Put simply, 3D printing makes a physical object from a digital file.
There are many different types of technologies that fall under the heading of 3D printing but they all work using an additive manufacturing process. This means that at the start of the process there is nothing and material is continuously added until the final object is ready. This is different to a subtractive manufacturing process (such as CNC machining) because those processes start with a raw block of material and material is taken away until the final object is ready. See a time lapse video of a 3D printer in action and the basic steps required for 3D printing below.
Three steps for 3D printing
The first step in the 3D printing process is to get a digital file (usually an STL file) of the object you want to make. There are two ways to get this digital file, either by creating the digital file from scratch using Computer Aided Design (CAD) programs or by scanning an existing object you want to replicate. Below we have made a CAD model of the Joint Strike Fighter jet.
CAD model of a Joint Strike Fighter jet
Then this digital file is fed into a slicing program that converts the digital file to a form that the 3D printer can read. This contains all the information the 3D printer needs to make the object and you can change the parameters of the physical object in the slicing program. Generally most 3D printers sold come with their own slicing programs or you can buy specific programs (Eg Simplify3D) or even use open source programs (Eg Slic3r). Below is the Joint Strike Fighter jet in the slicing program. The slicing program calculates how the 3D printer will move around and create the physical object.
STL of the fighter jet in the 3D printing slicing program
Finally the 3D printer takes this information and, in the case of Fused Deposition Modeling (FDM) machines, a layer of melted plastic is extruded onto the bottom (build plate) of the 3D printer. Then the next layer of melted plastic is extruded onto the first layer and so on until the object is finished. The plastic will cool and solidify almost as soon as it has left the 3D printer nozzle so you have a finished object as soon as the printer has finished. Below is the finished Joint strike Fighter jet that has been printed in ABS plastic.
Most desktop 3D printers use FDM technology because it is cheaper, requires less power and does not require expensive material as the input material. However it is also generally less accurate and is more limited in the materials it can use. If a digital file is suitable for use on a desktop 3D printer then it will be suitable for use on any other type of printer. This is why all the designs on Trinpy are guaranteed to be printable on a desktop 3D printer. Many CAD files are simply un-printable on desktop 3D printers because they were not designed with desktop 3D printing in mind. For example they may have very thin sections, extremely fine details, large unsupported sections, incorrect tolerances between parts and other problems that mean they will be un-printable on desktop 3D printers.
For more information see Different 3D printing technologies and Materials for 3D printing (coming soon).