Creepy Primo

As I’m sure most folks are aware, plastic parts that are under load will “creep” over time. It’s just a fact of life with most materials to some degree or another.

My Primo is now several years old and it’s clear that things have loosened up, in some cases, considerably. Bearings no longer contact rails, gantry wobbles, etc.

I’m going to re-print some parts and make a few more changes, but, this leads to a question.

I’m interested in opinions regarding which parts of the Primo are the most stressed and therefore may be most susceptible to creep.

My “engineering instinct” suggests that the most severe effect would likely be the clamps attached to the core, followed by the core itself, perhaps. The least likely might be the corner supports, although the belt tension adjusters have definitely loosened up.

The parts that are not so obvious to me are the trucks. They are clearly stressed, but to me at least, the degree to which this occurs is not obvious.

I know that some of you are mechanical engineers. My own background is Materials Science and Engineering but it was mostly electronic and optical materials, with less emphasis on mechanical properties, so my intuition in this area is not as developed as some of yours no doubt are.

I’m hoping that some of your insights may allow me to focus on reprinting the parts that are most susceptible to this problem of creep.

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+1 to your engineering background with a similar major and emphasis.

My primo trucks and core clamps all cracked causing excessive slope and ease to get out of square. Whether this was from heavy use last summer or when it sat unused the year before, it is hard to say, though I was not kind to it last summer. I’ve long thought it was only the use until I read your post and I wonder if its not being used in tension had some to do with it. Possibly it is creep or it might be due to deformation from thermal cycling with the mismatched CTE of the conduit metal the plastic is clamped around, so perhaps a fatigue mode with it. Winters get very cold around here and the parts were in an unheated garage. Regardless of the exact cause, we are in similar situations. I too would be interested in the answer to your question. There has to be a published study of the creep and thermal fatigue properties of PLA somewhere that might help with this.

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I have a burly build that is several years old. it sat in my workshop without a need to use it until recently. all my bearing appear to me making contact with the proper surfaces; both pen and paper drawings and cuts are reproducible. this machine has not seen heavy use, and I am guessing that’s why.

I wonder if printing with carbon fiber pla would help mitigate creep/stretch. It’s supposed to be more stable.