1. Uneven temperature of extruder: Uneven heating temperature of extruder may lead to uneven thermal expansion and cold contraction during extrusion, thus making the extruded pipe irregular in shape.

2. Mold design or manufacturing problems: Inaccurate mold design or manufacturing may lead to out-of-round extrusion pipe.

3. Material properties: poor fluidity or melting characteristics of materials may lead to unstable extrusion pressure, resulting in out-of-round extruded pipes.