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ABS parts tension

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Hi!

I am printing big parts that need  to have some mechanical strength from ABS. The printed parts have a lot of inner tension, making them want to delaminate, when they come out of the printer they already have tiny cracks in the corners, especially when the part is big & printed with high speed.

How can i release this inter-layer tension? I figured out that painting them with ABS-acetone-slurry somewhat bonds the shell quite nice but i would prefer a less handcrafty method if possible. I think maybe cold acetone vapour or boiling the parts in hot water could help but didn't try yet.

Update: Boiling ABS in water converts tension into shape :-(

IMAG1649.thumb.jpg.3f98b23e9da523ef84e4454d7dd1e72d.jpg

IMAG1649.thumb.jpg.3f98b23e9da523ef84e4454d7dd1e72d.jpg

Edited by Guest

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I am printing big parts that need to have some mechanical strength from ABS. The printed parts have a lot of inner tension, making them want to delaminate, when they come out of the printer they already have tiny cracks in the corners, especially when the part is big & printed with high speed.

A few things:

1) As far as I can tell ABS is not stronger than PLA. If PLA truly is not strong enough you might want to go with Nylon - I recommend "taulman bridge" but perhaps it is not stiff enough for your needs. It is certainly much stronger.

2) Usually the weakness in ABS comes from bad layer bonding. ABS has much higher "glass temperature" than PLA so it's harder to get the new layer to melt into the lower layers. To get this to happen you need to print slower, thicker, hotter. Everyone disagrees on ABS settings and I'm not an expert but here is what works for me:

2a) almost no fan. The part you show above has absolutely no overhangs so I would do no fan but usually I do 30% fan.

2b) 0.2mm layers. I find that .3mm layers are much too weak due to "beading" and 0.05mm layers can be weak because the layer being applied doesn't have enough thermal mass. I'm not sure the best thickness - around .1mm or .2mm but I think .2mm *might* be better.

2c) Print hot. I have gotten clogs at 255C (too hot if you print slow) so I usually print at 250C but I am still not good at ABS so listen to some other expert who has printed 1000 things in ABS. Even at 250C make sure you print at least .2mm layers and at lest 25mm/sec.

2d) Enclose your printer.

3) Enclosing your printer will also help with the shrinkage stresses. The glass temp for ABS is about 105C. Cooling to air at 20C means 85C of cooling/shrinking issues. If you cover the front of the printer with plastic and put a box on the top (big box to allow bowden tube in the back) then you should get about 40 or 50C air inside. This helps a lot! This brings your 85C of cooling down to 55C of shrinking. Don't go much hotter than 50C as the steppers might get too hot but 50C should be fine.

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The main reason i prefer ABS is that printing other materials literally takes forever. Maybe something is wrong with my nozzle, don't know. I can do the speed test tower with PLA at max ~ 2 mm³/s, XT and PET+ at 3 mm³/s, PLA with sewing machine oil almost 3 mm³/s before getting skipbacks. ABS doesnt seem to have a relevant speed limit, 10 mm³/s -> no problem (actually there is a small problem that the build platform starts to vibrate when printing flat roofs too quick. it helps to position the print at the back of the platform). It was like this from the beginning, atomic pulls are impossible. I ordered "the olssen block", once able to use a bigger nozzle I will definitely go for PLA because it has nice strength+rigidity indeed. Another thing i will try is to cast the ABS print in gypsum and heat it in the oven slowly. I have a small old oven here that i can afford to treat with blasting cement bricks ;)

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I checked the PTFE isolator and it was in pretty bad shape - quite surprising since the machine is almost new. :-/ Also the small silver tube between the isolator and the brass nozzle was not fully screwed down onto the brass. I treated the "elephant foot" of the PTFE coupler by "polishing" the inside with a 3.5mm drill for a while. Now i can PLA-print the famous test tower with 4mm³ at 228°C and 5mm³ at 238°C (Innofil PLA) completely skipback-safe - horray! :-)

Here ist what i concluded today:

1. the UM2 with a straight PTFE isolator can print with PLA very nice!

2. the PTFE isolator does not like it hot.

3. the PTFE isolator is also a bit short (when i push down the filament while the print head cools down for an atomic pull, i get softened PLA above the coupler - if this happens at the end of a print you probably have your filament blocked the next time you are starting a print.

4. the spare PTFE isolator you can buy is diamond filled or so. :-(

5. At the Ultimaker factory the hex screws of the print head are tightened by a gorilla.

Meanwhile, i brushed thin ABS-Acetone-Slurry on my ABS parts. It seems to be quite failsafe when you need strong and dimensionally stable parts where the surface look is not so important. The slurry makes the shell of the part up to about 0,5-1mm depth very soft (depends on how much slurry you brush on) and fully fuses the material, while the infill and inner layers are somehow not affected and keep the shape of the part exact. On some large "ceilings" a few top layers delaminated and got bumpy but they somehow auto-repaired over night. The downside: Respirator or fume extraction = Mandatory

Edited by Guest

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Another consideration as to strength:

For the parts I manufacture (sporting goods) I print in ABS rather than PLA, because the failure mode of acetone is much more benign - it splits between layers and deforms slightly in response to stress.

PLA, by contrast, shatters like glass when it suddenly breaks, and ejects very sharp shards at high velocity.

Note that the machine and some of its parts are not rated for 250C and higher.

With the Olssen block and a .8mm nozzle, I've printed ABS 242C, .6mm layers 1.0mm wide, 15cu/mm/sec at 50 mm/sec, and can go faster.

Edited by Guest

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2. the PTFE isolator does not like it hot.

 

True!

 

4. the spare PTFE isolator you can buy is diamond filled or so. :-(

 

It's actually glass. Glass filled. It is supposed to keep it stronger when it is at higher temperatures but I'm not convinced it helps. You can tell the glass filled ones as they are slightly grayer than white.

 

5. At the Ultimaker factory the hex screws of the print head are tightened by a gorilla.

 

lol! That's not good. but I'm sure nothing broke.

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