Vivek Rohatg, Gregor Hiesgen, Mark J. Lamborn, Ashish M. Sukhadia, Douglas E. Simpson, David W. Borrego, Pamela L. Maeger
Papers # 2018 Las-Vegas
Maintaining dimensions within specifications is problematic for the extrusion of large diameter thick-wall polyethylene (PE) pipe (> 75 mm wall) due to sag caused by insufficient resin melt strength. However, sag can be controlled through careful resin selection and optimization of the pipe cooling process. This paper describes the use of a new bimodal PE 4710/PE 100 HDPE resin which has been used to produce pipes up to 2,000 mm diameter and 127 mm wall thickness. The focus of this study is the production of 610 mm DR 7.3 (83.5 mm minimum wall) pipe per ASTM F2619. The cooling process was simulated using the chillWARE® software. Pipes were produced with all dimensions within specifications. Predicted temperature and viscosity profiles through the wall thickness are in agreement with the observed low sag. Analysis of the residual stresses are in line with the observed low toe-in and good pipe quality, which passed the quality control tests for short term hydrostatic pressure and fusion bend-back testing. Distribution of the carbon black was measured to be uniform across the wall thickness and good carbon black dispersion was achieved when analyzed per ISO 18553.
The extrusion of large diameter thick-wall polyethylene (PE) pipe is particularly challenging due to resin slump, or sag. The tendency for thick-wall pipe to sag due to its own weight during post-extrusion cooling makes it difficult to control pipe dimensions within specifications. The work discussed here examines the extrusion of 610 mm DR 7.3 pipe manufactured from a new bimodal HDPE PE 4710/PE 100 resin with excellent sag resistance. Analysis of the cooling process was performed using a software program, utilizing the thermo-physical properties (density, heat capacity, thermal conductivity) of the HDPE resin as its inout. This simulation software features an engine based on the finitedifference and finite-element methods to simulate axially-symmetric extruded polymer products such as pipes. The current work investigates a post-extrusion cooling process involving internal cooling for large diameter pipe. Results provided by the simulation are estimates of the through-thickness temperature distribution along the pipe during the cooling process, residual stresses, and resistance to sag. Simulated temperature profile results are verified by comparison with process temperature measurements. Simulation results in combination with the resin’s temperature dependent low shear melt viscosity are used to determine a sag indicator. It is shown that the geometrical stability of the pipe is greatly improved through the use of the new low sag resin. The influence of the cooling process on predicted residual stresses and resistance to sag are determined. Analysis of pipe quality in terms of hydrostatic pressure testing and bend-back on fusion joints, carbon black distribution and dispersion, and density across the pipe’s wall thickness are also discussed.
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