G’day Gustavo. One thing that you have to look out for when burning
out certain types of Plastics / Acrylics is that they do not burnout
completely. An ash is lift in the void after burnout, this is
sometimes the cause of the porosity, which in fact is not porosity
at all but inclusions in the metal from the trapped ash.
Some of the plastics may have a Silicon content or be Glass
reinforced or have some other non combustible content and no matter
how long the burnout the ash will remain.
When I cast using plastic components I make sure that there is only
one component in the Flask with a large straight sprue so that after
burnout I can have a look down the flask to ascertain wether there
is any ash. If ash is present and loose the a gentle blow of
compressed air is used to remove the ash, the flask is then put back
into the furnace an allowed to come back up to temperature.
If the ash is in a tight mass then at times I have had to resort to
poking at it carefully with a stainless steel poker to break it up
then blow it out with the compressed air.
This goes the same when burning out timber models and even insects.
There will always be some ash present and when not removed will
cause Inclusions in the metal.
PS any further please do not hesitate to contact me.
Michael W Kohlleppel (Metallurgist)
Art Tech Castings Australia.
@MWKohlleppel or Investmentcast@aol.com