TECHNICALITIES AND AESTHETICS OF CLAY EXTRUSION FOR THE ARTIST / POTTER

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1 A thesis submitted in partial fulfillment of the requirements for the degree of Master of Fine Arts in Art in the Graduate College of The University of Iowa July, 1981 TECHNICALITIES AND AESTHETICS OF CLAY EXTRUSION FOR THE ARTIST / POTTER by Lucien M. Koonce

2 Extruder Types Chapter 1 Plunger A plunger, or ram, extruder is usually manual operated, but there are pneumatic types on the market. The principle behind these extruders is that a force is directed downward on a plunger, either by torque on a drive lever or by pneumatic pressure, which is contained within a barrel. In front of the plunger would be a plastic clay body. The plunger, when applied with force, directs the clay load through the barrel and out of a die attached to the end. This results in the extrusion of a clay shape to the conformity of the die's design. Being rather simple in mechanics, a person can build a plunger extruder for manual operation. An article on this subject is Making a Bumper Jack Extruder, Ceramics Monthly, September 1976, pages This tool can aid in the production of many clay shapes and is used widely by artists/potters in the studio. They are simpler and less expensive than auger extruders. If a need for greater production in volume is needed, a pneumatic plunger would be in order. A number of companies manufacture plunger type extruders, the majority selling manually operated ones. Probably the two most popular are Robert Brent Corp., 128 Mill Street, Healdsburg, California and Baily Pottery Equipment Corp., 477 Hasbrouck Avenue, Kingston, New York The Bailey Corp. also makes a pneumatic model. Chapter 2 Auger An auger drive extruding system is associated with extruders powered by motors, ranging from less than one horsepower to twelve hundred horses and greater. These machines utilize an auger, or multiple augers, to mix and discharge the clay through a die. A factor to consider with an auger drive extruder is that a pressure cone of clay will be created due to the forward thrust of the rotating auger and the retarding effect of the liners around that auger. The cone's base is in the same plane as the face of the auger, with its apex at some distance to the front. The length of this cone depends on the diameter of the face of the auger, the nature of the clay, the moisture content of the clay, and the pitch or displacement of the auger and its r.p.m.

3 This pressure cone is of concern because it creates an area within the entire round, solid column with excess pressure at the center tending to make it move faster there. To extend the barrel and liners around the auger out past the end of the auger, then any point past the apex of the pressure cone the entire round, solid column would move solidly as a unit. The length of an auger barrel is dependent on the material being extruded and the size of the extruded product. Ideally, the clay should be compressed from auger until it is out of the die in as short of a distance as possible. Conditions are rarely ideal and people must work with the machines that they have. Therefore, it is sometimes necessary to extend the column. This works best when the height can be reduced in relationship to the length. In other words, if the cross-sectional area of the die is kept smaller than the area of the auger. Another problem encountered is auger memory. This is the rotary stress developed from the auger appearing after the product has left the die. This has been overcome by the use of directional guides placed in front of the auger parallel to the extruding slug attempting to stop the twisting action before the compression begins. Among the leading firms making extruders for use in a studio situation are the following: Bluebird Manufacturing Company, P.O. Box 2307, Fort Collins, Colorado 80522; A.J. Wahl, Inc., 8961 Central Avenue, Brocton, New York 14716; The Keller Corp., 3275 Penn Avenue, Hatfield, Pennsylvania 19400; The Bonnot Company, 805 Lake Street, Kent, Ohio 44240; Fate International, Bell and High Street, Plymouth, Ohio

4 The Die Chapter 3 Materials The materials used in dies have been varied over the years and the changes that have been made usually been made for improving the life of the dies. The die wear has a relationship to the material, rate of production, whether it is lubricated, and the pressure of the clay on the die. If the artist/potter has the dies made commercially, the particular company could offer the most advanced material, but for quite an expense. I shall discuss materials most suitable for the making of one's own dies. Plywood is easy to fabricate. Baltic birch plywood is the sturdiest available, about 75% over A-C grade fir. Wood will warp with pressure and wetness, and a heavily grogged clay will wear it rapidly. The thickness to use depends on the material's tensile strength and the amount of applied pressure by the extruder. Plexiglas wears harder than wood and fabricates as easily. It can break, costs more per square foot than plywood, and cannot withstand a lot of exerted pressure. Ceramics is very wear resistant, but it is fragile. Clay seems to want to adhere to it causing in some cases a balance problem. Cast iron can be cast into special shapes easily, as well as fabricated. It is suitable for auger extruders, being able to withstand the force. It will rust and pit which adds to friction. Cast hardened steel is more wear resistant than cast iron and also reduces friction slightly more. Aluminum is much softer than iron or steel and therefore can be shaped more easily. At the same time it wears greater and does not have as much tensile strength to withstand excessive pressure. Chapter 4 Design The biggest problem with designing a die is to achieve balance so that it will extrude uniformly. The natural tendency is for a die to run slow along the die surfaces (frictional surfaces) and fast between these surfaces. Die positioning and balance are less critical for plunger extruders than for auger extruders. The bulk of this chapter will deal with the auger system. Except where the auger causes a problem, the same information will hold true for the plunger types.

5 The ideal die through which to extrude would be a relatively thin-walled round tube of clay having the same outside diameter as the auger. Such a tube is symmetrical around both horizontal and vertical axes. A square, tubular column is symmetrical about both horizontal and vertical axes, and is probably the next most easily balanced shape. A rectangular or even more irregularly shaped die usually needs special provisions in the "neck" or lead-up portion of the die to counter the tendency of those portions of the clay column farthest away from the center of the auger to flow more slowly than at the center. As previously mentioned, large solid extrusions tend to flow faster through the center than on the edges. Die surfaces thus need to be sloped to assist extrusion and lubricated to reduce surface friction. Hollow extrusions require a more plastic clay body than solid extrusions. The cores in a hollow die (which form the voids) help to balance the flow of the material. Cores symmetrically placed with reference to the center of the die will have the beneficial effect of slowing down the center of the clay column relative to the outside. The walls of an extruded column should be thicker the farther they are from the center of the die. Thus the walls around the outer periphery of a die are usually about twenty percent thicker than those somewhat nearer the center. Cores themselves should have well-rounded corners, and the inside corners of the die itself should be likewise rounded or filleted to reduce the tendency to pull the clay. The faces of all cores in a die should be in the same plane, and this plane should be one-sixteenth of an inch to one-eighth of an inch behind the plane of the face of the die, never out in front thereof. Generally speaking, the simplest horizontal or vertical core bridge that can provide adequate support for all of the cores in a die is the most desirable, being no thicker than necessary to do the job. The bridge pattern, just as in the case of the core pattern, should be symmetrical about both vertical and horizontal axes. The largest dimension of a die opening should generally be smaller than the diameter of the extrusion auger. It is possible to extrude shapes which are larger than the diameter of the point auger. This is not very difficult where the extrusion die is cored. It is much more difficult where the extrusion has to be solid. In both cases, the die usually has to be mounted farther away from the end of the auger. Chapter 5 Extrusion Problems A common problem arising when extruding columns, which have corners, is "dog-earing. This is represented by a tearing of these corner edges running the length of the column. Bad corners appear when friction retards a smooth flow of clay in these areas of the die, and when the die has been placed too close to the

6 auger to insure that the corners fill out sufficiently. Too, a non-plastic clay is much more susceptible to dog-earing than a plastic one. When a clay column "flowers" outward from the center, or there is evidence that the center of the column, generally, extrudes farther than the sides or corners, the die is too close to the auger. Sometimes semi-solid products will warp just after they are out, the warp often being evident only after drying or firing. The position of the die relative to the auger may need to be changed, or the design or thickness of the core bridge, or the number, size, and closeness of grouping of the cores may have to be reevaluated. Cracks in the surface of a clay column at right angles to the column length is generally caused by excessive friction of clay with the interior of the die, often associated with extreme imbalance of the clay flow. Separation of the hollow clay column parallel to the length of the column at the sides or top or bottom, depending on the verticality or horizontality of the core bridges, is usually caused by a failure of the clay to knit back together after it passes over the core bridge. Roughen up, by means of short, heavy beads of welding, the top and bottom surfaces of the core bridge, or serrate the surfaces, to create turbulence as the clay passes over the bridge, which aids in the knitting back together of the clay. In some instances there may be a need to put more distance between the bridge and the cores, i.e., extend the length of the core pins.