Infill and travelling issues

[friedl_basson 5]

Hi - 

 

I am experiencing some issues when printing larger part (5+ hours usually).

 

I am by no means an expert, but have pretty much exhausted most of the options under the "Travelling" section in Cura.  The problem I am experiencing is when the print nozzle seems to be travelling back to the 'Seam' and start printing the next layer of the shell.  The Nozzle moves over the already printed infill which seems to work just fine for the first couple of hours.

 

Later during the print (± 4hours in) the print nozzle seem to be colliding into the infill layers causing the build plate to vibrate quite drastically.  Almost like a car hitting speed bumps.  The print seems unaffected (other than some terrible infill patterns) but I am more concerned abut the printer and possible damage to the Z-axis lead-screw and bearings. I have attached a picture of what is happening to the infill.

 

For this current print I have the following settings:

 

1. Enable Retraction - YES
2. Combing Mode - Not in Skin
3. Avoid Printed parts when travelling - YES
4. Avoid Supports when Travelling - YES
5. Travel void Distance - 2mm (have tried anything from 0.65 to 2mm
6. Z-hop - YES
7. Z-HOP Height - 4mm
8. Nozzle - 0.4mm
9. Layer height - 0.15mm
10. Speed - 60mm
11. Travel Speed - 120mm
12. Material - PLA (Proto-Pasta)
13. Nozzle Temp - 205C

 

Would appreciate some help.

 

Regards, Friedl.

 

[aglyons 0]

I'm having the same problem. I suspect this had something to do with combing. My next print I'm turning that off. I'll let you know.

 

UPDATE: Turned off combing and that issue went away. I suspect there's a bug with combing. Either that or I just don't understand it correctly. It appears to me that if combing is turned on it skips z-hop all together.

[gr5 2,167]

1 hour ago, aglyons said:

It appears to me that if combing is turned on it skips z-hop all together.

Yes.  You got it.

 

Combing is a feature to speed up printing.  It can make a big difference.  But it can also lower quality.  Normally combing moves do not retract (although there is a "max combing distance with no retract" feature to fix that).  zhop only happens on retractions.  So A implies B implies C therefore A implies C (combing means no zhop).

 

By the way, zhop is only recommended for delta printers and 99.9% of the printers out there are not delta printers so you might get ugly horizontal lines if you have zhop on.  Or maybe not.  Or maybe you will get them in a few months when your z screws get dirty.

 

yes the print head can slam (hard) into the edge (or infill) particularly on overhangs like the bow of benchy.  But so what.  If you have everything figured out correctly (like excellent bed adhesion)  that won't be a problem.

[dawsonwebuk 0]

I'm getting exactly the same. Printing is generally great but when it gets over a couple of inches it starts to catch the infill whilst it travels. I have Ironing - Not in skin turned on so suppose my Z retraction isn't working either. Just worried it knocks the print off one of these days, and on a 4 day print that's not good.

 

@gr5 you state "combing is a feature to speed up printing". I don't get that. Combing keeps the motion inside the print boundary, meaning it doesn't travel the shortest path meaning that it will slow down printing. Furthermore, because it doesn't leave the skin at a tangent it makes for a higher quality finish, not lower.

 

Presumably I can get rid of the top surface contact by taking off ironing which will in turn enable Z-hop? But i'm more likely to get surface bobbling?

 

References: 

Cura Combing Mode - Lukas Pomykal (lpomykal.cz)

 

I'm up for any great ideas to reduce this behaviour but maintain quality.

[gr5 2,167]

There's a lot to unpack and unfortunately I don't have much in the way of answers.  It would help to see a picture.

 

Usually the head hits the print on overhangs - particularly think of the bow of benchy the boat.  The overhang curls upwards because the liquid PLA is acting like a rubberband which has cooled enough to shrink but not enough to be solid and this pulls the new overhang layer back inward into the print and causes a lip that can get caught.

 

If it's hitting the infill then that's more rare.  Sometimes it can happen if your bed is vibrating.  Is it?  If so you can move the print to the back of the bed where it might be more stable?

 

Generally the answer to the print head hitting the part is to ignore it.  If the part ever comes loose then you can watch this video: https://www.youtube.com/watch?v=t58-WTxDy-k

 

 

Regarding zhop - this usually causes ugly horizontal layer lines because with zhop you need a super high quality Z screw (not some cheap chinese printer and even most Ultimaker's don't do great with zhop - you get z banding).  If the Z only moves one direction it's easier for the printer to get evenly perfectly spaced layers but if you add zhop then you have play (in the Z axis) and some layers a slightly squished and stick out and some are a little high and are underextruded.

 

Finally about combing - what you say is true - especially if the part is very large and has lots of walls and the combing has to take a very long path -then combing can be slower.  But usually combing is faster because you don't do the retraction.