Petg Cracks

[Nina Zahra]

Iwas 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).

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.

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.)

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.

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.

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.

@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 60C 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...

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.

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.

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.

After the print is finished and begins to cool it starts making crackling sounds and begins to pull apart at the layers. Over the next 5 to 6 hours the separations continue to be more severe and the crackling continues.

Save your parts and settings in a 3MF project file, zip it up, and attach it to a reply here so we can see your settings. I print large PLA pieces and don't get delamination after printing. Has this happened with multiple spools? What brand of PETG are you using?

Since I print functional units, my goal is not the joy of tinkering with the printer for a weekend. I find that to be about as interesting as tinkering with a tape dispenser all weekend long. So my attitude is mostly the same as with a broken tape dispenser...move on and solve the problem another way.

You sound awful busy. I'm busy too so don't have time to deal with downloading files from an unknown web site. If you'd care to save your part and settings off in a 3MF project file, zip it up, and attach it to a reply here as already requested above, maybe somebody can find the time to check it out. We're all very busy though, and working on somebody else's tape dispenser is no more exciting for us. Perhaps you'll have more time to help us help you at some point in the future

That comment is just a little snarky, don't you think? Brushing your teeth requires knowledge and experience, as does using a screwdriver or a microwave. But most tools require a reasonable learning curve and have reasonable predictability.

I keep three Prusa machines pretty busy around the clock printing my designs. I don't know how many unique designs I have done and printed, but it is many hundreds by now. They range from birdhouses to trash can mounts to switch brackets to artwork. It's pretty rare that I run into such difficulties as I have experienced with this particular print.

You sound awful busy. I'm busy too so don't have time to deal with downloading files from an unknown web site. We're all very busy though, and working on somebody else's tape dispenser is no more exciting for us. Perhaps you'll have more time to help us help you at some point in the future

My expectation was only that if somebody had a quick solution ("Hey, I have seen that and you just need to kick the machine in this spot") that would be wonderful. I didn't expect that anybody would spend much time debugging this issue. I have alternatives, namely, breaking it up into several pieces and making the backbone out of wood or aluminum.

So I was only hoping there was a well known, easy, straightforward known answer to this class of problem. That's all. Seems like it is a fiddly, complex problem...and knowing that is enough for me. I only need four of these brackets, and I can design a hybrid material design in less time than iterating this 18-hour print.

I wonder if this is another case of @SeattleDavid confusing the Prusa PETG and Prusament PETG profiles. The Prusa PETG profile prints at 10-20C cooler than the Prusament PETG preset. Printing PETG at 230C instead of the manufacturer's suggested 240-250C would certainly explain the delamination.

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