How did we get here?...

The most common type of 3D printer uses FDM technology. This stands for fused deposition modelling where the nozzle of the 3D printer extrudes filament in layers to build up an object.
This technology has been around for longer than you might realise.
They filed the first patent for this type of manufacturing in 1984, and this set the groundwork for 3D printing today.
Despite the introduction of new technologies, FDM hasn’t changed allot in the past decade.
When first invented 3D printing typically used a plastic called ABS. ABS is a thermoplastic heavily used in the injection moulding industry.
The trouble with ABS is that it’s derived from fossil fuels, is not biodegradable, and has a greater environmental impact than other filaments.
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So what now?...

While ABS is still a common material for use on 3D printers, PLA is much more widely available than it used to be.
PLA is also a thermoplastic, but it’s derived from renewable sources like sugar cane, beets, cereals and corn, making it biodegradable. These sources are by-products of the growing process, therefore using existing waste.
PLA biodegrades slowly and could still take up to 1000 years to degrade in a landfill. However, in the right conditions, like those found in industrial composters, PLA will completely break down in around 90 days.
Unfortunately, you can’t put PLA into regular recycling bins as it has a lower melting point than other plastics.
Therefore it needs to be sent to a specialist or ground down and extruded it into a new filament.
Some projects have made it possible to grind up old prints and extrude them back into useable filament at home. Check out this post by All3dp on filament extruders you can build or buy.
However, if you don’t want to build an extruder yourself, the other options are still relatively expensive.
Alternatively, some companies are producing recycled filament & filaments produced from other waste and organic compounds. For example, check out Filamentive and Fishy Filament 
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What about other manufacturing methods?...

Because 3D printing is an additive process, it only uses the materials needed to make the object. Compare this with subtractive manufacturing like CNC where excess material is cut away, causing sizeable amounts of waste.
Even with failed prints, 3D printing can reduce waste when compared to other manufacturing methods.
3D printing can also save between 41% - 74% more energy compared to other large-scale manufacturing methods as found by this study.
With other manufacturing methods organisations often make parts offsite and then ship them for assembly. With 3D printing, organisations can do this production on-site, avoiding transport costs and emissions.
3D printing typically uses an infill grid to save on material without comprising on strength. With injection moulding, this is not the case as these are solid.
You might think denser parts provide more strength, but this is not the case! Infill percentages of 25% or fewer can be just as sturdy as solid parts saving material and energy.
3D printing allows engineers to explore more efficient ways of producing parts, optimising the material used without sacrificing strength.
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So is 3D printing sustainable?...

In short, yes.
In industry 3D printing offers an alternative solution to more traditional manufacturing methods that can be more efficient and produce less waste. Also, the cost to make parts on mass has been significantly decreasing. Therefore, 3D printing is a more reasonable choice against injection moulding than it has been in the past.
At home, 3D printing has made it possible to custom-make parts without outsourcing to costly engineering firms.
However, industrial composting of waste remains inaccessible to most people and desktop filament recycling is still in its early development.
In the UK you can purchase this box from Terracycle as a solution for recycling 3D printed waste.