Warping vs curling ABS: two different failure modes?

[mhotze 7]

I've started using ABS (Innofil3D) on my first UM3 since a month, and had to fine-tuning the settings to get fairly good results. I'd like to share some insight about my last 2 prints, and wonder about the background of my observation.

My second last print looked like the picture below. I've printed it with

- 95 degrees heated bed,

- 260 degrees nozzle,

- no fan,

- print speed 20mm/s

- 0.8mm walls

- enclosed system (door + top except for the back; I'm monitoring temperatures by the way, both near the print tip, and one of the stepper motors, let me know if this is interesting)

The result is both slight warping at the first layer, but also curling of the edges of the top layer. Note that this is only the top part of the model where the model is smallest. My reasoning is that the time the layer is built is relatively short compared to the other layers with larger surface area.

Therefore I concluded I needed to prevent putting the next layer onto a relatively hot previous layer. So my next setting was to adjust the "minimum layer time" in Cura from 5s to 20, and enable "lift head". I also reduced the extruder temperature from 260 to 250, since one of my earlier observations was that I has some light oozing (even with retracted filament). This resulted in solving the curling at the top of the model, see picture below:

The funny thing though, is that warping did not noticeably reduce (but was not very bad either).

So now here's my question: what could be the physical explanation of what's happening? It seems like curling of corners at the top layers is a different phenomenon than warping at the bottom layers. It looks to me like curling at corners is caused by relatively rapid cooling of the top of the filament, but still hot bottom part of the filament. With the coinciding bigger shrinkage of the top half of the filament, you get the curling. To prevent this from happening, I managed to get rid of this by cooling down the previous layer enough, to get more evenly cooling down of the filament that is layed down next. The weird thing though, is that for preventing warping at the bottom, the general advice seems to be to get the heated bed close to the glass temperatures (for ABS this is 105 degrees). This is more or less the opposite of preventing curling at the top, where the previous layer needs to be cooled down enough??? I wonder how this works, or maybe I assumed wrong somewhere. The only thing I can imagine is that curling of the top is caused by the previous layer still being above glass temperature, and also curls up a bit since it's still deformable. If this is the same phenomenon as warping at the bottom, then this would mean that having a bed temperature near or above glass temperature would enhance warping instead of reduce it?

[gr5 2,172]

To fix the problems at the top of your print you need some fan. Just a tiny bit. The slowest where it reliably rotates is good. Around 30% on UM2 or 3% on UM3. If you do too much fan the layers won't bond well (not a problem with PLA). Bad layer bonding is not obvious until you break the part. It will break along layer lines. But if you enclose the machine and set the fan to minimum you should be fine.

The bottom corner curling/lifting/warping is a completely different unrelated issue. It is caused by upper layers cooling and pulling inward which rotates the corners up with tremendous force.

Keeping the bed at 105C helps because being above the glass temp means the ABS is more flexible and can warp the tiniest bit and yet still stick to the glass. This is MUCH preferred over the corners lifting off the glass.

Another trick is to make the filament stick like hell. ABS glue, rounded corners, brim, squishing the filament into the glass are all good ways to assure it WILL NOT COME OFF the glass.

260C is dangerously hot. Sometimes ABS will get all clogged in the nozzle and you have to take it apart. You don't want ABS at above 240C for more than a minute or so. Higher temps are worse (more likely to clog) but if you haven't had any clogs I guess your temp is fine. So once the nozzle is up to temp keep things moving/printing.

To really get your part to stick like hell to the glass I explain all the tricks in detail here:

 

[mhotze 7]

So after checking various sites, here's some things I've found as well regarding warping. It does point into a shear force issue.

There’s however quite some differences in the build plate temperature set-point and motivation for it to combat warping:

 

• Reduce warping by reducing the temperature difference between extrusion temperature and build plate: https://ultimaker.com/en/resources/158-print-not-sticking-to-build-plate
  
• Set build plate temperature to the heat deflection temperature (HDT): https://bootsindustries.com/heat-bed-3d-printing/
  
• Go for the glass transition temperature: https://ultimaker.com/en/resources/19537-how-to-fix-warping
  
• Interestingly, this site also mentions not to go too high with HBP temperature (at least for PLA): https://rigid.ink/blogs/news/3d-prints-warping-why-it-happens-and-how-to-prevent-it
  

 

Looking at my earlier prints, I tend to go for a too high heated build plate at first, as these show some inward deformation of the first layers:

Train of thought goes like this: the first few layers are kept above HDT, which causes a shearing force with each successive layer. With adequate build plate adhesion, this results in successive shrinkage of each layer added, but also the layers already deposited, as they are kept above HDT.

Warping did not really change by going from 99C to 85C, but the effect of inward shrinkage of the first few layers up until say 10mm did. Note that all points into the direction of a shear force related phenomenon.

The above also suggest there's an optimum HBP temperature to look for:

 

• not too high (causing inward deformation of the first few layers)
  
• not too low (causing too much cooling and going too much below HDT)
  

 

At least that’s what I found, but a might be wrong still there. I wonder if there’s support for this by observations from others?

Other measures to reduce warping (I think in order of importance):

 

• getting as best as possible adhesion to the build plate,
  
• reduce shear forces by setting lower infill, smaller bottom thickness, smaller wall thickness,
  
• prevent sharp corners,
  
• going as low as possible with the extrusion temperature to reduce temperature drop and related shrinkage.
  

 

For the curling thing at the top, I have not looked that well yet as what happens here exactly. It seems less of a inter-layer shear force phenomenon compared to warping?

Edited March 22, 2017 by Guest

[gr5 2,172]

I think you are getting a pretty good understanding but a few minor points. Not everything is caused by shrinkage. The issue where it "warps inward" a few layers above the glass is something else. The filament is like snot as it comes out of the nozzle and it cools in milliseconds a bit and shrinks but it is still liquid at this point but it's like a stretched, liquid rubber band. As it is layed down there is tension on this liquid rubber band which when going around a corner pulls inward. *That* is what causes the "warps inward" issue that is a few mm *above* the bed.

I suppose it's somewhat related to shrinkage but not in the normal way.

Note that very flexible materials like ninjaflex and taulman bridge nylon do not need to be printed above their "softening temp" because they are *already* flexible even at room temperature. For those materials you only need the bed hot enough such that the bottom layer doesn't harden up before it has a chance to make very good contact with the bed.

Also be aware that for almost every material other than PLA you also have to worry about layer adhesion or weak layer bonding. When laying down ABS on a solid layer below that has cooled too much the new layer doesn't melt the layer below enough for a good bond. The part seems completely fine until you put some serious stress on it and it breaks along the layer lines. To deal with this you want less fan with ABS (and nylon etc) and enclosing the printer helps also.