The following code quick-renders fine (F5) but when I try a full render (F6)
nothing shows up  Any thoughts on why?  Using: 2018.03.17 (git 3e7fbfff)

trayHeight = 1.425.4;
maxCutHeight = 0.425.4;

baseThickness = 2;
wallThickness = 2;
cornerRad = 2;
text1 = "1.4"";
text2 = "Subtract 0.0"";
text3 = "For 0 to 0.4"";
textThickness = .6;
textHeight = 4;

riserHeight = trayHeight-baseThickness;
baseLength = 2*riserHeight;

$fn=10;

difference()
{
minkowski()
{
union()
{
linear_extrude(baseThickness)

polygon([[0,0],[baseLength,0],[baseLength/2,sqrt(3)/2*baseLength]],[[0,1,2]],2);

        rotate([0,0,30])
        translate([0,wallThickness/2,baseThickness])
        rotate([90,0,0])
        linear_extrude(wallThickness)
       

polygon([[0,0],[baseLength/sqrt(3),0],[baseLength/sqrt(3),riserHeight]],[[0,1,2]],2);

        translate([baseLength,0,baseThickness])
        rotate([0,0,150])
        translate([0,wallThickness/2,0])
        rotate([90,0,0])
        linear_extrude(wallThickness)
       

polygon([[0,0],[baseLength/sqrt(3),0],[baseLength/sqrt(3),riserHeight]],[[0,1,2]],2);

        translate([baseLength/2,sqrt(3)/2*baseLength,baseThickness])
        rotate([0,0,-90])
        translate([0,wallThickness/2,0])
        rotate([90,0,0])
        linear_extrude(wallThickness)
       

polygon([[0,0],[baseLength/sqrt(3),0],[baseLength/sqrt(3),riserHeight]],[[0,1,2]],2);

    }
    sphere(r=1);
}
translate([0,0,riserHeight])
cube([baseLength,baseLength,5]);

}

--
Sent from: http://forum.openscad.org/

Your polygons are bad, use View/Thrown-Together, purple is bad, see  wiki FAQ
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/FAQ#Why_is_my_model_showing_up_with_F5_but_not_F6?
.

Admin - PM me if you need anything, or if I've done something stupid...

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

The TPP is no simple “trade agreement.”  Fight it! http://www.ourfairdeal.org/  time is running out!

Sent from: http://forum.openscad.org/

Yes I notice. It appears not to be the polygons...

Admin - PM me if you need anything, or if I've done something stupid...

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

The TPP is no simple “trade agreement.”  Fight it! http://www.ourfairdeal.org/  time is running out!

Sent from: http://forum.openscad.org/

It appears to not like the co-incident points at the corners of the base.

It works if you make the base so:
itsy=0.05;
#linear_extrude(baseThickness)
polygon([[0,0],[baseLength+itsy,0],[(baseLength+itsy)/2,sqrt(3)/2*(baseLength+itsy)]],[[0,1,2]],9);

I'm suspect CGAL should handle that. Anyone want to call BUG on this?

Admin - PM me if you need anything, or if I've done something stupid...

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

The TPP is no simple “trade agreement.”  Fight it! http://www.ourfairdeal.org/  time is running out!

Sent from: http://forum.openscad.org/

Oops, not my day today.

I had the cube commented out. Still an issue...

Admin - PM me if you need anything, or if I've done something stupid...

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

The TPP is no simple “trade agreement.”  Fight it! http://www.ourfairdeal.org/  time is running out!

Sent from: http://forum.openscad.org/

Something weird is going on.

I went back to the source in the OP.
Tried it in 2015.03-02 and it works:
http://forum.openscad.org/file/t359/td23839_2015-03.jpg

Then same source in 2018.04.01, which was what I used when I initially
looked with above posts. It now works:
http://forum.openscad.org/file/t359/td23839_snapshot.jpg

Note that these were both fresh run OpenSCAD instances, and F6 was the first
thing I did.
I then did preview, and that displays badly in both versions.

I definitely got the CGAL error mentioned above in my earlier testing.
Weirdness...

Admin - PM me if you need anything, or if I've done something stupid...

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

The TPP is no simple “trade agreement.”  Fight it! http://www.ourfairdeal.org/  time is running out!

Sent from: http://forum.openscad.org/

Thanks, for looking at my problem!  Did you have a recommendation on how I
should proceed?

FWIW, what I'm trying to do is make an OpenSCAD version of this:
https://www.thingiverse.com/thing:2829836

My next step was to clean up the base with the following added to the
difference after the part that cuts off the top:

    difference()
    {
       translate([.5*baseLength,.5*baseLength,0])
       cube([2*baseLength,2*baseLength,10],true);
       minkowski()
        {
            linear_extrude(5)
           

polygon([[0,0],[baseLength,0],[baseLength/2,sqrt(3)/2*baseLength]],[[0,1,2]],2);
cylinder(r=1,h=5);
}
}

In case that triggers a different issue...

--
Sent from: http://forum.openscad.org/

As I can't reproduce the CGAL error on both 2015.03-2 and the 2018.04.01
snapshot, I suggest you try either of these versions, or the latest  here
http://www.openscad.org/downloads.html#snapshots  , first.
I seem to recall a somewhat recent snapshot had a bad regression.

If the error persists, please post the specific error message
(Design/Flush-Caches will reproduce the F6 error message - it sometimes
doesn't get repeated due to caching). And the latest source code if you have
advanced.

There are two issues BTW, whatever is causing the CGAL error, and the really
garbled preview.
The preview can be 'fixed' via Preferences/Advanced-Tab/Enable-Goldfeather.
(I always want to type GoldFinger...for some reason, No Mr Bond, I expect
you to die!)
If that fixes the preview for you, could you post the info from
Help/Library-Info, it often points to a graphics driver problem (different
to the CGAL issue). You could try updating the driver.

For the CGAL error, it should not error when processing normal OpenSCAD
code, imported STLs are usually the cause.
I suspect co-incident faces/edges/points could be the contributor. Like
where the three sides touch each other in the center, and the corners. I'll
look there if you still get the problem.

Admin - PM me if you need anything, or if I've done something stupid...

Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above.

The TPP is no simple “trade agreement.”  Fight it! http://www.ourfairdeal.org/  time is running out!

Sent from: http://forum.openscad.org/

Here's the error message I see:
Rendering Polygon Mesh using CGAL...ERROR: CGAL error in
CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation! Expr:
pe_prev->is_border() ||
!internal::Plane_constructor::get_plane(pe_prev->facet(),pe_prev->facet()->plane()).is_degenerate()
File: /usr/include/CGAL/Nef_3/polyhedron_3_to_nef_3.h Line: 251
OpenSCAD version 2018.03.24

--
Sent from: http://forum.openscad.org/