Design for the process
A plastic part is only as good as the process it was designed for.
The same bracket is a different part when it is printed, molded, thermoformed or extruded: different walls, draft, radii, tolerances and cost. Designing for the wrong one is how a part that worked as a print fails as a mold. Each design here starts with the question "how will this be made, and at what quantity," and the geometry follows.
1
3D printing
FDM or SLA. Orientation, supports, wall and infill choices, tolerances that printers actually hold. Right for one-offs, fixtures, enclosures and short runs.
2
Injection molding
Draft, uniform walls, radii, parting line, gate location, ejection. Designed so the printed prototype mold and the production tool see the same part.
3
Roto, thermoforming, extrusion
Rotational molding for hollow parts, thermoforming for large thin shells, profile extrusion for constant cross-sections. Each has rules; I have designed for all three.