When I started this project long ago, I naively thought that I'd just collect together the many extruders I'd seen, including plastic filament, clay, concrete, and chocolate. Well, what works for 100mm to 200mm cubes (or thereabouts) is not always appropriate at 1m to 10m (or more) cubes.

Plastic filament

I even bought a direct filament extruder to use, but before I got far enough to begin construction, I figured that it was impractical on two counts.

• I calculated the amount of plastic needed to print a 1 square meter panel just 5 millimetres thick requires a bit over 5 kilograms of ABS plastic and about 6 kilograms of PLA plastic. Mmm...at $40/kg need something a lot less expensive!!
• Printing with a 0.4mm extruder is going to take a very, very long time to print at scales of 1 to 10m, and I suspect I won't live long enough to see it finish!

Besides, what was I going to do with the failed prints!!

Clay

So the first extruder I used was a 60ml syringe, and I filled it with a clay mix. However, I couldn't squeeze the clay out. I thinned the clay enough to get it to squeeze out the 2mm nozzle, but it was too thin to stay where it was extruded. A bit of digging around the internet exposed lots of untested ideas, and even more failures. The few examples of successful printing gave no indication of the clay formulation, and many seemed to use nozzles of 10 to 20mm, high pressure air pumps and heavy hoses. So I put the clay idea aside for the time being.

I tried using a flour mix in the 60ml syringe. It proved the ability to position the effector, but really only made very thin models. Like clay, the flour mix was either too thick to get out of the syringe or was too thin to really allow building in the vertical direction.

Concrete

I've seen concrete poured out of truck and pumped, so it cannot be hard to do can it? Well, concrete is poured into a form for a very good reason: it cannot be pumped if it is anything like thick enough to hold its shape unsupported in the vertical direction. Well, not unless you get a special formulation and who has published such a special formulation in the public domain. Besides, once you have printed an object that is a couple of metres in each direction, how does anyone move the object easily?

In the second installation of rd3dp, I was getting ready to try concrete when I finally figured that the resulting print was going to weigh several tonnes. I suspect that if I had attempted the print that it would probably have fallen through the floor into the carpark underneath: even concrete slabs have weight limits.

Besides, disposing of print failures is a huge problem for concrete.

Chocolate

Best eaten in smaller quantities than blocks of a cubic metre, chocolate is also a bit too expensive to use as a test material at that scale. 'Nuff said.

What else?

At a Show and Tell event at MakersPlace (makersplace.org.au), I asked the question about what to use as a material for rb3dp. I got several interesting answers:

• Ice, since it turns to water and just flows away
• Coffee grounds, since they are a waste product (=free) and lightweight
• Paper, since it is a waste product and also lightweight

Well, I've done some experiments with coffee grounds and paper without finding a reliable extrusion method for either. The experiments are still ongoing, but only sporadically (when another idea crops up).

Dirt

I was researching binding materials (for the paper or coffee grounds), when I found that the road building industry used water soluble polymers as dust suppressants and binders for road base materials (basically, dirt). There is still some work to be done but it gave me enough confidence to actually put up the idea to the public (see https://hackaday.io/project/13442-3d-print-emergency-accommodation).

If anyone can point to a URL with a reliable effector useful at the 1m to 10m scale of printed model, please comment.