1. Sculpteo’s 4th Annual Report on 3D Printing and Digital Manufacturing
https://www.sculpteo.com/en/get/report/state_of_3D_printing_2018/
2. Wohlers Report 2018: 3D Printing and Additive Manufacturing State of the Industry Annual Worldwide Progress Report;Wohlers,2018
3. Wohlers Report 2019: 3D Printing and Additive Manufacturing State of the Industry;Wohlers,2019
4. The RepRap Project
https://reprap.org