Model looks ok, sliced wrong and prints wrong. How do I fix?  

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

Your model is very likely not valid.  You did use the 3dprint tool that is part of blender didn't you to check before exporting it ?  Did you check the normals ?

From your pictures it even looks wrong before slicing, it does NOT look fine like your caption states.  Notice those dark areas ?  the missing geometry that should be enclosing the sides in the cutout ?  

If you are on windows 10 you can right click the stl once loaded into Prusa Slicer and use the fix through netfabb option.  That usually does a good job if the model isnt too broken.

If you want to post your Blender file and/or the stl I can take a look.  I use Blender for my designs so I've gotten 'ok' with it.  You will need to zip the files to attach them to this forum.

You have to be very very careful with boolean operations, they can easily make your models non manifold.  They look ok but they really really aren't.  

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

P2_joined.blend

.zip attached, any help appreciated. I'm new to Blender. It looks (to me) like the unions went ok, but my eyes are not great and I could be missing something. I did straightforward unions and differences with no funny tricks; I don't see what could have gone wrong, but I know that's a rookie assumption.

Thanks for any help!

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

Wow, where to start.  The built in 3d print tool shows , 329 Non manifold edges, 12 bad contiiguous edges, 31 intersecting faces and 1 zero face.

For a model to be 3d printable it must be 'manifold'.  In easy terms that can be equated to water tight.
The bottom disc area is not sealed, I can see the inside of it (the red areas) , also one of those outer post & ball features isnt connected properly as theres inside showing through there too.

Blender normals view. - 

When you boolean something if the edges align perfectly or the vertex do then it can actually break the model.  You want to have a slight difference.  Either cut all the way through or leave a thin section.  Also you should be learning how to model non destructively.  Do a youtube search for 'blender non destructive', you should gets loads of hits.  That allows you to reposition things, including all your holes.  You just leave modifiers in the stack rather than applying them.  

They get applied automatically on export (if the option is ticked).  The stl then has multiple parts but that doesn't cause the slicer any issue.

I cant even begin to describe how badly broken that model is.  It would probably be easier to redo it than fix it.  Unless you have a saved version with your basic shapes in before you did any of the booleans ?

Give me a few hours and I'll try and replicate it, but in a non destructive modelling way and then you can open the blend and use it as an example.

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

@neophyl Thanks! I clearly have a lot to learn about Blender. I have no idea how I managed to make something non-manifold - I thought all boolean operations on closed forms that weren't degenerate left closed forms - but I guess not. I'll look at non-destructive operations; because yeah, I've had to start over a few times when I had to move parts around, and it is not fun.

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

@scott-a-mayo

Here you go.  I have attached a new blend file.  There are 2 collections, Main and Cutters.  Main contains all the basic shapes and Cutters, well contains all the shapes used to cut things away 🙂  Your object is also in there as my reference piece too.

When you open it (done in 2.83 btw) you will notice that NONE of the booleans have been applied and that none of the objects have been union-ed together either.  The closest I came was joining a cylinder and a ico sphere for the short posts.  Even that I could separate later if needed.

You can adjust the size or position of any of the cutters still and that will move the corresponding 'cut' into the parts we want.  Same with resizing any of the basic shapes.  I've tried to keep it to the same as you had it but I rounded off things like the slot size to 1mm and 3mm as that was the closest whole value from what I could see and due to the low side count on your cylinders I got the closest I could get.  

Basically you  make sure to only select the parts you want to export, select export stl and then when it brings up the dialog you make sure 'Selected only' is ticked and also apply modifiers.  That way the output stl file contains just what you need and the modifiers all get applied as its created but they DONT get applied inside the blend so you can still work on them.
The zip file also contains an exported stl file.  I used a separate stl viewer to double check that what was exported was correct (always a good idea).  The other thing I tend to do is place it in slicer, slice it and check the preview.  If its got issues , like you had I can then go into Blender, edit, export and then use the reload from disk option in slicer to update the version on the plater.  With a bit of practice it becomes a fairly quick workflow.

Have a look and if you have any questions I'm happy to answer them.  Blender does have a learning curve but you can get there.  It can do just about anything once you figure out how 🙂  

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

@neophyl

I have to say both wow and thank you. 

I have to ask: did you have to sit there and measure each individual shape? Because it looks like you nailed it pretty much exactly.

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

Your original was placed in the centre of the ‘world’ like you originally had it, so for most things I could just add a new mesh and then scale it using the snap functions, snapping it to areas on your  original. The widths of the slots I measured using the measuring tool, created them and then rotated them until they aligned. As the slots origins are located at 0:0:0 still that’s easy to do. 
The posts with balls I measured the post diameters before creating the meshes but I manually positioned the one in the x axis over the original and created the balls and did that by eye overlaying it on top so they aren’t perfectly placed but close. Once the long and short posts were positioned in x then they were just duplicated and rotated 120 and 240 degrees to give the others. 
While I’ve now gotten familiar with the basics for hard surface modelling in blender I’m no where near fast. Some of the guys on YouTube could have probably replicated that part in 5 minutes. If I was doing it for me then when creating the meshes for the ico spheres and cylinders I would have upped the segments lots to make them much smoother.

RE: Model looks ok, sliced wrong and prints wrong. How do I fix?

@neophyl

I made a few changes and got it working. I do have a few questions, but I'll start a new post.