PETG brittle after printing

[P3D 46]

I just wanted to share some experience I had printing with PETG, in this case specifically 3Djake transparent PETG. 

 

I was printing a model that is subjected to some flexing, and that also has supports in tight places that I needed to remove. Two times, the model cracked during cleanup, in different places. I was quite at loss about how that could have happened, as I was following all the manufacturer specifications regarding temperature and also the cracks were straight through the part, not along the layer lines.

 

I found out, however, that for my printer (Anycubic i3 Mega), the fan doesn't even turn on with a 10% fan speed set in the print profile. I printed the same part with 30% fan (and the fan did turn), it came out with a bit more stringing and the optical transparency wasn't as good, but most importantly, the part didn't crack during cleanup.

 

Quite often, I have seen people recommend printing PETG with no fan at all - seems that this is not always a good idea, my guess is that the material somehow became brittle by being heated for quite a long time and not cooling fast enough (probably crystallization, but I'm not a materials scientist).

 

Did anyone have similar experiences?

[geert_2 557]

Does it crack along the layer lines, or does it completely ignore layers and go diagonally through them?

 

My experience with PET is that it is less strong than PLA, it will break sooner under high loads, but it can flex more. It will fracture without warning, all of a sudden. Fractures ignore layer lines (indicating good bonding). Also, it has less creep due to permanent loads than PLA. So for keychain mechanisms, carabiner hooks, and snap-fit locks it is better than PLA, since they need to flex to function. However, the PET keychain will break much sooner when it gets stuck in your pocket when you sit down. So it is far less forgiving than what you would expect, based on experience with cola/water PET drink bottles.

I print PET without fan, but at low speed and very low temp: the absolute recommended minimum, or even less: 210...215°C (recommended: 215...250), for 0.1mm layers, and 25...35mm/s. A higher temp did not improve quality nor strength: it caused bubbles and more stringing.

 

The downside of no-fan is that overhangs and bridges are terrible, but for my long and flat models this usually is no problem. Using fans decreased layer bonding, and caused a tendency to warp in long flat models. (I print on bare glass, wiped with a tissue moistened with salt water.)

 

Printing extremely slow (10mm/s) causes the material to get brown, thus indicating beginning decomposition in the nozzle, but it gives excellent bonding (plenty of time).

 

So I do think that some decomposition occurs, and that the indents caused by extruded sausages may concentrate stress and initiate cracks, and make it fail much sooner than injection moulded parts. Not sure about crystalisation: I don't see it getting opaque. But I am no chemist, so maybe I can't recognise it.

 

Anyway, try various settings on small test pieces, before doing large models. That is why I do the keychains, and I use them every day in my pockets.  :-)

 

See my comments and photos in this thread:

https://community.ultimaker.com/topic/30778-printing-airtight-with-petg/?tab=comments#comment-254606

 

 

[P3D 46]

Hi, thanks for chiming in!

 

As I wrote, it cracked straight through, with no visible evidence of layer lines at all. 

 

Do you dry your PETG before printing? I have experienced bubbling only when using wet filament.

 

 

[Elfonerio 5]

I found alot of cracking and shattering issues were brand dependent, some are just different then others.

[P3D 46]

51 minutes ago, Elfonerio said:

I found alot of cracking and shattering issues were brand dependent, some are just different then others.

 

Sounds like you did some experimentation already - is there a brand you could recommend?

[geert_2 557]

3 hours ago, P3D said:

As I wrote, it cracked straight through, with no visible evidence of layer lines at all. 

 

Do you dry your PETG before printing? I have experienced bubbling only when using wet filament.

 

 

Ah, okay, I missed that "straight through" part.

 

I don't dry PLA and PET. Although I do store them in a box with dessiccant. But while printing I just leave them in the printer, sometimes for several days or even weeks. Doesn't seem to make much difference.

 

PET seems to be a bit water-repellent. And PLA seems to get a little bit duller over a couple of years time due to degradation, but I am not sure if that is mainly UV-light, or due to moisture, but it is going *very* slowly over years, not days or weeks.

 

If I would use nylon or ABS, I would dry them though.

 

[Elfonerio 5]

2 hours ago, P3D said:

 

Sounds like you did some experimentation already - is there a brand you could recommend?

I make end use control panels for our product and have tried MH, both build and Pro, and Esun trying to save a few bucks, but in the end Ultimaker material proved to be the best, but most expensive.  It had much better flexibility (before cracking) and print properties.  I have switched to ABS as of late, seeing that I enclosed my printer, and don't plan to ever turn back.

 

As far as moisture, i never found PETG to get waterlogged, but I also use it when I get it, generally it never sits around longer then a month for me.  I ran my fan speed at %20 and never had a problem with inter-layer adhesion, it was strong and great, I just quit using it because I need a higher glass transition temperature.

 

Hope this helps

[P3D 46]

Thank you both for sharing your experience! 

@geert_2: Just for reference, I had a very moist spool of transparent PETG delivered by a local supplier (3DJake - who just didn't publish my review of the material on their website, seemingly because I didn't give it at least four out of five stars, so I will think twice before I buy anything from them in the future...). When I printed it without drying, it bubbled like crazy, the part dimensions were completely off and the parts had a frosted look. Some hours in the oven at 60°C and the print results were completely different, the dimensions were much better and the parts came out really clear.

@Elfonerio: Thanks for sharing! UM materials are not an option for me, unfortunately (just too expensive and the wrong diameter for my personal printer). I am eyeing ASA at the moment for its UV resistance and general toughness, but first I'll have to get an enclosure with an activated carbon filter. I'll also try PP as for many parts I could actually use a little flex, and it will be hard to beat its toughness. Now I need to get it to print decently, though...

[DivingDuck 104]

My PETG (mostly from DevilDesign) absorbs over the time moisture too. Less in the winter times and more in summer depending how much humidity we have. But this isn't a big deal as you can get rid of it easily in an oven.

[Oliveros 11]

I had this problem and I turned up my printing temp, now it prints like a dream, even watertight prints.

[P3D 46]

2 minutes ago, Oliveros said:

I had this problem and I turned up my printing temp, now it prints like a dream, even watertight prints.

 

Interesting, so the brittleness was solved by printing at a higher temperature?

[Oliveros 11]

Yes, for me at least.

 

I had problems with layer seperation, poor adhesion and just poor quality overall, turning up the temp did it for me.

 

[TrueNorth 3]

On 1/16/2020 at 1:41 PM, Elfonerio said:

I make end use control panels for our product and have tried MH, both build and Pro, and Esun trying to save a few bucks, but in the end Ultimaker material proved to be the best, but most expensive.  It had much better flexibility (before cracking) and print properties.  

 

Thank you for sharing your hard-won experience @Elfonerio. I just got done with 4 rolls of MH Build series PETg in Black. Layer adhesion was excellent, but flexibility was nearly non-existent and my parts broke easily. The breaking characteristics of the PETG was much like flint or glass:

 

After reading your post @Elfonerio, I ordered some UM CPE+ to do a re-print. So far, so good. It is has much better manners in the printer.  But you're right, @P3D, it is expensive. But my alternative was reinforced (abrasive) filament, so UM CPE+ is a halfway point to see if we can avoid the uber-expensive structural materials. 

 

 

Edited July 21, 2020 by TrueNorth

[JayWatt 0]

This is an old thread but I wanted to chime in.  I'm currently running a Creality CR-10v2 equipped with an E3D Hemera.  I've had great luck with PETG at a variety of temperatures, but it isn't perfect.  I believe that higher temperatures cause it to become brittle.  I am designing 3d-printable AFOs (leg braces) and have made some out of PETG, thinking that they would have lots of strength and a bit of flex.  Well...no.  I printed at 245 degrees for good layer adhesion, and I got it.  After printing, I have to heat the braces up with a heat gun so that I can form them to my legs/feet.  There lies the problem.  If I get it too hot, the plastic bubbles.  I don't see any steam or smoke from that.  I don't know if it's water or a component of the plastic itself that's boiling off, but it absolutely DOES get more brittle after I do that.  Each pair that I have made from PETG will begin to crack along stress points after a day of use.  They should have been plenty strong, but that constant back and forth stress is just too much, and cracks start to develop.  I am now researching new materials to use.  I really like TPU, but the standard 95A shore hardness variety has too much flex.  I'd like something that still has some rigidity on thick, solidly printed parts.  I'm also thinking of trying Nylon now that I have the Hemera installed.  But anyway, heat can absolutely make PETG more brittle.  High temperature printing may indeed produce the same effect.