floor.scad

 
union()
{
  translate([0, 0, 6])
  scale([0.9842519685, 0.9842519685, 1])
  import("floor.stl");
  cube([50, 50, 6]);
}

Repairing bad STL using AngelCAD polyfix.md

Original problem using OpenSCAD to import an STL fils

union()
{
  translate([0, 0, 6])
  scale([0.9842519685, 0.9842519685, 1])
  import("floor.stl");
  cube([50, 50, 6]);
}

Error message Rendering Polygon Mesh using CGAL... ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation! Expr: e_below != SHalfedge_handle() File: /usr/include/CGAL/Nef_3/SNC_FM_decorator.h Line: 426

Fix the problem using AngelCADs polyfix utility:

$ polyfix floor.stl 

Parameters:
  input_file = floor.stl


polyhedron 0 ================= volume=10890.5, dtol=0.01, atol=1e-06, maxiter=10
iteration 0: vertices=92100 faces=30700
         warning: 1 zero area faces.
         warning: nonmanifold edges: uc(1)=92100 
         merged 76748 vertices
         removed 1 collapsed or zero area face
         split 1 face
         total changes=76750
         no warnings

iteration 1: vertices=15352 faces=30700
         total changes=0
         no warnings

Summary: 
         polyhedron 0: vertices=15352 faces=30700 : no warnings

Writing: floor_1.stl

... polyfix finished, time used: 0d 00h 00m 01s

Then import the repaired file into OpenSCAD and press F6

union()
{
  translate([0, 0, 6])
  scale([0.9842519685, 0.9842519685, 1])
  import("floor_1.stl");
  cube([50, 50, 6]);
}

The problem is now gone

floor.stl

floor_1.stl

floor_angelcad.as

You are welcome, please tell your friends/colleagues about the software so it becomes better known.