Thixotropic: Viscosity decreases over time given a constant shear rate. As shear decreases, the material gradually recovers to its original properties. Examples include yogurt and paint. Pseudoplastic and thixotropic have similar characteristics and are often, incorrectly, used interchangeably [7].
6.4.1
Thermoplastic Fluid Properties
Most thermoplastic materials in their fluid (molten) state are considered non-Newtonian Class I, pseudoplastic fluids. As indicated in Fig.6.1, as shear rate increases,the fluid’s viscosity decreases.In thermoplastics, typically as shear increases, the curve becomes more linear, and in rare cases, it approaches Newtonian behavior [8]. In any event it is not time-dependent. Most pseudoplastic curves also become more linear with extremely high shear rates. Nevertheless, shear has a greater influence on viscosity than temperature for thermoplastics, as indicated in the two graphs in Fig. 6.2. The viscosity of thermoplastics is di fficult to measure in standard terms such as Poise or Pascal · seconds. Viscosity is most often given in a number derived from a melt flow index machine (see Fig. 6.3). This machine measures the rate of molten material that is extruded through a given diameter and length of an ori fice. Temperature, force (weight), and piston position are variables in the timed test. Melt flow is covered in ASTM D1238 and ISO 1133. Barring any chemical degradation, the viscosity of plastics can fluctuate back and forth as established by the temperature and shear of the melt.Furthermore, residence time (time spent in the injection molding machine or hot runner), again barring any chemical degradation, is important, but not to the critical extent exhibited in thermosets (as will be described Bingham plastic
