Friday, December 10, 2010

Plastic Extruder for Growing Media

For several months, I’ve been developing a plastic extrusion system that has been able to take virgin HDPE resin pellets, or shredded milk jugs, and properly melt and extrude them into a shape that could be used as a low cost growing media for my Aquaponics system.

There was a lot of trial and error to get to this point. The biggest problem is that the plastic retains little moisture. If the seeds aren’t directly placed in the flood/drain cycle, they won’t get any moisture to germinate. I typically grow in stone and some of the stone above the water line is able to wick and retain enough moisture to provide water to new seeds.

Another issue with HDPE is that it’s extremely smooth (again, works well to repel water). Even adding texture to the media during the cooling process, the media still is smooth, which makes it difficult for bacteria to stick to it. I was also a bit surprised to see that the roots didn’t really care to grow in it and they would grow around the edge of the net pot instead.

On the plus side, the plastic is light, fairly inexpensive, clean, and easy to work with.

I hope some of the info in the video is useful to some of my fellow aquaponic/hydroponic growers in their quest to find a better, more cost effective growing media.

EDIT:  There is now a second blog entry with some more details about this.... Click on the "Newer Posts" link near the bottom.

Below is the transcript for the video…no need to read it if you’re going to watch the video… I just included it so some of the search engines could pick up on the keywords. ;-)



Hello Everyone. Today I’m going to show a plastic extruder system that I built. The end result was to produce a synthetic, cost effect growing medium for my Aquaponics system.

The extruder consists of a hopper for high density polyethylene pellets. An auger then forces the pellets through a dual zone heating chamber. The heated material is forced through a small die at the end of the chamber.

The temperature in each zone of the heating chamber is controlled by a Teensy AVR microcontroller which is monitored and adjusted through its USB port connected to a laptop.

The auger is driven by a windshield wiper motor and it is geared-down using an old bicycle sprocket and chain.

The hopper is filled with HDPE pellets where they are slowly forced into the heating chamber. It can also be filled with shredded milk bottles or shredded milk bottle caps to add color.

The heating chamber is covered in some fiberglass insulation to conserve heat. There are two thermal probes mounted near the middle and end which provides accurate readings to the controller as the material is heated. The heating elements draw around 16 amps at 12 volts.

The molten plastic that is extruded from the die is squeezed through a set of rollers which embed a texture into the material. A small tube blows air onto the pressed material to cool it, and to keep the rollers cool.

This is one of the rollers after I turned it on my lathe with a close-up view of the texturing.

And this is a close-up video of the material being extruded and pressed through the rollers.

Here is a close-up view of the finished material once it has been cut to length. There is a waffle pattern embedded into the plastic which provides plenty of surface area for bacterial growth. The media lies flat which helps to retain moisture during a drain cycle. The pieces have plenty of spaces between each other for water and root growth.

Most HDPE plastic is classified as food-grade. However, one problem is that nothing likes to stick to it. Even though a texture has been embossed into the plastic, a small amount of movement can disrupt anything that was clinging on it.

This is a time-lapsed video taken with my PlantCam over a 30 day period. There are 3 bean plants growing. I also planted lettuce seed which didn’t germinate, probably because the top inch of the media doesn’t retain moisture like stone or expanded clay.

After 30 days, I removed the beans from the aquaponic system. I had the net basket wrapped in foil to prevent the roots from wandering into the surrounding stone. The roots seem to have an aversion to growing in the plastic and mainly grew between the basket and foil.

Thanks for watching. If you have any questions or comments, please leave them in the comments section below. Also please subscribe to my YouTube channel to see future videos!

19 comments:

Anonymous said...

You should turn the plastic into a filament so you can use on a 3D printer like a RepRap or MakerBot.

fuzzywolf said...

where did you find the extruder?

Web4Deb said...

I built it from scratch!

zzzomb said...

Very nicely done. I've been chopping up milk bottles for months gearing up to make a similar set up.

You may find this website also interesting:
http://recyclebot.tumblr.com/

A few questions for you though. What do you use for your heating element, and where did you obtain it?

Also.. I have taken the lead screw from a car tire jack, and found a pipe that it fits inside but not really as tightly as I would like. Does your auger have a bit of room surrounding it or does it fit snug inside the pipe? Where did you obtain your auger from?

Web4Deb said...

You can get Nickel-Chromium wire at mcmaster.com. Wrap your pipe in a high temperature tape and it will work fine. Don't overheat the wire since it can melt the tape. I used a standard 3/4" wood auger bit (bought a set from Harbor Freight) and cut it down to length. It fits nicely into a piece of 3/4" black pipe you can get at any hardware store. There's a little bit of slop between the wall and the bit, but it didn't seem to make a difference. I was concerned about the molten plastic oozing backward, but it never did.

zzzomb said...

Thanks heaps for your response. I had experimented with nicrome wire but couldnt think for the life of me how to electrically isolate it without losing heat. Will give your high temperature tape method a go. Great to hear a loose auger doesnt cause pushing issues.

Capo said...

Hi

You’re granular extruder has caught the attention of quite a few people in the 3D printing community.

Is there any chance you could post some photos of your extruder setup? It can be a little hard to make out the details from the youtube video.

If you can find the time I also have a few questions relating to your extruder setup which I have put at the bottom of this reprap wiki page.

Feel free to add any detail to the wiki page you like.

I also have a suggestion for your aquaponics application: Try (carefully!) running your extruded HDPE under a hot flame for a split second. Something like a gas BBQ burner will do. This will oxidise the surface layer which may assist the roots in binding to it.

Web4Deb said...

@Capo Please send me a private message through my youtube channel (www.youtube.com/web4deb) that has your email and I'll send you more info about the extruder design. Thanks. Sorry for the delay, your posting was caught in the spam filter and I just discovered it!

BluffIT said...

Hi,

I'm very impressed with your setup and am attempting to replicate something similar down here in the South of NZ.

Am interested to know what sort of PLC, sensors, and relays you are using as to build this using my DirectLogic06 seems a little expensive.

henry said...

Really great project.Did you try adding fine grain saw dust (or something) to the plastic mix? It might make a more plant friendly surface. The textured rollers might expose just enough.

Loren said...

I love your setup. I have started making my own. I have a few questions for you.
1. What voltage are you using on the wire?
2. What temperature ranges are you able to get from the wire? I am curious as I was thinking of using heater bands but if the wire is considerably cheaper then it might be worthwhile.
3. Would you be willing to show the circuitry for your controller and pc interface? and what program did you use to run it?
4. Did you have a way of controlling the speed of your screw motor other than just the gearing?
5. Do you think the wiper motor would be powerful enough to run more viscous plastics like an abs or polycarbonate?
Thanks
Loren

Web4Deb said...

@Loren
1) 12VDC. I wanted to run fairly low voltage so I wouldn't kill myself!
2) These wire can get VERY hot. You have to experiment with the lengths and voltages to find something that won't burn the insulation. If the wire is red, it's far too hot. Heater bands would probably work fine.
3) The circuit uses a Teensy from pjrc.com which drives a relay used to switch on/off the power to the heater. I never drew a diagram for it. It doesn't drive the motor for the auger. If you know a little bit of C, you could program it.
4) The speed is fixed. You may see from the video that there's a handle on the large gear. Most of the time I just turned it by hand until I figured out what a good speed was.
5) It wouldn't have any problem with ABS. I don't know about polycarbonate....I think it has a fairly high melting temp? the HDPE melts at around 120C.

Loren said...

Great! Thanks for the info. I will definately go with 110v or 240v heater bands. I'll have to check into the teensy thing. Might be cheaper than buying seperate pic temp controllers. I am hoping to use this machine to recycle left over polycarb sheeting from my small greenhouse manufacturing business so I will need to get the high temps from the bands.

Anonymous said...

How about adding some sand on the surface of the molten plastic? That gives the plants something to hold on don't you think?

eried said...

Wow incredible, do you have more details about your machine? how do you clean it to start with another plastic color?

Web4Deb said...

there's a little more info about it at http://www.reprap.org/wiki/Web4deb. You can't clean it....if you wanted to change colors you have to get the other one to work it's way out. It's basically impossible.

eried said...

Thanks a lot! It is funny how when you think something you found it in the web

merlen hogg said...

Hi Web4Deb,
I loved reading this piece! Well written!

Merlen Hogg
slilkon

Polybase said...

Nice to explain how it works and even more pleasing the safety measures. It is certainly a worth reading post. Thanks for your valuable time.

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