Home Polymer Recycling Unit To Produce Filament

I am a university mechanical engineering student from the UK and for my masters group project I am looking to design and prototype a recycling unit to that can use home waste to produce filament for RepRap users. If I am successful there is potential for this product to be taken to market and for RepRap users to save money by making their own filament.

I am getting my head around the self replicating ethos and I am looking into the possibility of the RepRep being able to print a high percentage of the components needed for this polymer recycling unit.

If anyone can help with a shredding solution for turning plastic bottles and packaging into particles please do get in touch, idustrial shredding options are far too expensive for this small volume application.

Also if anyone has tried or is working on something similar to this it would be great to see what you have achieved so far.

Your guidance and help will be much appreciated.

Have a look at Capolight's blog: [capolight.wordpress.com]

He's done some filament making work with HDPE. Not directly recycling, but it's a good start.

There's also a few other people who have done similar stuff.

Check the Recycler team thread.

I used to work for a company that made pool furniture from PVC piping. The pipe was cut with a commonplace drop saw, and the swarf that the saw cut was granular, about 3mm in its greatest dimension, regular and clean. Chinese-manufactured domestic drop saws are inexpensive. I think one would be a pretty good starting point. Ganged common tungsten-toothed saw blades in the throat of a chute could grind effectively if one were to think about how a more specialised machine might work.

Milling cutters like these would work. Mcmaster is expensive, but older machine shops will have boxes of these they rarely use anymore. I've also seen many at surplus sales places. They were mainly used on big horizontal mills which aren't in much use anymore. Some careful scouting could turn up a bunch for cheap.

The milling cutters would work well and older machine shops may have some sitting around, that are used to the point
that they are not safe for steel, that they may give to you. I got a whole set that way. They still work great on plastic
and light metals for hobby stuff.

I have seen a design somewhere that had a row of 4 inch diamter circular saw blades, each separated with a couple
of washers, all on a threaded shaft and secured at each end with nuts. Theed thread shaft was driven by a quarter hp,
110V AC motor with a pulley and belt. I don't know how many blades it had, but the cutting area was about a foot long,
and only slightly wider than the blades. All of it was housed in a box for safety and it had a weighted top hopper on it,
so that the weight was constantly pushing down on the material. The design kind of reminded me of a homebuilt
yard debris shredder and it would shred wood and pvc.

I have also seen designs where there are two axles of circular saw blades that mesh together like a paper shredder. That would ensure that all the plastic was finely divided into small granules.

Wow, very helpful comments thank you.

Following the initial link to the Recycling forum seemed to turn up a lot of brainstorming for shredding methods, is this because granular extruders are a given?
I’ve seen someone working on a similar item somewhere on the wiki/forum.

Sawing or rotating discs seem a favourite, with the aim of reducing the body size to almost powder state. We also have department access to a Granulator, which uses a reciprocating shear method to gradually demolish the input into granules that will pass through a filter. Since it reciprocates any lumps that are too big will be continually picked up and sheared until they are small enough. Do you think it possible to determine consistently small enough chip size just with a set of rotating saw blades?

The main problems that we are facing with this design is to make it low cost (£500 budget, but the department can be stretched) and easily implementable by people like yourselves, domestic RepRap users. We’ve already gathered that using stock parts, easily obtainable from a local hardware store etc, make an open-source design easy to replicate, any further feedback on this subject?

Also the most readily available sources of polymer waste in the home are clearly food packaging, mainly consisting of HDPE, LDPE, PET and PP. Are any of these polymers viable as feed stock in the current RepRap machines? Also how much filament would you say is used averagely; say kg per day, by your typical user?

Thanks so much for your input, and we feel we have only just begun to ask questions...

I was wondering a similar thing- does anyone know of a commercial object made of PLA? ( not RP)

And how reusable is PLA and ABS second time around?

PP is low-cost but is very waxy, almost impossible to glue and would warp quite a lot if extruded in a RepRap extruded I expect.

Milk cartons should be a good starting point as everyone has masses of them, and it's a large volume of clean plastic in every one.

---

[richrap.blogspot.com]

Chip size with saw blades is roughly based on the tooth depth of the blade and clearance between the blade and housing.
If you use a rough cut circular saw blade with 3/4 inch long teeth, you will get bigger chips than if you use a fine pitched "plywood" circular saw blade with 1/8" long teeth.

If the clearance between the blade and the housing is only 0.001 inches then no piece bigger than 0.001 inches can slip past the blade.
If the clearance between the blade and the housing is 1/2 inch, well then 1/2 inch chunks can slip past the blade without being cut.

A vertical stack of blades with decreasing tooth sizes and clearance may be the best solution. The top blade could be a rough cut
and the bottom blade be one of those metal cutting/grinding disks for a fine powder.

I think the part that I'm missing in my mind with regards to a sawing method is that surely it should just produce ribbons? no matter how close you put blades to one another or to an edge of the casing there will just be an increasingly fine width of polymer ribbon. perhaps another device after the saw blades that will cut the ribbons in a horizontal manner, similar to a pelletizer on an existing extrusion line?

Thanks for the advice about PP, was struggling to find any information on it in FDM at all. Don't think we will pursue that one at all then. How about PET? There's alot of that about!

Thanks very much

I have a good friend who works at a factory that blow moulds HDPE milk bottles. They have a commercial re-grinder for all the defective bottles and he gave me a brief description of its workings. One of the features of this re-grinder is a blower fan that is used to separate the smaller granules from the larger.

This idea shouldnt be too hard to incorporate into a home built shredder. A small mounted fan blowing across the falling debri could deflect any granules small enough into a collection bin with the other larger scraps falling into a hopper ready to be pored through again.

As for the idea of stacked cutting blade on a rod, does there need to be a space at all between the blades? Could you use 5 or so cutting blades pressed flat against each other creating a single wide blade? The gap between the blade and the wall would then be the minimum size of the granules you require.

---

-------------------------------------------------------------------------------------------------
capolight.wordpress.com

Ah yes, now I see. Yes I think that that could be very effective, a stack of circular blades with a small gap thats the only way out. This idea could also be incorporated inot the rotary shear shredder I was discussing above, with the ability to pick debris back up from the bottom that for any reason hadn't passed through a screening mesh. I guess then you wouldn't need an air stream for chip size sorting, since they would be continually shredded until they were physically the required size?

Hmmm, now for testing this method without the H&S officer in the workshop screwing at us...

It seems with the shredding issue potentially solved, the focus will shift to an extruder.

We'd love to make a variable pitch screw method. Any thoughts on how hard this would be to implement on an open-source level. I've seen various screw designs on here somewhere... Are there any other recommended extrusion methods?

Thanks for your time!

Not hard to make a variable pitch screw. A straight strip wound around a rod makes a constant pitch. A curved strip wound around a rod makes a variable pitch. Multiple strips stack-wound to make deeper flutes but still easily formed.

@Capo. If the blades aren't separated, it would work better if the teeth were staggered, less propensity to jam than with a single throat.

So all we would need is a load of cheap blades like these: Circular Saw Blades
all on the same axle with minimal spacing and offset at a slight angle to one another. That would be awesome!

What sort of motor could be used to power the shaft? I seem to have a hard time finding low cost high torque motors. I was thinking that the motor from a low cost hand drill might be usable?

Sorry to double post, didn't see you first reply murrayd. What do you mean by 'strip', i'm guessing its soldered onto the shaft?

I liked the granular extruder design here and the idea of creating an RP mould for making an epoxy (or similar screw).

Thanks very much

If you want to save the hassle of casting your own screw you can try modifying an auger drill bit. I have successfully extruded HDPE granules using a fixed pitch auger drill bit designed for wood work. It looks a bit like this one here. The guide screw at the end was cut off before use. This type of drill bit produces more than enough pressure and now my biggest problem is delivering enough torque at a low enough RPM to get a constant flow rate.

It looks like you can even get variable pitch auger drill bits designed for masonry work , such as the one show here, which you could also modify.

I believe it has been suggested before, but a meat grinder already as the variable pitch auger, and the advantage of the grinding body to mount the auger in. Replace the plate with one with fewer openings and perhaps use a heated aluminum plate to bind the granules into a filament.

Mike

how about a coffee grinder? I dont know if its internals are strong enough or if it could even come close, but it is probably able to be modified.

Also a paper shredder comes to mind for a first process on the plastic, just take the protective cover off and watch where you put your fingers, after that use the coffee grinder.

You can even use a blender to turn rice in to an extremely fine powder, or destroy an iphone, look for blend-tec on you tube, this may work on plastic aswell

Edited 1 time(s). Last edit at 11/07/2010 12:48AM by Jeff B.

My initial impression is that the pressure grinding processes such as the meat and coffee grinders work best because of the nature of the material that they're crushing. I fear that the higher fracture toughness of HDPE for one thing may mean it just jams in these mechanisms (if it was still a whole bottle at any rate).

Therefore something like a paper shredder could be used to reduce the intial bulk of the bottles? The ribbons from which could then be input into a coffee grinder or blender to reduce final chip size to something more powder like?

The thing is that our project group will aim to realise all these lovely destructive ideas into a reality. It seems that manually making some HDPE ribbons or shreds on a bandsaw may be a good start, then obtaining a low cost coffee grinder or blender for research...

It's going to be a week of raiding campus recycling bins and destroying bottles... not the worst degree in the world!

I was re-watching some of the will it blend videos and the plastic parts the put in the blender were turned into a very fine powder, and these were phone shaped objects. I really think this would be a great way to go.

The only downside is that the blendtec blender is like 400$, but the real question is, will a cheaper model do it?, can we hook up an insanely large motor to a cheap blender and make it work. If I can find a cheap blender around I may give it a try, I even have a 2hp single phase electric motor sitting around, I could RR some parts and hook it up. Thats way over kill I know, but its what I have. No guarantees that I will get to this or even get to it soon, I have so many other things to do at this moment.

wow, those will it blend? videos are incredible. although you have to cringe when he cracks a working iPad in half just to show off his swanky blender!

a little bit of research has revealed that the main characteristics you want from a motor thats capable of shredding/blending things is its power and start up torque. a little more research turned up the electric motor man what do you think to these?

the problem is that we'll have to splash a fair bit of our *budget in the hope that blending would work, even to trial it.

Edited 1 time(s). Last edit at 11/07/2010 01:35PM by BenBrazier.

If you look around you can probaly find one of those motors for free, it may just have a bad cap, put a new cap in it and it may work.

also if you buy it new, it would probably also be powerful enough for the saw blade idea, one motor can help with many different experiments.

I dont doubt the capabilities of the blendtec blender in reducing plastic to a powder for our uses, it just costs to much for what we want to do with it.

My mother has a blendtec blender, next time I visit her I will take pictures and measurements of the blade, maybe it can be made?

Edited 1 time(s). Last edit at 11/07/2010 02:34PM by Jeff B.

i found a vita mix blade on ebay: [cgi.ebay.com]

RR a container for it (it threads in) RR the adapter to hook a motor straight to it clamp to bench, test

that vita mix blade looks mean, dare i say that it would seem we could manufacture one in the department workshop? doesn't look at all complex or even that specialist? Very exciting developments.

Thanks

I think the best bet for making powder out of scrap plastic is to use router bits or drill bits. I have a cnc milling machine, and it can easily reduce plastic to powder, especially HDPE. I think a row of rotating router bits would easily reduce the scrap into powder, even if it was a strange shape like a milk bottle. You could power all of the router bits with one router using belts and pulleys. I have attached a sketch to illustrate the idea.
The router bits would be straight cut, probably 1/4" would work. The pulleys could all be made on a reprap. The only problem I forsee with this is the torque of the router, and also the stiffness of the enclosure and the ability to hold the router bits straight. Otherwise I think this design could make a lot powder very quickly.

Ben,
you can probably downsize the motor by adding a flywheel to the power train. That will provide instantaneous power when something tries to jam the cutter/blades/grinder without depending solely on the horsepower of the motor. And much cheaper if made from scrap plate steel.

Blender:

The blender may work best with denser objects. An intact, empty milk bottle is fairly low density, and the first impact with the blender blades will send it flying. You will likely need a lid that can press almost all the way down. Or maybe counter sink the center to allow the blades to fit inside. Make the overall chamber bigger than a milk bottle. Heat warp plexiglass or find a large glass tube so that you can see what happens inside. Round will be better than square so that there are no corners for pieces to hid in. Maybe slope the bottom to feed into the blades in the center. A couple of 608 bearings in the center with the blades bolted onto a shaft with a pulley on the bottom to attach to the motor so that you can move the motor from one contrivance to another. Then make the lid from a block of wood with the center recessed to fit over the blades and the bottom shaped to fit the slope of the chamber. Run one at a time and then power off and remove the bits, or put a ring of wire mesh along the bottom and cover the outside with PVC tubing cut in half. Run the output of the PVC tubing into a woodshop dust collector, or a shop vac with a cloth cover over the blower. With smaller blades this could also cut ABS or other harder plastics.

Paper Shredder:
Borrow an existing paper shredder from someone. An office grade one should have no trouble cutting the milk bottle walls. You may have to cut the milk bottle into quarters to get the pieces into the paper shredder. The motor should be strong enough without using the big one you are talking about. I have a shredder that makes confetti, not strips. That would perfect for feeding straight into a meat grinder or other pressure feeder. If you can only get a strip style paper shredder, run the plastic through twice. The second time the pieces should be small enough to feed a meat grinder.

Coffee Grinder (blender style):
You will only be able to grind small pieces at a time. After cutting the milk bottles to 2cm squares, you can grind one or two at a time. The average home spinning blade coffee grinder is designed for very small batch sizes, and the hand labor will be much higher than with any other approach. This could also pulverize small pieces of ABS or other harder plastics, but this would rapidly wear out the small motor and bearings.

Coffee Grinder (meat grinder style):
You will have to cut the milk bottle at least into strips. You might be able to go straight from the strip style paper shredder to the coffee grinder if you can feed the strips longways in. The coffee grinder will have the steel equivalent of a stone grain grinder. This mill be very good at taking small pieces and making small granules of them. You may also get enough pressure feed to go straight into the heater and filament extruder. Remember that this style is also designed for small batches, although it can at least run in continuous mode without having to power off and power out the bits.

Garbage Disposal:
This has grain grinding like surfaces just like the coffee grinder, but has much more power to work with. However it may need either flowing water or suction to pull the pits into and through the grinding surfaces around the outside. The swinging arms are only intended to break up larger chunks, and could be replaced with a larger version of the blender blades to break up parts of bottles into small enough pieces to go through the grinder. If you cut off the top and replace the swinging arms with the large blender blade, you should be able to feed whole bottles in and get powder out. This one would be hard to power with the external motor, but easy with the included motor. Without the blender blades, this could also grind up small pieces of ABS or other harder or denser plastics.

Opposed Saw Blades:
once again use bearings and steel rod to make power shafts with pulleys to be driven by your external motor. Space the blades about 3/4 of a disk diameter apart so that they have a strong inward pull. Use washers or nuts to space the blades a little more that a curf apart. You may need suction here as well to keep the small pieces from flying in all directions. This will really benefit from a large, heavy flywheel to keep the blades spinning when something tried to get stuck between the blades. This should be an easy, plywood box build. The results should be ready to directly into a grinder.

Single Row of Saw Blades:
The big chipper/shredders used to reduce tree limbs to chips usually have a single spinning blade with a very wide curf. The branches are fed into through a narrow opening perpendicular to the rotation. The same thing can be down here. It might be easier to use many saw blades and a bar or plate between each one to prevent large pieces from getting though. But I don't think this approach will work as well as the opposed blades. The plastic is flexible and will tend to get jammed in the exit openings.

Meat Grinder:
This is already designed to take medium sized stuff and make it pellet sized, and can also directly pressure feed a heater/extruder. The augers usuall have a variable pitch, wide at the entrance and close at the cutting plate. Unlike the coffe grinder, it does not have grinding surfaces like grain grinder, only cutting between the auger and the body of the grinder, so the output will be larger and more varied in size. Connect to the external motor with significant gear reduction to turn slowly with lots of power. You may still need to cut the bottles into strips to feed a a standard meat grinder. Or you can make a pre-grinder big enough to take whole bottles and cut them into small strips that feed straight into the meat grinder. Both could be powered off the same motor. The pre-grinder could even be made from an old push lawn mower. The scissor action of the rotate blades would be wide enough apart to grab whole bottles and a small exit hole at the bottom feeding into the meat grinder would cause larger pieces to be re cut until small enough to fall through.

Lawn Mower:
An upside down powered lawn mower would be great at taking whole bottles and cutting them to small scrap ready to feed to the smallest grinder or heater extruder. The air flow caused by the spinning blade would keep the low density plastic bits spinning until they could fir though a metal mesh screen around the edges, like the large blender. Once again, a shop vac or woodshop dust catcher would be needed to collect the chunks. Build a large chamber around the cutting blade with a narrow feed tube at the top just big enough to put whole bottles in. A dead lawn mower could be scavanged, the petrol motor removed and replaced with a pulley to be driven by the large electric motor you are talking about buying. This would also process ABS and other types of plastic, too.


I hope some of these ideas prove useful.

Mike

Im going to add, that no matter what method is chosen, please be extremely careful as this project could remove a finger or arm in an instant.

@ Rocket Scientist, Mike
Wow! thanks for going to all that trouble to type out your brilliant ideas! It's definitely got me thinking and I think you're right that it's time to start salvaging parts!
I will keep you all up to date with the progress over the next week, most of the ideas in my mind are yours more than mine now, but hey, it is open-source right?

Which I guess is another thing. As part of the product development process we will need to produce a seven year business plan, profit and loss, cash flow, all that stuff. Does anyone have any experience as to how an open-source company would make any money?Just a hint of a company who has acheived it will do if anything,

thanks for your time all!

@Jeff B
Thanks for the warning!

I would look at makerbot industries, all of their products are open source, and they seam to be making money.

---

[boxedrr.blogspot.com]