Plastic Extruder - Continued

I was a bit surprised to received so many questions about my plastic extruder. Hopefully this info will answer some of these questions:

There are 3 sections: bearing chamber, hopper, and heating chamber. These are made with a section of ¾” black pipe (gas pipe) and ¾” wood auger bit fits nicely into the pipe. There is a little bit of gap between the auger and pipe. I was concerned the molten plastic would ooze back out of the chamber, but it was never a problem. This unit can consume virgin HDPE pellets or cut up milk bottles.

The bearing chamber is about 3.5” long and is mostly dead space. The shaft of the auger bit is long and I didn’t feel like turning it down shorter. At the end where the shaft comes out, there is a thrust bearing placed on it to minimize the friction.

The hopper has half of the black pipe cut away to expose the auger. This is 4.5” long and has a metal hopper which holds around 1 cup of pellets. As the auger turns, some pellets get forced into the heating chamber while some just sort of move to the front of the hopper. Between the hopper and heating chamber is ¼” of plywood…to act as a heat insulator to minimize the heat loss through the metal.

The heating chamber has two heating zones. The main reason was to attempt to heat the plastic evenly since I wasn’t sure how it would heat as it moved through the chamber. This chamber is 5 inches long. The first temperature sensor is placed about half way through and the auger bit ends just before it (so it won’t hit against the probe). The second reason was I didn’t want to run a high voltage through the Nickel Chromium wire…too dangerous and I had a couple of PC power supplies I could run 12 volts at 8 amps. The pipe is wrapped in Kapton tape which is heat resistant….but you can see in the photo it did eventually burn through. Each zone is independently controlled by a microprocessor to maintain a consistent temp.

The heating chamber has a flange on the end which allows different die plates to be bolted on to the end. Depending on the size of the “hole” in the die, the plastic will extrude at different speeds. For the 1/4” slot, the auger turns at about 18rpm and can produce a ribbon of material at roughly 200 feet/hour.

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!

Winter season - DONE

In a tragic turn of events, my winter season is over. The circuit breaker for the greenhouse tripped and it went through the night with no heat.

The outside temps went down to 21F (-6.1C) last night while the inside temps went to 27F (-2.8C). Everything got frosted except a few lettuce plants. Tank water is at 38F (3.3C)...goldfish are conserving energy by sitting at the bottom of the tank. They'll be perfectly fine this way until spring.