Polymer Process Partitioning: Extruding Plastic Pipe

When plastic pipe is solidified, it proceeds through a long cooling chamber. Inside this chamber, inside the hollow extrudate, the plastic is molten, and this inner surface solidifies last. Sag, the flow due to the self-weight of the molten plastic, then happens in this cooling chamber, and sometimes, thickened regions (called knuckles) arise in the lower quadrants, especially of large diameter thickwalled pipes. To compensate for sag, engineers normally shift the die centerpiece downward. This thesis focuses on the consequences of this decentering. Specifically, when the molten polymer is viscoelastic, as is normally the case, a downward lateral force is exerted on the mandrel. Die eccentricity also affects the downstream axial force on the mandrel. These forces govern how rigidly the mandrel must be attached (normally, on a spider die). We attack this flow problem in eccentric cylindrical coordinates, using the Oldroyd 8-constant constitutive model framework. Specifically, we revise the method of Jones (1964), called polymer process partitioning. We estimate both axial and lateral forces. We develop a corresponding map to help plastics engineers predict the extrudate shape, including extrudate knuckles. From the mass balance over the postdie region, we then predict the shape of the extrudate entering the cooling chamber. We further include expressions for the stresses in the extruded polymer melt. We include detailed dimensional worked examples to show process engineers how to use our results to design pipe dies, and especially to suppress extrudate knuckling.

Provenance of Holocene Calcareous Aeolianites, Eastern South Australia

The southeastern coast of South Australia contains a spectacular and world-renown suite of Quaternary calcareous aeolianites. This study is focused on the provenance of components in the Holocene sector of this carbonate breach-dune succession. Research was carried out along seven transects from ~30 meters water depth offshore across the beach and into the dunes. Offshore sediments were acquired via grab sampling and SCUBA. Results indicate that dunes of the southern Lacepede and Otway coasts in particular are mostly composed of modern invertebrate and calcareous algal allochems. The most numerous grains are from molluscs, benthic foraminifera, coralline algae, echinoids, and bryozoans. These particles originate in carbonate factories such as macroalgal forests, rocky reefs, seagrass meadows, and low-relief seafloor rockgrounds. The incorporation of carbonate skeletons into coastal dunes, however, depends on a combination of; 1) the infauna within intertidal and nearshore environments, 2) the physical characteristics of different allochems and their ability to withstand fragmentation and abrasion, 3) the wave and swell climate, and 4) the nature of aeolian transport. Most aeolian dune sediment is derived from nearshore and intertidal carbonate factories. This is particularly well illustrated by the abundance of robust infaunal bivalves that inhabit the nearshore sands and virtual absence of bryozoans that are common as sediment particles in water depths > 10mwd. Thus, the calcareous aeolianites in this cool-water carbonate region are not a reflection of the offshore marine shelf factories, but more a product of shallow nearshore-intertidal biomes.