Extrusion clay products

In the 1960s Messrs. Uniceram SA of Marseille, France, built a hydraulically-driven piston extruder for the stiff extrusion of structural clay products, combined with a de-airing double-shaft mixer, was operated as a combined de-airing extrusion unit (Fig. 5). On the whole however the piston extruder has been of relatively little importance in comparison to the auger extruder within the field of structural ceramics. 

For extrusion of very wide, flat clay ribbons the twin-shaft auger extruder was reintroduced in 1994 in the field of structural clay products, whereas co-rotating screw-extruders are used since the late 1970s in advanced ceramic for the production of honeycombs. 

This method was primarily developed for extruded structural clay products (9). Small dumbbell-shaped specimens (of approx. 30 mm length) are cut out of a plastic clay column in different directions and at different spots. After drying or firing the tensile strength of these specimens is determined. One can calculate a quotient of strength in two directions, for instance parallel and vertical to the direction of extrusion. This quotient is called the lamination number. This lamination Fig. 1 indicates that there are no laminations. The more the lamination figure deviates from number 1, the more pronounced will be the lamination within the structure. Very extensive tests have been conducted with the aid of this method, which is however unsuitable for regular production checks because of the work and time involved. 

Deairing pug-mill extruders which combine mixing, densification, and extrusion in one operation are available for agglomerating clays, catalysts, fertihzers, etc. Table 20-55 gives data on screw extruders for the production of catalyst pellets. 

Nylon blends, dyeing, 9 204 Nylon block copolymer, 19 762 Nylon carpet fibers, stain-resistant, 19 764 Nylon-clay nanocomposites, 11 313-314 Nylon extrusion, temperatures for, 19 789t Nylon feed yarns, spin-oriented, 19 752 Nylon fiber(s), 24 61 production of, 19 740 world production of, 19 7654 Nylon fiber surfaces, grafting of polymers on, 19 763-764... 

In its simplest form an extruder consists of a body with a rotating axis equipped with blades, the so-called worm which extrudes the clay through a die, an opening with a special shape and construction which varies according to product requirements. During this extrusion the pressure on the clay is gradually increased to a maximum value. Near the die the pressure drops to zero when the clay is extruded. 

Generally, polymer nanocomposites can be obtained through two routes the first one is the polymerization of monomers in contact with the exfoliated clay and the second one uses existing transformation processes to produce nanocomposites, for example, by a reactive extrusion. There are, however, problems present due to the lack of affinity of the clay-polymer system because of the hydrophilic character of the particles. It is then necessary to treat the clay chemically to increase its affinity with the polymer matrix. This constitutes another whole area of research in the nanocomposites production. 

Fadeeva VS (1960) Extrusion of ceramic products from plastic clays. J Glass Ceram 14, 3, 5/1/ pp 86-94, doi ... 

In household ceramics for the production of pugs, e.g. roller machines, in structural ceramics for the production of bricks of all types, for the production of lumps for pressing roofing tiles, for the extrusion of tiles and split tiles, for the production of stone-ware pipes with horizontal but usually vertical extruders (Fig. 48) for pelletising expanded clay, in refractory ceramics and in the further field of technical ceramics, for the production of pugs in the manufacture of isolators (Fig. 49) just as for the extrusion of filigree honeycombs for various applications. 

In most cases, either a convenhonal type of combination pressure head and die assembly is used, or, depending on the properties of the body and on the requirements to he met hy the end product, a hydraulically optimized version. Smooth, streakless surfaces depend on fine-processed clays, hut the lack of coarse fractions in such clays makes them susceptible to lamination and structurahon. Shale clays are especially critical in this sense, because their clay parhcles tend to become oriented in the extrusion direction, with restrahfication of the column - a problem that often remains undetected until after firing. Hence, this aspect must always be given special attention in connechon with pressure heads and dies for split-tile extrusion. 

The vacuum also deteriorates remarkably due to steam admission with a corresponding rise in the temperature of the extrusion compound. As the temperature rises, the effect becomes more pronounced, because the partial pressure of the water in the clay increases at an accelerating rate [2], [3]. For vacuum-sensitive bodies/products, the use of a nonbarometric vacuum measuring instrument is recommened to preclude misinterpretation. The formulae describing the relationship between barometric and nonbarometric measuring techniques are presented in section 13.4. 

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