Tom Richards wrote:
>
> But how can we cause the expanding material to stop at the 3D CAD surface of
> the part we're fabbing WITHOUT the need for a mold??? If we can answer that
> we're on to something great.
>
You ask the crucial question. Like a piece of popcorn the foam comes
naturally to its own final shape. Figuring out how to control that shape
by locating the nucleation sites seems at this point like groping in the
dark. I can only offer some scattered observations.
1. Any shape can be grown from a sphere. To have a mental picture of
this, imagine the shape under consideration has been instantly carved
from a viscous liquid, and it then collapses under its own gravity.
Playing the video in reverse shows a way of forming the shape. Of course
this is a constant volume process unlike foaming, but the two will look
just the same if the camera on the foaming process smoothly zooms out to
give the illusion of constant volume. (An interesting point with this
viscous growth pattern is that, in general, some of the final surface
gets pulled out from the interior of the sphere. This is not just a
deformation of the sphere's skin.)
2. Interior surface is the main thing. A one liter cube of programmed
foam may contain a quarter billion nucleation sites. If we can control
that sort of internal complexity we can readily imagine forming an
internal pneumatic structure or machine that can alter the exterior
shape.
3. If no nucleation sites are programmed the plastic does not foam at
all, it just slowly loses its gas by diffusion to the exterior. If just
one nucleation site is programmed, a very large bubble will form there
because the bubble competes only with the exterior for gas. If two
nuclei are placed side by side, their aggregate volume will be hardly
any bigger than the lone bubble because of competition for gas. There
will however be a thin membrane dividing the resulting hybrid bubble.
Means which include acoustic energy are used in the manufacture of open
celled foams to rupture such membranes. Some similar method could be
used to break membranes that are below a certain strength. This is very
necessary to form passage ways and cleavage surfaces.
Jim Mallos
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