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Plastics processing performance

by:CSSSLD     2021-01-23
Thermoplastics in the heating process, in addition to the usual improve the injection molding processing temperature is adopted to reduce fluid viscosity, also the change of the shear rate can be used to control the change of viscosity, in order to meet the process on the properties of melt flow requirements. As for thermosetting plastics, processing temperature control should be brought in a certain range, because continue to raise the temperature, followed by crosslinking reaction can make the system viscosity st cover, finally cause will not be able to process the molding.

under the effect of shear stress, fluid flow, called shear flow, shear flow transverse velocity gradient, also known as the shear velocity gradient, he is defined as the fluid flow in a direction perpendicular to the direction of fluid velocity on the rate of change. Because the speed is the direction of fluid flow movement distance and the corresponding mobile time ratio, thus derived by the velocity gradient shear rate this concept, namely within unit time, the shear stress under the action of shear strain produced fluid.

polymer melts are typical viscoelastic materials. Characteristics of viscoelastic material is, after the stop forces of flow stops, while elastic deformation part tries hard to recover. Most thermoplastics only at very low shear rate and high shear rate under will render Newton's behavior, and in the processing conditions are usually non-newtonian behavior, and in most cases is the melt viscosity decreased with increasing of shear rate.

habits often think of shear velocity gradient and the shear rate used as a concept, sometimes also called flow rate, the unit is the inverse of the time. Viscosity is a fluid internal attraction between the molecules and molecular specific response of irregular movement. Flow of liquid in a cross section perpendicular to the direction of flow on flow velocity is different, is actually caused by the viscosity of the liquid. By the viscosity size can determine the size of the fluid flow resistance. The better the liquidity, the smaller the viscosity of the liquid, and the greater the viscosity, liquidity, the worse.

shear flow is described in the viscosity shearing viscosity, on the value is equal to the shear stress imposed by the fluid flow and the ratio of the shear rate of. Different fluid, the viscosity changes have different laws, generally can be divided into two categories, one kind is Newtonian fluid, its viscosity change only dependent on the molecular structure of the material and at the time of the temperature, and has nothing to do with the size of the shear stress and shear rate, that is, to a certain temperature conditions, the viscosity value is constant, the relationship between shear stress and shear rate is proportional to, this kind of liquid viscosity Newtonian viscosity. Another kind is a Newtonian fluid, its characteristic is the viscosity changes in addition to related to the chemical nature of the liquid itself as well as the temperature changes, also changes along with the change of shear stress and shear rate, the viscosity of non-newtonian fluid become non-newtonian viscosity or apparent viscosity.

liquidity injection tooling characterization, in a certain temperature and pressure under the action of melt flow properties. In describing the melt flow properties, shear stress, shear rate and viscosity is very widely used term. Fluid flow in pressure, fluid flow, if the fluid velocity of each layer, the adjacent liquid layer contact surface tends to hinder the movement of the friction force. This appears within the fluid internal friction, usually expressed in force per unit area, called the shear stress on the mechanics.

is a high molecular weight polymer compound, its nature is different from ordinary low melt flow characteristics of the molecular material. The melt viscosity is bigger, is in commonly 50 - 10000Pa. S, in contrast, some particularly large polymer viscosity, such as ptfe, ultra-high molecular weight polyethylene, may cause considerable difficulties to processing, also often become the important reason for the application is restricted. Fluid flow in the process, in addition to the unrecoverable deformation, viscous part also contains a recoverable elastic deformation.

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