Cura doesn't complete perimeter movements

[PCLoadPLA 0]

I have Z seam gaps that I have been trying to diagnose with retraction, prime, and linear advance settings. I'm running Cura 5.6 with Marlin 1.9 and a 0.8 nozzle and hot PETG (for layer adhesion). 

 

When I watch the printer carefully, the gap is caused by a delay of the filament coming out. The nozzle moves 2-3 mm before the plastic starts flowing. 

 

Is there not a "prime" function that I could tune to prevent this? 

 

I currently have retraction off because it seems the opposite of what I need. But I noticed there is a "extra prime after retraction". Does this work even with zero retraction?

 

If I use "retraction extra prime amount", will the print head move to the start position, complete the un-retraction including extra prime, and then start moving? Or does the un-retraction happen as the print head is already accelerating?

[GregValiant 1,300]

If you have the Experimental setting "Group Outer Walls" enabled it may have something to do with the dry starts.  I'm not sure why that is, but there is some evidence that it has an effect.

Long travel moves contribute to the problem of dry starts.

Large nozzles are also a contributing factor because they ooze more during travels.

Another setting that can aggravate the problem is "Coasting" which is designed to allow the nozzle pressure to fall off during the last few millimeters of an extrusion.

"Retract before outer wall" can contribute to the problem.

 

"Retraction Extra Prime Amount" may help.  The material will be part of the regular prime so it will happen before the nozzle starts moving.

"Retraction Extra Prime" is fussy because if an extrusion starts after a short travel the problem isn't near as bad.  If the extrusion starts after a long travel then it might need more prime.  It's a balancing act.  The "Extra Prime" amount is in mm³.  Without going into the math, start with 0.4mm³ of extra prime (about 3mm of extrusion at .4 layer height and .8 line width) and see how it goes.

 

Edited April 21 by GregValiant

[PCLoadPLA 0]

Thank you for confirming some of the operation details. 

 

I've noticed the problem is worse after long travel moves. It means the gaps are variable. I print large parts so the head is frequently traveling many cm between layer changes. 

 

I do not use coasting, group outer walls or retract before outer wall but I will confirm.

 

I will try increase travel speed, z seam shortest, and retract extra prime. 

[GregValiant 1,300]

There is another option here.  I've gone over the script, but I've never had to use it.

It's a post processor called "Retract Continue" and it will do an initial retraction of "Retract Distance" before the head starts to travel.  During the ensuing travel moves the filament will continue to retract based on a percentage number that the user inputs.  When the print head arrives at the start of the next extrusion the prime is the original retract amount plus whatever additional retraction occurred during the travel move.

A down side to the script bundled with Cura is that it is fooled by Z-hops and consequently there is no additional retraction.  If you aren't using Z-hops then it should work fine.  If it doesn't work for you then I'll post my version which does work with Z-Hops.