Melt shear viscosity
What is shear viscosity?
Melt shear viscosity is a material's resistance to shear flow. In general, polymer melts are highly viscous due to their long molecular chain structure. The viscosity of polymer melt ranges from 2 to 3,000 Pas (water 10-1, glass 1020). Viscosity can be thought of as the thickness of a fluid, or how much it resists flow. Viscosity is expressed as the ratio of shear stress (force per unit area) to the shear rate (rate change of shear strain), as shown in the equation and diagram below:
FIGURE 1. The definition of polymer melt viscosity, illustrated by a simple shear flow
Newtonian fluid vs. non-Newtonian fluid
For Newtonian fluids, viscosity is a temperature-dependent constant, regardless of the shear rate. A typical example of Newtonian fluid is water. However, for non-Newtonian fluids, which include most polymer melts, the viscosity varies, not only with temperature, but with the shear rate.
When the polymer is deformed, there will be some disentanglement, slippage of chains over each other, and molecular alignment in the direction of the applied stress. As a result, the resistance exhibited by polymer to flow decreases with the deformation, due to the evolution of its microstructure (which tends to align in the flow direction). This is often referred to as shear-thinning behavior, which translates to lower viscosity with a high shear rate. Shear-thinning behavior provides some benefits for processing the polymer melt. For example, if you double the applied pressure to move water in an open-ended pipe, the flow rate of the water also doubles, since the water does not have shear-thinning behavior. But in a similar situation using a polymer melt, if the pressure is doubled, the melt flow rate may increase from 2 to 15 times, depending on the material.
Shear rate distribution
Having introduced the concept of shear viscosity, let us look at the shear rate distribution in the cavity during injection molding. Generally speaking, the faster the adjacent material elements move over each other, the higher the shear rate is. Therefore, for a typical melt flow velocity profile, shown in (a), it is clear that the shear rate is highest at the mold-melt interface (or at the melt-solid interface if there is a frozen polymer layer). On the other hand, the shear rate approaches zero at the center line because there is no relative material element movement due to flow symmetry, as shown in Figure 2 (b). Shear rate is an important flow parameter since it influences the melt viscosity and the amount of shear (viscous) heating. The typical shear rate experienced by the polymer melt during the injection molding process ranges from 102 to 105 second-1.
FIGURE 2. (a) A typical velocity profile with relative flow element movement and (b) the corresponding shear rate distribution in injection molding filling.
Effects of temperature and pressure
Since the mobility of polymer molecular chains decreases with decreasing temperature, the flow resistance of polymer melt also greatly depends on the temperature. As shown in Figure 3, the melt viscosity decreases with increasing shear rate and temperature due to the disentanglement and alignment of the molecules and enhanced mobility of polymer molecules, respectively. In addition, the melt viscosity also depends on the pressure. The higher the pressure, the more viscous the melt becomes.
FIGURE 3. The viscosity of polymer melt depends on the shear rate, pressure, and temperature.
Rheological material properties contains a mathematical description of the shear viscosity as a function of shear rate, temperature, and pressure. For a discussion on how high pressure increases the level of viscosity, see Pressure dependence of viscosity.