One more note: $6 and 18 bot hours isn't the whole story. RepRap is about having a machine that can make itself. It would be an easy thing to use no plastic. However, the idea is that you use plastic to replace things that you can't print (vitamins). So you have to consider the overall cost of the bot and the ease of acquiring parts. I am quite certain that Wally is cheaper overall than Morgan. (Don't get me wrong. Morgan is a great machine and more mature than Wally is.) My point is we should really not worry about the amount of plastic or the amount of time it takes to print it. If you are printing a machine for a neighbor what is the difference between a day or two. Plastic is getting cheaper every day. In the last 6 months, plastic costs have gone down by about 25%. Costs could still go down by 50% and the filament makers can still make money. That is not to mention the new hobby level filament extruders that have hit the market.

I am designing a version of Simpson called BOB Simpson. (BOltless and Bearingless) He costs about $20 in plastic right now with hardly any vitamins to speak of. If I could recycle plastic and turn it into filament I can get it for the cost of electricity.

Moral of the story: Minimizing plastic use should be done only after you minimize everything else if you are interested in the cheapest self-replicating bot. (If that isn't your goal then no worries. RepRap has a spectrum of bots. Those that are quick to print and assembly but aren't as self-replicating and those that aren't quick to print and are highly self-replicatable. Pick what works for you. In other words, there isn't really a default. You have a spectrum to pick from. In the future, you will only have more choices to pick from.)