MACHINERY | PROFILE DIES
PolyXtrue was used to improve velocity distribution between the initial (left) and new (right) die designs
Thickness profile For the profile die, the three sides of the extruded profile had different thicknesses: the left side was 2mm thick, the top side in the middle 1mm, and the right side 1.5mm. In the land portion of the die, near the exit, the channel cross-section was the same as the profile at the exit. However, the length of the land for the three sides was different. Near the entrance, the channel gradually changes from a circle to the three-sided profile shape. Flow in the optimised die, with extrudate length
of 3m, was simulated – including shrinkage due to extrudate cooling. The flow was also simulated without shrinkage, by only including distortion due to non-uniform exit velocity. Because of the heat generated by viscous dissipation near the die walls, the temperature in a thin layer near the die walls rises to 224°C. (Temperature distribution inside the optimised die is shown on this page.) Beyond the die exit, the temperature began to
decrease as heat was lost to the cooling water around the profile. The predicted temperature distribution at 1m and 2m from the die exit are shown in Figure 11. Maximum temperature in each case is on the left side of the profile, which is thicker. After leaving the die at 224°C, the maxi- mum temperature falls to 34°C (at 1m) and 24°C (at 2m). When it is 3m from the die exit, it has reached 22°C – the temperature of the cooling medium. Because the exit velocity of the optimised die is quite uniform, distortion is smaller than that for the original die.
In conclusion, said Gupta, thermo-mechanical analysis allowed accurate prediction of the cooling shrinkage of extrudate after the polymer exited the die.
“Even though non-uniformity in the exit velocity 18 PIPE & PROFILE EXTRUSION | June 2018
has the maximum effect on extrudate distortion, cooling shrinkage further increases the distortion after the die exit,” he said.
Open access João Miguel Nóbrega, assistant professor at the University of Minho in Portugal, told delegates at the latest Profiles conference – organised by AMI – how his team has used open source computa- tional codes in the design of profile forming tools. He said that Open Foam (standing for Open
source Field Operation and Manipulation) has been used in conjunction with techniques such as multi-physics systems and several pre-compiled solvers – which handle everything from compress- ible and incompressible flow, combustion, heat transfer and viscoelasticity. “We’ve used these tools to establish and assess
experimental procedures to support the design of profile extrusion dies – as well as calibrator/cooling systems,” he said. The die design procedure progresses through a
series of simulation phases – including geometry generation (using Free CAD), creation of boundary sub-groups (with Salome), mesh generation (using Snappy Hex Mesh) and modelling (using Open Foam). Then, it is assessed for balanced flow distribution. If it is not balanced, either the bound- ary conditions or geometry are modified. In one example, the technique was used to assess the production of a polycarbonate swim- ming pool cover – including testing the effect of non-uniform temperature distribution. This split the die into different temperature zones, which were adjusted in order to ensure the correct shape of the final part. Future work in profile extrusion dies is expected
www.pipeandprofile.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56
