Initial Layer Smearing Through Small Holes

[skjbTHI 0]

Hello,

 

Our office recently purchased an Ultimaker S3 to accompany our UM2E, and overall it's been great. One problem I've noticed though, is that when printing parts with small holes (in the neighborhood of 1 mm), the nozzle will smear the filament on the initial layer such that the hole is completely occluded. This effect seems to take place up the next few layers as well. It seems to be an issue with the nozzle being too close to the build plate, from what I can tell. I understand this is a desirable setup in terms of build plate adhesion, but I'm wondering if there's an adjustment to be made to allow for the printing of small holes.

 

I've adjusted some settings in cura in effort to fix this, but to no avail:

• Initial layer horizontal expansion has been minimized to -1 mm
• I've tried reducing initial layer flow
• Horizontal expansion (beyond initial layer) has been minimized to 0

 

It looks to be slicing correctly, and it seems that the issue comes when printing only on the S3. I'm able to print quite small holes on the UM2E, though there I'm using the 0.25mm nozzle rather than the stock 0.4mm AA nozzle that came with the S3.

 

Any suggestions would be greatly appreciated.

[Smithy 1,144]

3 hours ago, skjbTHI said:

I'm able to print quite small holes on the UM2E, though there I'm using the 0.25mm nozzle rather than the stock 0.4mm AA nozzle that came with the S3.

I think this could be the reason. If you really need such small things or holes that are in the neigborhood then you should consider to buy the AA 0.25 core for the S3 as well. 

 

But give it a try and print the same model on the UM2 with the 0,4 nozzle.

[Framar 38]

I would also recommand to set a negative inital layer horizontal expansion. As you noticed it the first layer is squish on the print bed that you cannot get accurate printed geometry. On my S5 I need -0,25mm for initial layer and 0,1mm for all the other layers so my prints can match the CAD geometry size.

 

[skjbTHI 0]

14 hours ago, Smithy said:

I think this could be the reason. If you really need such small things or holes that are in the neigborhood then you should consider to buy the AA 0.25 core for the S3 as well. 

 

But give it a try and print the same model on the UM2 with the 0,4 nozzle.

 

Thanks for this. I'll look into purchasing a 0.25 mm core, but looking through old prints I found a print of this same model (which failed for other reasons) which printed this hole successfully, which leads me to believe the nozzle isn't really the issue.

 

What we've noticed is that it seems that the printer is explicitly filling in this hole in the first few layers. By manually reducing the material flow on the printer itself and pausing mid-print, we're able to see that the hole is printed initially, but that the printer keeps going over the hole after outlining it initially. It's unclear if the code is actually telling the nozzle to extrude material over the hole or if there's just a large blob that's falling out over it, but it doesn't seem that smearing is the main problem. I'm now of the mindset that this could be a G-code issue, though I can't figure out what exactly. Printing with lowered material flow is obviously not desirable as a long-term solution.

 

I've attached photos of the successfully-printed hole from a previous print, a photo of the first layer as it's being printed now, as well as a few images from the slicer.

[gr5 2,172]

18 hours ago, skjbTHI said:

the nozzle will smear the filament on the initial layer such that the hole is completely occluded.

I think you got it exactly.  This is on purpose.  If you don't squish the first layer extra hard, the part won't stick well which can be a disaster (e.g. head flood).

 

So the solution (for me) is to always set the "initial layer horizontal expansion" to around -0.3 for a 0.4mm nozzle.  Typically you want it about half the nozzle width.  But negative.

 

Now there are cases (e.g. gears in one case) where setting this to -0.3mm causes problems.  But for me these problems tend to be rare.  It depends on the model.

[gr5 2,172]

Another solution that I use a lot is to drill out vertical holes of critical diameter.  The existing

hole in the 3d printed part makes for a good guide to get perfect holes every time.  This solution is for vertical holes in general (not just first layer issues).  Also I usually make hole diameters in CAD about 0.4mm larger than desired (for all sizes e.g. from 1mm to 100mm).

[skjbTHI 0]

Just now, gr5 said:

I think you got it exactly.  This is on purpose.  If you don't squish the first layer extra hard, the part won't stick well which can be a disaster (e.g. head flood).

 

So the solution (for me) is to always set the "initial layer horizontal expansion" to around -0.3 for a 0.4mm nozzle.  Typically you want it about half the nozzle width.  But negative.

 

Now there are cases (e.g. gears in one case) where setting this to -0.3mm causes problems.  But for me these problems tend to be rare.  It depends on the model.

 

This was my first thought as well, and I went as far as setting initial layer horizontal expansion to -1 mm, which was the lowest Cura would let me. I would also note that on the previous print in which the hole printed faithfully, I didn't use any of the advanced settings in Cura.

 

I've attached two more photos of a recent attempt in which it's clear the hole is printing on the bottom layer (extra wide, presumably thanks to the initial layer horizontal expansion setting), but there appears to still be filament printing over it in subsequent layers. (the print looks extra messy because I had reduced material flow on the initial layer and forgot to return it to 100% later).

12 minutes ago, gr5 said:

Another solution that I use a lot is to drill out vertical holes of critical diameter.  The existing

hole in the 3d printed part makes for a good guide to get perfect holes every time.  This solution is for vertical holes in general (not just first layer issues).  Also I usually make hole diameters in CAD about 0.4mm larger than desired (for all sizes e.g. from 1mm to 100mm).

 

This is probably the best long-term solution, but I don't have a 1mm drill bit on hand at the moment :/.

[gr5 2,172]

Okay so I feel like you aren't showing us something.  Please show in cura layer view what's going on?  Is the second layer covering the hole in the first layer?

 

Actually if you could save your project and upload the project file here?  In cura do "file" "save...".  This will save your stl and rotation, position, settings, printer settings, overrides... everything such that one of us can duplicate what you are seeing.

 

I can't tell if the second layer is infill or support but it surely seems to be printing something on the second layer.  I thought you were describing a problem on the first layer but now it seems it is a second layer issue?  Or is it a 3rd layer issue?

[gr5 2,172]

To save time, maybe also you could check the part for errors. 

 

Cura has an amazing plugin to test your model to see if something is wrong with it and can repair a very few of the many potential problems:
In the upper right corner of Cura click "marketplace" and make sure you are on the "plugins" tab and install "Mesh Tools".  Then restart Cura.  Now right click on your model, choose "mesh tools" and first choose
"check mesh", then "fix model normals" and "fix simple holes" to see if that helps.

[geert_2 557]

I would also check the part and STL for errors, and always inspect the layer view. Maybe design a new "known-good" test part that only has this problem feature. Design from scratch a cube of 10mm x 10mm x 10mm, with such a hole, in a decent editor (definitely not SketchUp).

 

For small holes, you need to calculate-in the narrowing caused by the molten material being pulled inwards. Try pulling a string of honey or yoghurt in a small circle, it's like pulling a rubber band into a circle. Drill-out the holes *by hand*, if necessary.

 

Often I add a rounding at the bottom edges of prints, with radius 0.3 to 0.5mm. This generally takes care of elephant feet and such, without me having to mess around with any settings, and it allows for a good squeeze into the glass.

 

See these pics with tiny holes (not post-processed, and ruler is in mm and cm):

 

 

 

 

 

[skjbTHI 0]

On 10/6/2020 at 10:10 PM, gr5 said:

Okay so I feel like you aren't showing us something.  Please show in cura layer view what's going on?  Is the second layer covering the hole in the first layer?

 

Actually if you could save your project and upload the project file here?  In cura do "file" "save...".  This will save your stl and rotation, position, settings, printer settings, overrides... everything such that one of us can duplicate what you are seeing.

 

I can't tell if the second layer is infill or support but it surely seems to be printing something on the second layer.  I thought you were describing a problem on the first layer but now it seems it is a second layer issue?  Or is it a 3rd layer issue?

 

Attached is the sliced file — I've replicated the settings to the best of my memory but as I'm out of office today I can't upload it from the USB directly.

 

From looking at the part as it's printing, it indeed seems to be a 2nd (or maybe 3rd) layer issue. The initial hole prints (and prints wider than designed, presumably due to the horizontal expansion settings), but it seems in the next few layers the nozzle extrudes directly over the hole. This seems to be happening on the 2nd and 3rd layers, and possibly further, but at some point the whole does extrude as designed. I've broken the part in half and it's apparent that at some point up the cylindrical base, there is a hole of ~1mm in the center.

UMS3_Slider Hub Extended.3mf

[skjbTHI 0]

On 10/6/2020 at 10:14 PM, gr5 said:

To save time, maybe also you could check the part for errors. 

 

Cura has an amazing plugin to test your model to see if something is wrong with it and can repair a very few of the many potential problems:
In the upper right corner of Cura click "marketplace" and make sure you are on the "plugins" tab and install "Mesh Tools".  Then restart Cura.  Now right click on your model, choose "mesh tools" and first choose
"check mesh", then "fix model normals" and "fix simple holes" to see if that helps.

 

When I attempt to select "fix simple holes", it tells me it cannot as it must slice first, even after the model is sliced. The "check mesh" tool indicates that the part is watertight.

[gr5 2,172]

I looked at your model (it would have been better if you saved the whole project - "file" "save...") but anyway the hole in the model is very tiny - visually it looks like about 0.5mm in diameter.

 

3d printers print vertical holes about 0.4mm smaller than what the model shows.  Which means - you will have no hole.  You can either add 0.4mm to the CAD model (which is what I always do) or there is a new feature in cura - search for "horizontal", "holes", or "expansion".  There is a way to make the holes larger.  It may mess up other parts of your model -- well not this model -- but other models.

 

Why does the printer do this?  PLA acts like a liquid rubber band as it comes out of the nozzle because it shrinks in milliseconds (before it is solid) as it cools rapidly.  So when it prints that hole the "rubber band" is pulling inwards and you get that vertical holes are all smaller than desired.  It doesn't affect the first layer as much but that has a different problem because it is oversquishing.  On purpose.  So that the part sticks well.