Plastic Melt Rheology

Time:2022-08-09 08:26:14 / Popularity: / Source:

Plastic melt rheology is science that studies flow and deformation of plastic melt.
In classical mechanics, flow and deformation are physical concepts in two categories. Flow is a property of liquid materials, while deformation is a property of solid materials. General liquid flow follows Newton's law of fluid flow - shear stress on a material is proportional to shear rate, and general solid deformation follows Hooke's law - stress on a material is proportional to amount of deformation. Newtonian fluids and Hooke elastomers are two dominant classical models.
As a branch of polymer materials, plastics can flow and deform, are viscous and elastic, have viscous loss during deformation, and have elastic memory during flow. This viscoelasticity is rheological property of polymers. Flow makes it workable, and elastic properties make it more useful.
Plastic melt is a typical non-Newtonian fluid. During processing, rheological behavior is directly related to temperature, pressure and shear rate it is subjected to. Optimizing processing conditions can effectively reduce instability during injection molding, extrusion and other processing, then obtain plastic products with stable processes and uniform quality. .

Compressibility of plastic melts

Under condition of a certain temperature difference or pressure difference, property that volume or density of plastic melt can be changed is called compressibility. As temperature increases, force between polymer macromolecules weakens, free volume increases, and volume of plastic melt expands. If external pressure is applied, this expanded volume can be compressed again. This can be confirmed from PVT curve of plastic material. During injection molding process, back pressure is applied during metering of stock material, pressure is applied during filling and holding pressure stage. It is through this characteristic that density of plastic melt and product is increased to reduce process volatility and make up for product contractions.

Pseudoplastic fluid

Non-Newtonian fluids are divided into dilatant fluids, pseudoplastic fluids and Bingham fluids. Plastic melts are pseudoplastic fluids, typically characterized by a decrease in apparent viscosity with increasing shear rate, also known as shear thinning.
Power-law formulation describing pseudoplastic behavior
Plastic Melt Rheology 
Flow index n represents deviation of fluid flow behavior from Newtonian behavior. For pseudoplastic fluids, n is usually less than 1.
Take logarithm of both sides of equal sign to obtain a linear equation
Plastic Melt Rheology 
Note: Slope, n-1, does not change significantly within 1-2 orders of magnitude of shear rate, but over a wider range of shear rates, n decreases with increasing shear rate (non-Newtonian region), and at very low or very high shear rates, plastic melt is close to a Newtonian fluid.
Different plastic melts have different degrees of viscosity decrease with increase of shear rate, that is to say, sensitivity of different materials to shear is not same. Shear thinning of plastic melt is due to fact that molecular chain of polymer undergoes orientation disentanglement and slip under high shear, and intermolecular force is weakened, also known as reduction of internal friction, reduction of flow resistance, and reduction of apparent viscosity.

Effect of temperature on viscosity of plastic melt

Temperature of plastic melt increases, molecular motion unit has enough energy to overcome force between molecules, and free volume between molecules increases, which further causes viscosity of plastic melt to decrease. It is generally believed that near glass transition temperature, viscosity and temperature of plastic melt conform to description of WLF equation. When temperature is much greater than Tg, viscosity of plastic melt and temperature dependence conform to description of Arrhenius equation.

Effect of pressure on plastic melt viscosity

During injection molding process, volume of plastic melt is compressed as pressure increases, reducing free volume between polymer molecules and reducing activity of molecular chain, resulting in an increase in melt viscosity and a decrease in fluidity.

In conclusion

In injection molding process, viscosity of plastic melt is not only affected by temperature, pressure and shear rate, but also by molecular weight and molecular weight distribution width of plastic itself. Melt viscosity effect may be more pronounced.
Understanding rheological properties of plastic melts and selecting a reasonable filling speed in injection molding process are of great reference significance. RJG recommends that customers fill mold cavity as quickly as possible under premise of ensuring that appearance of product is free of defects, which not only reduces instability of process due to large fluctuation of material viscosity, but also reduces pressure loss during injection molding process, thereby achieving energy saving and emission reduction.

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