Concentric billet

The coextrusion processes are the second group of methods for processing "unprocessable" polymers. They are divided into split billet coextrusion and concentric billet coextrusion. In principle they involve the extrusion of the unprocessable component in a sandwich with a processable polymer, to promote extrusion. A form of substrate drawing is involved. 

Concentric Billet Coextrusion. The concentric billet extrusion uses a billet of processable polymer with a hole drilled in the center. A rod of "unprocessable" polymer is placed in this hole and the composite extruded (Figure 4). The concentric composite billet was first used to prepare "one polymer composites" in which the two components consist of two samples of HOPE of different molecular weight (35). Many features of these composites have been studied (36,37). 

In the case of a concentric billet consisting of two different polymers, the outer sheath can act merely as a processing aid and/or polymer die. After extrusion it is removed to leave the core component. 

Evaporation is the removal of a solvent by vaporisation, from solids that are not volatile. It is normally used to produce a concentrated liquid, often prior to crystallisation, but a dry solid product can be obtained with some specialised designs. The general subject of evaporation is covered in Volume 2, Chapter 14. That chapter includes a discussion of heat transfer in evaporators, multiple-effect evaporators, and a description of the principal types of equipment. The selection of the appropriate type of evaporator is discussed by Cole (1984). Evaporation is the subject of a book by Billet (1989). 

If live steam were used instead of indirect heat, the bottoms concentration would be higher in water. This distillation is studied by Billet (1979, p. 216). Stream compositions are given below the flowsketch. 

The correlations of Billet (66) and Onda et al. (187) are valid for various mixtures and packings and cover both absorption and distillation processes. The correlation of Kolev (133) is obtained for RA and certain random packings. In general, the mass transfer coefficient correlations need to be compared to one another and validated using experimental data. This shows, in particular, the way the mass transfer correlations influence the concentration prohles of the components and other relevant process characteristics. 

Bhattacharya et al studied the effect of molybdenum disulphide concentration on friction in metal deformation. Their results (Figure 13.G) .iiOvv un optimum at 3 - 4 wt.% and a steady decrease at higher concentrations for unannealed aluminium alloy, and two separate optimum concentrations for annealed alloy. Nandi also reported a drop in extrusion load for lead billets at certain specif concentrations. 

Fig. 2. Billets for SSE (a, b, c) and shells for SSCE (d, e, f,) (a) solid plug, (b) pipe billet, (c) powder billet, (d) concentric shell, (e) full width shell, and (f) half width shell.

In order to obtain accurate wall thicknesses and clean surfaces when making tubes from difficultly extrudable alloys, it is appropriate to use short billets whose surfaces are machined and which have been provided with a central hole. Extrusion is performed with floating mandrel to ensure a smooth inside of the tube furthermore the mandrel is changed after each extrusion cycle. Upsetting is not necessary, as the billet lies concentrically in the container bore after the mandrel has been inserted. In this case extrusion is carried out on the without shell method. 

Stepwise increasing the section is rendered possible by using a number of dies mounted either in tandem arrangement or inserted concentrically one into the other. Fig. 130 illustrates this method in which after a given extrusion time - when the shoulder of the mandrel has come close to the die - the support of the first inner die is removed so that the billet... 

Figure 1-3. Energy cost ratio Cq/Cob relating to column operation under normal pressure pxB> as function of pressure at the column top py for various mixtures acc. to Billet, Lugowski, Mackowiak [1]. Curve 1 System toluene/ethylbenzene Curve 2 System benzene/toluene Curve 3 System methanol/ethanol. Assumption Concentration at the top xd= 1, at the bottom xw = 0, in feed xp = 0.5. Reflux ratio r/r i = heat steam cost Ch = 10 and 25 = /t, cooling water costs Ck = 0.05 = /t...

The monograph published by Billet [2] includes detailed information on additional energy saving measures, such as optimum feed concentration, use of heat pumps etc. 

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