Stiff Plastic Forming

The water content for stiff plastic forming techniques is between 12 and 20 wt%, which produces partial or full filling of pores by water [1,25]. Extrusion is the most common stiff plastic forming technique, although injection molding can also fall under this cat­egory [22]. The pressures required for stiff plastic forming are lower than dry pressing, ranging from ~3 to 50 MPa (~0.5-10 ksi) due to the higher water content, which results in lower plastic yield points [26]. Extrusion is used to form clay-based products with a uniform cross section such as pipe, tubes, rods, and bricks [1]. In addition, thin-walled products with fine structure details such as honeycomb supports for catalytic converters can be extruded [1]. Extrusion processes can either be continuous or batch type [30]. Continuous auger extruders mix raw materials in a pug mill, shred and de-air the result­ing plastic mass, and then force it through a die (Fig. 10a) [30]. The shape of the die opening and the positioning of “spiders” or other tooling in the throat of the die deter­mine the shape of the extruded part [1]. The piston extruders used for batch processes can be used to form the same shapes as continuous auger extruders and they have a much simpler design (Fig. 10b). However, piston extruders can only produce limited quantities of product from a premixed plastic mass [30]. Common defects in extruded parts include laminations caused by wall friction and crow’s foot cracks around rigid inclusions [1]. Nonclay ceramics can also be formed by extrusion, but require formula­tion of suitable binder/plasticizer combinations [1].