An FDM 3D print is not isotropic: strength varies radically depending on the direction in which force is applied with respect to layer. The orientation of the model in the slicer is therefore a critical design decision for any functional part. A correctly oriented part can be 5-10 times stronger than the same part that is incorrectly oriented.
The key concept: anisotropy
In FDM printing, each layer is a fused plastic ribbon deposited on top of the previous layer. The adhesion between layer (interlayer bonding) is always weaker than the strength of the material itself. This means that the print is strong in the plane of layer (X and Y direction) and weak perpendicular to layer (Z direction). Pulling a part in the Z direction causes the layer to separate. Pulling in the X or Y direction causes the material to break.
The basic rule
Orient the model so that the main forces to which the workpiece will be subjected act in the plane of layer (X/Y), not perpendicularly (Z). In practice: if a workpiece is working in tension, the layer must be parallel to the direction of tension. If it is working in bending, the layer must be perpendicular to the neutral axis of bending.
Practical examples
Hook or L-bracket
If moulded standing (vertical), the L-junction is subject to delamination under load. Moulded lying down (horizontal), the layer crosses the critical zone and resistance increases enormously.
Gear or pulley
Print with the axis of the hole perpendicular to the plate. The layer will be circular and the teeth will resist tangential forces. If moulded from the side, the teeth will come off easily along layer.
Container or box
Print with the opening facing upwards. The walls will have layer continuous from bottom to top, maximising lateral resistance.
Clip or bending spring
Orient so that the layer are parallel to the bending direction. If the layer are perpendicular, the clip will break on first use.
Other factors to consider
In addition to strength, orientation affects the need for media (less media = better surface and less post-processing), surface quality (vertical surfaces have the lines of layer, the upper horizontal surfaces are smooth), printing time (lower pieces = less layer = faster printing), and stability during printing (wide base = less risk of detachment).
Practical summary rules
Identify the direction of the main force and orient layer parallel to it. Minimise surfaces that require support. Prefer orientations with the widest base on the platen for stability. For cylindrical parts, print with the axis perpendicular to the platen. If in doubt, print the part in two different orientations and test which one resists more.
