There's no single "strongest" layer height in 3D printing; the ideal height depends on several factors. While thinner layers initially seem advantageous due to increased extrusion count, the relationship between layer height and strength is more nuanced.
The Complex Relationship Between Layer Height and Strength
The strength of a 3D-printed part isn't solely determined by layer height. It's a balance between several factors:
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Layer Adhesion: Thinner layers can lead to improved adhesion if proper adhesion between layers is achieved. However, more layers also mean more potential points of delamination (separation between layers). Sources like Reddit discussions https://www.reddit.com/r/3Dprinting/comments/17jxjr6/whats-stronger_fewer_thick_layers_or_more_thin/ and a Prusa forum https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-how-do-i-print-this-printing-help/settings-for-functional-strength/ suggest that thicker layers sometimes exhibit better adhesion and thus, stronger overall prints.
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Extrusion Width and Cross-section: Thicker layers have fewer, but thicker, extrusions. Thinner layers have more, but thinner, extrusions. The overall cross-sectional area of the material in each layer plays a crucial role. A balance must be struck—thin layers that don't bond well are weaker than thick layers with solid bonding. This point is made clear by Markforged https://markforged.com/resources/learn/design-for-additive-manufacturing-plastics-composites/understanding-3d-printing-strength/3d-printing-settings-impacting-part-strength.
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Infill Density and Pattern: The infill significantly contributes to overall strength, often more than layer height itself. Reddit discussions https://www.reddit.com/r/3Dprinting/comments/b4ov98/what_matters_more_to_the_strength_of_a_print/ confirm this. A high infill density with a strong pattern will compensate for less-than-optimal layer height.
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Material Properties: The material used (PLA, ABS, PETG, etc.) also has a dramatic effect on strength, irrespective of layer height.
In short: A higher layer height can result in stronger prints if it leads to better inter-layer adhesion. However, this isn't always the case. The optimal layer height is often a compromise that balances sufficient adhesion with minimizing the risk of delamination. Ultimaker suggests a layer height of 50% of the nozzle diameter as a starting point https://3dprinting.stackexchange.com/questions/6722/relation-between-layer-height-and-bond-strength.
