Device for assembling plastic objects, said plastic objects comprising caps suitable to be coupled to a base support that is substantially at least partially counter-shaped with respect to said cap, said device comprising at least part of an assembly line of said caps and said base supports defining an assembly path and an assembly plane including said assembly path and - said device comprising a spindle suitable to pick up one of a choice between said caps and said base supports in a controlled manner, and suitable to position and push one of a choice between said caps onto said base supports and said base supports onto said caps arranged along said assembly path on said assembly plane, - control means comprising an auxiliary control system suitable to translate and rotate said spindle along directions perpendicular to said assembly path and plane (3a, 3b) and comprising a third encoder suitable to detect the orientation of said spindle with respect to said assembly path, said device being characterised by comprising - a plurality of spindles suitable to be moved along said assembly path by means of an apparatus, suitable to pick up one of a choice between said caps and said base supports in a controlled manner, and suitable to position and push one of a choice between said caps onto said base supports and said base supports onto said caps arranged along said assembly path on said assembly plane, - a positioning apparatus comprising at least a first detector and a second detector, said first detector being suitable to detect said first orientation of said base support with respect to said assembly path, said second detector being suitable to detect said second orientation of said cap with respect to said assembly path, said control means being suitable to detect said first orientation of said base support and said second orientation of said cap by means of said detectors (70,71) and to operate said auxiliary control system in order to align said base support and said cap. - said device comprising a spindle suitable to pick up one of a choice between said caps and said base supports in a controlled manner, and suitable to position and push one of a choice between said caps onto said base supports and said base supports onto said caps arranged along said assembly path on said assembly plane, - control means comprising an auxiliary control system suitable to translate and rotate said spindle along directions perpendicular to said assembly path and plane (3a, 3b) and comprising a third encoder suitable to detect the orientation of said spindle with respect to said assembly path, said device being characterised by comprising - a plurality of spindles suitable to be moved along said assembly path by means of an apparatus, suitable to pick up one of a choice between said caps and said base supports in a controlled manner, and suitable to position and push one of a choice between said caps onto said base supports and said base supports onto said caps arranged along said assembly path on said assembly plane, - a positioning apparatus comprising at least a first detector and a second detector, said first detector being suitable to detect said first orientation of said base support with respect to said assembly path, said second detector being suitable to detect said second orientation of said cap with respect to said assembly path, said control means being suitable to detect said first orientation of said base support and said second orientation of said cap by means of said detectors (70,71) and to operate said auxiliary control system in order to align said base support and said cap.