Rationale
Rotational viscometers are essential tools for measuring the viscosity of resin melts, and their core principle is based on the measurement of the viscous resistance of rotating elements. During measurement, the rotor of the viscometer is immersed in the resin melt being measured and rotates at a set speed. The viscous resistance of the melt on the rotor surface creates a torque that is proportional to the viscosity of the melt. By measuring this torque and substituting the known geometric parameters and rotational velocity, the absolute viscosity of the melt can be calculated. For melts that follow Newtonian fluid behavior, their viscosity values are independent of shear rate; For most non-Newtonian fluids, such as resin melts, the apparent viscosity varies with the shear rate. As a result, modern rotational viscometers often have the ability to perform steady-state or dynamic scanning tests over a wide range of shear rates to comprehensively characterize the rheological properties of materials.
Correlation between viscosity and resin processing properties
The viscosity of the resin melt is a key indicator of its processing performance, which directly affects the feasibility and final product quality of molding processes such as injection molding, extrusion, and blow molding. The rheological curves obtained by the rotary viscometer allow the extraction of several parameters that are closely related to the processing performance. For example, the zero shear viscosity of a melt at a low shear rate is related to the molecular weight and molecular weight distribution of the material, which affects the melt strength and elongation behavior of the product. In the common medium and high shear rate range of the processing process, the shear thinning behavior of the melt determines the filling pressure, extruder back pressure and energy consumption. In addition, the complex viscosity, energy storage modulus, and loss modulus obtained by dynamic oscillation tests are useful for evaluating the elastic effects of melts, which are important for predicting warping, shrinkage, and surface defects.
Methodological considerations
To ensure the accuracy and comparability of measurement results, the test must strictly control the experimental conditions and refer to relevant technical standards. Temperature is the primary control parameter that needs to be precisely set and maintained according to the processing temperature range of the resin. The filling process avoids the introduction of air bubbles and ensures adequate contact of the melt with the measuring element. The test usually includes a pre-shear stage to bring the sample to equilibrium. Measurement modes can be selected as needed: steady-state rate sweep, dynamic frequency sweep, or time-dependent behavior test. When analyzing data, the test conditions and the model used should be clearly labeled. A number of domestic and foreign standards provide guidance for the rheological performance of plastic melts, which set specific requirements for instrument calibration, sample preparation, testing procedures and report content.
Rheological properties of typical resin melts
Different types of resin melts exhibit significantly different rheological behaviors, and the measurement data of the rotational viscometer provides a direct basis for process optimization. The following table summarizes the typical rheological characteristics of some common resin types and their processing effects:
| Resin type | Typical rheological features |
| Polyethylene (PE) | The shear thinning is obvious, and the melt strength is high, which is suitable for extrusion blow molding. |
| Polypropylene (PP) | It is sensitive to shear rate, and the viscosity changes significantly with temperature, so attention should be paid to the balance of mold filling. |
| Polystyrene (PS) | Viscosity is highly dependent on temperature, and the elastic effect of melt is relatively low. |
| Polycarbonate (PC) | The melt has a high viscosity, is sensitive to moisture, and needs to be thoroughly dried before testing. |
| Nylon (PA) | It has low viscosity and good fluidity, but is prone to "jetting" and needs to control the injection speed. |
By analyzing the complete rheological profile of a particular resin, it is possible to determine the appropriate processing window, including the recommended processing temperature range, reasonable shear rate range, and predicting the required equipment pressure to avoid processing defects such as degradation, underinjection, or erratic flow.
Conclusion
Rotational viscometers are an effective bridge between resin material properties and processing applications. By systematically measuring the viscosity of resin melts under different shear and temperature conditions, a comprehensive and quantitative understanding of their processing behavior can be obtained. These data are irreplaceable guiding value for material development, formulation adjustment, mold and runner design, production process setting, and quality problem diagnosis. In practical applications, comprehensive analysis of viscosity data combined with the chemical properties of specific resins and target process requirements is the basis for efficient and stable processing and high-quality products.
References
1. Compilation of Plastic Melt Flow Rate Test Method Standards, China Standards Press.
2. ASTM D3835, Standard Test Method for Determination of Polymer Material Properties by Capillary Rheometer.
3. Introduction to Polymer Melt Rheology, Chemical Industry Press.
4. ISO 11443, Plastics - Determination of rheological properties of plastics using capillary and slit rheometers.