FDM 3D printing orientation: strength, finish, cost

In FDM 3D printing (Fused Deposition Modeling), the same part, made with the same material and on the same machine, can be strong or brittle, smooth or rough, cheap or costly according to its orientation in the print volume. Orientation is therefore not a simple process parameter, but a design choice that affects the mechanical strength, the quality of the surfaces, the quantity of supports required and, consequently, the production times and costs.

This guide shows how the printing orientation affects four fundamental aspects: strength, finish, supports and cost, and how to identify the most suitable solution for each part.

Why orientation counts: the anisotropy of FDM 

FDM deposits the molten filament layer after layer. Each layer adheres to the previous one through the partial fusion of the polymer in the contact zone. This bond between layers (interlayer adhesion) is weaker than the continuity of the material within the same layer.

From this derives an anisotropic behaviour:

• in the XY direction, on the plane of the layers, the strength is close to that of the solid material because the filament is continuous;
• in the Z direction, perpendicular to the layers, the strength depends instead on their adhesion and can reach only 30% to 70% of the strength in XY, depending on the material, the printing parameters and the chamber temperature.

In practice, an FDM part subjected to tension along the layers resists better, while the same component stressed perpendicularly to the layers can break at much lower loads, separating along the deposition planes.

From this derives the fundamental rule for choosing the orientation: the critical load shouldn’t act perpendicularly to the layers.

Orientation and mechanical strength

The fundamental rule of FDM orientation concerns strength: the part should be oriented so that the main stresses act in the plane of the layers (XY), not through them (Z).

How to identify the critical direction

Before choosing the orientation, it’s necessary to understand how the part will be stressed during use:

• tension or flexure → the most probable fracture plane is perpendicular to the stress. The layers must be parallel to the direction of tension or oriented so that the zone subjected to flexure doesn’t delaminate;
• snap-fits and flexible parts → the flexing of the hook must take place in the XY plane, never through the layers. A badly oriented hook can delaminate already during the first assembly;
• holes and pins under load → the axis of the hole and the direction of the load determine the best orientation to avoid the stress separating the layers;
• components subject to fatigue → cyclic loads amplify the interlayer weakness, making it even more important to orient the main stresses in the XY plane.

The Role of the Material

The extent of the anisotropy varies according to the material:

• materials with good interlayer adhesion, especially if printed in a heated chamber, such as PEEK and ULTEM, present a more contained anisotropy;
• flexible materials, such as TPU, tolerate interlayer stresses better thanks to their elasticity.

In any case, no FDM material completely eliminates the anisotropy: orienting the main loads in the XY plane remains a fundamental rule.

Orientation and Surface Finish

The orientation determines which surfaces of the part will turn out smoother and which rougher.

Surfaces and Their Quality

• top and bottom surfaces (parallel to the print plane) → the smoothest, because formed by solid layers. The bottom surface (in contact with the platform) can turn out particularly uniform;
• vertical surfaces (walls perpendicular to the plane) → they show the layer lines, with a roughness linked to the layer height;
• inclined and curved surfaces → they present the typical staircase effect. The more the surface approaches the horizontal, the more the steps become visible; on almost vertical surfaces the effect is instead minimal;
• surfaces with supports → the zones in contact with the supports tend to remain rougher and more irregular after removal.

How to Orient for Finishing

If a surface has to have a good finish for aesthetic, mating or sealing requirements:

• orient it parallel to the print plane, as a top or bottom surface, to obtain maximum uniformity;
• alternatively, orient it vertically, accepting regular layer lines, often compatible with the application;
• avoid low inclinations that accentuate the staircase and, when possible, the downward-facing surfaces that require supports.

There’s often a compromise between finish and strength. When the two requirements come into conflict, mechanical performance is generally favoured and the surface is improved with post-processing.

Orientation and Supports

Supports are temporary structures that hold up the parts of the piece that would otherwise be printed in mid-air. In FDM, in fact, the filament must be deposited on material already present, and overhangs beyond a certain inclination require a support.

The 45° Rule

In FDM, surfaces inclined up to about 45° from the vertical can generally be printed without supports, because each layer is sufficiently supported by the previous one. Beyond this threshold, the overhang becomes more critical and can require supports.

The orientation therefore determines the quantity of supports required:

• an orientation that creates numerous overhangs beyond 45° requires more supports;
• an orientation that keeps the critical surfaces within 45° or vertical can reduce or eliminate them.

Why supports increase costs

Supports have a direct impact on production times and costs:

• additional material: the filament used for the supports is consumed without becoming part of the piece;
• printing time: their creation increases the duration of the cycle;
• post-processing: the supports must be removed, adding a machining stage;
• surface quality: the contact zones tend to remain rougher and more irregular;
• risk of damage: the removal of supports from thin or delicate geometries can compromise the piece.

How to orient to reduce supports

The main strategies are:

• orient the most complex geometry upwards, resting a wide, stable surface on the print plane;
• exploit the 45° rule, rotating the part so that the critical overhangs remain within a self-supporting inclination;
• assess the subdivision of the part into several parts printable without supports, to be assembled subsequently with interlocks or inserts;
• use supports only where necessary, favouring the non-functional surfaces and protecting those where finish and precision are more important.

Orientation and Cost

The cost of an FDM part depends above all on the printing time and the material consumed, including the supports. Both are influenced by the orientation.

Print Height

The printing time depends significantly on the number of layers, which in turn varies with the height of the part along the Z axis. For the same volume, a component printed low and wide requires fewer layers than the same part oriented tall and narrow, and can therefore be produced more rapidly.

Rule of thumb: orient the part to minimise the Z height, compatibly with the strength and finish requirements. Fewer layers = less time = less cost.

Volume of the supports

Supports require additional material and increase the printing and post-processing time. An orientation that reduces their quantity therefore contributes to containing the overall cost.

The compromise

Strength, finish and cost don’t always lead to the same choice:

• the strongest orientation can require more supports;
• the cheapest orientation can expose the weak direction to the critical load;
• the orientation that improves the finish of one surface can worsen that of another.

There’s no universal “perfect” orientation: there’s the optimal orientation for the priorities of that specific part. The typical hierarchy for a functional component is: strength in the critical direction → finish on the functional surfaces → minimisation of supports and cost.

The decision hierarchy for orienting an FDM part

A practical method consists of assessing the orientation following a precise order of priority:

1. Identify the direction of the critical load. Assess how the part will be stressed during use and orient it so that the main stress acts in the XY plane, avoiding it crossing the layers.
2. Identify the functional surfaces. Matings, seals and aesthetic surfaces must have a good finish. When possible, orient them parallel to the print plane or vertically, avoiding low inclinations with a marked staircase and downward-facing surfaces that require supports.
3. Verify the flexible elements. Snap-fits, hinges and tabs must flex in the XY plane. This requirement can directly determine the orientation of the part.
4. Reduce the supports. Among the configurations that respect the first three criteria, choose the one that requires fewer supports, especially on the critical surfaces.
5. Reduce the Z height. All other requirements being equal, a lower orientation requires fewer layers, reducing printing times and costs.

In case of conflict, the preceding criteria have priority: functional strength comes before finish, which in turn comes before cost. Reducing costs has no value if it compromises the functionality of the part.

Orientation in FDM vs MJF

FDM is strongly anisotropic, so orientation represents a critical design choice. The strength along Z can in fact be significantly lower than that in the XY plane.

MJF (Multi Jet Fusion), on the other hand, presents a more isotropic behaviour, because the fusion of the powder doesn’t depend on the directional deposition of a filament. The mechanical properties therefore turn out more uniform in the different directions and the orientation affects the strength less. In MJF it takes on greater importance for tolerances, finish and packing, that is, for optimising the arrangement of the parts in the print volume. You can explore the topic further in our guide how to orient an object for Multi Jet Fusion 3D printing.

Practical implication: when a component is subject to complex, multidirectional loads, or can’t be oriented while keeping the critical stresses in the XY plane, MJF can represent a more suitable solution thanks to the lower anisotropy. FDM, on the other hand, remains particularly indicated when super-polymers are needed.

Conclusion

The printing orientation is one of the most important design choices in FDM, because it directly affects four fundamental aspects: mechanical strength, surface finish, supports and cost. The guiding principle remains simple: the critical load shouldn’t act perpendicularly to the layers. From this criterion derive the other priorities and the choice of the most suitable orientation for the individual component.

Understanding the anisotropy of FDM also helps to understand when it’s more worthwhile to change technology. If a part is subject to multidirectional loads that can’t be kept in the XY plane, MJF can represent a more suitable solution thanks to the greater uniformity of the mechanical properties. Knowing how to orient a part correctly therefore means not only optimising FDM printing, but also choosing the most suitable process for the application.

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Frequently Asked Questions About Orientation in FDM 3D Printing