Binder Burnout Problem

[Ehsan]

I am working with alumina ceramic. During the binder burnout of
injection molded components, blisters appears on the surfaces of the
components. Also hard skin develops on the surface of components which
appears as thin laminates over the sintered body. Kindly help me in
this regard. Best Regards

[meta...@hotmail.com]

Since you did not bother to tell us what was in your binder, or what
heating rates and hold times you are using any diagnosis of your
problem must be generic and mostly hand waving. Blistering suggests
you are heating at too high a rate.

Regarding your hard skin, you may want to look at:
http://www.pelcor.com/library/hard_skin_2002.pdf

The recent book “Ceramic Materials: Processes, Properties and
Applications” edited by Phillipe Boche and Jean-Claude Niepce contains
a discussion of binders and resultant problems. See pages 184 to 188.
Particularly, see Fig. 5.40 for heating kinetics of the alumina-
polypropylene-wax system which is from

I.E. Pinwill et al, “Development of temperature heating rate diagrams
for pyrolytic removal of binder used for fowder injection molding”, J.
Mat. Sci. V27, p 4381-88, 1992

Pittsburgh Pete

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[Gregg]

A quick first test is to run a DTA experiment.
Put one thermocouple in a bique fired part - bore a hole and stuff it in.
Do the same to a green part and fire them both together. (use a data
logger to record temperatures)
Plot the temperature difference vs. furnace temperature and time.
The temperature difference will tell you where you need to slow down in
your firing curve.
Depending on the size of the part, the temperature difference can be
dramatic - 100's of degrees
If the exotherm spikes sharply and you can't decrease it significantly
by slowing down your heating ramp.......
you have three options - change binder systems, pump in an inert gas
during the binder burn out or put the part in a covered crucible (to
limit oxygen to the burning binder).
Keep testing with thermocouples stuck into your parts until the exotherm
is minimized.

it's cheap, easy and you can use your existing kilns
Gregg

[Gregg]

oh ya...........
If you fill your kiln up with smoke from the burning binder and then
oxygen gets in
Your kiln will blow up! - make sure the atmosphere in the kiln does not
approach the combustion limits!
This can be a real problem for electric kilns and any kiln using an
inert atmosphere.

If you have a large kiln and a full load - be very careful - hire a
consultant before you kill somebody.

Gregg