vertical inaccurate at the bottom

[spiruline 2]

Hello,

I am printing ABS at 255º, layers are 0,1mm, with a heated bed.

I am getting some vertical innacuracy at the bottom 10mm of the print, there is a narrowing of the walls of the part. After those initial 10mm the walls are nice and vertical, so hardware seems to be all OK.

¿what could be the reason?

 

[owen 19]

It's got to do with Heat Bed Temperature and when your fan comes on I think.

It's a tricky balance as having the fan come on too soon or too hard causes lifting off the bed.

My settings are 125C Bed, 245C Nozzle, Dual Fans on from the start at 30%. Heated build chamber (though I don't know if I need this or not as I had it before I started printing ABS but I think it helps). I've avoided what you are getting now and usually getting it to stick but still not every print sticks successfully.

 

[calinb 11]

I think Owen nailed the cause. As he said, temps are critical with ABS--both ambient air temp and bed temp. Are the parts in the photo oriented in the build direction? I'd try to print the part in a heated build chamber (I use a bubble wrap tent.). I'd use a high bed temp (about 125) for the first two to four layers to bring the tent up to temperature and then reduce the bed temp to 110. I'd try a light fan for the first 10mm (or as high as the plastic is shrinking inward) and then turn it off until the the layer times become very short at the top protrusion.

A narrower waist from shrinkage, as illustrated in you photo, early in the print is common. Some fan usually helps. If you over-cool with a fan, darker ABS colors will often exhibit thin lighter color lines or striations, due to poor layer bonds and partial delamination.

You could also try thinner layers (longer print time). In general, thinner layers seem to reduce this effect but there are so many process combinations, I don't think you can count on any rules.

I've not found anything better for ABS adhesion than either juiced (ABS/acetone wipe) Kapton or PET tape. Hairspray on glass can work and be very convenient, but juiced tape is the best. It's so good I've broken a grand total of three glass build plates trying to remove large and blocky ABS parts (and I'm careful and know all the tricks for removal). You could also add a brim to the part, if adhesion to the bed becomes a problem while tweaking your temps.

 

[spiruline 2]

Thank you for the tips.

Yes the photos are oriented in the build direction. I haven't used any fan at all, because it seems to cool the heated bed and affect adhesion. I may have to try with a lower fan setting.

BTW, I am using the new heated bed: http://reprap.me/Alu-Heatbed-MK3

which is working very well. But my thermistor must not be the right one because it doesn't show more than 90º C and I am getting good adhesion with large flat and fat ABS parts.

 

[calinb 11]

Your concerns that the fan cools the heated bed are well-founded. The trick is to set the bed temp extra high for the first few layers (at least two layers). I often use 130 C. Do not run the fan for these first layers. Reduce the bed temp and run a low fan only through the problematic region. Only run the fan high enough to preclude the distortion. Again, if you err on the side of too much fan, the layer bond strength will suffer. My fan is stronger than the OEM fan (160 mA vs. 100 mA) and I never need anything near full speed to solve this problem. The correct fan setting is touchy and varies with the part. My fan speed range can usually produce a part with the distortion you have, a part without distortion, or a part with small layer delaminations--all just by changing the fan speed at various heights but not changing anything else in the process!

I occasionally use a skirt (a thin wall) around the part to protect the lowest layers from the fan's breeze. I almost always use a brim, which I typically add to the CAD model myself, because most slicers cannot create brims.

I've experienced the problems you're encountering but I have successfully solved them and printed a wide variety of large, thick, and blocky parts in ABS using these techniques. I also use the bubble wrap tent and achieve chamber temps as high as 65 C to 70 C at nozzle height within the tent. I do'h need no stinkin', expensive, and patented Stratasys oven!

For the best possible adhesion to the bed, I recommend wiping Kapton tape with a paper towel moistened with acetone. Rub it on a piece of old ABS and smear the bed. If you "age" the "juiced" Kapton with a pre-print, and can remove the part without damaging the tape job, the adhesion will be even better on the 2nd and subsequent prints. The adhesion is so good, I can actually turn off my bed heat after the first layers (if I don't need it to heat the build chamber) and the part will stick well all the way down to room temperature. Actually, I've found that 130 is a bit too warm for maximum Kapton to aluminum or glass bed adhesion. PET tape is the same, but it needs a bit of heat to stick well to my bed. The PET is thicker and tougher than Kapton, however.

I really want to know how the new UM2 automatically solves all these problems, becoming a turn-key FFF solution for newbie makers! Okay--the heated bed will help a little, but plenty of 3D printers already have heated beds.

 

Thank you for the tips.

Yes the photos are oriented in the build direction. I haven't used any fan at all, because it seems to cool the heated bed and affect adhesion. I may have to try with a lower fan setting.

BTW, I am using the new heated bed: http://reprap.me/Alu-Heatbed-MK3

which is working very well. But my thermistor must not be the right one because it doesn't show more than 90º C and I am getting good adhesion with large flat and fat ABS parts.