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[Belated] Precious Plastic in CUHK

William wkhs

[Belated] Precious Plastic in CUHK

08/05/2017 at 14:19

Alright, so I guess I finally had some time to document this project. I am a pharmacy student. No you didn’t read me wrong… I guess everyone of us has to get some hobby, do we?

Great thanks to @sueyeekhor for writing a fabulous intro article for us. I am the technician of the team (and others are all girls, yay). I thankfully survived my final year, ward rounds and (nearly) a research project. Now this would be a long post and log of what we’ve done to build a set of machines in the CUHK (our uni). @sharma-sagar here you go!

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11/07/2018 at 06:28

@xxxolivierxxx: I didn’t have time to work on our injection machine since I graduated. Nth to post sadly.

: I tagged it belated because it was posted 1/2 year after the shredder was functional already. Now it comes to others’ attention after >1 year. Aha!

08/07/2018 at 12:52


Perhaps the [Belated] means these posts have only just appeared? I  don’t remember seeing them before even though they are dated over a year ago.


Anyway looks like an excellent build and I tend to agree with all the observations made. The auto-stop detection is very similar to how I did it.

07/07/2018 at 21:02

I don’t know why no one else has commented on your shredder built process, but your shredder looks awesome.
Congratulations @wkhs

Do you have any more info/videos/pictures you can share of your project?

08/05/2017 at 14:39

2. Frame

Never trust aluminium. Never, ever.

A strong and well-built Al frame can easily support the weight of motor, gearbox and the shredder (or yourself). It’s tempting to use them (esp for those who built 3D printers with 2020 Al profiles). But Al is definitely not for the torsion of your monstrous motor!

Whenever the motor tries hard shredding tough things, either the aluminium bends or the gearbox-shredder combo will slide away from each other (depneds on how tight you have tighten the screws and whether your profile is slotted to allow sliding). I couldn’t explain on this observation further (sorry I am a amateur :D).

May be there are better cross sections for the profiles that can do the job but I suggest using steel for precious plastic machines in mechanical parts. (Yes we don’t machine SS in our labs normally. So we either purchased or used the thickest block of Al to make the parts for the injector).

Bring home msg 3: Never trust aluminium. Never, ever.

08/05/2017 at 14:26

So I’m gonna start with the shredder first. We started designing the electronics back in Dec last year, and finished the whole thing functional in Mar. It’s hard but great fun too!

1. Laser-cut parts
We just sent out all the files to a manufacturer in Mainland, China. They offered us it done in 150-200 USD (transportation fee inclusive).

It was a great deal until I found they have the wrong thickness… yes it’s 5.43mm not 6mm.

You see that machining? It’s a mechanical engineerer from a lab helping us out to make some extra spacers to make up for the thickness gap. Then we burnt out two drill bits… (weird moment when you see a slot in the drill bit because the steel was too hard that it milled a track on it!)

Bring home msg 1: make sure you know the tolerance before placing orders (until we have parametric designs like OpenSCAD).

Bring home msg 2: buy G-O-O-D drill bits.

04/07/2019 at 10:21

Well done

11/07/2018 at 07:37

Just in case anyone is interested in the programme, here you go:

08/05/2017 at 15:21

3. Motors

Men love motor – I just can’t tell why. We started using a combo with 1.5kW 1400RPM on 1:40 But it couldn’t shred PET bottles and kept on jamming. So we changed to 1:80 (works just well) with forward-reverse switch. [how should I post video though?]

And we thought it’s good if we have some auto-stop mechanism, just in case the machines jam and stop. So we added an optical encoder and did some programming. We have 3 relays in total (1 SSR for main power, 2 mechanical relays for forward-backward control). The [yet-to-be-finished] programme would stop any power to these relays and halt the motor.

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