Extrudate velocity

For different polymers the results can be more readily appredated by examining the change in pressure with extrusion ratio R for a constant extrudate velocity. Results for different polyethylenes are shown in Fig. 17, where the rapid upturn occurs at comparatively low extrusion ratios. For different polymers results are shown in Fig. 18, together with the best analytical Gts based on modifled Hoffman-Sachs analysis, which incorporates the strain, strain rate and pressure dependent flow stress according to Eq. (4) and the Avitzur strain rate field of Eq. (5). Figure 17... 

Fig. ZO. Extrusion pressure v. extrudate velocity for polyethylene - large scale R40 polyethylene extrusion (Tj, = 90 °C, product diameter 15.5 mm)...

The value of tq can be determined by plotting the extrusion pressure against the extrudate velocity V for extrusion through dies of constant value of L. The extrapolated value of extrusion pressure at E = 0 gives the value Pqvo at zero velocity, as shown by Eq. (15) ... 

Fig. 3. Variation of extrusion pressure with extrudate velocity. Extrusion temperature M °C, nominal draw ratio IS, die diameter 15.5 mm, sample R (for sample description see Table 2). R mes at lower and higher velocities are isothermal and adiabatic, respectively ...

The advantages of these instruments are that for feed zone properties they can operate at typical extruder velocities and normal loads. A further advantage is that the surface can be rubbed in," i.c., be contaminated by polymer as has been found to be the real situation [9]. 

Fig,9. Extrusion pressure vs extrudate velocity for LPE - large scale R40 extrusion (Tjg = 90°C product diameter 15.5mm). Reproduced from Plastics Rubber Proc. Applns., 2, 215 (1982) by permission of the publishers. Plastics and Rubber Institute (C). 

More comprehensive equations for stiff paste rheology have been developed from Griffiths original concepts [4,9,10]. The pressure required to extrude a paste body from a barrel of diameter Dq, through a die of cross-sectional diameter and length L at an extrudate velocity can be considered in two parts the pressure in convergent flow at the die entry (Pi) and the pressure required to push the paste along the die land in parallel flow P2). From plasticity theory of metals it can be shown that... 

Extrudate swell, commonly known as die swell, shown in Eig. 12.52, is not always visible at the die exit because the extrudate is pulled away from the extruder, causing draw down or neck down. If the extrudate is allowed to droll on the floor or is pulled from the extruder very slowly, die swell becomes very obvious. Polymer molecules in the die land area are oriented in the direction of flow. The extrudate velocity profile is higher at the center of the flow front and lower near the die walls. Immediately after exiting the die, the extrudate velocity profile is identical across the enter cross section. Consequently, the velocity at the surface of the extrudate outside the die... 

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