Flow characteristics of polymers during molding

In various molding processes of polymers, most of the processes require it to be in a viscous flow state, because in this state, the polymer is prone to flow and deformation under the action of external forces. Flow and deformation are the most basic process characteristics in polymer molding.

Polymer fluids come in two forms. One is that the solid is heated to the viscous flow temperature or above the melting point, and becomes a molten liquid, that is, a melt. Also known as dry plasticizing, it is characterized by the use of heating to turn polymer solids into melts, and only simple cooling during shaping. However, local overheating is easy to occur during plasticization. The other is to add a solvent to make the polymer reach a viscous fluid state, that is, a dispersion. Also known as wet plasticizing, it is characterized in that the plastic is fully softened with a solvent, and the solvent must be removed when setting, and the recovery of the solvent should be considered at the same time. The advantage is that the plasticization is uniform and the overheating of the polymer can be avoided. In the molding process, both polymer fluids have a wide range of applications.

The flow and deformation of polymers during processing are the result of external forces. When the polymer is formed, under the action of external force, a stress that is in balance with the external force will inevitably be generated inside it. There are three main stresses: shear stress, tensile stress and hydrostatic pressure. For example, the flow of the melt in the nozzle of the injection molding machine or the flow channel of the mold generates shear stress; the melt is stretched during extrusion blow molding to generate tensile stress. In the actual processing process, the force of the polymer is very complex, often These three stresses occur simultaneously, such as the flow of the melt under the condition that the cross-sectional area of ​​the runner changes when the material enters the die, gate and cavity in extrusion and injection molding, but the shear stress is the most important, because the polymerization The pressure drop of the fluid flowing in the molding process and the quality of the plastic parts are all restricted by it. The hydrostatic pressure is generated by the compression of the melt, and its influence on the fluid flow properties is relatively small, generally negligible, but has a certain influence on the viscosity, and the hydrostatic pressure is the main stress during compression molding.

Polymers have fluidity under certain temperature and pressure conditions, and there are two forms of fluid flow in a flat circular tube, laminar flow and turbulent flow, as shown in Figure 1-4. Figure 1-4(a) shows laminar flow, which is the sliding of adjacent thin layers of liquid along the direction of external force. The particles of the fluid move along many parallel flow layers. The same flow layer moves forward at the same speed. Although the velocity of each flow layer is not necessarily equal, there is no obvious mutual influence between the flow layers. Figure 1-4(b) shows turbulent flow, also known as "turbulent flow". Except for the forward motion of the fluid particles, the magnitude and direction of the velocity of each point change with time, and the streamline of the particles is in a disordered state. Laminar flow and turbulent flow are determined by the critical Reynolds number (Re). Generally, when Re is less than 2100~4000, it is laminar flow, and when it is greater than 4000, it is turbulent flow. During the molding process, the Reynolds number of polymer melt flow is often less than 10. , the Reynolds number of the polymer dispersion will not be greater than 2100, so its flow is basically laminar.

Laminar flow is regarded as the relative slip of adjacent thin layers of fluid along the direction of external force, and the flow layer is a completely parallel and straight plane.

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