Hi,

I get error "ERROR: The given mesh is not closed! Unable to convert to
CGAL_Nef_Polyhedron." at the render stage when using either of the scad
files shown further down in this email.
This happens even if I run it through
https://www.formware.co/onlinestlrepair

OpenSCAD version: 2021.01
OS: Windows 10
'Normal' stl file :
https://drive.google.com/file/d/1ifjqNpaKD4BoEhCJbF08Vpw2lHTa48WM/view?usp=drive_link
'Repaired' stl file :
https://drive.google.com/file/d/1HtTx3R_td5oH-A6fGRcz7PK-mCRDLI4F/view?usp=drive_link

If I take out the translate cube lines, it renders fine.

Any ideas please?

Thanks.

Richard.

depth_to_cut=25;
width_to_cut=40;
thickness_of_key=3;
distance_beyond_key=9;
depth_of_key=1.0;

difference(){

translate([-8.5,12,-10]) rotate([94,-11.5,0])

import("172079254104-OSC_fixed.stl", convexity=3);
#translate([-width_to_cut/2,thickness_of_key/2,-depth_to_cut/2])
cube([width_to_cut,width_to_cut,depth_to_cut]);

#rotate([0,0,180])

translate([-width_to_cut/2,thickness_of_key/2,-depth_to_cut/2])
cube([width_to_cut,width_to_cut,depth_to_cut]);

}

One piece of the puzzle is easy:  if there's only one object, the
imported STL, then most of the CGAL processing doesn't get done and so
any problems with the STL are not detected.  It's only when it's
necessary to do Boolean operations that the problems are detected. 
That's why it renders fine when you remove the cubes.

For example, here's a malformed cube.  (It's created with polyhedron()
rather than as an imported STL, but the issues are the same.)  The top
face is wound backwards.  (Use View/Thrown Together to see this.)

There are no complaints as long as it's the only object.  In fact, you
can remove one of the faces and there will still be no complaints.

But as soon as you add in the tiny cube(), CGAL hates it.

Note:  if you experiment with this sort of thing, Design/Flush Caches
before each render attempt.  There's something of a bug in that OpenSCAD
will cache subassemblies that had render errors, and so subsequent
render attempts will get the empty result but will not show an error
message.

points = [
    [0,0,0],
    [0,0,1],
    [0,1,0],
    [0,1,1],
    [1,0,0],
    [1,0,1],
    [1,1,0],
    [1,1,1],
];
faces = [
    [2,3,1,0],
    [2,6,7,3],
    [4,5,7,6],
    [4,6,2,0],
    [5,7,3,1],  // incorrect
//    [1,3,7,5],  // correct
    [1,5,4,0],
];

polyhedron(points=points, faces=faces);
cube(0.01);

I'm not an expert at diagnosing STLs, but this one isn't hard to find
problems with, using View/Thrown Together.

The un-fixed version has great gaping holes in the top and bottom, and
smaller problems elsewhere.  (Purple is bad.)

The "fixed" version is much better, but still has some - and all it
takes is one.  Here's a wide shot to establish context, and then a
zoom-in on the central pillar.  Note the tiny purple slices on the right.