Thermoplastics viscosities are described as having a pseudo-plastic non-Newtonian behavior. This type of behavior means that as you push the material at a faster rate, the polymer exhibits a reduction in viscosity. There are two critical factors that affect the viscosity of plastics: temperature and shear rate.
In our material characterization lab we measure viscosity utilizing two primary, capillary based systems: Autodesk Moldflow Injection Molding Rheometer (IMR) and Capillary Rheometer. (We will touch on Therma-flo™ separately in a later article.) Each piece of equipment and associated test method has its own benefits and drawbacks, but the IMR remains the preferred method for material characterization to simulate the injection molding process.
Most of our customers have some familiarity with capillary rheometers. They are widely available and are frequently used in the plastics industry as a method of material comparison. Test methods for capillary rheometry have been standardized in this industry for decades and can be utilized for most polymers.
Standards aside, melt preparation is critical for any rheological measurement. The capillary rheometer setup allows for an isothermal chamber to bring a material sample up to a desired temperature. Plastic materials are good insulators, so this may take several minutes. From the loading of the machine to the time of the last test, material could see over 10 minutes of residence time in the barrel. Due to residence times of this duration thermal stability of a given sample may come into question.
For melt preparation consistency, the IMR utilizes a standard injection molding machine. Every measurement made on the IMR is from a fresh shot of material with a very low residence time. The preparation of the melt is identical to the injection molding process. The melt travels through a reciprocating screw and barrel, developing a thermal and shear profile. The material has now been prepared with a near identical thermal and shear history to what it would see in production.
Shear viscosity looks at the materials viscosity relative to a specified shear rate. Typical capillary rheometers are benchtop units that may be fairly limited in their ability to push plastic materials. Most systems are limited to 10,000/sec of shear if they even have enough allowable force to push them that fast. The utilization of the injection molding machine for the IMR allows for higher shear rates to be tested, with standard runs seeing upwards of 60,000/sec.
Any capillary system may struggle to provide accurate measurements as more fiber reinforcement is added to a given material. Common designs used for capillary rheometers inhibit the ability to measure viscosity accurately with even moderate additions of fiber reinforcement. The IMR may measure fiber reinforced materials better, however, it is important to note that the plastification process of a material in an injection molding machine can be subject to fiber breakage.
It is crucial to understand what information these tests are really giving you, and why one method may be chosen over another. Overall, the capillary rheometer is excellent for comparing viscosities between materials and material lots. However, the IMR is more relatable to the injection molding process and gives us more information to better predict the viscosity of a material at a given temperature and shear rate.