The opposite extreme for plastics manufacturing in terms of scale is injection molding, by far the most practical means of manufacturing with plastics in bulk. This is very low cost for high volume but insanely high cost at low volume, with a minimum investment for a mold being a few thousand dollars or a few tens of thousands. Making a new mold takes time, energy, skills, and is difficult to automate due to the craft aspects of getting the mold right. It can take a few tries to get the correct mold design which is expensive in both time and energy and money. This is also not relevant for truly free technology for several reasons: scale breaks at small number, it requires high levels of technical expertise, it's energy intensive, requires that high quality metals machining also exist along side it to make the molds, and again cannot be edited after being cast. One can recycle it easily, yes, but not edit or repair an existing part.
The other major technology for manufacturing plastics which is mostly not used at scale but is useful for prototyping is CNC machining. This can make high quality parts fast, but is very expensive, needs heavy fixed infrastructure, is hard to build from trash, and needs a lot of specialized expertise.
My methods of plastic fabrication involve abandoning the paradigms of both additive(3d printing) and subtractive manufacturing(CNC), by converting a feedstock of mixed plastic trash into useful industrial products by cutting(not melting or shredding) them up into standard geometric shapes, then using heat welding and simple manipulation tools like tweezers and pliers to create three dimensional parts from 2 dimensional components. As with 3d printing, this can be used to make either mostly hollow structures or structures reinforced with an arbitrarily high density of structural lattices(typically honeycomb for most 3d printing systems).
I have been experimenting with various plastic trash feedstocks and have found that HDPE and LDPE(high and low density polyethylene) are by far the most tractable with these methods. In theory this can be extended to any thermoplastic, such as PMMA, ABS or PP, but for now I'm using HDPE and LDPE for all work here as those are easy to work with by hand and easy to find both in litter in the environment and in my personal trash feedstock based on the milk I consume daily with my breakfast cereal. Most milk containers are made from HDPE, as well as a lot of bottle caps for PP bottles(which I've found very frustrating to work with but the caps are great). The big orange traffic barriers(not cones) which can often be found trashed in urban swamp areas after they get hit by trucks, smashed and kicked off the highway are also a great source of LDPE.
At present my method is extremely simple: I cut plastic milk bottles up with a box cutter, wash them in a sink cut them into shapes with scissors, then use tweezers and a candle flame to make the weld joints. I developed these methods initially using a hot air gun with temperature control set to 130 C which makes the process much easier but have found that with practice the candle or even a lighter can be used just as well. Milk bottle work all calls for tweezers, and I use very flat bladed ones from the eye makeup section of the grocery store or pharmacy, which are intended for eyelash related work(hence the flat blades).
I have documented some of the processes for building useful stuff with these methods on Instructables and Youtube:
Unlike 3d printed parts it has proven to be easy to cut up and reform or just make small modifications to existing parts. I have also used HPDE bottle caps as structural elements for electronic circuits, and then integrated those via welds into various motor coil geometries. It's also relatively easy to integrate this into cardboard fabrication techniques from Action Geometry, making various tabs and slots as needed, and welding things into place. Finally I've found that connecting plastic parts with wood is simple without threaded fasteners or nails or wire by cutting holes into sticks and shoving glassy plastic into the holes, which harden into fasteners which can be easily broken off later and reformed.