TLDR:

This post is quite long and detailed because it involves the findings from hundreds of experiments.  If you're just looking to try the best settings, download TestSuite_OrcaSlicerOnly_V6.3MF , make sure to set your own filament profile and note the per-object settings that are used along with the global settings.  

• This plate was tuned against Prusament Simply Green PLA.  
• 3x Test objects are printed for 2 sets of optimized settings and a reference “traditional seam” set (for my Bambu X1C). One set of settings is Arachne for general use, the other set is newer and is optimized on curved overhangs, which are quite challenging, it may also work well for general use. 

For details and to optimize for your printer, read below!

Updates: 

2/29/24 PM:  New best settings!  Excel sheet and test suite updated to v2.  Using extrusion rate smoothing now helps a little bit more with quality. I think I'm finally done optimizing.

3/1/24: Youtube Video featuring this is now up: 

3/8/24:  Significantly reworked this post.  Updated Binary build files to latest main branch build.  Added new whole section on learnings about overhangs from the test pot model.  Retired old test plate and introduced a new best starting settings test plate.

3/12/24:  Removed builds, please use Orca 2.0 Beta or newer now, which has this feature.

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Scarf seams are a cutting edge feature in Orcaslicer that allow you to do amazing things, primarily enable hiding of seams on curved surfaces, which has always been a difficult problem with FDM 3D printing.  This guide is intended to show you how to use the feature and get better prints.  If you enjoy my hard work, please download + like my model, it makes me smile ! 

Scarf Seams - Overview

The basic idea with scarf seams is that they work by "ramping the seam" over itself, which results in a printed wall that prints on top of itself like this:

Because this is like wrapping a scarf over itself and is also like the common scarf woodworking joint, we call this a “scarf seam”.

What are the pros?

• Scarf seams solve a hard problem in 3D printing of hiding the seam, particularly for prints where the seam must lie on a curved surface. This is actually incredibly common, as inside round holes or external curved surfaces are present in a large number of prints.
• Scarf seams can also work well on flat surfaces, but in some cases traditional seams may be a better option.

What are the Requirements / Gotchas ?

Scarf seams can be good for many prints, but particularly in the cases of sharp corners it may be better to use a traditional seam. Scarf seams do have some requirements for use to get good results.  Extensive optimization has been done to find good general settings and another set of settings in particular that may work better for overhangs.  The general principals are:

• Use of a slower outer wall speed is particularly important to get good results without wall distortions.  How important this is may vary by printer's ability to precisely control extrusions at very low flow.  Going too slow can be bad too, as it causes localized heating issues.  Optimal speeds are generally in the range 50-100 mm/sec for the scarf speed.  My suggested settings use 75 or 100 mm/sec.
• Use of a wider outer wall helps decrease extrusion rate error.  For a 0.4 nozzle, I recommend 0.6 outer walls.
• Use of a higher layer height than 0.2 may be helpful, again, because it reduces extrusion rate error.  In some cases, however, it can hurt.  My settings are optimized on the common 0.2 mm height.
• Use of inner/outer/inner is the recommended wall order.  Outer/inner appears to provide worse results, as does inner/outer.  This means that overhang performance may suffer, but this is challenging in general with scarf seams (see considerations below).
• Use of extrusion rate smoothing can help edge out just a little more quality on my Bambu X1C printer.  I set mine to 300mm^3/s^2 and this got me the best settings.  Non X1/P1 printers may need other settings.. read the mouse-hover tooltip in OrcaSlicer for an idea of what to use for your printer.
• Toggling certain features make things ugly.  Do not use the following:  
  • Using "Wipe on loops" or “Wipe on external loops” make things bad.
  • Combination of (Retract on layer change off + Wipe while retracting off) also no good.

Overhangs optimization / additional considerations.

If possible, it is recommended to paint a scarf seam away from an overhang on a model.  However if it is not possible to do so, then there are some things you can do (see bullets below).  Certain overhangs can be a challenge with scarf seams, and it seems to depend on geometry strongly.  I have designed and tested a difficult “pot” model to optimize overhangs, and have run an additional 80+ tests on this to try and find good values.  Here's what I learned from this mess of little pots:

• Curved overhangs are particularly sensitive to both the number of inner walls, their width and their print order.  It's important to note that changing certain values like inner wall width can make a meaningful difference.  
• Situations where model thickness is so thin that there's no infill to relieve any extra over-extrusion can be particularly sensitive (my test pot does this). Single or double walls with no infill or anything touching them on the inside performed the very best, but this limitation is unrealistic for most prints.
•  Classic mode can sometimes help, because it stabilizes the width of the walls touching the outer wall, keeping them from varying as they do in Arachne.  This is particularly useful in the case where you don't have any infill between an inner and outer suface of the object, as is the case with thinner walled objects.
• Speed (outer wall/scarf speeds) is particularly important for overhangs because of the fight between good cooling and adhesion.  For me I found with my filament that 75 mm/sec produced OK results, 100 mm/sec produced better results, but 150 mm/sec was far worse.  There is likely a sweet spot for any given filament/temperature/hotend combination.  You may need to try a few values.
• Use of inner scarf joints is often beneficial to overall quality for the outer scarf seams, but can be a challenge for overhangs, as it causes cause an internal nozzle “landing mark” right next to the outer scarf lead-in start.  This is problematic because the hot nozzle sits there while a de-retraction happens, and leaves a visible inner “smoosh” which distorts the outer walls.  This doesn't seem to be a problem normally, but with overhangs it appears to cause issues.  Consider using staggered inner seams to help combat this.  Unfortunately the staggering distance is presently fixed and small.  I believe if it were larger, it might help quality even more.  I have opened a feature request for this.  Here's what the circular nozzle “landing mark” from an inner scarf seam (inner-outer-inner wall order) looks like, for reference:

What about setting xxxxxx ?

• Use of Conditional Scarf Joint is strongly recommended to be on.  This applies the scarf seam only when a round surface (vs sharp corner) is encountered and blends the best of both worlds.  You can also adjust the threshold angle which determines the crossover point between traditional and scarf seams.  In my pot tests and optimized settings, I use a value of 115 to force the scarf seam on everywhere for testing reasons.  This may or may not be optimal for your use case.  Adjust as you like.
• Use of Contour mode will give you scarf seams on the outermost wall only.  Contour+Hole will include inside “hole” walls in the part.  One mode vs the other may be desirable for different reasons.  My settings use Contour+Hole by default.
• Use of Inner Scarf often (but not always !) improves results, but will slow down the print overall. 
• Scarf around entire wall makes the nozzle do two passes as it slowly spirals around the entire perimeter.  This has not significantly improved results for me, but some people claim it works better.
• Making the scarf length shorter than the default 20 starts to become a particular quality problem, especially below ~10-15 mm.  I recommend leaving it at default of 20.
• Changing the scarf steps doesn't seem to matter much.  The default 10 is fine, although my settings use 20.
• Changing the scarf flow ratio doesn't seem to help, despite hopes that it would. I worked together with SoftFever on this feature in the hopes it would help overhangs, but it didn't improve anything dropping all the way down to 90% flow.
• Slowing down inner wall speeds can sometimes also improve quality. 

How-To:

1.  You will need to use a OrcaSlicer 2.0 beta or newer.  Find this here. 
2.  Download the latest TestSuite_OrcaSlicerOnly and print it, to validate the settings on your printer. Make sure you have picked your own filament presets and tuned your filament for temperature/pressure advance/etc before trying this ! 
3. Pick the best settings from the test-plate scan of parameters. If you don't get good results with one of the two settings i've offered for you here, you may need to start tweaking values based on the explanations listed above.  Note that each set of parameters has 3 test objects:  A cylinder, a double-hex, and a little pot, to test major geometry types. 

1. Test 1:  Original Base recommended settings
2. Test 2: Newer overhang optimized settings.
3. Test 3:  Scarf Seam off.. reference print.

Don't expect perfectly invisible seams.   While scarf seams are a drastic improvement in many cases, it doesn't mean they're always perfectly invisible on close inspection.  This feature is cutting edge.

What optimization work has been done ?

Stage 1:  Cylinders + Double Hexes

I first spent a good deal of time (60+ experiments) optimizing the scarf seam on simple models (cylinders and a double-hex).  You may see my work in the attached excel sheet and the huge number of parts (green pile of junk in pictures) with all the settings I tried.  If you wish to reproduce my work for this stage (SCIENCE!), you can do so by using these settings with the test plate I have included here.

 These were done on my Bambu X1C printer with a 0.4 Nozzle.  I do believe most of these optimization results will translate to most printers..  however some experimentation may be necessary for your printer.  I recommend running the 2x5 object test plate I've included here to validate that you will get good results with the suggested settings on flat/angular and circular surfaces before you use it on a whole print.

Stage 2: Complex curved pot

Not satisfied with the results of stage 1 on overhangs, I then sought to improve matters by designing and optimizing a small curved pot to find good overhang performance settings. It's small and cute and looks like this:

This took an additional 80+ experiments (see mess of pots image above), which unfortunately I haven't had the time to compile into an excel sheet.  You'll have to take my word for it that I tested a ton of things, and my general findings are summarized above in the Overhangs optimization / additional considerations section.  Basically, if you can help it, try to avoid placing the scarf seam on an overhang via seam painting.  If you can't, I've tried to formulate the best settings for you.  Find them in the updated test suite.

Other notes:

The hard work here is a collaboration with the broad 3D printing community and in particular many great contributors to Orcaslicer.  

On the model optimization, I worked with Michael from Teaching Tech.  He has another test model he designed, which is more expensive to print than my test plate, but is a better overall validation.  I recommend printing that after you run my test plate to pick the best parameters.