Energy Materials Testing Laboratory EVALUATION FROM

MoDEL FoR PowDER PRESSING AND DIE DESIGN, PART 2
A two-dimensional illustration of a
defective balloon eyelet produced by
stress-relief upon ejection from the
forming die.
the head of the part relative to the
bottom flange. To be able to eject the
part intact from the forming die, it
was determined that a critical minimum
pressure was required to compact
the powder in the bottom
flange to a density that would provide
sufficient green strength for
ejection. The head and bottom
flange would otherwise separate
upon ejection from the forming die.
A qualitative assessment of balloon
e ye l e t compac t i o n b a s e d o n
observed defects, die penetration
tests and sintering shrinkage showed
similar results for all powders examined.
To complete a more quantitative
analysis of the density gradients in a
balloon eyelet after pressing, a
94-wt% alumina part formed by dualaction
pressing at 12.9 MPa was
bisque-fired and subsequently characterized
using XRCT. A second
94-wt% alumina part formed at 56.9
MPa was fired and characterized
using microscopy and image analysis.
The lower forming pressure produced
a region of high-density in the
bottom flange and in a cone-shaped
region of the head of the part. This
dense region is strikingly similar to
the shape of the defective balloon
eyelet formed by stress-relief upon
ejection from the forming die in the
Dorst press. The density gradients
suggest that the forming pressure
Density gradients measured using XRCT
in a 94-wt% alumina balloon eyelet
formed by dual-action uniaxial pressing
with a top punch pressure of 12.9 MPa.
Density increases with color from blue to
green, yellow, orange and red. Note the
high density in the bottom flange and the
higher density cone that forms in the top,
similar to the defect described in the text.
The blue-green color outlining the part is
an artifact.
was higher in the bottom flange than
in the head of the part.
Results were different for the
94-wt% alumina compact formed at
56.9 MPa. The higher forming pressure
produced the highest density
(i.e., lowest measured porosity) in the
head of the part, particularly near the
through-hole. Density decreased axially
from the top of the part, and the
bottom flange had the lowest density.
The higher forming pressure produced
a higher, more uniform
pressed and sintered density in the
head of the part, indicating that the
forming pressure was higher in the
head than in the bottom flange.
The higher forming pressure also
produced a crack at the transition
radius between the head and bottom
flange, similar to that observed in the
parts formed at high pressure on the
Dorst press.
Generally speaking, forming with
increasingly higher applied pressures
from the top punch moved the highdensity
region from the bottom
flange into the head of the part and
produced a more uniform density
powder compact.
In virtually all cases, the density in
the head of the part decreased axially
from the top and radially from the
inner diameter. The lowest density
region in the head was always found
in the outer diameter, lower corner.
A sintered and polished quarter section
of a 94-wt% alumina balloon eyelet
formed by dual-action uniaxial pressing
with a top punch pressure of 56.9 MPa.
The figure also shows red dye penetration
in regions of open porosity and notes the
measured levels of porosity in different
regions as determined by image analysis.
Density and stress gradients at the
transition radius between the head
and the bottom flange contributed
to cracking, particularly at high forming
pressures.
Cracking was more prevalent in
dies designed with a sharper transition
radius between the head and
the bottom flange and/or when compacting
more difficult to press powders.
Model simulations of balloon eyelet
pressing were completed using the
properties of diatomaceous earth or
those of the 94-wt% alumina powder.
In this exercise, however, specific
compaction ratios were not used.
Systematic changes in the displacements
of the top and bottom
punches were used to assess pressing
balance and its effect on density
gradients in a balloon eyelet.
Unbalanced pressing from the top
alone produced severe density gradients
in the powder compact. Powder
was compacted in the head of the
part, but virtually no compaction
occurred in the bottom flange.
This result was consistent with the
general experimental observation: if
42 The American Ceramic Society Bulletin, Vol. 80, No. 2