Issue with exporting STL models inside a part back to STL for printing - A very technical question!

[Anonymous]

Hello everyone,

I think I have a very technical problem here. I create parts to print in 3D (resin) but the bases often have problems with one of the methods I use... I want to understand the difference between the two workflows I follow because this will impact the amount of work later down the line

I start with a base designed in F360 (profile, extruded).

I use a model of a jaw that was scanned and exported as an STL file.

workflow 1:

- open base model

- import mesh of STL file and reposition appropriately

- reduce mesh to 39000 faces

- convert mesh to BREP

- export the result in STL.

workflow 2:

- open base model

- import mesh of STL file and reposition appropriately

- export the result in STL.

This has consistently produced files that were up to 6 times bigger for workflow 2, and the bases (quite flat and designed on F360) do not stick together, whereas the printed jaw models all look good.

Logic would have it that the jaw models don't come out the same, considering that I am skipping steps on that front, but NO! It is the base instead...

So my question is: How does F360 export to STL? Does it look at bodies that are already 'meshed' for the maximum resolution and then match that when exporting everything? 
What could justify the bases not working, but the jaws are fine?

Attaching a model (mesh is the imported STL after reduction, adn the body is the converted mesh to BREP).

[etfrench]

It's not clear what you mean by the base not sticking together.  Is it not sticking to the teeth, or are the individual layers in the base not sticking together?  Here are the settings I use for creating STLs:

I would suggest a slightly different workflow and just use the imported stl instead of converting it to brep. 

https://knowledge.autodesk.com/community/screencast/842f29ea-1218-4682-b82d-4ccd357419ef

[TrippyLighting]

It makes virtually no sense for this work to be done in Fusion 360.

Converting a mesh into a BRep jsut to export it again is nonsense. In order for two meshes to print "combined" they don't have to be combined as solids, they simply have to overlap. The rest is done by the slicing software.

You'd be better off using Meshmixer!

[hfcandrew]

Agreed.

Also check out D3Mesh, it automates Meshmixer with the mmAPI. 

https://www.youtube.com/watch?v=Gy7IKnSThKE&t=2s

[TrippyLighting]

Thanks for posting this. I don’t have a 3D printer or work much with triangulated meshes. The entire channel this video comes from is very interesting and shows many interesting Workflows with Meshmixer. And it is entirely concerned with dental models, which should be super useful for the OP.

[Anonymous]

Hello everyone,

thank you for takign time to respond to my post. I am using Fusion360 because I design tool paths and extract GCODE for CNC machining from it. This is part of a long workflow, and this necessitates the conversion of imported STL files back into a body that Fusion360 can work with.

The issues with printing come from the base layers detaching on the edges (see picture below). We have two machines and both show the same problem with the models when the imported STL hasn't been converted to a BREP.

The bottom one in blue has issues on the actual teeth, but it is the case where that is present (out of probably 30 samples printed).

Any further suggestions, @etfrench @TrippyLighting @hfcandrew ?

[hfcandrew]

Are those wax models? Are those models being milled out? How is it separating? Is that just a mill line gouged out?

[etfrench]

@hfcandrew They are 3d printed probably with SLA printer. 

@Anonymous What settings are you using in Fusion 360 when exporting the STL?  How do they compare to the one I posted?  Have you tried maximizing the facet count?

[Anonymous]

Hello everyone,

@hfcandrew they are indeed 3d printed in acrylate resin

@etfrench the 'SAVE AS STL' option is only available for a single body... I need to export the two bodies, so I 'EXPORT AS...' from the main menu and there are no advanced options for that export method, unless I am mistaken.

[hfcandrew]

In Meshmixer you are export multiple shells (bodies) as one file, or combine shells.

I can't see any reason why the model would print with that peeling separation. Look through your slicer layer by layer and see if there is something funny at that point, such as a layer height that is too big.

I'd stop using Fusion for this. I'd use Deskproto as the CAM and Meshmixer for the CAD.

[TrippyLighting]

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@hfcandrew wrote:

I'd stop using Fusion for this. I'd use Deskproto as the CAM and Meshmixer for the CAD.

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Exactly! Use software that was purpose designed to work with your data format, triangulated meshes.

[hfcandrew]

Sorry I stated that first line poorly:

In Meshmixer you can export multiple shells (bodies) as one .stl file (ASCII stl) , or combine shells into one body and export (ASCII or binary).

[etfrench]

I'll have to disagree with @TrippyLighting  and @hfcandrew about using Fusion 360 for this.  While Meshmixer may be better, Fusion 360 seems to be more than adequate for adding a base to the stl.

@admin Save as stl is available by right clicking on the top node of the browser.  If your file contains other components which you don't want to include in the exported stl,  import the stl into a sub component and create another sub component for the base.  Use the parent component to create the combined stl.

[TrippyLighting]

When you state that Meshmixer might be better, but Fusion 360 is more than adequate, what grounds do you make that statement on ?

1. As I understand it, dental scans (and not only those) usually are relatively high in resolution and in that resolution cannot be converted into a BRep. Thus they have to reduced, which also reduces the level of detail. I don't know how that is reflected in the physical end result, but the fact is that you are technically machining a faceted solid.
   However, that reduction is not needed in Meshmixer at least theoretically yielding a better result.
2. This forum is full of problems resulting from importing .stl files. You might have gotten lucky with a single object and not encounter problems,  but chances are that might not continue. CAD software represents geometry with analytical and NURBS geometry. Mathematically precise descriptions of geometry, free of resolution. These descriptions of geometry require more computational resources than a mesh.
   When a triangulated mesh is converted into a BRep, each triangular facet is converted into such mathematically precise data. When you then combine the base with the faceted geometry Fusion 360 has to compute the intersection between hundreds, or thousands of such facets. Not only does that take longer, that might not be a problem, but very often problems with near coincidence and other such "things" are encountered resulting in error messages. With that many faces to intersect statistically the chances are high that an unfavorable condition exists. Often these problems cannot be worked around or require substantial trial and error.
   
   As such I stand by my recommendation to work with a software that avoids this stuff altogether because it was purpose designed for working with meshes!

[etfrench]

It should not be necessary to convert to brep.  The delamination of the base is probably a problem in the slicer or the printer.  It would be interesting to compare the original full resolution stl with the output of the combined base and full resolution stl from Fusion 360.

[Anonymous]

Thanks again to everyone for participating.

Sadly, I need to do all the workflow in F360 (Can't really discuss why). I cannot right-click to 'save as STL' for a file that has more than one body, as it won't combine it naturally. So I will just need to commit to one of my workflows and hope that the print reject rate is low.

[hfcandrew]

Boooooo!!! Anti-climatic result to a rather engaging thread.

lol, jj, gl

[TrippyLighting]

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@Anonymous wrote:

Sadly, I need to do all the workflow in F360 (Can't really discuss why). I cannot right-click to 'save as STL' for a file that has more than one body, as it won't combine it naturally. So I will just need to commit to one of my workflows and hope that the print reject rate is low.

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I believe in that case you are stuck. So you'll have to either fight with an application that unsuitable for your work or you pick an internal fight in your organization to allow using the tools you need.

Of course that assumes that the reasons you cannot discuss teh "why" this are external 😉

[etfrench]

Perhaps you just need a new slicer 😀