Wednesday, July 05, 2006


True Grit

Encouraged by the great performance of Ed's Z axis, I've fired up an experiment he's been asking for. I've done an extrusion on 240 grit sandpaper (some ask about the scale; the hexagon is 16mm (0.63") in diameter):

Note the relatively well-formed sides, extruded as 0.5mm thick layers. Between each layer, the fan was manually run while the outline was being printed. This seemed to cool things off enough, though there's no ducting to protect the heater yet. As expected, the polymer stuck to the sandpaper like, well, glue sticking to sandpaper really. Of course, the sandpaper is stuck to the glass sheet with glue to keep it flat. The Tower Of Ooze (right) came off OK, but the hexagon was stuck solid. So I end up trying to pull things off a sheet of thin, cheap glass and you know what'll happen if I persist in doing that... Fortunately the paper tore before the glass broke.

I'll do one or two more experiments with the sandpaper - might as well use it while it's stuck there - before washing the glue/sandpaper off and trying a different size grit.

Vik :v)

That is REALLY, REALLY looking good, Vik!
Very nice, layers are much more clearly defined! How about glueing it to that big ol' aluminium plate, it won't crack under removal stress. :)

I'm surprised that even at 240 grit, adhesion is still that strong. Maybe finer grit sandpaper, 600 or so, like the stuff used for sanding metal would still be sufficient.

If so, stuff like polystyrene foam as Plaasjapie suggested (sanded a bit to open it up?) and roughed wood would be comparable, and interesting to test.
Having difficulty getting a sense of scale. How tall is the object? It looks like there are 10ish layers, so I'm guessing 5mm.
That's about right.
Yup, 5.0mm total thickness - our layer thicknesses are pretty accurate.

Vik :v)
Have you considered DIY "sandpaper" made with say- spray glue and granulated sugar? then a little water would release your print, removing the paper could be done later by whatever means apply.
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I get it for 10 cents a sheet, so DIY sandpaper has limited appeal.

Getting relatively thin but perfectly flat sheets of wood is proving harder than originally anticipated.

Vik :v)
...hard time finding flat wood...

I had been thinking suggesting plywood, but more along the lines of 3/4 inch. Small squares (like at 20 x 20cm) that thick are extremely stable and flat, especially in the smoother finish grades.

In better quality grades (more ply layers) flatness and stability is very good all the way down to 1/2, sometimes even 3/8 inch, as long as one steers clear of the general purpose rough plywood. Good lumber yards have both low and high grade plywoods.

Now seeing you mention needing thin do you need? To fit under the extruder I'm guessing...? Hobbyshops sometimes have very good grade softwood plywood for model airplanes and boats, in all sorts of flavors under 5mm (1/4") thick.

briand, cool idea about the water soluble sugar sandpaper!
//Yup, 5.0mm total thickness - our layer thicknesses are pretty accurate.//

That helps put things in perspective. So, what we're looking at is an extreme closeup of something about the size of a pencil eraser. Those small imperfections are almost invisible at that size.
Looking good Vik,
Yesterday I saw a paper cutting cnc machine using basically contact cement for that (felt just like the back of sticky notes). 3M has a good product: F77 contact cement. The paper cutter was using a flexible plastic backer (with contact cement to hold the paper) and translating that under the head. Probably .030 inch thick plastic (maybe nylon) with a fine texture.
How about a block of Oasis instead of sandpaper (it's that stiff foam that florists use to put flower stalks in). That would be rigid, would have a rough surface to which the Capa would stick, and would be easy to separate things from.
I looked at oasis foam last weekend, and the quality seems dubious. It'll certainly need to be shaped before use. I'll give it a go a some point, but it'll have to take its place in the queue.

Vik :v)
What would be the optimal thickness of the support material?
How about if the construction surface just had a few blocks that could be clamped down (say magnetically) with their edges flush with the bottom layer of the extruded product to hold it in place?
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