Briefcase sized, portable plastic grinder/filament extruder/spooler

Doesn't the shredder/grinder/auger approach have a tendency to trap a lot of air (and perhaps wash water) in the recycled material as it gets pushed toward the heating chamber for the extrusion process ? A lot will likely be pushed back through the auger under initial compression but some will likely get trapped and pushed into the heating chamber and exit through the extrusion nozzle as a bubble in the filament. (may be an idea to detect for this in the overall design anyway ?)

For 3D printing to work successfully then to me it looks like the quality of the filament has to be pretty high. If there is an air (or water) bubble in the filament then not only is the filament likely to break it is going to result in an uneven layer being printed. With this in mind, a 'melting pot' approach may avoid the need for pre-shredding and help remove any air and water and allow for the skimming off of any floating contaminants (label residue) prior to extrusion. It does however use up energy for the heating as you suggest.

I suspect that part of the 'fun of the design' will be figuring out feed rates at each stage and fine tuning motor speeds/temperatures to keep the production flowing without interruption to the filament.

I know that my suggestion for a two part approach was less than ideal, however I was also aware that an overly complex project may not get featured on the show whereas a modular one might. Pellets sounds like a good halfway point to me - build a machine that can extrude from recycled pellets, then build a machine that can create pellets from shredding recyclables. Line them up in front of the 3D printer and then you have your portable factory which can be further refined.

Shouldn't be any water in the system, other than possibly vapor from any absorbed by the plastics.  Some plastics absorb and/or are more permeable than others.

How about a hopper to feed-auger/grinder, into a small melting chamber, then a second auger extruding off the bottom, so the extruder only gets liquified plastic?  Still needs to be a continuous process, once started, so the hopper needs a capacity large enough to allow for normal human activity and behavior.

Something like one of those coffee-grinder to fresh coffee makers.  where the feed stock is in a hopper, then goes through a grinder and into the rest of the process.

For that matter, w/PWM heat control, maybe insulate an automatic-drip coffee-maker heating element/tube, as the melting area, prior to extrusion?  Or wrap a heating element around a piece of pipe and insulate that, for the melt chamber?  Pot sits on top, like a coffee caraffe', preheating and melting, then auger feeds through the heat-tube, pushing out through the metered orifice, for the final filament size?  Might be able to insulate and convert one of those single-cup size ADC coffee makers?  Extruder auger would need a thermal control switch, as well, preventing it from operating below some suitable temperature.

Maybe, initially, have a designated plastic, and make only one kind of filament, such as collecting and cleaning only plastic milk-jugs, is the source?

Excuse the MS Paint, it's what I have and I'm no artist, by a long shot...

I was thinking of small amounts of water left over from cleaning the plastic. If it goes into a hot melt though then this should quickly boil off as the melt point of HDPE appears to be around 250F, well above the boiling point of water at 212F.

I thought about feeding liquid plastic into the auger as well. May need to have this at a slightly lower temperature (250F-300F) so that the plastic is still gooey at this stage rather than fully liquid so as then the auger doesn't have to be so liquid tight. Alternatively consider a near vertical auger so that leakage is less of an issue. Then the extruder heater can raise it up to the 350F+ required for extrusion. Another bonus is that if the plastic is in a liquid state in the auger then less torque will be required to turn the auger. The downside is if the liquid plastic is allowed to cool in the auger then it will require to be heated up to 250F to get things turning/flowing again.

High temperature glue guns appear to operate at around 380F so could potentially be used for the final extrusion heater and will have thermostatic controls built in.

Designated plastic initially sounds like a good idea to me as there is less chance of human error in identifying specific plastic types.

I thought, maybe by putting up a picture of someone my friend wants to be able to help, by printing a full arm prosthesis, it might help put a face on the problem.  my friend has a limited, public school department budget, and, w/o enough filament, can only help a few children.

This young lady is Kim, a paramedic, and was working an accident, when she was struck by an 18-wheeler.  Even a thin-shell, hollow, cosmetic whole arm would take vast amounts of plastic to make.  A completely portable system, could take the technology to every city, and producing their own filament could help make that a reality.

This video may be of interest.

https://www.youtube.com/watch?v=8J7JZcsoHyA

Shredder

Build a Shredder — Precious Plastic

Extruder

Build an extrusion — Precious Plastic

Might need a larger briefcase though.

A small shredder, hand-driven by a ratchet...

The "Filastruder" uses a soda bottle for a hopper...

These are part of why I think it CAN be done into a small briefcase-size unit, where most of the unit is actually a fold-out hopper that can double as reel storage.

The shredder might need a motor with quite a bit of torque though depending on the intended duty cycle. Just thinking of typical compact paper shredders here which can overheat after only a few minutes of continuous shredding of a few sheets at a time. Can gear it down but I suspect that shredder blades will have an optimum speed to chip the plastic rather than to chew it.

High torque drill, maybe?

Would perhaps first need to work out how much material needs to be shred in order to have enough raw plastic to extrude a usable amount of filament in one go before choosing an appropriate sized motor.

I'm assuming that if touring with this briefcase then time will be limited, so probably looking at stockpiling the bottles beforehand then straight into near-continuous shredding upon arrival, and extrusion into reels of filament, whilst consultations about prosthetics and 3D design take place. Then as soon as the first reel is ready 3d-printing can start.

I've just weighed an empty 2pint milk carton here and it weighs around 25grams. Googling and you appear to get just over 100m of 3mm diameter filament per kilogram of plastic.

Filament volume and length – ToyBuilder Labs

So that's around 2.5m of filament per milk carton, or around 40 such cartons requiring shredding for each 100m spool.

A high torque cordless drill motor is compact however it's not really designed to run continuously for any length of time. Entry level bench pillar drills tend to have 1/3HP or 1/2HP electric motors but they are large and heavy.

My crosscut office shredder here is rated at around 1/5HP and definitely would not be up to the job.

Looking at the Precious Plastic shredder build and they are suggesting a motor rating of around 2.7HP running at  @ 70RPM. Sounds a lot however perhaps required for 'just works' type operation.

Drills with the impact and hammer-drill settings are intended for heavier, longer operation, since that is for masonry and concrete.

The collection and prep phase might further be defined, as well.  Suppose that the collection and prep is only one type of bottle, since we've already specified one type of plastic, to reduce the need to disassemble and clean.  Clean and cut into relatively flat pieces, and run them through an office shredder, followed by low-heat, to insure dryness.  Caps, tops, bottoms and handles could be set aside, to be recycled using a larger grinder.  For purposes of airline mobility, restricting the plastic makes sense.  When using an automobile for transportation, it would be easier to have a larger grinder.

Once the smaller pieces are pre-shredded (think typical strip-shredder) for the final-grinder/extruder, the final grinder isn't having to do all the work.  Its' purpose is to make the final pieces small enough to feed into the melt chamber and extruder, not ground-down from chunks.

When things gear-up, it might be practical to have several interchangeable extruder heads, for different plastics, rather than trying to clean one out, for use with different plastics, so designing a setup with multiple temperature presets makes sense, either way.

Realistically, I'm trying to help a teacher do more.  Most of their printing is done in the classroom.  However, a fully portable mini-factory could be considered as a more prudent setup, for those with limited space.  If the extruder and the printer both close down to briefcase size, then it can be more practical for home use, mission trips, demonstrations, etc.