I mostly use solid basic shapes (like e.g. a cylinder) which I modify using extrude or moving edges/vertices and then combine using the boolean modifier, which can do union, difference and intersection.

Sometimes you have to switch to carve in the modifier to get it to work correctly.

The important thing is to only work with watertight objects. So for your hollow cylinder example I’d do the following:

• Create a cylinder with the desired outer radius and height.
• Create a taller cylinder with the radius of hole.
• Move both onto the same location. The taller cylinder should stick out on bottom and top
• Select the outer cylinder -> modifiers -> add modifier boolean -> difference to inner cylinder
• Select the outer cylinder and export it with the option “Export only selection”

Watch some freecad tutorials

First off I’ll say that what programs you’d want to use depend on what kind of things you’re wanting to design. If you’re wanting to design something artistic (a sculpture or miniature or model of a space ship or whatever), Blender’s a great choice. If you’re wanting to design something that’s more for mechanical use (a custom wall mount for something, a replacement gear for machinery, a case for a Raspberry Pi, whatever), you’ll probably be better served using a CAD (Computer-Aided Design) program like TinkerCAD, FreeCAD, AutoCAD, or OpenSCAD.

I do a lot of CAD sort of model designs and I personally love OpenSCAD, but it’s made very particularly for programmers. TinkerCAD has a much lower learning curve and is free to use.

I don’t do much artistic model designing for 3d printing. I dabble in Blender too, but mostly just to make minor tweaks to models I found online.

About your cylinder question, you’re right that any slicer would be pretty confused if you gave it a shape like that. To make it work, you’d need a shape that had volume. Not something that’s an infinitely thin surface.

To get a “tube,” you’d have to decide how thick you want the walls and deal with that in whatever program you’re using to make the 3d model. In a CAD program, the way you’d think about this is that you’d add a cylinder and then “subtract” another (concentric) cylinder with a smaller radius. Like, if you wanted a cylinder with an outer diameter of 3cm and a thickness of 2mm, you’d make a 3cm cylinder and “subtract” from that cylinder another cylinder with a diameter of 3 centimeters minus twice 2 millimeters or 2.6 cm. (The second cylinder would also need to be of equal or greater height than the first.)

The result would be an object with volume and with a very specific defined thickness that your slicer would be able to handle nicely.

Where CAD programs let you think in terms of “volumes” of various shapes and combining them in interesting ways, programs like Blender work more in terms of “polygons.” I’m not terribly familiar with Blender, but just off the top of my head, if I were trying to do the same thing in Blender, I might make a cylinder 3cm in diameter, another one 2.6cm in diameter (at the same location with the same height), remove the top and bottom faces of both cylinders, reverse the normals on all faces of the second cylinder, and then manually, one by one, create faces to fill in the “ring” around the top and the “ring” around the bottom.

Does that all make sense?

So it will ultimately come down to what file format you need for the 3D printer and if Blender can support that. Otherwise, you may need to generate your geometry using a CAD program.

Regardless, you will need to generate the solid model of your cylinder. In this case you’d extrude your cylinder surface to create a volume body and not a sheet or surface.

Start off with Thingiverse or similar. I recommend something like: https://www.thingiverse.com/thing:3553160 There are a lot of models there - those are .STL that you “slice” and send to your printer. There is this: https://www.thingiverse.com/thing:2187167 which is CAD models with the working left in - OpenSCAD in this case. You load it up, generate a .STL and then pass that to the slicer.

I have a large plastic cabinet in my garden for storing a lawn mower etc. The hinges died years ago. I have printed new ones. At least 20kg of plastic waste has been avoided being dumped a lot longer. I am well aware (now) that I should not have bought the bloody thing in the first place!

If you go the OpenSCAD route, you might like this: https://github.com/JustinSDK/dotSCAD - the author sadly passed away recently but his work is legendary in my opinion.

Once you get printing sorted out, then move on to your own stuff. … or not - give it a go! I have a large bag of very strangely shaped PETG experiments that went badly wrong and need recycling.

Google for TinkerCad to see an amazingly easy modeling tool. Put shapes together, and set some of the shapes to be a hole (negative shape).

It’s incredibly easy to make useful things.

i use openscad, for simple stuff is very efficient even without knowing much

1. Learn basic freecad
2. Learn basic openscad
3. Learn how to use openscad in freecad
4. Iterate

Side note, freecad is CPU bound, so use a good CPU.

If you are willing to pay the price, you can try Moment Of Invention. It has also a fully functional time limited demo. I worked with the demo and, while for now I use Freecad, I must admit that it is pretty good

To answer your question (and not just recommend another piece of software instead):

Making a cylinder and deleting the cap faces makes what’s known as a non-manifold mesh. To my knowledge this means you can see the backfaces without travelling through any existing faces (in your case, you can see them by looking through the holes you made when you deleted the cap faces).
That cylinder has walls that are theoretically infinitely thin, so you should thicken them up before attempting to print it. You can do this with a Solidify modifier. You can also extrude and scale them if you like, it achieves the same effect.

Given a manifold mesh, your slicer will treat the inside (as in, the direction the backfaces are pointing) as solid, and you can change the density and infill pattern to whatever you like.

Make a few of these shapes, both manifold and non-manifold, and see how your slicer reacts when you tell it to slice them.