It's well known that one of the most commonly thrown away plastics is HDPE (High Density Polyethylene). It is difficult to work with because it shrinks as it cools once formed so it is not generally used in 3D printing, however it's extremely useful in harsh chemical environments, that's why it's commonly used to store chemicals. This instructable will guide you in making a simple hand-crank extruder powered by nothing more than a simple kitchen appliance. No electronics know-how is necessary.

Warning: Wear safety glasses when using tools.
Warning: Use oven mitts when using the injector.

Step 1: Center End Caps

In this step, you'll need to create pilot points to drill the center holes on the end caps. It's important that this step be done, otherwise the piston may not fit correctly and the injection nozzle will look screwy. To get the most accurate center, we will rely upon the accuracy of the threads, so you'll want to use a nipple as the support.

If you have a lathe, all you'll want to do is to insert a 3/4" non-galvanized nipple into the lathe, put on a cap, and drill the hole. I don't have a lathe since they cost more than I'm willing to pay, so I create an adapter for use with my drill press. Here are the steps:

Find the biggest dowel that your drill press will accept.
Cut a short piece of the dowel off.
Wrap the dowel in masking tape so that it'll fit tightly into a 3/4" nipple.
Insert the dowel into the drill press / hand drill.
Insert the smallest 3/4" nipple you can, on top of the dowel adapter.
Screw a cap onto the nipple-adapter.
Place knife / blade onto cap while spinning.

With respect to step #3, it may require a long piece of tape. Once you finish, you may want to wind it back onto the tape-roll.

With respect to step #7, the rotation tends to push the cutting blade towards the center of rotation. If this doesn't happen, you may need to (roughly) level the cap-tops and try again. A fresh blade from a utility knife works great, though I used some sewing scissors.

Step 2: Tap the Caps

In this step, your goal is to tap the caps. Nozzle: 1/8"-27 MIP tap. Screw Cap: 3/8"-16 NPT tap. The steps are:

Drill pilot hole.
Enlarge pilot hole to fit tap.
Start tap using drill press (no power)
Complete tap manually.

Note: Don't forget cutting fluid, it'll reduce wear/tear on your tools.

With respect to step #3, I used a vice-grip to weight down the drill press up/down spokes. I centered the piece under the tap using a nipple. I rotated the tap using the drill chuck. I kept the work piece from rotating using a plumbing wrench. THIS STEP IS IMPORTANT. Again, if this is not straight, the screw-piston may not work as expected.

Step 3: Crank Arm

In this step, create the crank arm by putting two threaded holes through a piece of aluminum. You'll want a sufficiently long bar to make extruding the plastic easier by providing greater torque. I used the 3/8"-16 NPT tap. Do this for each end of the bar:

Make square.
Put an X through the square.
Intersection marks the center.
Scribe the hole for easier drilling.
Drill a pilot hole.
Drill hole for tap.
Tap the hole.

Note: Use cutting fluid.

Step 4: Bevel Body

For the body, I used a 3/4" x 4" non-galvanized pipe. When these pipes are manufactured and threaded, the outermost edges get compressed inward because of the threading process. To undo this, it is necessary to file the inside using a rounded needle file. Also, during manufacture, there is a bead-line which is difficult to remove, mark it by adding filed notches on either side of the bead line. I'll mention this in more detail in a later step.

Step 5: Piston Preparation

In this step, you'll be preparing the piston. Again, this would be easier using round aluminum stock and a lathe if you have one. This is the alternative method.

Find a 1/2" thick square piece of aluminum that is wider than the inner diameter of the body pipe.
Center-mark the piece using a thin permanent marker.
Drill a shallow pilot hole.
Use a compass to draw a circle around the pilot hole, slightly wider than the inner diameter of the body pipe.
Trace the circle using a standard permanent marker.

Once complete, drill and tap the hole using 3/8"-16 NPT.

Step 6: Rough Rounding

In this step, you'll want to rough-round the piston. Basically, you want to turn it into an octagon. If you don't have a band-saw, skip going to the gym. I used a hack saw and it took a long time to saw...

Using a vice, square around the circle that was drawn in the previous step.
Using a hex screw and 4 nuts, secure the squared piece.
Using the hex screw adapter, you can cut at any angle after securing in a vice.

Regarding step #2, the piece is secured by two jam nuts on either side. Two addition nuts keep the jam nuts from unjamming. Jam nuts are just nuts used against each other keep things from unscrewing. You can then take this adapter and hold it in the vise by the hex screw's head. This will allow sawing at any angle.

Step 7: Make Piston

In this step, transform the octagon from the previous step into the piston for pushing plastic through the extruder.

Secure the piece to a screw using two jam nuts.
Insert into drill press.
Rough-file into a cylinder slightly smaller than largest inside diameter of body pipe.
Use sand paper to finish the cylinder.
Add notch using a needle file to compensate for the body pipe's inner weld bead.
Strip threads by drilling with a sufficiently large drill bit.

With respect to step #3, to keep the filing cylindrical, use a flat file and use a square block to keep it level the drill press table.

With respect to step #6, the smooth cylinder is difficult to work with because it is smooth. Use some rubber to get a grip on it. I used rubber from an old bicycle inner tube to hold the piece when removing it from the screw and when drilling the threads. DO NOT USE PLIERS WITHOUT A BARRIER, they will dent/deform the cylinder.

Step 8: Shrink Washer

In this step, you'll need to shrink the washer. This washer should fit over the piston-cylinder and be slightly smaller. Do so by using a hex-screw-jam-nut adapter, like previously, and with some needle files/sand paper.

Step 9: Prepare Carriage Bolt

In this step, I prepared the 3/8"-16 NPT - 6.5" carriage bolt for accepting the piston. Use a rotary cut-off tool for this step.

Remove any writing/logos from the head.
Round off the square underside of the head.
Thread to the top of head using a die.

This will allow the piston to slide to the very bottom of the head and to rotate freely. Make sure to be thorough with step #2 otherwise step #3 is EXTREMELY difficult.

Step 10: Assemble

This is just a step on how everything was assembled. In the first picture:

Piston slides freely onto modified carriage bolt.
Washer slides over piston.
Jam nuts keep piston in place, but allow it to rotate freely.

In the second picture:

Cap added to the carriage bolt.
Two jam nuts keep piston from leaving the tube when all allowable plastic is extruded.
Two standard nuts connect crank arm.
A long spacer with a hex screw is used for the crank handle.
A standard nut is used to secure the handle.

Step 11: Usage Guide

This step will deal with how to use the extruder. You'll need to wear oven mitts or use pot holders for this because the entire unit will be heated. The stock material are pieces of HDPE gathered from a bottle of empty laundry detergent and from a bleach bottle. Obviously, you should wash the bottle(s) you plan to use, dry, and cut up into small pieces using scissors / gardening shears.

Although the uniformity doesn't matter that much, I aimed for 1/4" pieces that I could (somewhat) pour into the extruder.

Loading: Load the extruder by first placing the nozzle onto the body pipe. Fill with as much plastic as possible. Attach piston-screw portion and twist as much as possible. Repeat to add more plastic.

Heating: Put the extruder, loaded, into a toaster oven at 400°F allow 20-30 minutes for it to heat. If you plan on changing the nozzle(s), heat them too and keep them in the toaster oven until ready for swapping. Using a paper towel is a quick/easy way to change them.

Extrude: Do this as quickly/safely as possible. With oven mitts, hold/extrude the plastic. If you want it to cool instantly, extrude it into a water bath. You can also extrude it into a form. Once the plastic runs out, you'll need to reload and reheat.

Pictured below are 3 different nozzles and the pieces that they produced. Also, pictured is a very rough injection molded piece which I produced by injecting with the 1/8"-27 MIP brass nipple into a 1/2" copper sleeve. It's not very good quality because I did not use enough pressure in the sleeve.

Note: When coming out, the consistency of the plastic is like bubble gum. It's not liquid, but it can be squeezed.

Step 12: Cross Sections

Pictured below are the cross section results of extruding from three different nozzle options. It seems difficult to completely get rid of bubbles. It's most difficult to extrude from the smallest nozzle, but it seems to have the smallest bubbles.

Step 13: Nozzle Cleaning

Although you don't need to, you can clean out the nozzles. It's generally easy because HDPE shrinks as it cools. Here are the methods:

Small Nozzle: Use a hook screw, wiggle, and pull (when cooled). The piece inside the nozzle shows that with the right pressure, that HDPE can adapt very well to the shape of its container.

Medium Nozzle: I only used a hook screw to push the plastic through.

Large Nozzle: Because of the rough surfaces of cast iron, it is difficult to clean, so I did nothing. You could heat/pick off pieces, but it's not worth it.

Step 14: Final Thoughts

Some issues that need to be addressed, please comment if you have any ideas/suggestions:

Smaller nozzles take more effort to turn. I'm not sure what the practical limit, but to overcome this, a longer crank arm is needed. This makes holding onto the body difficult. So far, I think holding the body with a vise or vise-grips is the way to go. The bigger concern is the force of the carriage bolt on the cap. I'm not sure what the limit of this is, but I think that a finer threaded bolt may be the way to go.
Air bubbles are present whenever plastic is extruded. I'm not sure how to get rid of them.

Thanks for reading, enjoy!