Introduction
The flow characteristics of polymer melts are a key factor affecting their processing and molding process. As a widely used measurement tool, rotary rheometers can accurately characterize rheological parameters such as the viscosity of melts by applying a controlled shear field. These parameters are closely related to the behavior of materials during extrusion, injection molding, and other processes, providing an important basis for process optimization and material development.
Measurement principle
Rotary rheometers typically employ a flat or cone plate measurement system. When one of the components rotates, a shear flow is created in the melt sample being tested. The instrument calculates shear stress versus shear rate by measuring the torque required to maintain a specific rotational speed (or strain rate), or the rotational speed generated when a specific torque is applied. The apparent viscosity of a melt (η) is defined by the ratio of shear stress (τ) to shear rate (γ̇):
η = τ / γ̇
By changing the shear rate or applying oscillatory shear, information such as the viscosity change curve (flow curve) and viscoelastic modulus can be obtained.
Viscosity is related to processing
Polymer melts typically exhibit non-Newtonian fluid properties, and their viscosity varies with shear rate. This shear thinning behavior has a direct impact on processing. The high zero shear viscosity at low shear rate (such as when the material stays in the storage area) is related to the plasticization and transport of the melt. The viscosity at high shear rates (e.g. through the die) determines the pressure and energy consumption required for machining. The flow curve allows you to evaluate how the material behaves at different stages of processing.
| Rheological parameters | Processing significance |
| Zero shear viscosity | Reflects material conveyance, melt strength and sagging tendency |
| Shear rarefaction index | The sensitivity of viscosity to the shear rate was characterized, which affected the difficulty of mold filling |
| Melt elasticity (normal stress difference) | It is associated with instability phenomena such as extrusion expansion and melt rupture |
Key points of the test methodology
To ensure reliable measurement results, you need to pay attention to multiple links. Sample preparation should ensure that the material is fully melted and non-degradable. A strain scan should be performed prior to testing to determine the linear viscoelastic zone. When performing steady-state flow scanning, the shear rate range should cover the actual machining area as much as possible. Temperature control must be precise and stable, as viscosity is highly sensitive to temperature and often follows the Arrhenius relation:
η ∝ exp(Ea/RT)
where Eais the flow activation energy, R is the gas constant, and T is the absolute temperature. In addition, the potential impact of wall slip and thermal degradation on the test results should be concerned.
Data application analysis
The resulting rheological data can be used in several ways. Quality control is possible by comparing flow curves for different grades or batches. Combined with capillary rheological data, the measurement results of the rotary rheometer can be corrected at high shear rates. Viscoelastic data (e.g., energy storage modulus G' and loss modulus G'') help analyze the structural relaxation time of the melt, which is related to the internal stress and dimensional stability of the product. Correlating rheological data with specific machining process parameters (e.g., screw speed, die pressure) can establish a machining window to guide production.
| Common problems with machining | Possible rheological correlating factors |
| The extrusion surface is not smooth | The melt elasticity is too high or the flow is unstable at low shear rates |
| The injection molded part is warped and deformed | The melt relaxation time is too long, resulting in high freezing orientation stress |
| The interface between the co-extrusion layers is unstable | The melt viscosity of each layer is not properly matched |
Epilogue
The use of rotary rheometer system to measure the viscosity and other rheological properties of polymer melt is an effective bridge to connect the intrinsic characteristics of materials and the actual processing properties. Through scientific interpretation of rheological data, various phenomena in processing can be deeply understood, and solid technical support can be provided for optimizing process conditions, improving product quality, and developing new materials that meet specific processing requirements.
References
1. Fundamentals of Polymer Melt Rheological Measurement, Journal of Materials Testing Technology, Vol. X.
2. Rheological Problems in Polymer Processing, Proceedings of the International Society for Polymer Processing.
3. Rotary rheometer operation and data analysis guide, instrument analysis manual series.