly 50%-80%), indirectly accounting for the space occupied by the screw. For example, if the barrel total volume is 100 liters and the filling rate is 60%, the actual material filling volume is approximately 60 liters.

To quickly assess demand: In most scenarios, as long as you know "total barrel volume ÷ volumetric flow rate," you get the theoretical maximum residence time (the actual time will be shorter), which allows you to preliminarily determine whether it exceeds the material's critical thermal endurance time, thus meeting the basic requirements for process control.

For example:

Taking the extrusion processing of PBAT material as an example:

Screw parameters: diameter d = 65 mm (6.5 cm), effective length L = 260 cm, effective filling volume V ≈ 3.14*6.5²260/4000 ≈ 8.7 liters (simplified calculation, ignoring filling rate differences)

If the filling rate is 60%, the effective filling volume of the material V = 8.7 * 60% ≈ 5.2 liters.

Processing parameters: Mass flow rate G = 5 kg/min, PBAT molten density ρ = 1.25 kg/L, then volumetric flow rate Q = 5/1.25 = 4 L/min.

Dwelling time calculation: T = 5.2 / 4 ≈ 1.3 minutes

Under this process, the actual residence time of the material is approximately 1.3 minutes, which is much lower than the thermal degradation critical time of PBAT (usually 5-8 minutes), ensuring process safety.

The role of scientifically managing dwell time.

Avoid thermal degradation: For polymers such as bio-based biodegradable materials that are sensitive to heat, prolonged residence time can lead to chain scission and performance degradation. Scientific management can effectively reduce these issues.

Ensuring product stability: Stable residence time ensures uniform heating and shearing of materials, reducing fluctuations in product performance (such as mechanical properties, appearance, etc.).

Improve production efficiency: By optimizing residence time, it is possible to reasonably increase processing speed without causing degradation, balancing quality and capacity.

Reducing production costs: minimize waste caused by degradation, reduce rework rate, and indirectly enhance production efficiency.

By mastering the calculation method of residence time, practitioners can more accurately adjust parameters such as screw speed and feed rate, providing a scientific basis for the extrusion processing of polymer materials. This is especially crucial for ensuring product quality in the case of heat-sensitive bio-based degradable materials.

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Calculation and Application of Residence Time of Polymer Materials in Screw Extruders

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