machinery feature | Flat dies
Plastic Flow’s optimisation software improved the design of a sheet die
machined, which can reduce development time by 40-50% according to Mahesh Gupta, president of Plastic Flow, and a professor of mechanical engineering at Michigan Technological University. “However, virtual fine tuning of extrusion dies still
requires designers to modify the die geometry them- selves after each flow simulation, using their past experience,” he said, in a presentation at Antec last year. He says that the new software eliminates the need
for this ‘virtual trial and error’. After each flow simula- tion, the geometry is improved, and the simulation is run again. This cycle of geometry improvement, followed by a flow simulation, is repeated until a design with a uniform exit velocity distribution is obtained process. As well as eliminating the need for designer interven-
tion for die improvement after each flow simulation, the new software is expected to further reduce the develop- ment time for extrusion dies, he says. Before an extrusion die can be optimised, its geometry is parameterised. That is, various dimensions of the die are defined as adjustable parameters which can be changed to modify the die geometry. This is done in SolidWorks, because for complex profile dies the geometry cannot easily be broken in simple elements such as bricks and prisms. The parameterised die geometry created within
SolidWorks is then passed through the extrusion die simulation software – Plastic Flow’s PolyXtrue – for flow simulation. PolyXtrue is available as an add-on to SolidWorks.
The sheet die analysed had a coat-hanger type of
manifold, defined by 19 specific die geometry param- eters. Of these, die width, manifold centre depth, inlet length, inlet width, and land gap are fixed. The three angles (manifold backwall angle, manifold angle and secondary manifold angle) – along with preland side length and manifold side flat length – were not changed during optimisation as they had only a small effect on the velocity distribution at the die exit.
16 FILM & SHEET EXTRUSION | April 2017
Keeping the value of secondary manifold side length the same as that of the secondary manifold centre length, the remaining eight geometry parameters were optimised using the new software. The sheet die was optimised for flow entering at 684kg/hr, and the polymer temperature at die entrance as well as the die wall temperature was 230 °C. It was evident from calculations that exit velocity near the middle of the die was much larger than that near the two ends. If this initial die geometry had been used to extrude the sheet, the thickness of the sheet near the middle would have been a lot larger than the thickness near the ends. The predicted pressure drop across the die was 12.6MPa. “This software is expected to reduce the develop- ment time for extrusion dies by more than 50%,” said Gupta.
Viscoelastic effects At the same event, Hyunwoo Kim of Dow Chemical presented details of a study showing the effect of viscoelasticity on flow uniformity in a film die. Flow simulations were conducted on flat die geometries based on the rheology of different resins with different viscoelasticity characteristics. The results showed that viscoelasticity can have a
significant influence on flow uniformity – and that flow distribution becomes less uniform as the elasticity of the resins increases. In the study, viscoelastic simulations were compared using the Polyflow CFD code to evaluate the elastic effect on the flow distribution in the existing flat die designs. Resins with different amounts of elasticity were chosen for the viscoelastic modelling and the resulting flow rate distribution from the same die geometry was compared to determine the effect of viscoelasticity on flow uniformity. The study found that, at similar flow conditions,
more elastic resins such as PS or LDPE showed less uniform flow distribution than more viscous resin like polycarbonate. With an increase in elasticity of the same resin (with the viscous rheology kept the same), the flow distribution became less uniform. “The results emphasise the importance of capturing
the viscoelastic nature of polymer melts for flow simulation, and optimising the die design to create a more uniform flow from the flat die geometries,” said Kim.
Click on the links for more information: ❙
www.nordson.com ❙
www.cloeren.com ❙
www.plasticflow.com ❙
www.dow.com
www.filmandsheet.com
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