Abstract <p>This paper presents a design-oriented framework for engineering internal surface roughness in additively manufactured (AM) polymer pipes intended for mixing and flow conditioning. The framework takes advantage of the natural layer surface texture generated by AM processes, with a focus on thermoplastic fused filament fabrication (FFF). The proposed approach integrates geometric design rules, AM and material constraints, and both computational and experimental evaluation. The objective is to design functional roughness patterns on internal pipe walls using only the printed layer lines, while the constraints come from the limitations of the manufacturing process and material used. Because of this, the framework incorporates manufacturability filters and material selection guidelines to ensure that proposed roughness features exceed the geometric noise of the printing process and remain stable under operating conditions. An illustrative case study was done to explore and demonstrate the framework, which included computational fluid dynamics (CFD) and experimental tests. The pipes used were polylactide (PLA), manufactured with uniform ridge heights and tested under transitional flow conditions; the effect of ridge height at different height scales was the main exploration variable. The results illustrate the major elements of the framework and suggest the existence of a minimum effective roughness threshold for transitional flow. The study establishes a foundation for the design of more advanced roughness patterns for applications in mixing, heat transfer, and general flow conditioning.</p> Graphical Abstract <p></p>