Shear flow and shear viscosity
DATE：2019/12/3 8:45:53 / READ： / SOURCE：This station
I. Shear flow and shear viscosity
During the molding process, the basic knowledge of the viscosity characteristics, causes and influencing factors of plastic melts under shear stress is extremely important.
The flow and deformation of liquids are caused by stress. The stresses that plastic materials are subjected to during the forming process are mainly shear stress, tensile stress, and compressive stress. Of these three types of stress, shear stress is the most important for plastic material forming. In the extruder, the drag of the screw and the barrel causes the melt to flow; the melt flow in the die is generated under pressure. During extrusion molding, the pressure drop and the required power of plastic materials in plasticizing and mixing equipment, or in molding equipment such as extruder or die, are limited by shear stress.
Tensile stress is very important for stretch-oriented products and fiber spinning in plastics, and often occurs together with shear stress. The compressive stress is far less important than the first two and is generally negligible because plastic materials have less compressive deformation. Of course, compressive stress cannot be ignored in some cases. In extrusion molding, with the exception of extrusion-blow molding, the effect of gravity is usually ignored.
(I) Flow state of plastic melt
As is known to all, low-molecular liquids flow in straight ducts under shear stress in the form of laminar and turbulent flow.
Plastic melts are laminar in most cases, but turbulence also occurs in a few cases. The reason for this is the elastic effect of plastic melts during the flow process. Draw (II) Effect of Shear Stress
Figure 1-2 discusses the simplest ideal drag flow (ie, shear flow). In Figure 1-2, a certain amount of liquid is sealed in two parallel plates, one plate is fixed and the other is given.
A pull (or thrust) F of the plate makes it move along the x axis at a stable speed. The area of the parallel plates is A, and the distance between the parallel plates is h. Because the flow is stable, there is no unevenness
Therefore, each layer of liquid in the middle layer is subjected to the same shear stress.
That is, x = F / A. The shear stress generated by the layer of liquid under this layer's shear stress is the flow rate of the shear liquid, υ / h, called the shear rate. small
It can be seen from Figure 1-2 that during the shear flow, the direction of the velocity gradient and
The direction of flow is vertical.
During the molding process, the basic knowledge of the viscosity characteristics, causes and influencing factors of plastic melts under shear stress is extremely important.
The flow and deformation of liquids are caused by stress. The stresses that plastic materials are subjected to during the forming process are mainly shear stress, tensile stress, and compressive stress. Of these three types of stress, shear stress is the most important for plastic material forming. In the extruder, the drag of the screw and the barrel causes the melt to flow; the melt flow in the die is generated under pressure. During extrusion molding, the pressure drop and the required power of plastic materials in plasticizing and mixing equipment, or in molding equipment such as extruder or die, are limited by shear stress.
Tensile stress is very important for stretch-oriented products and fiber spinning in plastics, and often occurs together with shear stress. The compressive stress is far less important than the first two and is generally negligible because plastic materials have less compressive deformation. Of course, compressive stress cannot be ignored in some cases. In extrusion molding, with the exception of extrusion-blow molding, the effect of gravity is usually ignored.
(I) Flow state of plastic melt
As is known to all, low-molecular liquids flow in straight ducts under shear stress in the form of laminar and turbulent flow.
Plastic melts are laminar in most cases, but turbulence also occurs in a few cases. The reason for this is the elastic effect of plastic melts during the flow process. Draw (II) Effect of Shear Stress
Figure 1-2 discusses the simplest ideal drag flow (ie, shear flow). In Figure 1-2, a certain amount of liquid is sealed in two parallel plates, one plate is fixed and the other is given.
A pull (or thrust) F of the plate makes it move along the x axis at a stable speed. The area of the parallel plates is A, and the distance between the parallel plates is h. Because the flow is stable, there is no unevenness
Therefore, each layer of liquid in the middle layer is subjected to the same shear stress.
That is, x = F / A. The shear stress generated by the layer of liquid under this layer's shear stress is the flow rate of the shear liquid, υ / h, called the shear rate. small
It can be seen from Figure 1-2 that during the shear flow, the direction of the velocity gradient and
The direction of flow is vertical.
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