By M.J. Stevens, J.A. Covas
This e-book is meant to fill a niche among the theoretical experiences and the sensible event of the processor within the extrusion of thermoplastic polymers. the previous have supplied a foundation for numerical layout of extruders and their parts, yet as a rule supply scant consciousness to the sensible functionality, specifically to the clash among construction fee and product caliber. In perform extruders are usually bought to accomplish a number tasks; on the other hand, the operator can have to take advantage of a computing device designed for an additional goal and never inevitably compatible for the polymer, technique or product in hand. The operator's event permits him to make stable product in unpromising conditions, yet lots of variables and interactions usually supply it appears contradictory effects. The wish is this booklet will offer a logical history, according to either concept and adventure, with the intention to support the commercial processor to acquire the easiest functionality from his gear, to acknowledge its boundaries, and to stand new issues of self assurance. arithmetic is used in simple terms to the level that it clarifies results which can't simply be expressed in phrases; ifit is omitted, not less than a qualitative realizing should still stay. The approximate idea won't fulfill the purist, yet this turns out to the authors less significant than a transparent illustration of the actual mechanisms on which rather a lot of the polymer processing relies. M. J. STEVENS J. A.
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Considered uniform across a normal cross-section, and will fall in the direction offlow. Using the subscripts y, rand R for values at the corresponding radii, the shear stress and shear rate at radiusy are Ty and 1'y = dVyjdy, respectively. If the pressure falls from P + dP to P over a length dL, then equating forces on a cylinder radius y and length dL (Fig. 13) Note that: • shear stress is proportional to pressure gradient; • shear stress is proportional to radius, since dPjdL is independent of radius; • shear stress is independent of fluid properties; • shear stress at the wall TR = (Rj2) .
6 X 10- 6 x om x 10- 8 m 3 The energy dissipated in passing die is Q. 5 X 106 =65W Then 65 . e. as if the strain energy at the wall persisted over the whole capillary-see Fig. 9. 97°C To compare these shear strain energies with that in e1ongational flow, consider a die entrance, reducing in diameter from 50 mm to 5 mm in 20 mm length - a sharp taper in order to exaggerate the e1ongational component. Evidently, due to the lower average shear rate, the shear strain energy in the taper entry will be less than in the capillary.
G. by increasing die length, the greater the dissipation and the less the die swell (Fig. 10). For a circular converging channel, the extensional strain at constant volume V (Fig. 2) is: e=L+dL_ I L =~. 73) Note that a constant strain rate and constant extensional stress require a parabolic rather than linear convergence, and the former is preferably approximated in practice by a series of decreasing taper angles, as explained in Chapter 5. 2» and F3 = 1 - (Rd Ro)3. He also states that this extensional pressure drop is small for small taper angles.