Single layer failure - no or poor adhesion  

The commonest reason for poor adhesion is a contaminated print-sheet.   Look up threads here about cleaning.

Cheerio,

RE: Single layer failure - no or poor adhesion

Thanks for the information but it’s about 2-5mm of  printing, not the first layer. 

Ah, I missed that, try a cold pull in case there's a partial clog.

Cheerio,

RE: Single layer failure - no or poor adhesion

Thanks, I’ll give that a try and let you know how it goes next print. 

RE: Single layer failure - no or poor adhesion

I did the cold pull; 1st print was ok, 2nd print was worse. It has layer separation at 3 heights, 2-3mm, 70-75mm and 95-100. 

As the cold pull had some effect try a couple more.  If the problem persists come back with a picture, preferably on the print sheet.

Cheerio,

RE: Single layer failure - no or poor adhesion

I did 6 cold pulls then a test print, the problem is still there. It's not all the way through the layer but it's noticeable (IMG_3338). I notice that there's a small glob of PLA at the opening on the bad layer.

RE: Single layer failure - no or poor adhesion

Here's photo of previous prints that the layer was completely bad and another that wasn't all the way through.

RE: Single layer failure - no or poor adhesion

One possibility (if you are using Prusament) is the filament snagging on the filament retaining slot in the side of the filament reel
this can momentarily cause filament jams leading to no extrusion. 
One cure for this, is a filament guide that keeps the filament in the centre of the spool... 
regards Joan

RE: Single layer failure - no or poor adhesion

I have the filament guide... I thought of that too so I checked to see it was not scored, but thanks!

Work through:

https://help.prusa3d.com/article/regular-printer-maintenance-mk4_419000

Pay particular attention to the nextruder section.

Cheerio,

RE: Single layer failure - no or poor adhesion

I did the cleaning and the nextruder was clean but I did find a problem with the lower x-axis smooth rod, there's a gouge! I didn't find it during my last lubrication (200hrs) because I wasn't looking for it.  I purchased my MK4 pre-assembled, what do I do now? Do I need to ask for warranty repair?

Thanks for the last link, I hope this is the root cause and can be fixed easily.

RE: Single layer failure - no or poor adhesion

I forgot to mention;  the problem happens when I print at 0.2mm (structural), I printed an object last night at 025mm (speed) and it printed without any faults.

Sometimes smooth rod gouges can be negated by rotating the rod 45° - but take some clear pictures and contact Prusa support by chat.

Cheerio,

RE: Single layer failure - no or poor adhesion

Thanks for your help.