Monday, January 23, 2006


Concrete ideas

One low-cost way of speeding up the construction of large parts might be to produce a hollow part and fill it with some gunk. Epoxy is quite expensive, metacrylate resin is a bit better, and concrete is relatively inexpensive. Environmentally, all have issues and are not recyclable but there are low-carbon concretes being developed.

The part would need to have built-in keys to ensure the filler remained in place, and cross-bracing to ensure it did not bulge when filled.

Vik :v)

Wouldn't crossed latticework inside hollow cavities be quick enough to make? That's what I've seen the machine in the department using, though I've not calculated how strong it would be.
It works to a point, then don't. Stability is particularly important as we are striving for an accuracy of 0.1mm over a distance that could be almost 500mm. Any bending or compression will throw us off. Concrete is not particularly noted for being compressible, and so might work quite well in vertical load-bearing columns.

Vik :v)
Sounds like a plan, and concrete's dead easy to get hold of just about anywhere to boot! If you left some latticework in place, you'd have a nice form for reinforced concrete too, of course.
The problem with doing that sort of thing is getting your polymer separated from the concrete and where you put the waste concrete when you decide that you want to recycle your device into something else.
With the exception of the shrinkage issue which it is well known how to compensate for, Ceramics, ceramics, ceramics. They can be produced from almost any part of any continental material. They are universaly available and have been handled for countless centuries. They cast great slip molds which would seperate very nicely from any 2 part polymer mold or a flexible 1 part (latex or silicon.)
And ceramics can be ground down in a ball mill and recycled.
They do unfortunately usually require firing but that can be done with a number of means such as large
fresnel lenses for solar firing, propane, electric oven, wood or coal.
For small firing jobs (or as an alternative to a laser for sintering), one could use an arc-image furnace. I found the design in an old NASA report in the library - you get two parabolic lenses pointing at each other with a shutter in between for control, stick an arc in the focal point of one and the workpiece of the other (The prototype in the report was made from two old WWII arc searchlights pointed at each other), and the workpiece in the other. I guess parabolic (or fresnel) lenses would be really easy to make with a fabricator, although a certain amount of heavy polishing would probably be needed to wear down the ridges formed by the melt layers.
For the issue of separating concrete from plastic for recycling; if you've a polymer with a density < ~ 1 g/cm^2, you pummel the entire part until the cement is in near-powder form (the plastic probably won't be - deforms more than shatters), then mix it into water. Plastic floats to the top, cement sinks. Skim, dry, and reuse...
There is the matter of what to do with the concrete itself, though.
The Taig (Peatol in the UK) miniature metal lathe has a steel top bed, screwed to an aluminum channel that is filled with a mixture of 1/2 plastic portland cement and 1/2 fine sand. The bed is cantilevered and some testing that I did showed no deflection even with 50 lbs on the cantilevered end vs. the fixed end.

They have been making the lathe beds like this for the past 20 years or so.
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