In the general polymer production processes, polymers must be melted to process them. So, a very good understanding of molten plastic’s nature to obtain successful plastic production processes.
The most prominent features of polymer melts are; viscosity that is very different compared to other Newtonian fluids. And viscoelasticity is the special feature of polymer melts.
Viscoelasticity is a very important physical phenomenon that is observed in polymers. The most important result of the viscoelasticity of polymers is the ‘die swell’ phenomenon that is observed.
Die swell is the expansion of polymer melt when quits from die orifice. There is a constant diameter of the die orifice which restricts the polymer melt to take the shape of this orifice. Before the pushing of polymer melts into the orifice, there is a distribution of polymer molecules. This distribution is restricted by the orifice channel. When polymer melt quits from this orifice channel, it ‘remembers’ its molecular distribution before the orifice, then proportionately takes the first diameter or shape.
This die swell phenomenon must be considered and compensated with a proper solution. Solution or compensation of die swell phenomenon is the designing of the required length of orifice channel.
The orifice must be enough length to give the new molecular distribution to polymer melt inside the channel. In this case, very small which is negligible die swells may occur.
First of all, we need to understand the term ‘viscosity’. In simple, viscosity is the fluid’s resistance to flow. When the viscosity is high for a fluid, the flowability of this fluid is low. Because of the very high molecular weight, the viscosities of polymer materials are very high compared with other known fluids such as oil, water, etc.
The viscosity of fluids is generally explained with the required shear stress that is needed to be applied to a fluid to flow. When this shear stress is high, viscosity is also high.
For Newtonian fluids, viscosity does not change with the changing rate of shear, which depends on the rate of flow. The difference in the viscosity nature of polymers emerges from this point. Unlike other Newtonian fluids, when the shear rates increases which are also dependent on flow rate, the viscosity of polymers decreases. With the increasing rate of flow of polymers, the force and pressure requirements to push the polymers increases though. This fact must be calculated and considered in the design of polymer production processes. This feature of polymer melts is also called ‘pseudoelasticity’.
Temperature also a very important parameter for polymer viscosity. Like other Newtonian fluids, with the increasing temperature, the viscosity of polymers decreases.
This is the summary information about the die swell, pseudo-plasticity of polymer melts. You can also find various kinds of information about polymers in Mechanical Base.
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