Methods, particularly automated methods, are provided, as well as 3D-printed composite and hollow objects and 3D-printing systems for printing them. Methods comprise deriving a central line of a hollow 3D object model, calculating reference point(s) and/or line(s) along an inner surface of the hollow 3D object model, and filling the hollow 3D object model with material that comprises a thread defined with respect to the central line and the reference point(s) and/or line(s) and with filling material surrounding the thread. The support construction thus formed may be removed from the 3D object by pulling on the thread, extracting it and the surrounding support filling from the hollow object, thus enabling 3D-printing of convoluted or elongated hollow objects and objects with narrow openings. The parameters of the thread, such as type of curve and thickness, are selected to ensure thread extraction without risk of tearing or knotting the thread.

A double-layer cushioning structure and a preparation process thereof, relating to technical field of packaging products. It includes a buffer cover. Hollow double-wall structure in buffer cover play a good buffer role, when an outer wall is deformed inwards by impact of external force, reactive force will offset impact of external force to achieve buffer effect; first reinforcing ribs and second reinforcing ribs strengthens strength of inner and outer walls, thereby reducing wall thickness, softening product, and saving processing materials; first connecting holes, second connecting holes, and support holes between inner and outer walls can better support inner and outer walls, and longitudinally strengthen buffer cover, and further ensure overall structural strength of buffer cover; soft material is provided on a wall surface of a product protection groove, to further soften wall surface in contact with the product to protect surface of product from being scratched.