Tiny black dots under the first layer  

additional information

If they were burnt marks or some burnt stuff from inside the hotend, they should appear in other layers as well. However, they only show up at the layer touching the smooth PEI sheet. Could they be from the smooth PEI sheet?

RE: Pyrolysis

They are likely to be burnt / pyrolysed PL.

PL pyrolyses differently to PET; with PET the pyrolysis breaks down the molecule to give a build-up of a lower viscosity but sticky material that comes out in blobs during the printing.  With PL the pyrolysis products soon become more viscous than the molten material and so get stuck around the nozzle, after a few heat / cool cycles they become near carbonised and so break off (and are then pushed out through the nozzle by the incoming fresh PL) early in the next print cycle.

It's possible that you're using a nozzle temp that's a bit too high for the type of PL that you're using.  In the past we've had this problem in production, particularly with a couple of cheaper brands of PL; these were also the worst brands for blocked nozzles (when the carbonised material remained in the nozzle and just built up instead of being ejected by incoming fresh PL).

RE: What are the solutions?

Thanks. The nozzle temp is within the recommended range from the manufacturer. So what shall I do now? 

Why the system keeps asking me to enter something in the Title?

Performed a cool pull and the filament came out extremely clean. Not a single black dot at all.

Inspect your heater block

Take a moment and inspect the heater block. You might have to remove the part cooling fan duct to get a good look.

1. Build-up of filament on the block is a common problem, particularly with PETG. PETG likes to stick to whatever's hottest, so any little threads or loose strands tend to gravitate to the block.
2. Also be sure you don't see a puddle of filament at the top of the block, as this indicates a leak due to improper nozzle tightening.

If either of these are happening, you'll get little bits of burnt filament dropping off onto the build plate or into the print at odd intervals.

1. You can scrape off any build-up with a strip of corrugated cardboard using a squeegee action, or a wooden toothpick or brush. Watch out for the delicate wires.
2. Consider using a silicone sock if the problem is simply accumulation of filament on the block. 

same title

Thanks. I will check as the printer is printing now.

Additional information: Since I replaced a brand new hotend, I only use PLA. As the hotend came with the nozzle, I did not do any tightening or loosing of the nozzle. Strangely speaking, those dots only show up in the bottom most layer. If it were cause by burnt stuffs from the heater block, dots should also appear in other layers but only the layer touching the smooth PEI sheet.

I checked with Support and the guy had no idea. He asked me back where those dots came from? Then, he suggested me to check the bearing on the x-axis as some rubber might have fallen off!  This should not be the case as those dots are only on the bottom-most layer.

BTW, why the system keeps asking me to enter a title?

2¢ of suggestions

Posted by: @prusanewuser

Additional information: Since I replaced a brand new hotend, I only use PLA. As the hotend came with the nozzle, I did not do any tightening or loosing of the nozzle.

That may well explain your problem. If the hotend assembly rattled around during shipping, it would be very easy for the heatsink to have rotated, thus loosening the heatbreak as it is inserted into the heaterblock. Unless you have tightened the nozzle against the heatbreak at 285C, there's a very good chance you have a leak. Look for the puddle atop the block to check.

Strangely speaking, those dots only show up in the bottom most layer. If it were cause by burnt stuffs from the heater block, dots should also appear in other layers but only the layer touching the smooth PEI sheet.

Well, that's an interesting perspective, but I would still urge you to do the basic diagnostic check rather than second-guessing things. If you have crap on your block or a leak, theoretical musings don't help much. One possibility is that some shakes loose as the print starts. It could be build-up that accumulates on the nozzle and gets pulled off on the 1st layer. Who knows? If you have either problem, it can cause this symptom and you want to fix it.

I checked with Support and the guy had no idea. He asked me back where those dots came from? Then, he suggested me to check the bearing on the x-axis as some rubber might have fallen off!  This should not be the case as those dots are only on the bottom-most layer.

That's another good, obvious thing to check. Again, I wouldn't spend too much time thinking about why it's NOT a specific problem, and instead concentrate on doing the basic checks to CONFIRM it's not a simple specific problem. It's a bit like checking the power cord. It's stupid, but it's a part of a thorough procedure and you may well spot something else amiss as you check the basics.

BTW, why the system keeps asking me to enter a title?

It's a "feature" of the recent upgrade. Nobody likes it. You can tap space or insert some funny unicode. The system will also insert an annoying "Re:" to the front of the subject line if you edit your post (in the very short edit window), which makes no sense at all.

RE: Tiny black dots under the first layer

Posted by: @bobstro

Take a moment and inspect the heater block. You might have to remove the part cooling fan duct to get a good look.

1. Build-up of filament on the block is a common problem, particularly with PETG. PETG likes to stick to whatever's hottest, so any little threads or loose strands tend to gravitate to the block.
2. Also be sure you don't see a puddle of filament at the top of the block, as this indicates a leak due to improper nozzle tightening.

If either of these are happening, you'll get little bits of burnt filament dropping off onto the build plate or into the print at odd intervals.

1. You can scrape off any build-up with a strip of corrugated cardboard using a squeegee action, or a wooden toothpick or brush. Watch out for the delicate wires.
2. Consider using a silicone sock if the problem is simply accumulation of filament on the block. 

I checked but did not see any leakage. The top of the block is very clean. Anything else to check?

No picture, no service

Show a picture.  We're all curious

PL: Clean Pulls & Blockages

Yes, on some printers (before the change of filament, but still within the manufacturer's suggested range) we've done that also with a clean result, and still soon after had a nozzle blocked by carbonised PL.  Since swapping to a better (more reliable brand of PL (ColorFabb Economy) the problem has disappeared.

This was our experience while running a small print farm.

RE: Tiny black dots under the first layer

Hi, I did more tests of printing a big square of 1 layer, the one used for calibration test. Now I found that the dots sometimes appear on the top of the first layer but sometimes on the bottom of the first layer (the layer touching the smooth PEI sheet).

As you can see, there is no leakage and the hotend is very clean. Sometimes I got dark dot the same size as the one shown in the photo but sometimes the size is about 1/2, 1/5 or 1/10. Strange thing is even I added a silicone sock and did a PID calibration today and tried again, this kind of black dot still shows up. This time, it also shows up in support materials. So where are these dark dots coming from?

https://www.flickr.com/gp/183206770@N08/AH42F0

So far it seems to be happening only to the three spools I bought together recently. Leftover of the same product bought previously and other products do not seem to  have no problem. Could it be somehow those three spools are defective? 

I also performed three cool pulls. All came out clean without any dark dot.

Something mechanical perhaps?

I agree your block & nozzle look clean. At this point, look for something mechanical like a belt wearing. I'd put a sheet of white paper on the bed for visibility, then try moving the XYZ axes between min & max with no heat. See if the dots appear then. They might be bits of belt or housing wearing off.

RE: Tiny black dots under the first layer

OK. Thanks. I will check. Do you know why only the 3 spools I bought recently have problem? Strange thing is as soon as I switched to other filaments, problem disappeared. Let's call these 3 spools S and other filament spools V. So far, if I printed in the order of:

S->V->S->V->S->V

I got:

D->OK->D->OK->D->OK

where D means dark dots showed up and OK means no dark dots.

Also, shall I leave the Silicon sock on?

More a process of elimination

If swapping the spools matters, it's unlikely to be mechanical, but I'd still do the test just as part of a process of elimination. If it only spots with some spools, well, that's pretty damning for those spools, but that's not a normal occurrence. Try the sock off again just to eliminate things from consideration. If you get spots with no heat, no sock, and no filament involved, it's clearly something on the printer. If not, you can eliminate that from consideration.

If I were you and this test doesn't show anything wrong with the printer, I'd consider avoiding those 3 spools and see if anything starts up again. It's always possible to get duds. 

It is getting a little tiresome to play cat & mouse with this. Take some useful pictures, show us the various spools, provide whatever information you can to help us help you. Name names, brands, filament details. You can drag individual pics into the edit box or upload a batch with the Add media button above.

Defective material

This further suggests that the black dots are pyrolysis of the material during the longer residence time under heating during the hot end heating phase before printing begins.

Suggestion:
Place apiece of white paper on the build plate.
Raise the Z axis to 150mm height (enough to see a good stream of extrudate).
Preheat the hot end.
Go to Settings - Position - Extruder in the menu and extrude 200 mm or so, check for black dots.
Extrude another 100mm while still hot.
Cool and repeat.
Check for black particles on the paper.

This should allow you to see any problem without involvement of the build plate.

Incidentally, how are you cleaning the build plate?
I recommend a nonionic surfactant (e.g. automatic dishwasher rinse aid) and warm water (rinse well and dry well after cleaning).

RE: Tiny black dots under the first layer

I started getting the same thing. Only on the first layer. Prusament PETG, dry, no previous problems.

Live Z calibration seemed to be OK, but I tried raising it by 0.02 and the dots disappeared. The nozzle must have been burning the filament on subsequent passes, either the previously laid down filament, or tiny amounts that were building up on the nozzle tip from unintentional ironing.