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Archive for January 10th, 2016

Sienna Hood Rod Pivot: Failure Analysis

Our Larval Engineer returned the remaining chunk of the failed PLA hood rod pivot from “her” Sienna minivan:

Sienna hood rod pivot - PLA fracture

Sienna hood rod pivot – PLA fracture

A closer look at the top surface (facing you in the picture above) shows the threads didn’t fuse into a solid mass across the entire object:

Sienna Hood Pivot - PLA fracture - top

Sienna Hood Pivot – PLA fracture – top

The darker region in the middle comes from the infill pattern, which should have air gaps.

The bottom surface (on the platform during printing) shows how the threads spread out when the nozzle is closer to the platform than the layer thickness:

Sienna Hood Pivot - PLA fracture - bottom right

Sienna Hood Pivot – PLA fracture – bottom right

That’s more pronounced on the other side of the pivot:

Sienna Hood Pivot - PLA fracture - bottom left 1

Sienna Hood Pivot – PLA fracture – bottom left 1

The infill looks like a separate wall inside the two perimeter threads. That’s pretty much what you get in the space between two close-set walls: there’s not enough room for the full infill pattern.

A slightly different focus plane shows the mashed bottom layer, infill sitting atop the bottom layer, and fused perimeter threads:

Sienna Hood Pivot - PLA fracture - bottom left 2

Sienna Hood Pivot – PLA fracture – bottom left 2

Because 3D printing doesn’t (and really can’t) produce a solid block of plastic, the object will fail much more readily than an injection-molded part. The threads in the most highly stressed section fail first, after which the remainder will just rip apart. In this case, the hood rod provides a huge lever that easily overstresses the plastic; I’m surprised the original part lasted as long as it did.

We all knew PLA wasn’t the right material for the job, right from the start, so we’ll see how the enlarged PETG version works in the field.

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