David Hendley on mon 25 feb 02
It is well-known, I thought anyway, that clay flows faster in
the center of an extruder than at the edges. The phenomenon
is even more pronounced when an expansion box is used.
If your die is radially symmetrical there are seldom any problems
with curling extrusions unless the die is installed off-center.
When irregular shapes are extruded, the flow of the clay needs
to be taken into account when designing the die.
Of course, this uneven flow can be used to your advantage,
if you know how to work with it. For instance, the 'extruded
bowls' that are sometimes shown in advertisements for Bailey
extruders are made with a U-shaped die. When installed, the
'bottom' of the U is near the center of the extruder, and the
tops of the U are near the outside. The result is that much
more clay goes through the center and the extrusion curls up
so much that a bowl shape is created.
If your die makes extrusions come out curled and you want
them to be straight, you can 'tune' the die to make it do what
you want. For example, I wanted to make a hollow vase extrusion
about 5 inches wide on 2 sides, but only 2 inches wide on the
other 2 sides.
During test extrusions, the wide sides would always come out
rippled, because more clay was passing though the center of the
long sides than the short sides out at the edges of the die.
To correct the problem, I drilled 2 'bleed holes' near the center
of the die. The holes allow the extra clay to go through holes
rather than forcing too much through the walls that are near
the center. The 2 coils that are also extruded are simply removed
and wedged to be used again.
Extrusions that are just a little off will start out curled a little
but then straighten up when the weight of the clay that has
already come out pulls it down. Extrusions can also be made to
come out either straight or crooked by guiding the clay with
your hand as it comes out of the die.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
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