FDM parts are inherently anisotropic — they are stronger within layers and weaker between them. This becomes critical for load-bearing parts. If tensile or bending forces act across layers, the part is more likely to fail at layer interfaces. Wherever possible, parts should be oriented so that primary loads run along the layer direction. In real applications, poor orientation is one of the most common reasons for unexpected part failure.

Thin walls and slender features may look fine in CAD but often behave differently during printing. Walls below 1.2 mm can become inconsistent or fragile, especially in taller geometries. For functional components, 1.5–2 mm provides better reliability. Thin vertical features, even if printable, can become brittle or vibrate during printing, leading to poor surface quality or dimensional deviation.

While FDM can handle moderate overhangs, angles beyond 45° typically require support. Unsupported regions may sag or show poor surface finish. Bridging (printing across gaps) is possible for short spans, but longer bridges can droop depending on material and cooling. Designing with self-supporting angles or breaking complex geometries into multiple parts can significantly reduce post-processing effort and improve consistency.

Sharp corners are natural stress concentrators and should be avoided in functional parts. Internal corners should include fillets (typically ≥1 mm radius) to distribute stress and reduce the risk of cracking. This is especially important in parts subjected to repeated loading or vibration. Small design changes here often make a noticeable difference in durability.

As thermoplastics cool, they shrink — and this can lead to warping, especially in large flat areas or long walls. Materials like ABS are more sensitive to this. Adding fillets, ribs, or breaking large surfaces into smaller sections helps reduce internal stress and improves dimensional stability. Ignoring this often leads to parts lifting from the build plate or deforming after printing.