Our second key machine is our Precious Plastic-designed injection machine. This open-source design has been around awhile, and while it's far from perfect, it has made our recycling operation possible.

In theory, a plastic injection machine is pretty simple. Imagine a giant syringe with a heater on it... that's how plastic injection machines work. In practice, it's a little more complicated.

Our injection machine has a long lever arm connected to a steel plunger that fits into a seamless steel tube. The tube has a pair of band heaters around it, along with a pair of thermocouples to monitor the temperature. These are connected to a pair of PID temperature controllers- one for the upper part of the tube, and another for the lower part. The tube has a threaded end for common 1/2-inch pipe fittings. The dies screw on to the threaded tube.

The advantages of this design are that it's relatively simple, so that it's easily repairable if something goes wrong. It also has- comparatively- a very large capacity for this type of machine. Typically, a small benchtop design used by industry will have a capacity of 5-30 cubic centimeters. The Precious Plastic design can inject 150 cubic centimeters. This additional volume is important, since we are making thicker items that are designed for a long lifespan. Conversely, the plastics industry typically produces thin plastics (think water bottle thin) that require very high pressures and very uniform, pure raw materials.

And while overall, this injection machine is a great design, there are a couple of dumb things about it. Foremost is that it take two people to operate- one to screw on the mold and cap the barrel, and a second person to operate the lever arm. Another is that the ergonomics of this machine are terrible- it was obviously designed by young people with great knees and hips. You need to work on your knees in order to screw on the dies, and since you're screwing the dies on essentially upside down, it's very easy to cross the threads and damage either the die or the barrel. The barrel also needs to be capped with a screw plug while the plastic is heating, otherwise it runs out.

Overall, though, the advantages far outweigh the downsides. It's pretty essential to have a certain volume of plastic per shot, and this machine can inject enough for a small flowerpot, two coasters, or multiple stacks of sea animals- these are our primary products, I'll write more about these in a future post. If you are thinking of setting up your own recycling workspace, I believe this design would probably be the best choice, design flaws and all. I've been thinking about ways to improve the design- it's one of those projects that I'll get to as time allows.

In our shop, we have three key machines that are pretty much essential for recycling plastic: a shredder, a "large" injection machine, and our new "small" injection machine.

First off, the shredder. Some kind of way to cut the plastic into smaller bits is necessary in order to remelt and fit into most machines (with an exception, more on that in a minute). 
Our first shredder was the Precious Plastic design. These resemble an oversized paper shredder driven by a large gear motor. This worked fine for a while, though it did have a tendency to jam every once and a while. But after a few months of pretty regular use,  the large and expensive motor burned out. We aren't sure why this happened, though I suspect it's a 240 volt motor, and we were feeding it 220v or something like that.

Whatever the reason, now we were in trouble. We were collecting a LOT of #5 plastic, and we had to have a shredder in order to keep up with it. We were unable to find a local electrical shop to rebuild the burned-out motor.

So we ordered a small commercial shredder that is used in the plastics industry. It's one of the smaller ones made, but for us, it's a monster. It operates at high speed, and has large, strong cutterheads that can chew up just about anything we can throw into it. Though just to be safe, we usually pre-chop the really big stuff into smaller bits... just in case.