Every Plastic thermoplastic object manufactured in the world will have gone through the melt stage during its processing. Normally solid plastic polymer pellets will be feed into an extruder or injection moulding machine, where the polymer is melted and then shape formed to create a fibre, film, tube or injection moulded article. Thermoplastic shape forming is a highly developed technology with a huge economic turnover; however a deep scientific and engineering understanding of how thermoplastics behave whilst being processed has only recently started to emerge. With the aid of sophisticated numerical modelling and the choice of a correct viscoelastic constitutive equation it is now possible for engineers to model the way thermoplastic can be processed. This in turn means that existing processes can be improved and new processes discovered not by just trial and error, but by engineering design.
The polymer melt rheology story is one that has been with me all my academic life. It has involved first observing, using flow birefrengence techniques, the processing flow behaviour of molten polymer. N.Checker, M.R. Mackley and D.W. Mead. On the flow of molten polymer into within and out of ducts Part 1 , Part 2 Phil. Trans. Roy. Soc. (Lond.). A308, 451-477 (1983).
Then, the polymer had to be characterised in the correct way. M.R.Mackley, R.T.J.Marshall, J.B.A.F.Smeulders and F.D.Zhao. The rheological characterization of polymeric and colloidal fluids. Chem Eng Sci. 49,16, 2551-2565 (1994).
Then systematic experiments were carried out using the Multipass Rheometer (MPR) that was specifically developed for the task of following polymer processing flow.
Below is a Powerpoint presentation given about the MPR in 2007 at the University of Braga, Portugal.
Then others developed numerical code that was able to model the processing behaviour of molten polymer. T.D.Lord, L. Scelsi, D.G. Hassell, M.R. Mackley, J. Embery, D Auhl and O.G.Harlen The matching of 3D Rolie-Poly viscoelastic numerical simulations with experimental polymer melt flow within a slit and a cross-slot geometry. Journal of Rheology 54, 2. 355-373 (2010)
The final part of the story is told in a seminar presentation given in 2009 at The University of Science abd Technology, Hong Kong.
There are two movie sequences in the presentation which appear on this page after the powerpoint presentation.
The MPR movie below shows the flow birefringence processing behaviour of molten polyethylene flowing within a slit. Each dark retardation band measures a certain level of stress within the polymer
The movie below shows the flow birefringence processing behaviour of molten polyethylene flowing within a Cross Slot geometry. Each dark retardation band measures a certain level of stress within the polymer.