In an effort to try and control the internal fill material of a part (both direction and amount) I’ve been experimenting with some ideas. (I am using a PP3DP UP! Plus printer, but this applies to any 3d printer)

If the generic cross pattern is either not strong enough for you, or you want to tailor the direction of strength a little better what you must do is model some features in your part that the slicing software can use to put material where you want it.

The simplest method I’ve found is to model internal “cuts” or “slits” in the part. For the UP printer, the slicer software will do the 2 walls around the opening, but if the cut or opening is modeled narrower then 0.1mm, the opening never really materializes (is not present in the internal structure of the print) since the material from both sides will fuse together when being printed. I’ve been using 0.05mm for the slit width.

So if your complex geometry part needed to be stronger in one direction then in another, you could model some “slits” in the direction you want more strength* and less (or no slits in other directions).
* More strength in comparison to the generic fill of the slicer software.

This method can not only generate strength in desired directions, you could use it to just create thicker walls (more around the perimeter, more walls if the slicer software gives no control of that like the Up! Plus software)
Another thing this method can create (which is not currently possible with the UP software) is a solidly filled object. Just create offset surfaces of the outside perimeter surface (towards the inside of the part) every 1mm or so. Then turn those offset surfaces into “cuts” into the part. At each one of those location, you’ll get another set of perimeters.

A few things to note:
– With ABS, I found that walls shift slightly when more material is deposited due to the extra material the “slits” introduce. On my Up! Plus walls shift about 0.15mm to 0.2mm. I can compensate by adjusting the CAD model to account for this. Your millage will vary so do some testing.
– With PLA I’ve noticed a lot less of this wall shift. I haven’t done any extensive testing, but from the few parts I’ve printed I’d say under 0.1mm of wall movement is observed.
– Depending on how smart (or not so smart in the case of the Up! Plus) your slicer software is, it may not know that

I think the best part of all this the ability to internally “truss” a part in the Z direction with solid material. Normally in the Z direction the only material that can resist breaking is the strength of the inter-laminar bonding of the plastic fibers(deposited filament). And really only the perimeter 2 layers, since I would not count on the infill to resist much when stressing the layer bonds.
Imagine printing a tall (in the z direction) skinny tower. If you try to snap it, it will break at the layer bonds.
Now I can make internal diagonal solid pillars (reinforcements) that can resist forces in the designed directions.

More on this as I do more experiments.