machinery feature | Profile dies


layer, there was a large difference in the relative velocities (ABS is more viscous, so extrudes more slowly). This led to the ‘C’ section (PS) being thicker. When switching the materials


around (using the PS as the substrate), the


velocity profile was very different. The ‘J’ section is now thicker, and the bending of the final profile is now in the opposite direction. “For a multi-layer profile, balancing the


velocity distribution at the die exit can be difficult if viscosities of co-extruded polymers are very different,” said Gupta. “Post-die distortion in the extrudate shape was found to be very different for different polymer combina- tions.”


Gupta modelled the final shape of the extrudate (in red) against the actual shape of the die exit (in blue)


Reworking dies


Researchers at Aachen University in Germany have used visualisation and simulation to improve the way in which profile dies can be re-worked for individual products. Producing lots of individualised products, in small lot sizes, is a major challenge for profile manufacturers. “The conventional design approach in the profile


Salamander can extrude five-chamber


window profiles at speeds of 6m/min


extrusion industry is based on the empirical knowledge of die manufacturers,” say the researchers. “They iteratively rework the flow channel (up to 15 times) based on the flow through the die as it is determined in running-in experiments. For small lot sizes, these itera- tions are the major cost driver.” A method developed within the Cluster of Excellence by the chair for Computational Analysis of Technical Systems (CATS) and the Institute of Plastics Processing (IKV) is done in two stages: first, reworking iterations are transferred from the running-in experiments to the design stage of the die; and optimisation techniques are used in order to automate the design process. Together, this leads to a potential for significant cost reductions. The passage of the plastics through the flow channel of the die is simulated using highly parallel Computa- tional Fluid Dynamics (CFD) software called XNS, which was developed by CATS. XNS has also been included in an optimisation framework, offering a variety of optimisation algorithms. From a given starting geometry, the flow channel is


automatically modified until it matches the predefined optimisation goals. The chosen design objective for the extrusion die is a homogeneous velocity distribution at the die’s outflow. The resulting simulation data can be used to explore


the results in an interactive, immersive manner in a virtual environment. An application was developed at


16 PIPE & PROFILE EXTRUSION | May/June 2012 www.pipeandprofile.com


the Virtual Reality Group based on the Vista software framework and its FlowLib add-on. Visualising ‘streamlines’, in the form of tube-like


‘particles’, helps to visualise the homogeneity of the flow field. At the same time, the cross flow is visualised by an interactive plane that slices the cross flow field orthogonally. Combined with the 3D visualisation of streamlines, this is a useful way of identifying the regions of the extrusion die that are responsible for inhomogeneous outflow velocities. This information is then used to construct the


geometry parameterisation for a particular extrusion die, say the researchers.


Faster production Co-extrusion tooling from Greiner Extrusion has helped a window profile manufacturer to speed up production. Greiner has built a 20m long co-extrusion tooling


unit for Salamander Industrie-Produkte. The system allows Salamander to extrude plastic window profiles at 6m/min. The profiles are 76mm deep, with five cham- bers. The process achieves both high precision and high speed, by means of the rotated tooling structure – which also secures straight inner bridge geometry. Jürgen Diemer, production manager at Salamander,


said: “We have significantly reduced scoring on the profile surfaces.” Conventional tooling units of comparable dimensions


have a vacuum energy consumption of around 31.5kW/h. At Salamander this has been cut to 6.2kW/h – an energy saving of over 80% – thanks to a patented Greiner development that seals the slits between the seven dry calibrators with ‘closed vacuum rings’.


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