Strength of printed ABS plasic parts 1

[PrintScaffold]

Hello everyone!

How to calculate strength of part printed on a regular 3D printer from the ABS plactic?
Is it calculated the same way as a steel part, using tensile strength? Does yield strenth have meaning in case of such parts?
Or is the technique different?

http://www.cadroad.com

 

[ctopher]

As far as I know, the same.
I did a quick Google search, came up with this: Link

Chris, CSWA
SolidWorks '15
ctophers home
SolidWorks Legion

 

[silentbobo941]

You could try using the regular strength formulas but compensate the values with the % infill of the part. For example, if you had a diameter of 1" with a 75% infill you would take the diameter in the calcs to be 0.75". Might be a good start.

 

[MacGyverS2000]

The same... assuming your layers have good attachment to each other. If your machine needs tweaking on the temp profile, you can very easily split at a ring.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[jhardy1]

Just remember that 3D printed plastic parts are VERY orthotropic, depending on many factors such as the precision of the machine, the print settings (% fill and pattern of fill), orientation of the printed part during building, etc.

The layer-to-layer strength and stiffness (especially in vertical tension) is likely to be limited by inter-layer adhesion, while the in-plane (horizontal) strength / stiffness of the perimeter shells can approach the properties of the virgin stock filament.

If your 3D printer is well set up, you can get very good layer-to-layer adhesion generally, but it only needs one area where the adhesion didn't quite "take" (stepper-motor "skipping", a bit of dust or dirt, or the lower layer cooling too quickly before placing the next layer, for example), and you can get a local horizontal crack in your part which can act as a stress raiser.

In general, solid parts are printed using a relatively thin perimeter "shell" and a honeycomb fill or similar, rather than solid fill. This saves weight and material, is faster to print, and can also result in better quality prints (less blobbing / warping / melting as the part can cool down better), but the strength of a honeycomb is less than the theoretical strength of a 100% fill. The pattern of fill you use (rectilinear, hexagonal, etc) will also affect the bulk strength / stiffness.

http://julianh72.blogspot.com

 

[jhardy1]

@silentbobo941

You could try using the regular strength formulas but compensate the values with the % infill of the part. For example, if you had a diameter of 1" with a 75% infill you would take the diameter in the calcs to be 0.75". Might be a good start.

Rather than using 75% of the diameter to estimate the properties of a rod printed with honeycomb fill, it would be more realistic to calculate the stiffness based on a hollow tube with the full external dimensions, as the moment of inertia (which matters for bending calculations) varies with the 4th power of the diameter, and it is the outer shell which provides most of the overall stiffness.

E.g. imagine a 10 mm rod that is printed using a 2 mm thick perimeter shell and a 25% honeycomb core. I would assume something like a 10 mm hollow tube with 2 mm wall, plus 25% of the 6 mm diameter core:

For a 10 mm solid rod: Ixx = pi * 10^4 / 64 = 491 mm^4

For the 3D printed rod, I would suggest effective Ixx ~ pi * (10^4 - 6^4) / 64 + 25% * (pi * 6^4 / 64) = 443 mm^4, which is 90% of the bending stiffness of the solid rod. You might need to reduce this somewhat to allow for the build quality of the 3D printer - eg assume an outer shell of 1.0 to 1.5 mm thickness to allow for build imperfections.

Even if we ignore the infill altogether, effective Ixx of the hollow tube ~ pi * (10^4 - 6^4) / 64 = 427 mm^4, which is still 87% of the bending stiffness of a solid rod.

If you use 75% of the diameter, you get effective Ixx ~ pi * (7.5^4) / 64 = 155 mm^4, which is only 32% of the bending stiffness of a solid rod. This might be conservative, but it may be difficult to calibrate against actual performance when you have parts with varying dimensions and fill proportions.

http://julianh72.blogspot.com

 

[PrintScaffold]

Thanks for the interesting and useful responses!
Has anyone calculated and tested the strength of such parts in practice?
Any safe estimates of strength based on experience?

http://www.cadroad.com

 

[jhardy1]

Yes, lots of people have done it, but you NEED to test the parts made on the specific hardware and firmware, with the specific filament stock, and with the specific slicing arrangement intended (layer thickness, fill pattern and density, etc), as the results are HUGELY variable!

E.g. take a look at

https://www.lulzbot.com/blog/measuring-3d-printed-material-strength

In a recent study, Pearce and his team examined the basic and tensile strength and elastic modulus of printed materials in acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) using a range of open source hardware. They found average tensile strengths of 28.5 MPa (megapascals) for ABS and 56.6 MPa for PLA, with average elastic moduli of 1807 MPa for ABS and 3368 MPa for PLA.

The study concludes, "It is clear from these results that parts printed from tuned, low-cost, open-source RepRap 3-D printers can be considered as mechanically functional in tensile applications as those from commercial vendors."
[My emphasis]
Copy of full paper here:

https://www.academia.edu/6209168/Me...ters_under_realistic_environmental_conditions

- the scatter of the data in Figures 2 and 3 is VERY illuminating!

Typical values for "virgin" ABS stock are a tensile strength of ~ 44 MPa and an elastic modulus of around 2.3 GPa (eg Ref:

http://teststandard.com/data_sheets/ABS_Data_sheet.pdf

), and for PLA the corresponding values are around 50 MPa and 3.5 GPa respectively (eg Ref:

http://www.makeitfrom.com/material-properties/Polylactic-Acid-PLA-Polylactide/

)
(Unfortunately, the referenced paper by Pearce et al does not seem to give the mechanical properties for their stock filament.)

This suggests that with a well-calibrated machine under ideal (laboratory) conditions, you can get something like 75% of the "virgin" stock tensile strength, and up to 90% of the elastic modulus (stiffness), but actual results can be very variable (as the paper demonstrates), and can be MUCH lower in "real-world" conditions.

Note that most hobby 3D printers have a certain proportion of "failed" prints, which are obviously defective and are discarded, but an indeterminate number of "possibly defective" prints, where the part looks OK, but may actually be marginal on inter-layer adhesion etc. In a test situation, these parts might actually achieve very little test strength / stiffness, but might be perfectly acceptable for non-critical components, depending on the design and how they are loaded in service. (Part of the "art" of design for 3D printing is understanding the mechanical properties of the printed parts, and setting them up to minimise the risk of failure - e.g. don't apply tension across layers, avoid "bridges" and the like which can create regions of lower strength material, etc.)

http://julianh72.blogspot.com

 

[EHudson]

Some claim higher layer to layer strength is gained by bathing the ABS in acetone.

 

[MacGyverS2000]

Some claim higher layer to layer strength is gained by bathing the ABS in acetone.

Layer-to-layer strength is based upon adhesion between the layers, which means controlled temp of both layers as they are laid down. An acetone wash after the fact will do nothing to promote between-layer adhesion.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[TSnowden]

Yes and no. On a part of any significant size, it will not really increase inter-layer adhesion. HOWEVER ... the common perception has some truth to it, in that a good acetone wash will merge the outer 'rim' of the layers together into a more or less solid shell - increasing the effect jhardy mentioned. It also seems (empirically) that under use of most 3D printed ABS parts, the layers start separating from visible areas, and then propagate inwards, making the acetone wash an effective way of printing separation.
That said, it doesn't 'merge' the layers as a whole, so as far as calculation is concerned would make it even more complicated...