Continuous mixers screw machines

Extmders with modified barrels and screws have been used to carry out the function of a continuous mixer of mbber compounds. The early machines of this type had both barrels and screws fluted, but at different parts of the chamber. One of these machines is the Transfermix. The mbber commences travel down the barrel of the machine in the flights of the screw, but transfers to... 

Most of these machines are based on two adjacent screws, but some, like the continuous mixers, have a different geometry, which is better defined as consisting of two rotors. 

Fig. 10.7 Schematic view of the CM/single screw extruder combination used in polymer compounding operations. The chute, connecting the two processing machines, transfers the molten and mixed stream at low pressures to the throat of the melt pump SSE, which generates the pressure needed for pelletization. [Reprinted by permission from E. L. Canedo and L. N. Valsamis, Farrel Continuous Mixer Systems for Plastics Compounding in Plastics Compounding—Equipment and Processing, D. B. Todd, Ed., Hanser, Munich, 1998.]...

Another difference between the extruders and continuous mixers, pointed out by Valsamis and Canedo, is that, in the former, channel solids and melt flow dominate, and flow over screw flights is a secondary effect (except for power calculations). In contrast, the circumferential flow in CMs (and internal mixers) over the wing tips is the major flow component. Thus, while wing tips and screw flights appear to be equivalent machine elements, their role and function are quite different. The wing tips provide high shear... 

The continuous mixers make it possible to control the feed rate, screw speed, temperature, as well as the discharge orifice setting and temperature. The new machines that operate at high screw or shaft speeds require a short residence time. However, care must be taken that high... 

Anonymous, Plast. Machin. Equip., 53, June (1981). Anonymous, Twin-screw Continuous Mixers, ComacPlast, Techn. Inf. (1989). 

The machines mainly used for compounding plastics, chemicals and food are co-rotating, closely intermeshing twin-screw extruders. Some 30 000 such extruders are in use worldwide, about 1/3 are ZSK from Coperion Werner Pfleiderer, Stuttgart. In the chemical industry increasingly more and more batch mixers are being replaced by continuous twin-screw knea-ders. 

Originally developed for natural rubbers, foodstuffs, and elastomers, at present these machines are used as a stirred tank reactor or a twin-rotor continuous mixer. Their bearings are exposed mainly to radial, not axial, forces. By using one screw longer than the other, one can separate the mixing and pumping functions. 

Many modified screw designs for improved mixing have appeared in the patent literature [72-76]. Some of these designs are shown in Figure 1.13. In the last 30 years of the twentieth century it was realized that twin screw machines were better continuous mixers. 

Continuous mixers consist of pairs of machined shafts each with a screw and rotor sections. The earliest machines of this type were developed by Ellermann [5, 6, 196, 197] of Krnpp in Magdeburg, whom we met in Section 9.2.3.1. This was probably associated with the l.G. Farbenindustrie synthetic rubber program. It was an intermeshing coimter-rotating machine (see Fig. 9.30). After the war, FUermann was associated with Josef Fckimd Sohne in Diisseldorf, where he developed a modified machine that was marketed as the Fck extruder [5, 6,197]. FUermann s Fck machine was also intermeshing counter-rotating. 

What about the machinery The proliferation of different rotors made available for batch mixers over the last few years leads one to believe that this part of the industry is in its infancy, rather than approaching old age. It is likely that further designs will appear, but some specialisation may creep in where particular rotor designs are matched to particular parts of the industry. In continuous mixing machinery, the corotating twin-screw machine appears, at last, to be achieving inroads into the rubber industry, and this is likely to continue. Other continuous compounding machines which have had a measure of success in the past are likely to be reintroduced in modified form, particularly those, like the MVX, which were shown capable of outputs in terms of tonnes per hour. 

Many extruder manufacturers now produce dump extruders which are fitted beneath the discharge door of an internal mixer and receive into their feed hopper the full charge of the mixer. The speed of extrusion of the machine is governed by a series of sensors in the feed-hopper, to ensure that the process is continuous and that the screw will not be starved of compound, thus ensuring a continuous production of the mixed rubber compound. Product from such a machine can be slab or pellets. 

The first operation of chocolate processing is to homogenize the cocoa mass, ground sugar, milk powder and part of the cocoa butter in a mixer which can be a kneading machine with a Z-shape mixing arm or a continuous screw mixer. 

Paul Troester started manufacturing single-screw extruders (SSE) in 1892, but only in 1935 the first SSE for thermoplastics was introduced. Four years later, Paul Leistritz built electrically heated, air cooled SSE, having L/D = 10, automatic temperature control, variable screw speed, and nitrided barrel. The machine is considered prototype of the modem extmders. The SSE offered continuous processing capability, but it was notoriously poor as a mixer. Furthermore, under the standard processing conditions, a significant non uniformity in the shear history had resulted in large temperature differences of the melt, AT = 60°C. To alleviate the... 

Ceramic feedstocks for extrusion can be produced either in a batch process, or continuously. Which approach is taken depends on the details of the application and the material to be prepared. In the ceramic industry at present, four different types of machines are generally used High shear mixers, roll mills, screw extruders and shear rolls. 

Continuous compoxmding and mixing [23,24] is accomplished by a variety of machines ranging from single-screw to twin screw extruders with variations in design and additional equipment units to achieve a range of end products. The type of compounder or mixer chosen depends upon the specifications and properties required of the end product, volume requirements, overall economics, and whether the compounding is done in-house or by a custom compounder. 

In the case of batch mixing, the resulting compound is manufactured in discrete quantities. Typical examples of batch mixers are two-roU mills and internal mixers. Continuous mixing is usually carried out using either single or twin-screw extruders, the components of the mix being fed on a continuous b lsis into the machine. 

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