Cylinder on box. Why does cylinder seem to start printing early?

[DavePSB 0]

(Using Cura 4.5.0)

 

I ran across this behavior, which I'm sure is correct, but I would like to know what Cura is thinking...

 

Relevant screenshots are collected in the attached .jpg.

 

I modeled a simple cylinder sitting on a box in Blender.  They are two separate objects but meant to be printed as one object (they are not boolean'd together or anything).  The bottom face of the cylinder has the same z coordinate as the top face of the box. 

 

Both objects were exported together to  a single .stl.  (Select both objects and export selection).

 

(To be clear in what follows below:  I always expected Cura would print these two objects as a single object.  That is, I never expected to get two loose objects off the printer!  I know that Cura seems to do its own union of multiple objects before slicing)

 

Now, when sliced, I thought that the four top-layers of the box would have been completed before any sign of the cylinder showed up. But it seems the cylinder starts appearing in the first top-layer of the box.  The screenshots will show what I mean.  It is not like the cylinder ends up printing 4 layers shorter.  Rather, it looks like Cura is somehow anticipating the start of the cylinder 4 layers ahead of time.

 

I am happy with the behavior, but just want to know why it happens as it may affect structural decisions in future prints.

 

Thanks!

 

[Cuq 162]

When you import a model into Cura  where several part are in the same STL file, the geometry is considered as a single part. That's why you get this result. But if you stack the same part from two different model in this case Cura knows that it's two part and you get the second result . Hereafter the same geometry but on the right it's  one STL import and 2 in the first left case.

Edited June 25, 2020 by Cuq

[DavePSB 0]

Thanks for the quick response.  Its good to know that I can get either result depending on how it is exported.

 

I can now understand why Cura is creating the hole.  When the two parts are exported in one .stl, it helps to imagine that Cura wants to be able to fill the entire part (cylinder + box) with water in one filling (rather than filling the box then the cylinder).  This explains why the hole pierces all four top-layers of the box.

 

Cheers!