A device for assembling plastic objects including caps for coupling to base support at least partially counter-shaped to the cap. The device includes at least part of an assembly line defining assembly path and assembly plane including the assembly path and plurality of spindles moveable along the assembly path by an apparatus and configured to controlledly pick up the caps, the spindles position and push the caps onto the base supports along the assembly path; positioning apparatus including first and second detectors, controller with auxiliary control system to translate and rotate spindles along directions perpendicular to assembly path and plane and third encoder to detect spindle orientation relative to assembly path. First detector to detect first orientation, second detector to detect second orientation and controller to detect first orientation and second orientation by detectors and operate auxiliary control system to align base support and cap.