Extruders, extrusion pressure flow

Chiruvella, R. V., Jaluria, Y., Sernas, V., and Esseghir, M., Extrusion of Non-Newtonian Fluids in a Single-Screw Extruder with Pressure Back Flow, Polym. Eng. Set, 36, 358 (1996)... 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extruded through the capillary by means of a piston to which a constant force is applied. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

Capillary Viscometer. The end products from the liquid mixtures are usually obtained by extruding the liquid mass through narrow tubes or slits (e.g., spinning of fibers, injection molding, or film extrusion). Therefore, the pressure flow through a capillary is of technological interest. Hence, we analyzed the flow of a liquid mixture through a capillary with circular cross-section and compared the results of theory and measurement. 

It is also possible to determine the viscosity by extrusion methods. This is analogous to the determination of viscosity with fluids, except that the material is extruded under pressure. For a fluid, the slightest pressure P will produce motion, and the volume of flow will be proportional to the pressure applied. Thus for fluids, we have the simple form of Maxwell s equation... 

In addition to the torsion plastograph mentioned above, a number of other methods are used for appraising the properties of plastic bodies in practice. Plastic strength (rigidity) is determined by means of the Rebinder penetrometer which measures the depth of penetration of a cone forced into the body under constant load. The respective quantity is related to yield point. The commercial Brabender plastograph measures the resistance of mixture to kneading. The extrusion viscometer, from which the material is extruded under pressure, is analogous to flow-out viscometers. 

Sometimes, a space is left between the end of the screw(s) and the die plate. If rheological properties of the material are such that further densification takes place in this transition space, a denser extrudate is produced. In most cases, a large gap is used in high pressure screw extruders (see Section 8.4.3) where the necessary forces to obtain extrusion are solely developed by the screw(s) and high hydrostatic pressure is required to induce hydraulic flow through the extrusion channel(s). In low pressure axial extruders, extrusion blades are commonly attached to the end of the screw shaft (Fig. 8.27). In those cases, the gap is small and the plastic mass is compressed in the... 

The separate determination of two specific extrusion pressure components is particularly informative - the pressure loss caused by powerful forming processes of the ceramic compound at the feed end of the extruder die (so-called Bagley pressure ) and the pressure loss ( flow resistance ) inside the die. 

The swell of the extrudate as it leaves the extrusion die is an important phenomenon in polymer melt extrusion [14]. Die entrance angle plays an important role in the die pressure in the extrusion pressure. Die pressure is affected by flow rate of polymer melt, cross section of the die, die temperature, and material viscosity [15]. Controlling the die pressure can achieve a maximum production rate and an optimal application of extruder. Therefore, the entrance angle in die should be optimum to minimize the pressure drop [16]. Die entrance angle has no considerable effect on mechanical strength of the product, but the land length has some effects [17]. 

Because the working of self-wiping twin-screw extruders is mainly based on drag in an open channel, the equations derived for single-screw extrusion can be used. Combination of drag flow and pressure flow in a single chaimel leads to an equation for the throughput per channel ... 

The Velocity of Extrudate Exiting. It has been already pointed that the rate of extrusion is an output variable only in the case of the compressor version of hydrostatic extrusion. For the plunger processes, under the normal flow, the rate of extrusion is an independent quantity. If the working fluid is pumped to the chamber by a compressor, and in the chamber there is a control valve for releasing unnecessary liquid, then it is possible to maintain the extrusion pressure at a preset level. In this case, the velocity of product exiting depends on pressure and temperature. According to theoretical estimates, and to experiment (38), the extrusion rate exponentially increases with pressure. 

White, J. L. and Huang, D., Extrudate swell and extrusion pressure loss of polymer melts flowing through rectangular and trapezoidal dies, Pofym. Eng. ScL, 21, 1101-1107 (1981). 

The important process parameters include gas temperature and flow rate, extrudate temperature and flow rate, and pressure. While the applications for extrusion and hot gas welding are similar, extrusion welding can improve productivity when welding thick members because it can fill the even large V-grooves with one or two passes instead of applying multiple beads. 

The solidification process of the extruded material also influences the filament diameter. Materials that solidify quickly would not lead the molten material to spread over i.e. the dimension of the deposited filament remains same as the diameter of the filament as extruded. The appropriate selection or setting of process parameters is crucial for successful extrusion of polymer scaffolds. Many different factors contribute to successful deposition. The extruded material must flow consistently through the nozzle when pressure is applied to avoid inadequate dispensing. However, it is also essential that the pressure should not set too... 

When the pressure is increased much above 0.23 GPa, stick-slip extrusion occurs. At an extrusion pressure of 0.23 GPa, the extrusion stops when the viscosity increases sufficiently that the extrusion force cannot overcome the resistance to extrusion. During slip in stick-slip extrusion, a spiral fractured extrudate is produced that is similar to spiral products found in shear fracture of polymeric melts during extrusion (12). The flow profile in the solid state extruded rods is a deep shear parabola (10, 13, 24) (Figure 2) which suggests that stick-slip may arise from shear fracture of the extrudate. 

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