Extrusion pressure flow

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 extmded through the capillary by means of a piston to which a constant force is appHed. 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). 

Rotational flow and pressure flow rate calculations for the screw geometry and process conditions are performed for the injection-molding process in the same manner as for an extrusion process. Since the plasticator of an injection-molding process is not a continuous process, the instantaneous rate must be calculated based on the time that the screw is actually rotating. The instantaneous rate is the rate that is compared to the calculated flow rates for the screw. The instantaneous specific rate is calculated as follows ... 

Fig. 10. Pressure/flow-rate characteristics ( tentative flow curves ) of angular circular extrusion head (the design is given in Fig. 8) in molding of polypropylene with MFI = 0.5 g/10 min under conditions of core rotation at a speed of, min-1 1 — 0 2 — 10 3 — 20 4 - 30 5 — 40 6 — 50 7 - 60 8 - 70...

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. 

Pressure flow of two immiscible fluids with different viscosities that flow as separate layers between parallel plates are often encountered inside dies during co-extrusion when producing multi-layer films. Such a system is schematically depicted in Fig. 6.16, which presents two layers of thickness h/2 and viscosities m and //,2, respectively. 

Equations 6.3-19 is the well-known isothermal Newtonian extrusion theory. Since it was obtained by the solution of a linear differential equation, it is composed of two independent terms, the first representing the contribution of drag flow Q,j, and the second, the pressure flow, Qp. The net flow rate is the linear superposition of the two. 

In Eq. 6.6-16 the first term on the right-hand side is the drag flow and the second term is the pressure flow. The net flow rate is their linear superposition, as in the case of the Newtonian model in single screw extrusion. The reason that in this case this is valid for non-Newtonian flow as well is because the drag flow is simply plug flow. 

These rheometers are widely used to study the rheological behavior of molten polymers. As shown in Figure 3.35 the fluid is forced from a reservoir into and through a fine-bore tube, or capillary, by either mechanical or pneumatic means. The fluid is maintained at isothermal conditions by electrical temperature control methods. Either the extrusion pressure or volumetric flow rate can be controlled as the independent variable with the other being the measured dependent variable. 

The pressure gauge, a so-called flow pressure meter, came into existence in the 1950s, and was installed in the pressure head. This instrument continuously measures the extrusion, or flow, pressure, which is dependent on the moisture content of the body, and so initiated automatic moisture... 

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]. 

Melt polymers have low thermal conductivity. As the volumetric flow rate decreases, the pressure gradient increases and hence the back flow, that is, negative velocity zone becomes more pronounced. The flow in the down channel direction is a combination of drag and pressure flows during extrusion. 

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 ... 

Note that the pressure flow reduces the output when the pressure gradient is positive, which is often the case in actual extrusion operations. 

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. 

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