Digital Wanderings

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3DP Strength Test

Putting to the test a bag holder to see how much weight it can take, the degenerescence / failure mode and ability of FEA to detect the issues.

The piece :

Printed at 250°C with a colorfull but problematic ABS filament. The filament is bubbling when extruded, probably due to moisture that tries to find its way out when heated by the hotend. Or maybe it’s an intended effect by the filament maker, so that it gives an rugous finish and hide the filament stripes (works quite well, dont know if it was intended, but hey it gives a nice finish, so, let’s say it was intended 🙂 ).

Walls thicknesses are ~0.9mm and grid type infill of 20% (cura settings. gives a grid spacing is roughly the thickness of the piece, so, it’s not much); Layer height is 0.3 (for a 0.35 nozzle). Printed as displayed so that the plane of filament deposition is aligned with the efforts.

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A first test was carried up to 10Kg of load. At which point buckling appeared on the outer walls. That was expected, as the material is not homogeneous. Still holding well though, but now all the stresses are on the top and bottom walls.

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Other reloading unloading and light twisting has been carried just to see if this degraded state can hold. It does, not too much, but it holds a little. No pics. Sorry.

Then a second test has been done. Full failure happened at 21Kg of load. There’s room for improvement with better capacity of taking shear stresses on corners.

Pictures of the second test. Note the delamination on the wall, revealing a weak bonding between layers. That’s probably aggraved by the bubbling that was occuring in the filament.

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Zoom on the broken part

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Finite Elements Analysis 1

Load : 100kN load (10Kg, 1st test), off center (induces a couple)

The model represents well the failure areas, but looks unrealistic regarding stress levels, there might be something wrong with the model (50MPa is a bit too much, some research studies suggest that printed ABS will take around 20 MPa before failing). Todo: redo it with a centered load.

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FEA #2 – simulating the degraded mode, (the section has ruptured, only 2 flanges are taking the stresses)

For a load of 10Kg (centered), we endup with 8MPa stresses. Model is linear, hence, means that failure happens in tension, at about 16MPa (20 Kg load). That’s consistent with the studies on printed ABS strength.

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Dimensions

Overall dimensions (approx): 100mm long, 50mm wide, 10mm thick

Section dimensions : 10mm x 8mm ; walls ~0.9mm thick

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Part Slicing

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Structural

ramkam • August 2, 2014


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