How do I make partial bottoms?

[Inquisitor720 1]

Another Noob question.  I've had my printer for about a week.  I use a CAD package to create models and I have discovered that I can use shells and Cura (3.3.1) will use the wall thickness parameters (in the Shell section of Cura) and flesh out the walls growing in the Z direction.  I also discovered that I can create inner walls and define just about any shape and have variable thickness walls in any location.  I have some rather complex shapes that I really just want to constant thickness walls.  So just using Shells with no inner walls would be just perfect!  Unfortunately, I would like the bottom to be only partially covered.  So my shell models have shells on the bottom surface in the locations where I want thickness.  Unfortunately, when I Prepare my model, the entire bottom is covered.  If I zero out the "Bottom Thickness" value in the Shell section of Cura, I get no bottom.  Here is a simplified sample... basically a petri dish with a large hole in it.  Just an outer ring of shells and the bottom partially defined.

 

Is there some setting that I can use to achieve this?

 

Thanks for your help.  

[gr5 2,093]

So your model is a cylinder?  But you want the result to look like the upper photo?

[gr5 2,093]

I mean the quick answer is to model what you want - model a hollow cylinder instead of a solid one.  It won't be as perfect as if you model a solid part and turn off infill but it should be pretty close.

 

Then use default settings in cura.

 

[Inquisitor720 1]

Thank you gr5. 

To answer your first message...

1. My actual models are more irregular and difficult to get a precise parallel inner wall.  That is why I would like to take advantage of Cura's ability of using a shell and simply adding the number of layers from its settings.  The above images are just a simple model to illustrate my problem.
2. Yes, I would like the finished print to look like the first image.

From your second message...

Is there a non-quick answer ☺️ (to successfully getting what I need with a solid model / no infill)?    I'm gathering... from your station in the community that I have not "just" overlooked some setting(s) of what seems like thousands to my Noob eyes.

 

I have done as you suggested (making a hollow model) but the results are not good on several issues.

 

Again... thanks for your reply.  

 

[gr5 2,093]

Well you can create a second model that has the inner shape (in your example, a smaller cylinder) and you can load both STL files and you can have cura modify infill settings such that you have different infill settings inside the smaller cylinder than outside.  And you can set one of these to use solid infill, and the other to use no infill.  LIke in this video:

 

But I don't think that will get you what you want.  I'm not sure.  You'd have to play with it.  But the added complication is you would need to model the inner "cylinder" shape (which is not a cylinder in your "real life" examples) anyway.  So at that point you might as well model everything.
 

Anyway I think you are out of luck but maybe someone else has an idea.

[Inquisitor720 1]

This is a great idea!  Didn't think of using multiple models.  And I can certainly apply it to other situations.  But I ran into another snag...

 

I took the test ring model (no bottom) and created a second model of a horizontal stiffener.

 

I loaded both models in Cura and Prepared them...

 

They even printed and bonded together as one. 

 

Unfortunately, when I tried it with one of my real pieces, I ran into a problem.

 

1) I have this as a solid model with no bottom and no fill...

 

 

2) and have this stiffener as a separate model...

 

3) However when I combine them in Cura and Prepare them, the stiffener is ignored...

This shows the outer shell about 200 layers up with an outer shell of 1.2 mm (3 lines).  You can also see the shadow of the stiffener inside, but its not getting prepared (or printed). 

 

Why does the first test work, but not the second case?

 

Thanks for your help!