Advantages of FDM vs injection moulding :

• Cheap equipment costs
• No need to make moulds/adjust the machine for different parts
• Very complex structures possible
• In some case material usage can be optimized
• Comparatively small machinery

Disadvantages of FDM vs injection moulding :

• Super slow (a 1x1x1cm dice might take in the order of 1h to print, while the injection moulding machine can spit out a whole lot of them in a minute or so)
• Print times scale with the cube of the size. A 20x20x20cm print will take in the order of one week to complete.
• Small details are not so pretty
• Rough surface
• Parts might need some post-processing (e.g. removing supports)
• Filament (e.g. PLA, ~€20/kg) is much more expensive than pellets (e.g. PLA, ~€1-5/kg) used for injection molding
• Many materials that are available for inejction moulding are not available for FDM
• Only very limited foam materials available
• FDM parts are much less robust than injection moulded ones, especially along the layer lines
• FDM has more issues with exact dimensional accuracy. It’s much easier to e.g. make a perfect ABS part with injection moulding than with FDM. In injection moulding you don’t have to worry about warping and stuff like that.

TLDR:

FDM is great when you want to make very few pieces, the part is neither too small (<1cm) or too large (>15cm), you don’t need perfect mechanical properties and it’s fine that the finished part looks kinda rough.

Or if you can only have one device because you have limited space. Then it defaults to the 3D printer due to its versatility.

3D prints are great for those one off things. In the past I’ve printed replacement parts for a swimming pool, a g-clamp I needed for a one off task, I’ve fixed a rotary clothes line…

My most recent print was an AirTag holder for my wallet. Aviator who make the wallet wanted €35 for their one. Mine cost me probably 5p in plastic 😂