Open-source 3D printed parts often stronger than proprietary versions

Open-source 3D printed parts often stronger than proprietary versions
Joshua Pearce, an associate professor of Materials Science & Engineering and Electrical & Computer Engineering and Michigan Technological University.
[www.3ders.org]

Pearce and his team conducted a study on mechanical properties of components fabricated with open-source 3D printers. The study used standard test methods, except for changing the environmental conditions to be equivalent to those used for home-printing rather than controlled lab settings.

Pearce says, "The results of the study are clear - 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. In many cases they were actually stronger than the results reported in the literature."

"This study only looked at the tensile strength in the plane of the print bed, next we need to expand this study to look at interlayer adhesion."

This seems more a test of the filaments than a test of the printers. I'd like to know which filaments were tested, but I'm not going to sign up with academia.edu to read the paper. I'm looking forward to their layer adhesion testing results, but expect similar.

At first, the result seemed like a no-brainer, but there are a couple interesting items:

The first interesting thing is that ABS has the reputation for being stronger, but PLA tested with both higher tensile strength and elastic modulus. My guess is the heat resistance of ABS makes is seem stronger when building printers and other things that get warm.

The second interesting thing is that if filaments corresponding to the vendor of each machine were used, then this seems to be a corroboration of the idea that filament from the closed source manufacturers is significantly more expensive, but not significantly better.

Link to the paper:

Mechanical Properties of Components Fabricated with Open-Source 3-D Printers Under Realistic Environmental Conditions
B. M. Tymrak, M. Kreiger, and J. M Pearce
[www.academia.edu]

One reason for higher values is most likely due to performing the tests at a higher strain rate than other studies.
Higher strain rates have been shown to result in higher tensile strength values compared to a lower strain rate in printed parts [20].

To gather a comprehensive data set covering a wide range of 3-D printers and their settings, a .STL file (as shown in Figure 1) of a tensile test specimen conforming to the ASTM D638 standard was created and distributed online for anyone to print and send to the researchers for testing

[14,15]. An extra, unattached cylinder was added to the .STL file to aid in proper printing, but was not a part of the specimen.


A complete set of 10 specimens of each of the combinations of variables shown in Table 1 was printed on a variety of open-source 3-D printers including an original Mendel RepRap, a Prusa Mendel RepRap, a Lulzbot Prusa RepRap, and a custom MOST RepRap.


For ABS, average values among both layer height and orientation showed no large discrepancies as all averages stayed with a reasonable range of the overall average. Specimens printed with a 0.2mm layer height had the greatest tensile strength, while specimens at 0.4mm layer height had the greatest elastic modulus. Between the 0º/90º and +45º/-45º orientations, +45º/-45º was the strongest, while 0º/90º had the greater elastic modulus.The PLA specimens showed greater variability between parameters. For layer heights, tensile strength averages varied by 11.9 MPa, or 22%, between 0.3mm and 0.2mm layer heights while elastic modulus varied by 194 MPa, or 6%, between 0.4mm and 0.2mm layer height. Differences between values based on orientation were much smaller. The tensile strength for the 0.3mm group was brought down by a set of 10 specimens with an average tensile strength of 35.4 MPa yet maintaining an average elastic modulus of 3342 MPa.

Quote
Dale Dunn
The first interesting thing is that ABS has the reputation for being stronger, but PLA tested with both higher tensile strength and elastic modulus. My guess is the heat resistance of ABS makes is seem stronger when building printers and other things that get warm.

Characterizing strength is tricky thing. In a complex part, the kind we love to print on 3D printers, there are typically all sorts of stress concentrations. For example, if you put a notch in a part that is in tension, the stress right at the notch can be maybe 3x higher than in the rest of the part. In a more brittle material like PLA, a crack will form, propogate, and the part will break altogether. In a more "plastic" material like ABS, the highly stressed material at the notch, before breaking, will deform more significantly, and the deformation will actually alter the stress flow so as to reduce the stress concentration. With the stress more evenly distributed across the part, it then requires more force to actually break it.

Thus, in a smooth part without stress concentrations, PLA will be stronger. But in a part with significant stress concentrations (typically sharp internal corners) ABS may very well be stronger.