Blobbing at Some Z Seam Locations

[CodeFalcon 0]

I have a problem with blobbing at the start of the outer layer on some prints. Shown here:

 

 

I am using Cura 5.8.1. I have attached a project file and a .obj of the bolt I am trying to print.

 

I have tried using the Overhang Wall Angle setting under experimental to force it to put the Z Seam in a better place where it is supported but that hasn't helped. It only happens when Cura puts the Z Seam of the outer wall at a place not supported by the layer below. For example the top layer here:

 

 

Has anyone experienced this before or have any ideas on how to fix it?

 

 

Bolt.obj Nut_and_Bolt.3mf

[GregValiant 1,409]

I've printed a lot of threads.  Since your models are all about the threads I would suggest going to a 0.10 layer height.

 

I will assume that you are using a standard thread form with an included angle of 60°.  The absolute maximum "overhang angle without support" is 63° so you are very close to that.  In addition, since the previous layer is inset from the current layer (no real support for it)  when the nozzle makes it's shuffle over to start the outer-wall - oozing pushes a little material out and down.  Add that to the lack of any support and you get the blob.  A lower layer height will help support the area of the blob and and the blob should be more of a zit.

Regardless of the material, you might want to insure you have some decent layer cooling as well.  You'll be bouncing off the Minimum Layer Time and cooling that feather edge of the threads could be a key.  

 

I ended up altering the thread form (for printed threads) and changing the included angle to 70°.  That allows the previous layer to provide a little more "shoulder" so the start of the next layer is better supported.  It isn't perfect, but it works for me.  

 

I had high hopes for the new Scarf Seam when printing threads but so far as I've tested, it hasn't provided the big bang that I had hoped for.  It may just be a matter of dialing it in better....or a snake oil salesman put one over on somebody.  

 

I see you are using a little bit of "coasting".  I'm not a fan of "Coasting" for anything other than large flat skins.  It tends to bounce the nozzle pressure up and down.  That can make dialing in other settings harder to do.

 

Finally, there are no corners on the threads to hide the seam at so I would set the "Seam Corner Preference" to None.

[Slashee_the_Cow 479]

Some settings of note:

• You have your retraction distance set very high and your retraction speed set very low. This is almost guaranteed to result in blobs since with Retract at Layer Change turned in, the nozzle sits at the seam for a whole second and pushes out probably more than enough filament to fill the chamber resulting in a blob.
  For PLA on a printer with a Bowden extruder such as yours, the retraction distance should be about 6mm and the speed about 45mm/s.
• Disable Travel > Retract at Layer Change. This results it in having to have to prime the nozzle (unretract) at the start of a layer, which can produce a blob, since it just moves into position and extrudes the retraction distance which shouldn't make it out the end of the nozzle, but it can (especially with your extrusion settings). Disable this even with changing your extrusion settings because the travel is rarely very far.
• You have both Walls > Outer Wall Wipe Distance and Experimental > Enable Coasting turned on. These settings are direct opposites.
  • Wiping makes it travel past the seam without extruding filament (just getting rid of anything in the chamber) in case of underextrusion at the start of the seam so it fills the gap. If you're not underextruding at the start of the seam, this will cause overextrusion and blobs.
  • Coasting makes it stop extruding shortly before it reaches the seam in case it doesn't stop extruding immediately when it reaches the seam and just relies on any filament left in the nozzle chamber to finish the wall. It needs to be carefully tuned for your printer if you're going to use it. This will never cause overextrusion, but can leave holes in your print.
• Set Infill > Infill Wipe Distance to to 0mm. Infill wipe makes the print head travel past the end of infill lines but it can force walls outwards resulting in blobs.
• If your bolt is going to need to withstand any force, increase your infill density, and possibly change the pattern. Cubic infill produces very little infill in cylinders:
  
  Even leaving it as cubic but turning the density up to 20% gives you much more internal structure:
  
  There's no point trying to save filament if your result is so weak it's just going to break and you need to print the stronger version anyway.
  • Speaking of strength, PLA is a bad choice of material for something like this. It's very brittle and I can pretty much guarantee that if you grab your bolt with one end in each hand and bend it, it will snap where your threaded section starts coming out of the body as that's the narrowest point:
    
    PETG is a little bit of a pain to work with a lot more of a pain to store (as it absorbs moisture from the air so you either need to store it with desiccant or dry it before your print it if it's been a few days since you last dried it) but it's a little bit flexible and less likely to snap with force applied to it. I can vouch for this personally since I've printed a couple of tripod plates for cameras - and in the end I just bought some 1/4" screws, cut the heads off and glued the threaded bits into the plate (but PETG still worked better for the plate as it will sustain force when you're screwing it into something).
• Material > Printing Temperature is set higher than I've ever seen it done practically for PLA. Most spools of filament will come with recommended printing settings printed somewhere on the side of the spool, but for regular PLA I just run it at 200°. PLA+ (which is mostly PLA but with some additives, depending on brand) I run at 210°.
  • Material > Build Plate Temperature is set a bit low. I don't know you're using the raft because you need some extra adhesion, but a higher temperature will aid adhesion. For PLA (or PLA+) I have it at 65° for the initial layer and 60° for the rest of the print.
• You have Material > Vertical Scaling Shrinkage Compensation set to something which isn't 100%. Setting it to a value lower than 100% (you have it set to 98.9%) makes it slice it assuming the material expand as it dries. PLA is incredibly stable and will rarely shrink, expand or warp as it dries. Very few common printing materials expand as they dry (though higher temperature materials are more likely to shrink). At best, this causes underextrusion because the printer expects the material to expand a bit (which it won't) and at worst it'll throw off your geometrical accuracy because it's being printed a little shorter than it should be, which is important for things like nuts and screws because they need the threads to be the right distance apart.
• For smoother curves, set Mesh Fixes > Maximum Resolution to something lower, like 0.1mm. This setting mostly exists because older printers couldn't keep up with the number of commands to do things more accurately to the original model on things like curves. Not really a problem these days.

Edited November 16 by Slashee_the_Cow
extrusion > retraction, my brain hurts

[CodeFalcon 0]

@GregValiant, @Slashee_the_Cow,

 

Thanks both for your responses, they both contain a lot of useful information. Turning off coasting and reducing the layer height helped a lot.

 

 

@Slashee_the_Cow I agree; my material choice and model aren't great for this application. It was more of a test print to dial-in settings for printing threads on my printer. Disabling Travel > Retract at Layer Change and increasing the bed temperature to 60 helped a lot. Thank you for pointing out the Material > Vertical Scaling Shrinkage Compensation. I am not sure why that wasn't set to 100%. I am not sure I understand what you mean by Extrusion Distance or Extrusion Speed. It was my understanding that Cura would calculate these based on the filament diameter, the intended line width and the layer height.

 

I have attached a new project file with my new settings. Here is the newly printed result:

I am very pleased. All the blobs are gone and unless you specifically look for it, you can't see the seam either.

 

My last question is, for test prints and prototypes, is there any way to speed the print up without causing blobs, as this print now takes much longer?

Nut_and_Bolt.3mf

[GregValiant 1,409]

I just printed an oval handle and the print time was seriously impacted by "Scarf Seam".  Speeds were down as low as 2mm/sec.  At a set print speed for all features of 50mm/sec, the average speed (of just the extrusions) was 24.5mm/sec.  That's a pretty big hit.

On the other hand, the seam came out really good.  "If ya wanna dance you gotta pay the piper."

Here is the Zseam on that handle.

Edited November 16 by GregValiant

[Slashee_the_Cow 479]

4 hours ago, CodeFalcon said:

I am not sure I understand what you mean by Extrusion Distance or Extrusion Speed.

Sorry, I've been having some problems with my brain lately (sadly not sarcasm 😞). I meant retraction distance and speed.

 

4 hours ago, CodeFalcon said:

I agree; my material choice and model aren't great for this application. It was more of a test print to dial-in settings for printing threads on my printer.

It's always best to test with the material you're going to use since they behave differently. For example PETG is significantly more likely to have (and by that I mean almost impossible to prevent) stringing so you need to dial in your retraction settings a bit better.

 

4 hours ago, CodeFalcon said:

My last question is, for test prints and prototypes, is there any way to speed the print up without causing blobs, as this print now takes much longer?

Slashee's Silver Rule™: Small Scale Testing is Your Friend

If you're just testing how well you can print threads, you don't need to print the whole bolt. Just do a section like this:

If you want to test how well the thread connects to the head of the bolt, just print a section like this:

You can also reduce the infill as strength isn't important. You could remove a wall (but just one) and see how that goes but it does change how things are going to go a little bit (if you take away one wall to test and then you get different results when you print the full thing... don't take away one wall to test).

And don't forget Slashee's Golden Rule™: Slow print > bad print.