Shear thinning materials

Orifice viscometers should not be used for setting product specifications, for which better precision is required. Because they are designed for Newtonian and near-Newtonian fluids, they should not be used with thixotropic or highly shear-thinning materials such fluids should be characterized by using multispeed rotational viscometers. 

SBCs and TPVs are shear-thinning materials. Therefore, as such they are benefited from the flow restrictions at the entry of molds and this allows the provision for small diameter, short landed gates. For TPVs and SBCs, it is suggested that gates should be 20%-30% of the wall thickness of the part to be filled. 

Fig. 4.2.2 Dimensionless shear rate and viscosity as a function of radius for a power-law fluid under the conditions shown in Figure 4.2.1. For a highly shear thinning material, the shear rate is large near the wall and close to zero near the center. The viscosity can vary by several orders of magnitude in the pipe.

Polymer rheology can respond nonllnearly to shear rates, as shown in Fig. 3.4. As discussed above, a Newtonian material has a linear relationship between shear stress and shear rate, and the slope of the response Is the shear viscosity. Many polymers at very low shear rates approach a Newtonian response. As the shear rate is increased most commercial polymers have a decrease in the rate of stress increase. That is, the extension of the shear stress function tends to have a lower local slope as the shear rate is increased. This Is an example of a pseudoplastic material, also known as a shear-thinning material. Pseudoplastic materials show a decrease in shear viscosity as the shear rate increases. Dilatant materials Increase in shear viscosity as the shear rate increases. Finally, a Bingham plastic requires an initial shear stress, to, before it will flow, and then it reacts to shear rate in the same manner as a Newtonian polymer. It thus appears as an elastic material until it begins to flow and then responds like a viscous fluid. All of these viscous responses may be observed when dealing with commercial and experimental polymers. 

Before the viscosity can be calculated from capillary data, as mentioned above, the apparent shear rate, 7 , must be corrected for the effect of the pseudoplastic nature of the polymer on the velocity profile. The calculation can be made only after a model has been adopted that relates shear stress and shear rate for this concept of a pseudoplastic shear-thinning material. The model choice is a philosophical question [11] after rheologlsts tried numerous models, there are in general two simple models that have withstood substantial testing when the predictions are compared with experimental data [1]. The first Is ... 

The form of the viscosity function is of practical relevance for processing plastic melts and other shear thinning materials. Shear rates can differ greatly depending on the process (Fig. 3.5). Sedimentation and trickle processes under shear force give shear rates of far less than 1 s . In the case of extrusion and in pipe flows, shear rates can reach several hundreds of reciprocal seconds, particularly if a portion of the volume is flowing over the tip of the screw (see Chapter 9). In this case, shear rates can reach several thousand reciprocal seconds. 

Figure 8.10 Conveying characteristic of a screw element with Newtonian and shear thinning materials...

Mathematical models, which can predict the shape of a flow curve of a shear thinning material including lower and upper Newtonian regions, require at least four parameters. The Cross model is one such model ... 

A more general mathematical description of shear thinning materials comprising an apparent yield stress is given by the Herschel-Bulkley model ... 

This equation is analogous to the previous equation, but includes correction for shear thinning materials. 

This equation is also used to calculate volumetric flow rate through an annular die gap, but includes correction for shear thinning materials. The correction factor is obtained graphically from Fig. B.l [53] below. 

The dynamics of the molten material in the extrusion head is a quite complex process involving the melting point of the materials coupled with flow processes and stresses which are in part related to the viscosity and surface energy as well as the design of the extruder. Most 3D printers use shear thinning materials that follow power-law viscosity models (7.4) ... 

Thixotropic A shear-thinning material that exhibits a time-dependent recovery in viscosity after removal of the shearing force. 

Some shear-thinning materials deform like elastic solids until a certain stress, known as the yield stress, is reached, after which they deform as normal shear-thinning liquids. These materials are described as shear thinning with yield value, and have a consistency curve like curve 2 of Fig. 6.3. Flow ceases when the stress falls below the yield stress. Materials which show this behaviour include toothpaste and muds. These substances also tend to be thixotropic. 

Representations of pseudoplastic or shear thinning material response... 

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