Tag Archives: viscosity

Measuring Viscosity: Capillary Rheometer vs. Injection Mold Rheometer

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.

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Viscosity, Part 3- Therma-flo™

In the previous two tech tips, we began a three part discussion on how our industry looks at the viscosity of a polymer. We reviewed capillary rheometers and melt flow index machines up to this point. We pointed out various benefits and flaws for each method. Now we are going to take a look at a newer approach to evaluating a polymer’s viscosity called Therma-flo™.

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Viscosity, Part 2: Melt Flow Index

In our last tech tip, we started a three phase discussion about viscosity. The first phase talked about how capillary rheometers are used to characterize the viscosity of plastics at a wide range of shear rates and temperatures. However, due to a variety of reasons most people in the injection molding industry prefer to use a simpler device when talking about the viscosity of a plastic – the Melt Flow Index machine. One should be aware that even though MFI is the dominate method used to characterize how plastic will flow in a mold, it does not actually provide a measure of viscosity.

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Viscosity, Part 1: Capillary Rheometers

Viscosity is a term that people in the plastic injection molding industry talk about quite often, so it is something we will discuss in more detail during the next three tech tips. Viscosity measures the thickness of a fluid. The thicker the fluid, the higher the viscosity. In other words, viscosity is resistance to flow. There are various methods used to measure viscosity and all three will be talked about in the upcoming tech tips, but this tech tip will start with capillary rheometers.

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(IR)Relative Viscosity – Trouble with the Curve

The Relative Viscosity (RV) test is often used to help identify an ideal fill speed/time for the mold. Fill speed and the resulting fill time are an indication of how fast the plastic is injected into the mold. The speed affects how much shear heating and shear thinning the plastic experiences, which in turn affects the material’s viscosity, the pressure, the temperature of the plastic inside the cavities, and the overall part quality (dimensional, aesthetics, strength, etc.). Though there are many different approaches to selecting the idea point, those who teach this method generally agree that for most parts the fill time should be located somewhere on the flatter portion of the curve (Figure 1).

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