5.2 Rules for printed structure
The B2 review and the v2 design turned into a set of rules. They now apply to every structural part in this project.
Orientation
- Load along the layers. Layers bond at 50 to 70% of the in-layer strength. Pick the orientation before adding features.
- A side print repeats the profile in every layer. A feature as wide as the part is a T-junction inside each layer. A narrower one starts above the bed and overhangs.
- Keep a side-printed part at constant width. A taper adds new, unsupported area to every layer in the lower half. That is why the v2 spine stays 60 mm wide and does not flare into the foot.
- Walls set stiffness in a side print. The curved faces are perimeter walls. Two 0.42 mm walls each side give about half the solid stiffness of an 8 mm band. Top and bottom layers only thicken the two narrow ends.
- 45° is not a free lunch. Tilting a part 45° evens out the weak direction at the cost of the strong one, needs support under the whole part, and steps every face and hole. Use it only where two loaded legs cannot both lie in the layers.
Fasteners
- Heat-set inserts go in flat-printed parts, axis vertical. An insert presses outwards all round. With its axis in the layer plane, part of that pressure pulls layers apart and can split the wall along a layer line. The side-printed spine has only clearance holes; its mates hold the inserts.
- A blind insert hole needs the insert's length plus about two thread pitches: about 6.7 mm for a 5.7 mm M3 insert.
- Countersunk screws are measured head included.
- Shoulder screws in keyholes hang a part at a fixed height with no tools.
Joints
- Laps, not butt joints. Spread screws along the direction of the moment.
- Design joints before trusting a split. A fused check solid hides overlaps.
Creep governs PLA stands
Stresses in these stands stay near 1 MPa, far below breaking. The limit is creep under constant bending. Keep the support under the centre of gravity, and keep warm parts (the PC's base) off structural PLA.
Can FreeCAD do the analysis?
Partly. FreeCAD's FEM workbench meshes with Gmsh and solves with CalculiX. It can apply gravity and point loads to a PLA part and report stress and deflection. That is a good check of the hand integration above.
Layer orientation is harder. The FEM GUI takes an isotropic material only. CalculiX itself supports orthotropic elastic constants, so the weak Z direction can be modelled by editing the solver's .inp input file with the print's axes. Even then it is linear and short-term: it does not model creep, layer adhesion failure or the infill pattern. The useful approach is to compare orientations and layouts, not to predict a life.
I have not run FEA on the stand yet. The hand method was enough to show a 25× difference between layouts.