These two parts were printed from the same .stl file. The one on the left was printed at .010 layer thickness while the one on the right was printed using .007 layer thickness. The wall was designed to be .050 thick at this point. As can be seen the one on the left has a gap between the inner and outer walls while the one on the right does not. The reason for this is the way an FDM system lays down material when building.
Each bead or “road”, as it is called by Stratasys, is twice as wide as the layer thickness that the model was processed in. For example if you select .010” layer thickness each road will be .020” wide. The same rule holds for all resolutions, with some adjust-ability available for those using Insight and Fortus systems.
When your model wall thickness is exactly four times the layer thickness each road supports the other forming a strong structure. This means the minimum acceptable wall thickness is four times the desired resolution, i.e. .040 for .010 resolution.
When your wall thickness is six times (or more) the layer thickness, you get a strong wall with each road supporting the others. Any wall thickness greater than six times the layer thickness allows the FDM system to “raster” or zig-zag to fill the gap. With much thicker walls you may get double walls on the outside (exposed to air, rather than against more material) of the model. This means .060 or greater walls for .010 resolution.
There is a zone between 4x and 6x layer thickness where you will see a problem like the picture at the top (left side), having a gap between the roads.
Any wall thickness in this range will create weak parts due to no contact between the inner and outer roads. This also allows the Waterworks solution to get trapped in between the walls which will take a long time to drain and may cause discoloration of the parts, particularly if printed in a dark material.
When designing parts for FDM production you need to think about this limitation while designing and either size the walls accordingly or use a different resolution. The example at the top of the post was repaired by printing at .007 resolution, which would have a minimum thickness of .028 (4 times) or .042 or greater (6 times). Since the required wall thickness, .050, is greater than 6x the layer thickness, it was able to fill the gap, while printing at .010 put the wall thickness right in the middle of the zone which we need to avoid, at 5 times the layer thickness.