Most consumer 3D printers on the market today utilize the FFF (Fused-Filament Fabrication) process, which means they require plastic filament. It doesn't take long for most makers to realize that that filament is much more expensive than the same type of plastic in bulk. ABS, for example, which is one of the most common filament materials, costs just a couple of dollars per kilogram when purchased in bulk pellets, but starts around $18 in filament form. You have the potential to save a lot of money if you can make your own filament from bulk pellets, and this video from CNC Kitchen explores the feasibility of doing so at home.
3D printer filament is a plastic material, like ABS or PLA, which is colored, stretched, and wound onto a spool. That sounds simple enough, but it is actually difficult to make good filament. That's because the filament's cross section must retain circularity and its diameter has to be consistent. If the diameter is inconsistent, it will result in under-extrusion or over-extrusion. Both of those conditions will yield poor prints. We usually see those problems with cheap filament, but you'll need to worry about diameter consistency if you're going to make your own filament.
To ensure that the filament is consistent, CNC Kitchen used a 3DEVO Desktop Filament Extruder. Those machines start at $7,450, which means that you'd have to make a lot of filament to justify the expense. But the machines are practical for those of you who are looking to sell your filament. As expected, this machine produces nice, consistent filament when working with virgin pellets and colors. But the more interesting test was to see how it could handle recycled material from failed prints. CNC Kitchen only touched on this subject, as they plan on going into more detail in a future video, but they teased that they had some good results. They used 3DEVO's SHR3D IT motorized shredder machine to turn failed prints into pulp that could then be fed into the extruder.
There are far more affordable filament-making machines out there, like the Filastruder that costs around $300. But if you want to sell filament or just really want to make your own custom filament, 3DEVO's machines seem to be very capable.