What can be said about the seam lines in FDM parts?

The correct assertion regarding seam lines in Fused Deposition Modelling (FDM) parts is that they are a visible feature between layers. In FDM printing, the process involves extruding thermoplastic filament layer by layer to build up an object. Each layer is deposited on top of the previous one, and the transition where one layer ends and the next begins is what creates the seam line.

Seam lines occur due to the nature of the additive manufacturing process, where the filament cools and solidifies as it's deposited. These lines can be more pronounced depending on various factors such as print speed, temperature settings, and the type of filament used. Additionally, the visibility of the seams can be influenced by the design of the part, the slicer settings, and how the print head travels, especially during retractions and layer changes.

While sometimes these seams can be minimized through settings adjustments and strategies like changing the starting points of layers, they are inherently part of the FDM process and serve as an indicator of layer adherence. Therefore, recognizing that they are a visible feature is key when analyzing the quality and aesthetics of 3D printed parts.