De-airing extrusion

In his Types of extrusion units , Willy Bender investigates all the various kinds of extruders and combined de-airing extrusion machines. 

Table 2 Boundary parameters of extruders and combined de-airing extrusion units used in structural clay industry comparing the levels in 1960 and 2005...

Fig. 3 Single-shaft de-airing extrusion unit used for the production of slugs...

Multiple shaft extruders. Some publications quote a classification for multiple shaft extruders referring to combined de-airing extrusion units defined by the sum of their auger shafts, infeed shafts and mixer shafts. This method of categorization is, however, disputed. 

Hot extrusion. Hot extrusion, which involves heating of the clay by the introduction of steam into the combined de-airing extrusion unit, is primarily aimed at reducing the drying time. Most of the factories in the structural clay industry use the benefits of hot extrusion. The steam is usually added at the mixer via a bottom plate in the mixing trough. As a rule the aim is to reach a column temperature of 50-60°C. 

Classification by the Design Version of the Combined De-airing Extrusion Unit... 

The type of combined de-airing extrusion machine can be defined by, amongst other aspects, the configuration of the auger and mixer shafts, by the method of coimection between the extruder and de-airing mixer, as well as by the construction of the de-airing mixer. One differentiates the features accordingly, as shown below ... 

Basically there are two options for connection of the machines that form the combined de-airing extrusion unit ... 

Compact version combined de-airing extrusion unit. With this version, the extruder and mixer form a conjoined single unit with one only common drive, the latter representing a cost benefit and probably the main motivation for the development of this type of construction (Fig. 13). 

The compact de-airing extrusion unit is only available in a longitudinal arrangement it has lost much of its importance in relation to the combined de-airing extrusion machine. 

Any combined de-airing extrusion machine consists of an extruder and a primary pug sealer, of which several versions do exist, and therefore the following combinations are possible ... 

Fig. 14 Combined de-airing extrusion unit with vertical primary pug sealer and expansion barrel...

Final observation. Only the most important and commonly used types of extruders and combined de-airing extrusion machines have been listed in this contribution. Bearing in mind the vast variety of options available on the market, there is no pretence to completeness. Especially prototype... 

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. 

Fig. 5 Combined de-airing extrusion unit with de-airing double-shaft mixer and piston extruder, built by Messrs. Uniceram, Marseille, about 1960...

The practice of de-airing extrusion began in the United States about 1920 and in Europe from 1932 onwards. 

The schematic illustration already shows the principle of later de-airing extrusion machines, i.e. a primary pug mill A1 and a main extruder B3, which are coimected by a low-pressure chamber B, the actual vacuum chamber. In this system, the clay cushions formed inside the auger flights of the primary pug mill and main extruder serve as a sealing for the vacuum chamber, in which a negative pressure is produced by the vacuum pump E. This patent was acquired by Messrs. Chamber Brothers in Philadelphia, and was used to build de-airing extruders (Fig. 43). 

Fig. 44 De-airing extrusion machine of Messrs. Fate-Root-Heath Company, Plymouth, Ohio...

Fig. 47 Combined de-airing extrusion machine comprising of vacuum doubleshaft mixer and auger extruder, 1937...

First systematic work in America in respect of de-airing extrusion... 

Dr. Diirst of Switzerland receives a DRP (German Imperial Patent) for de-airing of clay, which serves as a basis for the construction of de-airing extrusion machines made by Messrs. Rau-... 

Messrs. Handle of Miihlacker/Germany build their first de-airing extrusion machine... 

To summarise, it can he established that both values improve positively with a high de-airing of all bodies, whereas with a partial de-airing the lamination figures deteriorate with all kinds of bodies and in certain cases result in even worse lamination figures than with non-de-aired extrusion. 

It is a wrong approach, often applied in practical operation, to use overdimensioned vacuum pumps with a high rated throughput instead of amply-dimensioned vacuum pumps with a higher ultimate pressme, except for such cases where the vacuum collapses frequently as a result of insufficient material feed into the combined de-airing extrusion unit or inadequate sealing of the vacuum chamber. 

Piston extruders use the same vacuum pumps as employed for de-airing extrusion machines see the contributions of Laenger and Redmann under chapters 15 and 16. 

Quite a few experts entertain fairly vague notions about how much vacuum a combined de-airing extrusion machine can achieve as a function of ... 

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