v4 Shredder Development
I’m Friedrich from Germany – a Industrial Designer and Engineer. I will work on the next revision of the Precious Plastic shredder in Eindhoven until the beginning of the next year.
As most of you know there are some things about the shredder which can be improved to build a more reliable/productive and safer machine. The recent weeks I ran some tests with the current shredder, a fast-spinning shredder and a double axis shredder to compare them against each other.
Some points which we are sure we want to improve:
Right now we have a max output of ~10kg/h – to run a workshop or small business with our shredder we need at least 40-50kg/h. The goal should be to shred in half a day (or less) the complete plastic for a week.
The next version should not jam that often as the current shredder does. This is not just a problem of the motor power only but also concept related. Furthermore we want to implement automatic reverse (Torque detection) and back to normal operation.
This is very related to the topic of reliability, due it is not possible to build a save shredder if it is not processing the input material in a reliable manner. The shredder should run without any correction of the user during the shredding process. The shredding blades must not be accessible during the operation, this is ensured by mechanic constraints or sensors.
Some of you report a fast wear down of the shredding blades. The new version should have a way to reduce the necessary time for maintenance and changing of blades. I will try to increase the lifetime of the blades by optimizing tolerances, cutting angles and material selection.
The revision of machines will come with a guideline for a easier part-selection process depending on personal needs.
So how will we start?
Our biggest problem right now is the huuge variety of plastic we recycle – small/big, thin/thick, flat/geometric and different materials. And most of the shredders are specified on one particular task. E.g. slow running Granulators to recycle runners of the injection process.
I found a different concept from our existing one which i want to adapt to a smaller size shredder. I think it can solve the problem about the big variety in the best way. The biggest benefit of this shredder type is that, the process is more reliable and you can throw in even bigger rests of plastic like left-overs from the extrusion machine or half injected parts.
Its called single shaft shredder which looks like those ones here:
Of course our shredder will be a bit smaller 😉
The build will be similar to the current one (stacked blades on a hex-shaft) – just the blades will be between 10-20mm thick, to assemble the teeths on them. The width will be the same for all, but the length will be adjustable to your needs (education vs. production)
What do you think about this concept?
I you look at the Weima video 0:06 it appears to be using carbide inserts (such as used for lathe tooling) for the teeth. This would allow using a cheaper softer material for the bulk of the cutter and you can just replace (or rotate) the inserts when necessary.
I would recommend a dual shaft approach though, they seem to grab material and feed it through much faster. Also as you can make the cutters half the diameter to cover the same shredding area, you need less torque = less power. My dual shaft shredder is only 1/2Hp (375watts) and it rarely jams unless I really push it.
I also implemented an auto jam detect/reverse/restart feature, quite easily done with a microcontroller and an encoder on the shaft. What you can’t see on the video is that it will try several times to clear a jam (almost always works first time) but if it can’t it will turn off the machine.
(For some reason the video doesn’t start in the right place, skip forward to 3:15)
Looks like a very nice shredder!
I would be interested to hear why you decided for an encoder based solution to detect jams?
I tested also a 1.1kW dual shaft shredder quite extensively, they work perfect on thin materials such as bottles, single use plastic products and especially flat items.
My big problem with them was, as soon as I inserted harder and bigger pieces it tended to jam due as soon as they cut deeper into the material, the direction has to be reversed, the part removed and repositioned by hand. This is always a dangerous process, due the machine is running while you have to hold the part in the optimal position to be feeded inside.
Where in other cases the parts just are not feeded and jam the inlet of more material (see picture below)
You would need enough pressure from a pusher which breaks those parts. You cannot just fill the hopper with material and walk away -> so in a workshop there needs to be always one person standing next to the shredder and feed the shredding material in the best direction – piece by piece.
For bottles and parts that are under a certain ratio to the blade thickness/size a double axe shredder is propably the best choice, but for random plastic I don’t belive its the best choice. Unless you build a really big one with a blade-diameter of >20-30cm.
Nice shredder 🙂
I saw a shredder that used saw blades a couple years ago. Used in Central America.
Cant find the video anymore.
I did see the video by Jeremy Fielding. It looks promising but needs to feed from the top thru a long hopper to keep fingers out and flying chips from being flung out backwards
Hi, yes I have seen the saw-blade based “shredder”.
Dave made experiments before the V1 of the shredder already. It is a very inexpensive and tempting solution to go for!
The problem about the setup is that the user has to feed in the material, take it off again and feed it in in another direction. And as soon as you want to divide small granulate into smaller granulate you have a real safety issue.
It might be a perfect solution for flat sheets or to divide thicker pieces for shredding. But overall this solution is not universal enough for our huge variety of plastic.
Have a nice day
I used an encoder because it’s a simple, cheap and reliable way of providing speed feedback to the controller. The one I used is really simple (made it myself) only 4 pulses per revolution, no quadrature, I just time the interval between pulses, longer than a set time and the motor is bogging down or has stalled. I also put the encoder on the far end of the driven shaft, furthest from the motor, so any problem anywhere along the drivetrain will show up.
Thank you for the information!
I was already planning to measure the current of the motor consumes, to create a torque-amp relation. So there you would see the torque curve raising up if the motor stalls.
I will look into the electronics more after I finished my current mechanical build. Propably it’s worth to check which option is cheaper and easier to build. I assume the output will be usable in both ways for our purpose..
alternately. I have also seen someone using a push lawn mover hacked to be a shredder.
Basically you enclose the bottom with your mesh, for example 6mm holes.
the outpout chute is also closed off.
the input chute is fed from the top at a angle so the plastic is not flung out backwards.
to be honest in my opinion there is only one solution regarding these problems.
To be independent of the shape and rigidity of the waste input we need a shredding unit with different stages.
To safe money by using only one drive you will need a gear (belt or pinion type).
This will be the only solution to reach a maximum throughput in the same time.
What do you think?
Yes I agree with you!
The question in the end is if the ratio is reasonable, between how much extra money the user has to invest for the machine versus the gained output/h. One option would be indeed to create two separate shredding units which can be mounted on top of each other. Or depending on the mainly type of processed plastic could be also specificly picked and used as a single unit.
The clear advantage is that we probably cut the costs maybe by using motors with belts instead of high-ratio gearboxes.
One option would be to go with 20mm insert-cutters for very rough particles. And then granulate those in a smaller ready-to use particles with a double axis/ fast spinning shredder under it.
Update: Proof of Concept – Shredder with Blade Inserts
Hey together – here is my current build of the next shredder version.
1.) The User feeds the plastic into a hopper (not displayed here), from where it falls down.
2.) A pusher feeds the material towards the single shaft with inserted blades.
There can be different types of pushers (manual-direct, manual-spindle, electric-spindle, pneumatic, hydraulic) depending on available resources and performance requirements.
3.) The plastic is squeezed between the pusher and the shredder blade and gets shredded downwards. The short blades and the closed cylinder enable the material to reposition itself during the shredding process, rather than getting stuck between the blades.
The blades can be exchanged easily and used from four different sides or resharpened when necessary. The final geometry might change to triangular blades (to set up the distance between static and rotating blade evenly) in a later step.
I like this design a lot, though the shape of the cutters look a little hard to resharpen, and like you wouldn’t get many sharpenings before they are too short to work. Maybe square tool steel held by simple grub screws perpendicularly would be better? Just advance it slightly each time you sharpen. Some relief on the cutting edges would be good too.
For the pusher, you could use a ‘self reversing screw’ driven by the same motor that powers the shredder, saving on extra components. www.youtube.com/watch?v=9UbnvtJ2qMk
Thanks for the video, didn’t knew something like this even exists!
My plan is to use for the automatic systems pistons (cheaper) or electric driven spindles. So we will track the torque curve of the shredder shaft, and as soon as it hits a certain limit the pusher can be stopped and reversed everywhere. In theory this can enable to use a wider Range of motors, due the jamms can be reduced.
About sharpening I’am not that worried right now, but we keep in mind to design maybe a small sharpening station with it so everyone can do it in the workshop.
Hey people, just reading through and thought I’d share what I’m doing. So first bottles get soked in a chlorine rich water. Killing all bacteria cleaning and loosening labels. Then I remove labels and rings. Then use a guillotine to cut in half and then straight into a strip cut industrial paper shredder. This makes it easier to handle and literally stretches every bit of plastic so any stubborn dirt automatically gets detached ready for a final rinse and dry. Then finally put through the precious plastics shredder. simplistically the easiest way I’ve found to clean large amounts of plastic at once. Cheers
most shredded plastic is cleaned with more cheaper lye, not bleach… both are chemical base (not acid).
I used a more industrial shredder last week and it had a similar blade arrangement as this.
gravity is always your friend! maybe try rotating the linear actuator/pusher plane 90 degrees so it’s pushing the plastic down into the blades. the hopper can still come in around the sides of the actuator.
Standardization can be your friend! Three questions:
a) I noticed your ‘hardened’ teeth are an hourglass shape, so I assume your thinking of getting four sharp sides out of that tooth before you have to replace it with a new one? Great idea, but why not use standard carbide inserts? Finding someone to cut, then harden, those hourglass teeth is going to be frustrating for a lot of people.
b) The linear actuator is an interesting idea, are you referencing the auger feeder design used in a production granulators? if you haven’t seen them, you should search it. Feeding material in, is a proven industrial method. Bravo on that direction and good luck in keeping that simple and consistent. I look forward to that development!
c) Why is a hex bar still being used? A standard round axle with a keyway is not strong enough? That hex bar is strong, but requires machining to mate with a bearing or pulley. Machining in general – is a huge barrier to entry. Just sayin’…even a square bar would be more readily available to more people.
Nice to see thoughts from other people on this machine! Thanks for the update!
Hey thanks for your well-known reply, lets try to answer your questions 🙂
a) Indeed that was my first idea also. It’s still high in the list for the next iteration. I found out now, that there is a standard over different suppliers for those carbide inserts.
The question in the end is for which price we can buy them and how this will influence the machine price. I quoted yesterday the price for the teehts of this proof of concept. And right now one insert is around 0,30€ right now. The readymade inserts i found are in the area of 1-2€. Of course they are heat treated, sharpened, and professionally made. So its still on my mind but we have to consider also the worldwide availability of those and the price for ~150 Inserts.
I consider also to outsource it to a professional toolmaker and order from there in quantities of 10k+ to lower the price. But this would be also a major change of operation overall and is not discussed jet.
b) An interesting method! I have seen it on bigger shredders as well. Propably it can suit certain applications very well, e.g. if you have industrial waste and you know which size your parts have you feed in there.
c) Acutally i did not considered as a problem for now. For now the goal was to get up a running proof of concept as fast as possible, without spending to much time in optimizing the design for manufacturing, though the final shredder will be much different (bigger).
But i heard also from someone in South Africa, that he had big troubles to source the hex-bar.
A round axle with a/multiple keyway(s) might work as well, however you just shift the machining from a lathe to a mill.
From an engineering standpoint i assume its the best “solution” is to use a hex shaft, but I totally agree with you that a squared shaft might be a good alternative. I put this topic into the next revision research =)
Thank you very much for your input
Highly appreciate it!
I wonder if something like this could work. This is engineered for lettuce and cabbage, but perhaps a more robust prototype could press plastic bottles into small squares.
Uh, vergass ich, can Ich auch ein bisschen Deutch sprechen.
I was told that here is good to share this design I’ve been working on as a side project.
The concept is a higher volume shredder that is relatively straightforward to make, with a built in chopper to reduce particle size. It’s probably 60-70% mature as a design and there are still a number of small things here and there that need to be ironed out. It currently does not have safety covers, a hopper or electrics. All the bearings, gears and motor are off the shelf metric.
The general idea is that the shredding section takes the plastic and reduces it to ribbons, then the choppers break it down into smaller parts. Could change it to two shredder sections though.
Attached is an image or two (3D rendered, this hasn’t been built yet) to show what is looks like. Let me know your feedback. I’m happy to share CAD files if you like, this is intended to be opensource. Also happy to assist development of it.
Well, this seems to be almost an indusrial one, how many kilos of shredded plastic per hour ? Or you just have the blueprints in SolidWorks and have not make it in real ?
What if we are looking at the v3 shredder wrong? Aside from tooth profile.
In other projects I have done in the past, I used armor plate to make blades for hammer mills, high end knives, and hatchets.
What if we just waterjet the cut blades from armor plate and maybe change the teeth of the shredder to 4 or even 5?
I have used Ar500 and Hardox in the past. Both pretty much similar. They would work better than tool steel as they are pre tempered and the toughness and hardness is already proven.
for those looking to do a 2 stage machine, have you thought about using the saw blade design to shredd in strips and then then the standard precious plastic design beneath to cross cut it. similar to some paper shredders?
I was discussing with a engineer the other day about shredders and the high speed unit I posted pictures of. We was thinking that it could be made with off the shelf parts (no cnc) pretty easily and instead of using expensive tool steel blades, one could use leaf springs sharpened with a angle grinder.
We already saw one unit built this way here in the Philippines powered by a small diesel engine.
remember the goal of this project is that it can be built in the third world. The extensive CNC machining isnt going to fly in the third world.
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