Cardboard

Cheap and plentiful material. I believe by laminating layers with opposing grain into "plycard" it will be very strong.

I'd like to try finishing the final product in paper. Paper is cheap and should look fine painted. Though with the MDF boxes, getting it flat was a challenge. It may not be as bad here because the paper was structural there which made it hard to work with when wet. Wesley Treat also has a great tutorial on painting surfaces to look like wood, I could try that. Acrylic paint is cheap. Could also try poly over that for protection.

I'd also like to try wood veneer and see if I can make something that looks nice.

My initial test was just 3 squares of cardboard with alternating grain. I used cheap, clear school-glue. The white glue is cheaper, so I should try that next time I need to purchase glue. I diluted it down about 2p glue to 1p water. The right amount of glue seemed to be stripes about 2" apart. The outer layers of cardboard were unevenly wet and wanted to warp, so they needed to be held down.

Water based Polyurethane costs about $40/gal. I'm guessing my best bet is to do one layer of cheap glue to coat the surface, then do a thin layer of poly on top of that.

I want to layout a 4'x8' area and time how long it takes to make a 5ply that large.

Once I've got a sheet, I want to try making a cardboard rabbet box.

Cyclone Separator

This would be an interesting test. Can I build a cyclone separator from cardboard?

As a model, the funnel has "petal" layers and "ring" layers. Probably do 5 ply. The inner most layer is a petal layer to allow for the ramp. Each petal has a top and bottom half with interlock and stick out to form the ramp. Complex geometry, but not too bad.

Pulp

The scale test (below) didn't work well. I'm curious if I could make panels by pulping the cardboard. From a 3D printing video, they mixed pulp with a binder (glue or rice). I could do this in a 5 gallon bucket using a drill as the mixer then pour into a largish mold, say 2'x 2'. I could also try adding sawdust.

Scale test (3/19)

Did 3-ply. First layer only took 20min to get down, but the second two layers (which required gluing) took about 70min each. The whole thing took 2:45. It also took about 1/3 gallon of glue.

I also put brown paper on one side by first putting down a mixture of glue and water, then putting down the paper and smoothing it, then wetting the paper and smoothing it more.