There are many different types of 3D Printing. But there are 3 types we often used :
FDM / SLA / SLS. What kind of product suit these 3D Printing.

In terms of materials, Plastic is suitable for FDM such as ABS, PP and so on. But
these materials must be the soft plastic. Photopolymer is suitable for SLA.
But Photopolymer is only a comprehensive concept for the photosensitive resin. It
has a wide range of materials. Please choose the right one. Metal Powder or
Ceramic Powder is suitable for SLS. SLS is the high end 3D Printing. Even for the
Chinese Stealth Jet Fighter-J20, there are many inside components which made by
SLS 3D Printing.

In terms of Product Structure, If the item infrastructure is too complicated such as
making a human skull for the model in medical teaching. We can use 3D Printing
technology to make it. Even the artificial human heart also can be made by 3D
Printing.

So when you designed a machine or device, we need the prototype part to
assemble the prototype equipment to check the technological feasibility. The 3D
Printing can be a good way. You can ask help for some companies such as
2Fprototypes from China. After all the suppliers are very professional on this.
They not only can provide you the engineering resolutions but also can provide you
the one-stop manufacturing services. Of course if you are very good at the
programs and know how to operate the 3D Printer, you can buy the 3D Printing
machine and do it on your own. That may have a lot of fun.