PLA distortion at the top of tall heavy prints

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rybrentd
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PLA distortion at the top of tall heavy prints

Post by rybrentd » Fri Oct 07, 2016 1:03 am

I have had the printer since March of 2016 and have been very successful in printing with multiple materials, especially with the help of these forums! :)
So I was printing with grey PLA using simplify 3D, .250 layer height, 4800 mm/min print speed (80mm/sec), 100% fan after 1st layer and noticed that as the print
got taller the distortion in the print got greater. I believe it is due to the printing speed and the effect of the weight of the object on the Y axis. The Y axis seems
to be the only axis that has "play" in it, i.e. you can grab the print bed and rotate it (VERY gently) as there is only one linear bearing. The weight of the print is
almost .5 kg and I am printing 2 at a time so ~1 kg total weight. Anyways, I have never had this distortion and was wondering if anyone else had issues with tall,
heavy objects at higher print speeds. I printed one at half the overall height and the same print speed with no distortion, but any input/advice is appreciated.
distortion1.jpg
I'm thinking I should:
(A) reduce print speed (my least favorite option as it is already a 36 hour print at 80 mm/sec)
(B) adjust print speeds slower as the object gets taller (not sure if this is possible in simplify 3D) so 80 mm/sec for the 1st 2 cm, 60 mm/sec for the next 2 cm, etc.
(C) find out what the maximum object weight is until distortion starts

I have attached the 2 .stl files as well for reference
Attachments
GL101214_Housing_2_Section_2.STL
(443.44 KiB) Downloaded 131 times
GL101214_Housing_2_Section_1.STL
(444.03 KiB) Downloaded 133 times

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ednisley
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Re: PLA distortion at the top of tall heavy prints

Post by ednisley » Sat Oct 08, 2016 2:07 pm

rybrentd wrote:you can grab the print bed and rotate it (VERY gently)
The complete Y+Z stage weighs nearly 7 pounds, so the plastic weight is pretty much irrelevant. In any event, the object will be pretty nearly centered on the plate, so there's not much torque applied around the Z axis to the slide bearings.
as the print got taller the distortion in the print got greater
There's the problem: that object is so tall that you're seeing what happens as the plastic shrinks during cooling.

In round numbers, PLA has a thermal coefficient of 10 ppm/°C, so a 175 mm tall object at 230 °C shrinks by:

Code: Select all

0.35 mm = (10x10^-6) x (230 - 30 °C) x 175 mm
Given that you're using a 0.25 mm (or whatever you're using) layer thickness, you can see the problem: a bit more than a layer has gone missing!

As the object grows, the initial 0.25 mm distance from the nozzle to the top layer gets progressively larger and the print quality gets progressively worse: the plastic threads must stretch along the Z-axis and get correspondingly narrower in the XY plane. As a result, although there's no sudden change (the top layer isn't really 0.35 mm below the nozzle), the plastic must extend over an increasing effective layer thickness.

Those parts are obviously designed as extrusions (with a bit of subtractive machining), which is an entirely different manufacturing process that produces uniform quality over the entire part length. If you're going for looks, rather than function, you could glue shorter sections together, perhaps with internal epoxy fittings (because PLA doesn't solvent-bond well with common solvents). The fins won't print in any other orientation, so you're pretty much stuck with a vertical orientation.

Bottom line: you're up against a limitation of fused-filament 3D printing.

[Edit: never attempt arithmetic early in the morning after a long drive. Fixed the numbers, added derivation in the hopes I got it right, doesn't change the conclusion.]

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rybrentd
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Re: PLA distortion at the top of tall heavy prints

Post by rybrentd » Sat Oct 08, 2016 6:28 pm

ednisley wrote:The complete Y+Z stage weighs nearly 7 pounds, so the plastic weight is pretty much irrelevant. In any event, the object will be pretty nearly centered on the plate, so there's not much torque applied around the Z axis to the slide bearings.
I didn't notice any play in the z-axis and it is built very solidly, but I was referring more to the y axis where the entire weight of the print is cantilevered off 1 linear slider. Even with no print you can rotate the print bed (out of level with the x-axis or nozzle) very easily. I don't know if this has any real life effect (especially at high print speeds) and I have never noticed an issue during prints, but saw it as a potential cause because as I haven't seen this issue with printing before.
ednisley wrote:As the object grows, the initial 0.25 mm distance from the nozzle to the top layer gets progressively larger and the print quality gets progressively worse: the plastic threads must stretch along the Z-axis and get correspondingly narrower in the XY plane. As a result, although there's no sudden change (the top layer isn't really 1.75 mm below the nozzle), the plastic must extend over an increasing effective layer thickness.
That makes sense and would explain it. I did notice the layers widths becoming thinner once the shrinkage distortion started. Perhaps PETG or a lower shrinkage material would be a better option? I'm not sure how PLA shrinkage measures up against other PET variants. It is a prototype to be made out of extruded aluminum, so you were right on the money on that one! I printed this in 4 sections and used gorilla clear epoxy to put it together, but I was tentative to do more sections as straightness was a huge parameter. I did try .3 mm layer height print (with a .35 mm nozzle) which was a mistake and it ended up worse than the .25 mm layer height print. After the fact I realized you don't want the layer height to be more than 80% of the nozzle width which I wasn't aware of before as I always print 50 µm to 200 µm layer heights. I appreciate the insight that was a thorough and well thought out response :D

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