First 3D Printing is an additive manufacturing process, where material is added upon by each successive layers in various forms namely solid plastics called asFDM Process. Also you can add resin and powder by stereolithography processes such as SLA & SLS.
To manufacture any products in 3D Printers, you need to design first in CAD (solidworks, Catia) or other softwares like SketchUp, FreeCAD and others as per your applications. Also, you can download files from file sharing platforms such as thingiverse or refer to 34 Best Sites for Free STL Files, 3D Printer Files & Models | All3DP.
Then you need to select the process as per your requirement and budget. For example if you need parts for direct casting purposes then you need either wax parts possible in SLA. Also, if you need direct replacement parts for high strength & precise parts SLS is the answer. If you need to check the design aesthetics, fitment & other parameters and need a solution in small budget FDM (Fused Deposition Modelling) can be the best solution. In my personal opinion every first prototype should be FDM.
There are many materials used nowadays for 3D Printing.
ABS, PLA, Nylon, HIPS, PCetc.
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