j.a.velez on mon 25 feb 02
Jim, John, Bonnie et al:
Jim wrote:
"the extrusion that comes out "curled" is due to uneven pressure forcing =
the
clay thru the dies... If you have a large extruder.. then you need to =
make
sure that the die is centered so equal pressure is exerted to all the =
surface
of the clay. and the clay extruded will be straight..."
John wrote:
"Now let's see...Lets just say: When you pull on the plunger you are
compressing the clay and the pressure is distributed equally through =
out. In
an extreme case, when the exit hole was just at the side of the barrel,
there would be no lateral pressure on the side next to the barrel and it
would curl that way. "
Although I do not have or use an extruder, here is my explanation:
The extruded form curls obviously because you are extruding more clay on =
the inside wall of the die than on the outside. I believe that happens =
because there is an initial "drag" (friction) on the walls of the =
barrel. Clay been a plastic material it adheres to the walls of the =
barrel, when the initial pressure is applied the velocity of the =
particles in contact with the barrel wall is zero while the velocity =
maximizes towards the center of the barrel. As the pressure increases =
the clay will "shear off" the barrel walls equalizing the velocity of =
the clay at the outer and inner wall of the extrusion form. I am not =
sure you could completely eliminate the curling phenomena. I have not =
studied flow of plastic materials, but it probably approaches the =
behavior of high viscosity liquids in laminar (low velocity, =
non-turbulent) flow. As others have suggested, it might be better in =
this case "to go with the flow" than to try to eliminate the curling.
Best regards, Jose A. Velez
John Jensen on mon 25 feb 02
Personally, I don't care to or expect to eliminate the curling...I can
easily adjust to it. I've just been curious about just what forces and
resistances are out of balance vis a vis the extrusion.
John Jensen, Mudbug Pottery, Annapolis
mudbug@toad.net, Toadhouse.com
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