SPECIAL PVC Processing. Counter-rotating, closely intermeshing twin-screw extruders are primarily used nowadays for the production of pipes, profiles and sheet of unplasticised PVC. This article traces the evolution of this machine technology from its origins down to the present. The Historical Development of the Counter-rotating Twin-screw Extruder Following the founding of the Anger company, the brothers Anton and Wilhelm Anger developed a process in the early 1950s for extruding powder into a finished pipe in a single working step (Fig. 1) [1]. Almost simultaneously, they also succeeded in solving the problem of how to join pipes by means of plastic. In 1954, Anger developed a Mapre extruder. Just one year later, the first self-developed twin-screw extruders were built. These were 3-section screws (without vent section) and had a length of 12D. Fig. 1. Technicians during early extrusion trials (Anger, 1953) [1] Fig. 2. Twin-screw extruder BT 150/11,5D (Schloemann, 1964) [2] HANS-PETER SCHNEIDER I n pipe and profile extrusion, conical twin-screw extruders are primarily used in the lower to medium performance range as principal and coextruders. Parallel twin-screw extruders are mostly used in the medium-to-high-performance area. For pelletizing, where the maximum output performance is achieved, parallel extruders are used almost exclusively. Translated from Kunststoffe 5/2005, pp. 44 50 Historical Development PVCu pipes were first laid in 1935 as pressure pipes for public water supplies. Pipes of PVCu suffer from neither corrosion nor incrustation. It was the search for efficient ways of producing these PVCu pipes for water supplies that sparked the development of counter-rotating twin-screw extruders. The family tree of extruder manufacturers, as it were, has three main branches: Anger (Mapre), Kestermann and Schloemann (Pasquetti). After a test period lasting several years, combined with extensive theoretical studies and practical trials along with a whole range of pipe-laying operations, Anton and Wilhelm Anger, together with stake holders, founded the company Kunststoffwerk Gebrüder Anger GmbH & Co. Its headquarters were in Munich and the production shop was in Bogen on the Danube, in the buildings of a former leather factory. The company s first extruders were launched in 1959. In 1960, the Schloemann company took over the Pasquetti company and al- Kunststoffe plast europe 5/2005 1
SPECIAL so began building twin-screw extruders. Models BT 50/12D, BT 80/12D and BT 100/12D were followed in 1964 by the BT 150/11.5D, which had an output rate for pipe of 200 kg/h. It had a 70 kw electric motor and a heating capacity of 48 kw (Fig. 2). Weber produced its first twin-screw extruder DS 60 in 1961. This had a processing section of 12D but no venting. It did not launch longer machines with venting, such as the DS 60 17D, DS 85 16D and DS 120 17D, until the period from 1964 to 1968. In 1962, the Rheinstahl Group acquired a majority holding in Kunststoffwerk Gebrüder Anger GmbH & Co. By that time, the plant had 30 pipe-production lines and was one of Europe s leading manufacturers. In 1964, Anton Anger eventually founded the company AGM (Anton Anger Allgemeine Maschinenbau GmbH), headquartered in Linz/Austria. His brother Wilhelm s company, located in Vienna, called itself APM (Anger Plastic-Verarbeitungsmaschinen GmbH & Co). While AGM built conical and parallel twin-screw extruders, APM concentrated on the production of one-stage and twostage parallel extruders (Fig. 3). From 1964 on, Kestermann also started offering single-stage twin-screw extruders without venting (K2-80, K2-100 and K2-120) and two-stage designs with venting (K2-80/86V, K2-100/107V and Fig. 9. Conical and parallel twin screws K2-120/130V). Later, the single-stage machines were also fitted out with venting. In 1968, Reifenhäuser built an extruder with a diameter of 125 mm and a processing unit of 16D (without venting) (Fig. 4). Reifenhäuser subsequently acquired the twin-screws (Bitruder) of Schloemann in 1972. In the year 1969, Cincinnati started selling its conical extruders CM 45 (Fig. 7), CM 55, CM 65 and CM 80. The program at that time was rounded out with a two-stage parallel machine (A4/ 125/125). Apart from the companies just mentioned, further producers of counter-rotating twin-screw extruders in the 1960s were Bausano (Italy), Bandera (Italy), Mapré (Luxembourg) and Leistritz (Germany). Rheinstahl eventually acquired Kestermann s activities in 1970. In so doing, it acquired the machine design in which the exchangeable breaker plate was secured against rotation in the barrel (Fig. 5). 1971 was the birth year of Krauss- Maffei Extrusionstechnik. Initially, it was headquartered in Munich. The chief engineers came from AGM and APM.Apart from a conical KMD50K, it offered two parallel extruders, namely the KMD90-20D (Fig. 6) and the KMD120-20D. In 1974, Krauss-Maffei Austria was founded and engineering and marketing were switched to Asten near Linz. Production of the extruders continued in Munich. The headquarters of Krauss-Maffei Extrusionstechnik were switched back to Munich in 1979, where they have remained ever since. Thyssen and Rheinstahl merged in 1972. Three years later, Thyssen Plastik Maschinen (TPM) was founded. This company evolved out of the former division of Thyssen Rheinstahl, whose activities were mainly based on Kestermann s extrusion technology. After a very short development period, the new twin-screw extruder series was presented in 1976. These were exclusively parallel models Fig. 3. Two-stage twin-screw extruder A4/80/84 (80 and 84 mm, 11D and 8D), (model APM) [2] Fig. 4. Twin-screw extruder 125-16D (Reifenhäuser, 1968) [2] Fig. 5. Twin-screw extruder (Kestermann, 1970) [4] Fig. 6. Twin-screw extruder KMD 90-20D (Krauss-Maffei, 1973) [6] Fig. 7. Twin-screw extruder CM 45 (Cincinnati, 1976) [3] Fig. 8. Twin-screw extruder TPM2-90-22V (TPM, 1980) [7] 2 Carl Hanser Verlag, München Kunststoffe plast europe 5/2005
SPECIAL Fig. 11. Conical twin-screw systems with screw diameters of 50, 60, 85, 107, 130 and 160 mm. The processing units varied with the application, and were either 16D or 22D. The TPM 90 (Fig. 8) was added to the range in 1979. The new Service and Development Centre in Dornach near Munich opened in 1979, only to shut again in early 1980. The company then moved to Bad Oeynhausen/Germany. This coincided with the birth of Battenfeld Extrusionstechnik. Development of Conical Extruders Two designs have prevailed for counterrotating, closely intermeshing twin-screw extruders: the parallel and conical types. In the parallel variant, the external screw diameter and the axial distance remain constant over the entire screw length, whereas, in the conical variant, it varies continuously (Fig. 9). Parallel extruders have been made industrially since the 1950s. In the formative years, the mechanical reliability of the parallel twin-screw extruders did not always come up to scratch. The main problems occurred in the region of the driven shafts. On account of the narrow axial distances, it was not possible, with the bearing technology available then, to accommodate long-term radial and axial forces by means of appropriate dimensioning techniques. It was not until the end of the 1960s that bearings became available which enabled parallel extruders to offer adequate operational reliability [8]. The problem of safely accommodating the radial and axial forces was solved with the development of conical twin-screw extruders. In these machines, the screw external diameter and the axial distance increase steadily from the screw to the transmission. This offers design advantages for shaping the distributor drive: First of all, the two bevel wheels of the distributor drive, viewed from the screw tip, can move back so far that their average diameter assumes an adequate size for the permanent design. The distributor drive consists of very few parts, which leads to cost advantages during manufacture (Fig. 10). Furthermore, this solution offers good scope for designing the receptacle for the axial bearings [9]. The first conical extruders were built by Anger (AGM) (from 1964 on), Cincinnati (since 1969), Weber (since 1981) and Krauss-Maffei (since 1973). These are so-called single conical screws. In this type of screw, the flight depth remains constant along the length of the screw. In 1974, Krauss-Maffei registered its patent for a socalled double conical screw [10]. In double conical screws, the flight depth decreases continuously from the feed section to the metering section (Fig. 11). Without any change to the axial angle of the transmission for the length of the barrel unit, the external diameter at the start of the feed section was enlarged under the terms of patent claim No 1 such that the ratio of overlapping with the local external screw diameter along the full length is approximately constant and, according to patent claim No 2 amounts to between 18% and 20% of the local screw diameter (by overlapping is meant the flight depth minus the flight clearance). This change to the diameter ratios produced constant overlapping at every point along the screw and, to be sure, in the same ratio as in the parallel Krauss-Maffei screws. The main reason for developing a double conical screw was to increase the output rate while retaining the barrel length and the axial angle, and thus to use the same transmission. Naturally, further performance enhancements in the pipe and profile area necessitated higher screw torques for the double conical extruders. Fig. 10. Conical distributor drive The inventor of the Krauss-Maffei patent co-founded Maplan in 1977 and circumvented his own invention. Maplan filed a new patent [11] in 1978 in which the overlapping ratio of the double conical screws, relative to the Krauss-Maffei variant, was altered slightly and not kept constant. Cincinnati had also been producing double conical screws since 1978, which were termed super-conical from 1985 on. In both screw systems, the degree of overlapping lies between 18 % and 20 % (as in patent claim 2 of the Krauss-Maffei patent), but the overlapping ratio varies along the length. As a result, patent claim 1 of the Krauss-Maffei patent was not infringed. Cincinnati never registered a patent for the double conical or super-conical screws. However, in 1983, Cincinnati registered a working model [12] in which the barrel features at least two axial sections of different conical angles. The use of different conical angles in the screws and thus in the barrel was never implemented, but would nonetheless have been possible since the conical Cincinnati cylinders consist of several segments. The conical screws designed by Weber have been of the single conical type since 1981. In 2000, Battenfeld, which until then had only produced parallel extruders, presented its so-called negative conical screw design [13], which today is called the active conical design. In this design, the flight depth increases continuously from the feed section to the end of the metering section. Development of Parallel Extruders The delivery rate of an extruder at a given screw diameter depends on three key factors, namely the installed screw torque, the maximum screw speed and the specific drive energy. Kunststoffe plast europe 5/2005 3
SPECIAL L/D ratios Fig. 12. Evolution of L/D ratios in twin-screw extruders of different manufacturers The introduction of venting and screw temperature control enabled substantial increases to be made in screw operational speeds from the mid-1960s onwards, but, to avoid greater wear, not to the same extent as in operating torque [14]. Practical experience over recent decades has shown that certain screw and peripheral speeds should not be exceeded in the screw designs used so far as, otherwise, this may lead to partial over-shearing of material and thus to temperature inhomogeneities. Further limiting factors on the screw speed are the recipe and, especially, wear on the processing unit. The delivery rate of a twin-screw extruder of given screw diameter can therefore not be increased by means of higher screw speeds. Instead, the extra performance must come from an increase in specific throughput. The specific output is the ratio of the output rate to the screw speed. For a given screw diameter, the specific output rate can therefore only be increased by increasing the screw torque or reducing the specific drive energy. Kunststoffe construction and a boom in the plastic windows sector increased demand for high-performance extruders which could be used non-stop for the main window profiles. Extending the Processing Unit Not only is shear energy incorporated into the dry blend, but also heat energy via the barrel heaters. A certain amount of energy is required to plasticate 1 kg of PVC dry blend. The specific total energy, i.e. the sum of specific heat and shear energy, is therefore almost constant. Heat energy and shear energy are accordingly closely related to each other. In practice, it is found that the shear energy fraction increases with increase in screw speed. On the other hand, more shear energy has to be incorporated into the material via the screws in the case of a short processing unit relative to a long processing unit. Given the same specific output and the same screw speed, the dwell time of material in the extruder with the long processing unit is greater than in the short extruder of the same screw diameter and axial distance. In other words, a longer processing unit can be used to reduce the specific drive energy, provided that all other construction factors are kept constant. In practice, this is accomplished by using screw geometries of lower compression. The processing units of twin-screw extruders have accordingly become longer as time has passed. The first twinscrew extruders had a processing length of 8D. Some 35 years ago, processing lengths were still 12 18D, whereas nowadays they are between 22 and 36D. The L/D ratio has risen more or less linearly from 1955 to 2001. Outstanding extensions to the processing units are the extension of the Krauss-Maffei pipe extruders to 36D in 2001 and the extension of the processing units of the Krauss- Maffei profile extruder to 32D in the year 2003 (Fig. 12). Overlapping Torque Increase The first twin-screw extruders still featured comparatively low screw torques. Continuous improvements to radial and axial bearings made it possible to more or less treble screw torques between 1960 and 1977 [15]. While the rise in screw torque between 1960 and 1990 was virtually linear, it increased dramatically thereafter into the mid-1990s. This was triggered primarily by an enormous performance increase in the area of window profiles. Innovative developments in die Fig. 13. Preheating device with vanes (Krauss-Maffei model) Extending the processing unit is not the only way to increase the dwell time of the material in the extruder. It can also be done by increasing the level of overlapping of the screws (D/a). While the early twin screws still had relatively short overlapping ratios of 1.15 to 1.20, these now lie between 1.19 and 1.23. This has proven to be the optimum value in practice as regards shear rate of external diameter and root diameter, maximum possible root drilling for internal temperature control of the screws, and screw strength. 4 Carl Hanser Verlag, München Kunststoffe plast europe 5/2005
SPECIAL Fig. 14. Multi-screw extruder (Krauss- Maffei model) Increase in Throughput The main engine for driving the development of twin-screw extruders was the efficient production of semi-finished plastic goods of PVC, initially pipes, but later profiles and sheet. Recent decades have seen steady increases in output rates, especially of the parallel twin-screw extruders. Material Preheating In the early 1970s, Krauss-Maffei looked for other ways of increasing the performance of twin-screw extruders. Increasing the output by extending the processing unit, coupled with a torque increase, seemed to be feasible only with a huge effort. More powerful transmissions were not available. Reasons of quality and wear protection prevented any consideration from being accorded to increasing performance by increasing the screw speed. Accordingly, a preheating device was developed, and presented to the public for the first time in 1975. This preheating device was mounted on top of the feed opening of the extruder barrel and its purpose was to incorporate into the material some of the energy required for plastication prior to the actual extrusion process. It features a motor that employs a speed reduction gear to drive a shaft on which vanes are mounted (Fig. 13). The vanes slide on heated, circular plates that are arranged in several levels. The powder passes from the hopper into the upper level, is transported further by the vanes until after 7/8ths of a revolution, it falls through an opening into the next level. This process repeats itself in subsequent levels until finally the heated powder is transferred to the extruder screws [16]. The preheating device also functions at the same time as a feed unit. The use of this type of preheating in combination with appropriate downstream processing unit can increase the output rates of the various extruder types by 20 35 %. The greater output is achieved at the same screw speed and motor load. Maplan later launched preheating devices that initially consisted of two individually driven screws which conveyed the material through a heated housing. Additional heating of the material was effected by means of the oil-heating system for the screws. Nowadays, Maplan offers preheating devices that consist of a heated barrel and co-rotating, oil-heated conveying screws. Apart from material preheating, these devices have a metering function. Multi-screw Extruders A further alternative to conventional twin-screw extruders was presented by Krauss-Maffei at Europlastique in Paris in 1974, namely a multi-screw extruder for large pipe production, i.e. for output rates of between 800 and 1000 kg/h. The maximum screw diameter that could be managed at that time was 130 mm. As yet unresolved wear problems made it risky to build machines of even greater screw diameter. Accordingly, multi-screw systems were developed that combine two pairs of screws in one barrel. One twinscrew assembly is replaced by the central screw (of 220 mm diameter) with which, from opposing sides, a small screw engages (of 110 mm diameter) (Fig. 14). The same peripheral velocity is obtained by halving the screw speed of the central screw relative to the side screws. The more favourable ratio of surface area to throughput obtained with smaller screw diameters enables a great deal of heat energy to be input from outside. Heat is also introduced via the heated central screw. The fact that the number of intermeshing zones is twice as high leads to better compounding of the dry blend. The material ejected from the chambers is collected in the mixing head, homogenised and fed to the adapter. Development of Throttle Designs Fig. 15. Comparison of different types of screws Plastication of PVC dry blend requires compression of the material in addition to the input of heat energy and shear en- Kunststoffe plast europe 5/2005 5
SPECIAL Fig. 16. Two-stage vent screws (Kestermann model, 1968) [17] ergy. In the multi-part vented screws, the various manufacturers have incorporated different compression elements or throttles. The purpose of the throttle is twofold: First, the material is compressed and slightly plasticised and, second, the venting section is hermetically sealed off from the feed section. Over the course of time, the machine manufacturers have devised a range of designs for the throttles. In the 1960s, Kestermann built socalled two-stage vented screws (Fig. 16). Each stage has the characteristic features of the single-flighted feed zone, the thread transformation zone and the multi-flighted delivery zone. Between the first and second screw flight, the area in the flightfree screws contains a breaker plate immobilised in the barrel such that it cannot rotate (Fig. 17). In the first screw flight, the material is drawn in and fed towards the breaker plate. A concomitant pressure builds up that causes the preheated, partly sheared material to agglomerate. The pressure build-up can be influenced by exchanging the perforated breaker plate for another with different flow-through cross-section. Back in the 1950s, Anger used a single flighted zone of relatively small pitch and Fig. 17. Non-rotating breaker disc in screw barrel (Kestermann model, 1968) [12] Fig. 18. Double flighted throttle in counterrotating twin screws narrow flight and roller gaps as throttle element. The compression could be varied via the height of the pitch. In the 1960s, APM manufactured fivezone screws with a compression zone, seven-zone screws with two compression zones (Fig. 15) and composite vent screws featuring five zones. In 1975, TPM acquired Kestermann s machinery programme. At K 1976, six completely new, parallel twin-screw extruders (50 160 mm) were presented. Instead of the breaker plate, the profile screws were fitted out with a double flighted, closely intermeshing throttle while the pelletizing and pipe screws featured so-called baffles. While the pipe screws were a one-part design, the pelletizing screws came apart so that the baffles could be exchanged. The degree of plastication could be adjusted to suit the material via the number and contour of grooves in the baffles. Nowadays, machine manufacturers usually use throttle elements of the same pitch as the other screw zones in their twin screws. The screws are normally made in one piece. The throttles vary in pitch, number of flights, effective length and the flight and roller gaps (Fig. 18). Extruder manufacturers now mostly offer different geometries for different application areas. For any particular application area, such as profile extrusion, the various geometries often differ only in the throttle zone. REFERENCES The bibliography can be called are up under www.kunststoffe.de/a012 THE AUTHOR HANS-PETER SCHNEIDER, born in 1955, works for Krauss-Maffei Kunststofftechnik GmbH, Munich/Germany, where he is project leader for the process-engineering of twin-screw extruders. 6 Carl Hanser Verlag, München Kunststoffe plast europe 5/2005