# Selective Solar Sintering

From: Alex den Ouden (alex@denouden.demon.nl)
Date: Sun Jan 30 2000 - 22:17:06 EET

Hello list -

Jim Mallos of Heliakon Solar Sintering Lab tells us about the site
on (selective) solar sintering he's just started.
http://geocities.com/solarsinter )

Some guesstimates and some thoughts.

Firstly, on a *spot* sintering method

accuracy. Say, a maximum "printing" volume of an A4 (30*20 cm
roughly) by 8 cm high. I interpret "none-too-high accuracy" as,
say, 0.5 mm characteristic length for the finest details (and thus
also for minimum layer height). That would mean a sintering spot
of roughly 0.5*0.5*0.5 mm3. Let's suppose a sintering speed of 1
m/s (rather less than commercial speeds, but consider the hardware
needed to control the path of the spot). This would mean we'd have
to heat roughly 250 mm3 (1/4 cm3) of material to sintering
temperature each second. Specific heat and "semi"-melting heat of
course depend very much on the material we want to sinter. Once
we've decided which material we'd like to use for the first
experiments, we could calculate the number of J/s (i.e. W) needed.
And from that, given a solar energy density of ... W/m2, the area
of sunlight to be concentrated into our sintering spot of 1/4 cm2.

A given spot speed of 1 m/s at a spot width of 0.5 mm means a
sintered surface area of 500 mm2 (5 cm2) per second. The full A4
would thus take 2 minutes per layer of 0.5 mm. Of course, it's
seldom we need the full A4 (better start with a sheet or slab,
then!). For the full working height of 80 mm, we would need 160
layers. So, to sinter the complete block of 30*20*8 cm3, the
machine would be working for over 5 hours. I guess that a
"typical" product would require about 1/10 of the total volume and
hence nearly one hour. Seems quite reasonable? Sunlight would
(with luck) be sufficiently stable over this period.

To concentrate sufficient sunlight is not all that difficult, see
the webpages Jim mentions on parabolic mirrors (composed of a
multitude of flat mirrors) and on Fresnel lenses. However. We're
talking about a spot of light focused to about 0.5*0.5 mm2 and a
facetted parabolic mirror will not be able to produce that
directly, i.e., without further lenses or an objective. The
Fresnel lens has another problem. It does not focus the light into
a focal point, but rather into a focal volume (due to the
differences in optical path - the "removed" sections of the lens).
I expect that the focal depth will be well over the required 0.5
mm for most lenses easily available. So again, we would need a
concentrating lens or objective. Remember, both the facetted
mirror and the Fresnel lens are designed for sun "furnaces" in
which a high energy density over an area of several cm2 is very
useful while small spots would not do at all.

The SSS-machine would comprise several sections:
- energy provision (lens/mirror plus concentrator)
- spot control (movement, possibly by small galvanometer mirrors)
- table mechanism (with a stroke equalling the maximum working
height)
- powder handling unit
- computer plus software and interface to operate the controlling
mirrors
- software to calculate the required sections

Just a suggestion - would it be an idea to work in polar
coordinates instead of rectangular? Just think of that discarded
two-speed CD-player waiting for us to put it out into the sun ....

Secondly, on a *surface* sintering method.

I've not yet considered this idea in more depth, it came up when
writing the above. Say we could use a more or less standard
printing or copying technology to produce a series of masks. And
then use these masks to sinter the full area of a given layer in
one go with concentrated sunlight? In fact, something in between
mask exposure stereolithograpy and selective sintering.

With very best wishes
Alex den Ouden (Eindhoven, Netherlands)

e-mail: mailto:alex@denouden.demon.nl
website: http://www.denouden.demon.nl/

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