Coextrusion Flows

Tzoganakis, C. and Perdikoulias, J., Interfacial Instabilities in Coextrusion Flows of Low-Density Polyethylenes Experimental Studies, Polym. Eng. ScL, 40, 1056 (2000)... 

In [91], Chen studies the long wave asymptotics of the concentric Poiseuille flow of two upper-converted Maxwell fluids under axisymmetric perturbations. He concludes that stability can generally be achieved by increasing the volume of the more elastic component , in agreement with the lubrication effect. The corresponding short wave asymptotic study of the same flow is done in [92]. In the case of coextrusion flows a study for arbitrary wavelengths and low Reynolds numbers is done in [93]. 

K.P. Chen and Y. Zhang, Stability of the interface in coextrusion flow of two viscoelastic fluids through a pipe, J. Fluid Mech., 247 (1993) 489-502. 

Svabik, Samsonkova, and Perdikoulias [45] proposed another explanation for the interface distortion in coextrusion of fluids with equal viscosity. They performed three-dimensional flow analysis of coextrusion flow and found that even in coextrusion with Newtonian fluids with equal viscosity layer distortion takes place. Obviously, this type of distortion cannot be caused by normal stress differences since these do not occur in Newtonian fluids. Also, with the viscosities being equal the distortion cannot be caused by viscosity differences. The predicted layer distortion is schematically illustrated in Fig. 9.43. The authors call the distortion resulting from purely viscous flow geometrical encapsulation. 

APPLICABILITY OF ADVANCED CONSTITUTIVE EQUATIONS FOR COEXTRUSION FLOWS OF POLYOLEFIN MELTS... 

The fitting/predictive capabilities of three models (extended Pom-Pom, PTT-XPP and modified Leonov model) are tested for both, steady as well as transient shear and uniaxial extensional flows of mLLDPE and HDPE. The applicability of these constitutive equations has been investigated from the coextiusion flow modeling point of view. Finally, the FEM and modified Leonov model has been employed for the stress analysis in the coextrusion flow domain and predicted stress fields have been compared with the stress measurements fl om the flow coextrusion visualization cell. 

Figure 6 Comparison between measured (6a) and predicted (6b) principle stress difference for coextrusion flow at the merging area. Details about the experimental/theoretical approach in [14],...

The design of a die for an appiication is a very complex process. The design must be such that regions with long residence times do not exist, and it must provide uniform flow at the exit. Moreover, coextrusion dies must be designed with iayer uniformity and without process instabilities known as the wave instability and the zig-zag instability [i2, i3]. These instabilities are beyond the scope of this book and thus are not discussed further. Like transfer lines, the velocities in a die shouid be at least i.5 cm/s for thermally stable resins (with antioxidant systems) and at least 3 cm/s for resins that are not thermally stable. 

Viscoelastic flow effects in polymer coextrusion. In this example we will present work done by Dooley [7, 8] on the viscoelastic flow in multilayer polymer extrusion. Dooley performed extensive experimental work where he coextruded multilayer systems through various non-circular dies such as the teardrop channel presented in Fig. 9.39. For the specific example shown, 165 layers were coextruded through a feedblock to form a single multiple-layer structure inside the channel. 

J. Dooley. Viscoelastic Flow Effects in Multilayer Polymer Coextrusion. PhD thesis, TU Eindhoven, 2002. 

Fig. 12.30 Schematic showing the velocity profile in the three-layer coextrusion through a rectangular channel. [Reprinted by permission from C. D. Han, Multiphase Flow in Polymer Processing, Academic Press, New York, 1981.]...

We now turn to the treatment of the die lip coextrusion analysis of Mavridis and Shroff (65). Their analysis treats the flow of an arbitrary number of layers of melts that are viscoelastic, flowing in a one-dimensional nomsothermal flow, shown in Fig. 12.35 and... 

Coextrusion can be performed with flat, tubular, and different shaped dies. The simplest application is to nest mandrels and support them with spiders or supply the plastic through circular manifolds and/or multiple ports. Up to 8-layer spiral mandrel blown film dies have been built that require eight separate spiral flow passages with the attendant problem of structural rigidity, interlayer temperature control, gauge control, and cleaning. Many techniques are available for coextrusion, some of them patented and available under license (Chapter 5). 

Similar results can be obtained for Couette or Poiseuille flows of several fluids in parallel layers these flows are important in particular in the modelling of coextrusion experiments. Le Meur [50] has studied the existence, uniqueness and nonlinear stability with respect to one dimensional perturbations of such flows. The behaviour of each fluid is governed by an Oldroyd model such as (16)-(17), where the nondimensional numbers Re and We are defined locally in each fluid. On the rigid top or bottom walls, the velocity is given—zero on both walls for Poiseuille flow, and zero or one depending on the wall for Couette flow. The interface conditions on the given interfaces are... 

Nucrel [DuPont]. TM for an ethylene-meth-acrylic acid copolymer resin available for use in conventional extrusion coating, coextrusion coating, and extrusion laminating equipment designed to process polyethylene resins. Higher melt flow index results in fewer resin leaks. 

Fig. 8 Coextmsion feed block manifold and sheet die (A) (1) sheet die with flow restriction (2) adapter plate (3) feed block asembly (4) core material layer inlet (5) upper material layer inlet (6) lower material layer inlet and coextrusion multimanifold sheet die (B) (1) lower melt channel (2) upper melt channel (3) lower choker bar (4) lower choker bar adjustment bolt (5) upper choker bar (6) upper choker bar adjustment bolt (7) flex lip.

Coextrusion is a process variant in which two or more extruders feed a single die. A laminar flow of two or more... 

In the present extrusion of HDPE ribbons, the deformation patterns were examined by the deformation of parallel ink marks preimprinted on the surface of a HDPE (Figure 1-d). At EDR >12, the low and high MW HDPE exhibited a typical shear parabola and a W-shape deformation profile, respectively, with both characteristics enhanced at higher EDR as shown in Figure 2. These characteristics of the deformation patterns are in well agreement with our previous observations (3) and further confirm the previous conclusion that there is no significant effect of cutting a billet into two halves and/or coextrusion of a film with the split billet halves on the deformation flow patterns. 

---