CellarDoor
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Getting started with PETG (on the Mk2.5)

Jun 19, 2018 8:12 pm

Hi there,
I am using a Mk2 for about half a year now and bought the Mk2.5 upgrade end of April. Since I was printing ABS until now, I wanted to learn to print PETG – but for the last few weeks I am only looking at failed first Layers (despite the very first print, a Benchy, with the Live-Z from ABS). Since then I tried nearly all values around the ABS-Live-Z (+-100), corrected the right side mesh bed leveling to be +50 now (the skirt was stopping on the right, so I raised the bed level correct until it nearly does not anymore, but also I am at the max). This is the first attached image, where the brim gets really squished at the right back side.
I also tried different temperatures for the original Prusa PETG and a little bit of flow. And – most of all, I looked at the print start with a speed down to 25% to slowly see, where it might fail, which I try to illustrate with the second image, that usually (with ABS) should read S-t-a-c-k-e-n – and I know it's a fine structure, _but_ it worked fine for all my test and prototypes for the last half year with ABS (using the same settings even for the first prints on the Mk2.5).

The problem seems to be that a little bit of PETG catches the nozzle and from than on a small blob smeares the PETG everywhere but not where it should got. And that seems to happen at a random point (sometimes it looks a bit better in the beginning, and as I said, the first Benchy printed well including the text on the bottom) – but I can't figure out why that is happening and it is not happening deterministically.

Any ideas what I can check besides 200nm of Live-Z checked through, temperatures, speed (all from 30% to 100% in 5% steps)? I am really lost in my approach to PETG and running out of ideas. I also tried with glue and without, with even more glue,...

Even more since the same Live-Z (even without the 50nm correction on the left) and the same 3D model, just sliced in Slic3r to the gcode for ABS works completely fine, that's what's wondering me the most.


First image
IMG_2509.JPG
IMG_2509.JPG (1.62 MiB) Viewed 685 times
second image
IMG_2510.JPG
IMG_2510.JPG (809.58 KiB) Viewed 685 times

ken.d
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Re: Getting started with PETG (on the Mk2.5)

Jun 19, 2018 8:15 pm

I'm using Inland PETG. I had to increase the temperature to 250C and slow it way down to get it to stick to glue stick. It still doesn't stick directly to the PEI bed.

CellarDoor
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Re: Getting started with PETG (on the Mk2.5)

Jun 19, 2018 8:19 pm

That's why I already switched from PrimaSelect PETG to Prusa PETG, because I thought for that one the slic3r (Prusa slic3r) PrusaPETG settings should be right. For me it seems - independent of what I am doing, PETG has the most preference to just stcik to the nozzle.

martin.w15
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Re: Getting started with PETG (on the Mk2.5)

Jun 19, 2018 11:48 pm

The trick to PETG is to put a Z offset of 0.15 - 0.2mm in the slicer and print the first layer very slowly at the bottom of the temperature range. You can up the temperature on the rest when it speeds up. You also may need to increase the extrusion multiplier slightly and the K value as the filament is just falling to the bed/print so the amount extruded must be just right.

Regards,

Martin

CellarDoor
Posts: 37
Joined: Jul 05, 2017 7:09 pm
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Re: Getting started with PETG (on the Mk2.5)

Jun 20, 2018 4:29 am

Dear Martin,
thanks for the ideas, what to try, too. Can you help me a little bit and guide me to where to set the Z offset in slic3r? I have found the extrusion multiplier, but I can only set that globally and not modify it for the first layer (for temperature I can do that, of course). And – what's K value? I never heard of it and google is not much help.

CellarDoor
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Re: Getting started with PETG (on the Mk2.5)

Jun 20, 2018 5:21 am

I did a short morning test with these squares https://www.thingiverse.com/thing:1776031 and the result actually does not look that bad. It's completely the same settings I ended my test with the above file, just in the version with a lot of glue on the bed (but that did not work on the above file either).

The middle square is not perfect, but in total this looks quite reasonable. I am totally puzzled, why the above file won't print, if this one looks that good. Somewhere I have a huge misunderstanding in my head concerning PETG.
IMG_2512.JPG
IMG_2512.JPG (1.95 MiB) Viewed 647 times

martin.w15
Posts: 415
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Re: Getting started with PETG (on the Mk2.5)

Jun 20, 2018 6:06 pm

The Z offset is found in 'Printer Settings/General/Size and Coordinates' the highlighted number in the screen shot below
PETG Z Offset.jpg
PETG Z Offset.jpg (104.52 KiB) Viewed 624 times
The K value is found at 'Filament Settings/Custom G-code/Start G-code' the highlighted number in the screen shot below
PETG K Value.jpg
PETG K Value.jpg (76.14 KiB) Viewed 624 times
K value is the ratio off extrusion on slow parts (corners) to fast parts (long straight runs).

You are correct that the extrusion multiplier is global. It would be nice if it could be set separately for the first layer. As a result my first layer is slightly under extruded but the rest is fine.
DSC_0358.JPG
DSC_0358.JPG (9.93 MiB) Viewed 624 times
Regards,
Martin

CellarDoor
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Re: Getting started with PETG (on the Mk2.5)

Jun 20, 2018 6:51 pm

Thanks for the heklp. May I asked, what the K value actually does?

I now first tried adapting your values (K60) (Z offset 0.2) and an additional extrusion of 1.05
(each enabled and disabled and one short print try with my first layer from above) none of that
yielded anything nearly as good as the one above; so for most cases the PETG had the favour
of just building a large blob at the nozzle.
Compared to my ABS approaches, it seems that the Z-Leveling (Z offset & Z Live) has to be :_veeeerry_ precise
and in total that's so many values which I don't know the effect of... is there maybe some visual guide somewhere saying: Your Print looks like this or does that THEN do this change to your settings?
I feel like turning something like 8 knobs randomly without getting any feeling for the effects.

martin.w15
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Re: Getting started with PETG (on the Mk2.5)

Jun 20, 2018 6:58 pm

Actually because of that Z offset PETG is a lot more forgiving on bed levelling. To quote Rigid Ink "It is like printing on air"

Regards,
Martin

martin.w15
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Re: Getting started with PETG (on the Mk2.5)

Jun 28, 2018 8:42 am

Since my last post I have changed my nozzle and heater block of plated copper ones. After some initial problems I managed to tune PETG in for this nozzle and block combination without sock.

I increased my Z offset from 0.2mmm to 0.3mm this completely eliminated any dragging. So I upped my first layer speed to 30mm/s (the default in Slic3r as set by Prusa.) A perfect first layer at speed.

Regards,
Martin

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