The ones I know about, e.g. submicron syringe filters, are made from
polyphenylene ethersulphone. The pores come from phase separation: you
use a mixture of polymer, solvent (e.g. NMP) and nonsolvent (e.g. water
or IPA). When you spin the mixture, it spontaneously makes hollow
porous fibres, which are what the filters are made of. Because of the
self-assembly, the holes are uniform enough to use for sterilizing water.
Bigger holes, e.g. styrofoam, are made by gas bubbles--either evolved
from the reaction itself, as in urea-formaldehyde foam, or blown in
during manufacture, as in styrofoam.
There are a number of cellular materials that you could use; I favour
polyurethane foam which is pretty controllable.
The foam would normally be processed into slabstock form then cut to shape,
which will expose the cells. You can control the amount of open-cells (i.e.
the porosity of the structure) with the formulation and moulding
In theory, you can achieve a cellular (porous) structure in any plastic via
injection molding or extrusion, by "using an appropriate foaming agent".
Have a look at http://www.cromptoncorp.com/foaming-agents.html , for example,
and Google for more suppliers of foaming agents for the plastic of your
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