Introduction: Stress Testing and Delamination: PLA Failure Experiment

About: Based in San Francisco I strive to incorporate custom lettering in the art that I design, print, and make.
This instructable reviews the stress testing after 2 weeks of use of a ~60% 3D printed over the door hook. I stopped the print early because of time constraints on a public 3D Printer. The full print would have taken almost 90 minutes which is a bit too long for testing. Given that the model was not completely printed this is not a review of how PLA does not work well under stress, but rather a review of how well PLA holds up even under daily stress when improperly printed. 

The stress testing was done by placing the model over a door frame (the model was a bit big by 2 cm) and using it for hanging lightweight objects likes travel towels. 

When stress testing consider:

1) what is the breaking point
2) what is the wear per 24 hour period
3) can you improve the model for better stress testing results
4) what is the failure point when it no longer works

I made it at Techshop on a Type A Machines 3D Printer
Submitted by SFlettering for the Instructables Sponsorship Program

Step 1: Model for Stress Testing: 50% Print of Over the Door 2 Hook Model

Here you can see one side of the hooks on the side that was left unfinished when the print was stopped early due to time constraints. Notice the edge is unfinished and has a rough appearance. 

The second image shows the side that was printed as finished with correct edging on the bottom side of the print.

Note: This model was printed in draft mode.

Step 2: Bending and Loosing Hold Power

Here is the view after stress testing this model for about 2 weeks. The failure point after 2 weeks of casual use was once the model could no longer support weight and remain hanging over the door. Notice the 2 areas where the PLA started to bend and the top area where it began to pull apart.  The back area had some broken strands and was the most "stressed" from use.

TIP: If you are clever maybe you can figure out how to reheat the PLA and "fix the model"

Step 3: Delamination and Splitting: What Happens When PLA Printed Models Start to Fail

Here is a closeup view of how the model began to split and the groups of strands actually began to break apart. The splitting was caused by severe stress and it is interesting to note that groups of ~4 strands of PLA split apart from each other on the back side of the hook. 

Note: The side also began to pull apart from the printed middle section. 

TIP: When testing your models consider using multiple print quality levels. In this example only one draft quality model was used for stress testing due to the 90 minute print time for draft quality.

Step 4: Original Print Issues: Coming Up From Print Surface

When I first began to print this model it failed on the print bed due to a Z-level that left the print surface a bit too far away from the nozzle. Usually a space between the print surface and the nozzle that is 1-2 layers of paper thick is best. In this example the model began to pull away and I let it continue to print until the nozzle pushed the printed layers out of position and I stopped the print early. 

TIP: Learning from your mistakes will make you better at learning.

Step 5: How to Improve Model Design to Mitigate Stresses

Here is the model after stopping the print midway due to time constraints. In the image you can see how the model when printed with the correct z-height is solid and connected to the print surface area completely on a good quality print. You can also see in this image how the hooks were meant to curve and are not just flat. 

The model design was well done and this stress testing experience taught me many things to consider when printing a final 3D print such as

1) when to stop a print
2) how to budget for total print time
3) what happens when PLA is stressed
4) design possibilities for areas under stress

To mitigate the amount of stress I would change the model design to use a specific width to match each door frame's width that is set  using thingiverse customizer app. A second design option would be to scale by x in KiSSlicer to get the door hanger to the right approximate size when matching the door width to the printed model.
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