Here is a publication that disconfirms your theory, saying that low layer thickness leads to high dimensional accuracy:
If you scale a calibration cube to 150mm long x 20mm wide x 15mm tall and print it with PETG with a skirt, it's very likely to pick up on the ends. It's a near certainty with cooling running at 100%.
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An inverted prism reliably causes severe warping, in all aspects: warping due to insuficient bonding to the glass, and warping due to steep overhangs. This shape has a very small bonding area to the glass, and huge overhangs that do cause huge shrinking and warping forces.
This is a very hard test: due to warping of the overhangs, they do curl up, and then the nozzle hits the curled-up edges very hard, knocking the part loose. I used that as bonding test in the beginning.
To prevent the prism from falling over, I used a cross of two prisms, instead of a single prism, and I added a bigger patch in the center.
Different bonding methods, printing-speeds, layer-heights, temps, materials and colors all have an effect on the warping, curling and bonding.
Pictures:
This one sticks pretty well to the bed, without too much warping (just a little bit at the left):
The basic concept: two inverted prisms, plus a central patch:
Testing the effect of custom skirts on warping and bonding. The idea was to minimise cooling of the glass due to fans blowing, but it didn't make much difference:
Severe warping from the bed, plus edges curling up, and nozzle banging into them (but this print could still be completed, although barely):
Bottom-view of another testprint, showing some warping at the edges:
Cross section. Main dimensions: contact area at bottom = 2mm wide; top = 10mm wide; height = 5mm; bottom-length = 45mm; top-length = 50mm:
If you would want even more warping, you could always make the prism top (=pointing towards the bed) smaller, so there is less bonding area.
In my experience:
- thick layers = more warping from the bed, but less curling-up of the edges
- thin layers = more curling-up of the overhanging edges, but less warping from the bed, and better accuracy
STL-file: warptest11.stl
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Upper layers cool and pull inwards. The best way to stop warping is to increase adhesion to the bed (you should use all the common methods you can: rounded corners, brim, squish (often most important), heat bed above softening point, glue). But another way is to heat the air close to the softening point. Of course if you do this then you need more fan.
I define the softening point (different from glass temp, different from HDT, different from melting point) as where the material goes from clay-like to solid-like. Above the softening temp you can bend the filament and cool it and it stays in the new position. Below the softening temp if you bend it, it's more likely to break. It's hard to describe exactly but you know it when you play with filament at these temps.
For PLA the temperature is usually 52C.
@geert_2 I do enjoy your experiments.
Here is a Geert Test Piece altered with the addition of two 1mm x 15mm diameter elephant ears on two adjacent legs. This is printed in PETG with the cooling fan at 100%. Basic settings were: Print Temp 235°, Bed 80° with AquaNet Super Hold hair spray, 0.2 Layer Height, .4 Line Width, 8 walls.
The two "earless" legs lifted just over 1mm. The elephant ears were being pulled up at the center but the outer edges held.
@GregValiant: a question, out of curiosity: is there a special reason why you made the elephant ears 1mm thick? When I design custom brims, I usually make them 0.3mm to 0.5mm based on gut feeling, but I have never tried 1mm. The standard single-layer brim in Cura is sometimes too thin to keep a stubborn model down.
Hi @geert_2,
I used 1mm because it was PETG and it would pull harder than if it was in PLA. Even at 1mm the ears on that test piece were pulling up in the center from the part yanking on them. I usually go with a minimum of 25mm diameter for just that reason.
If I design in the elephant ears into the part (they aren't always round of course) and I think they need to be thicker, then I usually add a chamfer at the part line to make trimming easier. I add the .4 thick ear and then copy the edges and extrude them .6 at a -45 draft angle. So the added thickness helps to keep them from deforming but the trim line is only .4 thick so the exacto blade goes through it easier.
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4 hours ago, GregValiant said:Hi @geert_2,
I used 1mm because it was PETG and it would pull harder than if it was in PLA. Even at 1mm the ears on that test piece were pulling up in the center from the part yanking on them. I usually go with a minimum of 25mm diameter for just that reason.
If I design in the elephant ears into the part (they aren't always round of course) and I think they need to be thicker, then I usually add a chamfer at the part line to make trimming easier. I add the .4 thick ear and then copy the edges and extrude them .6 at a -45 draft angle. So the added thickness helps to keep them from deforming but the trim line is only .4 thick so the exacto blade goes through it easier.
Thanks for this close-up of the trim line. Indeed, I was wondering how you could easily cut off such 1mm thick ears. That is why I was limiting myself to 0.3mm - 0.5mm custom brims, the maximum I could easily cut through. I am going to keep your solution in mind for future critical designs.
Thank you guys, I can confirm that @geert_2's part above also warps with PLA.
I think there is potential to minimize this example even further, I will try one fourth of @geert_2's test part.
Another kind of feature that are often affected are holes (as I've read), do you know about other feature-specific occurrences of warping?
Yes, feel free to try variations on this test model. My guess is that 1/4th is going to fall over easier, but this in itself is also a good test, of course.
Tiny holes are more difficult to lay down the first layer: the strand of filament tends to be pulled off towards the inside of the hole. I had that in the beginning with my typical models with lots of holes (see pic below). However, after I decreased the nozzle-bed distance, and after I started using the "salt method", I have never had this issue anymore.
The "salt method" is: wiping the glass with a tissue moistened with salt water, immediately prior to starting a print. This leaves a thin mist of salt on the glass, increases bonding of PLA a lot, compared to printing on bare glass. As long as the bed is hot, models stick well. But once cold after completing, models pop off by themself. You can see the salt on some pictures above. My "salt method" works very well for long, low models like rulers (see pic below). Thus 99.99% of the time I only use this salt method, without any glue.
Picture: typcial model with lots of holes (for reference: embedded watermark ruler is in mm).
Also, high but thin models like lantern poles are hard to print. They do not really warp, but they get knocked off the bed easily, due to the long lever-action, even with a brim. This is where a thicker custom brim is usefull too. The salt method does *not* work well here: you would better use glue, which has a bit of elasticity and can absorb shocks better than salt.
Picture: model knocked off the glass, due to long lever-action (high model and small base), plus overhangs that severely curled-up (more than 1.5mm), so the nozzle hit them very hard when traveling from one part to the other. Also notice the salt on the bed.
Less curling-up after changing printing settings, so less knocking-over, and the model could be completed. (This was a bridge test with hanging, removable supports.) So this is on the edge of what the salt method can do.
In my experience, big chamfers and roundings at the bottom also decrease bonding, and thus increase warping. As do single-wall hollow objects. In both cases, the bottom layer is peeled off the glass, sort of. That is why I developed the "inverted prism" test, it is a hard test.
A thick bottom layer of 0.3mm reduces contact area and thus bonding: the strands are too round. A very thin layer of 0.1mm is difficult to lay down evenly. I get best results with 0.2mm first layer. Printing this first layer too cold also reduces bonding and increases warping. Idem for insufficient priming of the nozzle. So, the first layer should be squeezed well into the bed, printed thin, hot and slow, after good priming with a skirt or brim.
Years ago I did some tests on the effect of bed temperatures on bonding and warping: lower bed temperatures reduce warping a lot, because the plastic is much stiffer and further away from the glass transition temperature. But a lower bed temp also reduces bonding a lot, so the models tend to pop-off suddenly with a loud plop in mid-print. Higher bed temperatures increase warping and thereby also reduce bonding: the bottom stays too soft, and it is peeled off the bed very gradually. This is what you see in the red test pyramid above. For PLA, there is a sweet spot inbetween these, where bonding is still good, and there is not too much softening: around 55-60°C. Below 50°C bonding is too poor, and above 65°C, models peel off and sag and get elephant feet. (This is in my experience, but this may differ from PLA brand to brand, and depend on circumstances and bonding methods, of course.)
Near the edge of the glass, temperature can be a lot lower than in the center. This can also reduce bonding and thus increase warping near the edge. Idem for small models printed with 100% cooling fan: this cold airflow cools down the glass locally with 10-20°C.
Moist air also reduces bonding, and increases any tendencies to warp or to come loose. Very dry air like in freezing winter weather, improves bonding, or at least does not reduce it. Poor bonding in rainy weather is what initially prompted me to test bonding temperatures and methods, and why I developed the salt method. See here for my old manual on the salt method: https://www.uantwerpen.be/nl/personeel/geert-keteleer/manuals/
So tendencies to warp, or to come off the bed, could be a combination of all this:
- tiny contact area, compared to model-size ("inverted prism" shapes),
- big models filling the bed,
- chamfers, roundings, overhangs, at the bottom of the model,
- thin high models,
- tiny holes,
- single-wall, hollow objects,
- too thick bottom layers,
- nozzle too far way from bed, not enough contact area and squeezing,
- too low or too high bed temp,
- too low nozzle temp,
- too high printing speed,
- too much local cooling, fan blowing too hard,
- moisture,
- non-optimal bonding method for the material,
- other things I forget?
The default settings in Cura take care of most of the machine-related issues.
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geert_2 557
Warping-tendency on the bed *while printing* seems to be lower with thinner layers: their bottom is flatter and has a bigger contact-area to the bed than thicker layers, so a better adhesion and less warping. Also, a very thin layer can excert less forces while cooling than a thick layer. A 0.3mm layer suddenly cooling down and shrinking will be able to pull much harder than a 0.06mm layer, I think.
But for warping afterwards, for example the completed part sitting in the sun or in weather, I have no idea. Prints with thicker layers have more entrapped air (=are less transparent) than prints with thin layers. But I don't know if and how that would affect internal stresses and warping afterwards? Interesting thought though.
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