T he cost-effectiveness of a pipe extrusion. The 36D Process Concept Opens up New Potential

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1 The 36D Process Concept Opens up New Potential Pipe Extrusion. Anew process concept increases the melt throughput and therefore the productivity of single-screw extruders. At the same time, thermal and material homogenisation were greatly improved. Significant advantages were achieved by lengthening the process unit. HENNING STIEGLITZ FLORIAN SCHNEIDER Translated from Kunststoffe 12/2005, pp T he cost-effectiveness of a pipe extrusion line principally depends on the efficiency of the individual components. A key component of this is the extruder. Better screw geometries and technical advances in the materials to be processed have greatly increased the extruder's effectiveness. Impressive boosts in output have always accompanied the lengthening of the processing unit (Fig. 1). When modifying its KME singlescrew extruder series, Krauss-Maffei, among other changes, also increased the length of its processing extruders. Demands on the Extruder The demands made on a plastication extruder have hardly changed in recent years. A uniform material quality depends on higher and pulsation-free throughput, thermal and materially homogeneous melt, controlled shear deformation and melt temperature; other benefits are low specific investment (price/throughput ratio) and operating costs, and a large processing bandwidth (PE 80, PE 100, PP- H/B/R, PB, PE-LD) [1]. New features are the requirements for low melt temperatures to avoid sagging at large dimensions, and to minimise the cooling length, direct extrusion of blends of different basic materials and additives and a high pro- 1

2 EXTRUSION portion of recyclate, regrind or regranulate. Demands for improved cost-effectiveness have greatly boosted the progress in process engineering in the last five years. An improvement in costeffectiveness in the case of a plastication extruder usually means higher throughput for the same machine size (screw diameter) [1]. Throughput and processing length High, Pulsation-free Throughput To improve throughput significantly, it was necessary to greatly modify the extruder. The new pipe extruder series therefore has a processing unit that is 6D longer than its predecessor. The processing length is now 36D. The feed system is still based on the grooved bush system, but has been completely revised. The standard barrier screw previously used has been replaced by a newly developed barrier mixing screw concept. The 36D process concept largely meets the demands made on the plastication extruder. There are two ways of achieving high throughputs. One is to increase the screw speed and the other is to raise the specific throughput, i. e. the conveying capacity per screw revolution.with viscous pipe materials, the mechanical work in the processing unit is converted into heat, mainly by internal friction (dissipation). High screw speeds must be avoided to prevent the melt from overheating. Higher melt throughputs must therefore be achieved by increasing the specific throughput. It has been found that the specific throughput cannot be kept constant when the screw speed is increased. A decrease in the specific throughput is usually observed [2]. This decrease is often described as non-linearity of the output and, in the case of virgin material, can be up to 15 %. It is a result of the plastic granule trickling into the screw chamber [3, 4]. However, the phenomenon of non-linearity must not be confused with the phenomenon of the screw speed limit. The phenomenon of the screw speed limit represents a much greater reduction in output (30 to 40%). The fall in output occurs immediately at a screw speed (limit) and is the result of excessive heating as a result of friction in the grooved bush. As a result, a melt film already forms in the grooved bush, which affects the entire conveying process. Fig. 1. Chronological development in pipe extrusion with the example of an extruder with 90 mm screw diameter for PE-HD (source: Krauss-Maffei product information) Feed systems To allow maximum throughputs for a given screw speed, the feed system should be designed to minimise the loss in the specific throughput as a result of trickling material. Here, at the sort of counter pressures found in practice, the speed limit must lie above the maximum screw speed of the extruder. The feed system of the new KME series was completely redesigned and redimensioned based on practical and theoretical discoveries (Fig. 2). The high-performance feed system of the new series is characterised by significantly higher specific throughputs and minimised loss with increasing screw speed. At the maximum speed, throughputs were 25 to 30 % higher Thermally and Materially Homogeneous Melt Fig. 2. Comparison of two feed systems (30D standard, 36D high performance) with the example of three conventional PE-HD pipe materials [5] In pipe extrusion, the thermal homogeneity is usually assessed by the appearance of the pipe interior surface (Figures 3 and 4). Thermal inhomogeneities lead to waviness. There is no objective measure of the thermal homogeneity. The inhomogeneities or surface quality that are still permissible in the pipe depend strongly on the processor and the customers. However, it is not known for insufficient thermal homogeneity to affect the mechanical properties of the pipe. Good homogeneity is generally achieved if the raw material, extruder and pipe head are well matched. The extrud- 2 Carl Hanser Verlag, München

3 Fig. 3. Pipe with poor thermal homogeneity er and pipe head are often considered as an indivisible unit. That is particularly true for the thermal homogeneity. The extruder only generates a raw melt. Breaking down the residual solid and completing the thermal homogenisation are the functions of the pipe die. To allow sufficient time for thermal equalisation processes to take place, the volumes of the pipe heads are kept large. Where the spiral mandrel distributor and consequently the much more compact pipe heads are used, the extruder must perform virtually the entire melting work and thermal homogenisation. From the point of view of the processor, this is welcome, because extruders can be combined with any dies and the advantages of the new pipe head series can be exploited to reduce the residence and purging times. This gain in flexibility with simultaneous increase in the mass throughput is only achieved if the melting work and the homogenisation capacity of the extruder can be increased to the same extent. The integral work balance on the processing unit must not change significantly, that is to say that the required material-specific enthalpy increase must be obtained in the processing unit. A traditional way of increasing output and at the same time maintaining the melting efficiency and homogenisation, is to lengthen the processing unit [6]. This offers significantly more possibilities for significantly improving the thermal homogeneity with a consistently lower melt temperature. In addition, the longer processing unit opens up new opportunities for direct extrusion of a basic material with colour masterbatch, additive, regrind or blends, e. g. PE-HD and PE-LD. For these processing tasks, material homogeneity is the main requirement. Material homogeneity means the quality of comminution and dispersion of additive components in the basic material. With the 36D process concept, this is possible without screw conversion and fitting additional shearing/mixing sections and 3D cylinder lengthening. It meets the requirements for low melt temperatures. Controlled Shear Deformation and Melt Temperature Fig. 4. Pipe with good thermal homogeneity Melt temperature as a function of throughput i Manufacturer Krauss-Maffei Kunststofftechnik GmbH Krauss-Maffei-Straße 2 D München Germany Phone +49 (0) 89/ Fax +49 (0) 89/ info@krauss-maffei.de Fig 5. Comparison of a 30D standard processing unit and a 36D high-performance processing unit identical cylinder temperature profiles; screw diameter: 75 mm; melt pressure: bar; material: PE 100 (grade: HE3490LS, manufacturer: Borealis) [5] The fear that lengthening the process unit inevitably leads to higher melt temperatures is not justified. This can only take place if fundamental process engineering principles, such as the energy balance are not observed. The comparison of the melt throughput in the case of a 30D standard processing unit and a 36D high-performance unit shows that, at high melt throughputs, the melt temperature can even be reduced by lengthening the processing unit and increasing the specific throughput (Fig. 5). In the newly developed barrier mixing screw concept, the strict separation between the solid and the melt in the barrier region has been eliminated in order to melt certain solid components in a disperse way. The disperse melting mechanism can be compared to the melting of an ice cube in a whisky glass. Heat is removed from the whisky, or in this case the polymer melt. There is a lower (melt) mixing temperature at the end of the process. Shear deformation, the product of the mean residence time and mean shear rate, can be used as a measure of possible material changes as a result of degradation 3

4 EXTRUSION Speed/throughput behaviour of a 90 mm screw or crosslinking. Changing the shear deformation by lengthening the processing unit would not be expected to increase material degradation. The mean residence time is derived from the free volume of the processing unit and the material throughput. If these parameters are increased to the same extent, the mean residence time remains constant. The mean shear rate tends to become smaller because of the greater channel depths. In practice, that can even offer potential for modest increases in speed. Longer Instead of Bigger Fig. 6. Output performance of a KME 90-36B at a constant melt pressure of 220 bar. Processed material: PE 100 (grade: HE3490LS, manufacturer: Borealis) [5] Speed/throughput behaviour of a 75 mm screw Fig. 7. Output performance of a KME 75-36B at a constant melt pressure of 220 bar. Processed material: PE 100 (grade: HE3490LS, manufacturer: Borealis) [5] Pressure/throughput behaviour Fig. 8. Output performance of a KME 75-36B at a constant screw speed of 180 rpm. The melt pressure was varied in a range from 220 to 430 bar by means of a throttle die. The amount of cooling water required was dimensioned for a setpoint value of 60 C. The processed material was a PE 100 (grade: HE3490LS, manufacturer: Borealis) [5] The comparison of the output rates of the old and new KME extruder series shows a significant increase of 25 to 30 % (Table 1). Provided that the requirement on the throughput does not change, processors now have the unique opportunity to use a smaller extruder size than formerly. That means that a 75 mm extruder can now be used where a 90 mm machine used to be used. This offers a variety of advantages offers to users. The residence time of the material in the machine is shortened. This reduces the thermal stressing and the purging time of the extruder. The cooling water flow rate required for the grooved bush and gearing is shorter. The increase in overall size saves costs. That applies for a new purchase as well as for replacement parts. Overall, the use of the smaller machine can greatly increase effectiveness. Practical Experiences The 36D process concept has proven itself in industrial use in a range of different applications, for example in the manufacture of smooth piping (PE-HD, PP- H, PP-R and PE-Xb), corrugated piping (PE-HD and PP-C), aluminium composite piping (PP-R), and the like. The capabilities can be demonstrated with some experimental results. At screw diameters of 90 mm as well as 75 mm, the material throughput is linear (Figures 6 and 7). Similar behaviour can also be seen in the processing of other materials or the addition of regrind. The melt temperature at maximum output rate is still at a conventionally low level. Taking the example of the 75 mm extruder, it can be seen that the specific throughput does not decrease even at high backpressures and maximum screw speed (Fig. 8). The extruder is pressure resistant; the speed limit is above the max- 4 Carl Hanser Verlag, München

5 Table 1. Comparison of the output rates [kg/h] of the 30B standard extruder and the 36B highperformance extruder with the example of virgin PE-HD imum screw speed. The cooling water consumption does not increase with increasing backpressure and has a very low level. Compared with the standard feed system, the cooling water consumption could be reduced by about 75 %. Summary Screw diameter [mm] The 36D extruder generation shows how much potential remains to be exploited in pipe extrusion by the use of a longer processing unit. Output rates for PE-HD can be increased by 25 to 30 % on average. Further improvements in performance can be expected in coming years. The lengthening Output of 30D series Output of 36D series of the processing unit does not necessarily lead to higher melt temperatures. Distinct advantages can also be found regarding thermal and material homogenisation. The modification of the grooved feed system has brought considerable processing improvements. Cooling water consumption of the grooved bush could be reduced by 75 %. Another important factor for customers is that the 36D technology achieves better cost efficiency. REFERENCES 1 Wortberg, J.: Anforderungen an Plastifizierextruder; Optimierungsziele. In: Einschneckenextruder Grundlagen und Systemoptimierung; VDI- Gesellschaft Kunststofftechnik; Düsseldorf März 1993; 2. Auflage, pp Wortberg, J.: Einschneckenextruder mit schnelldrehenden Schnecken. In: Extrudieren und Thermoformen von Verpackungsfolien; VDI- Gesellschaft Kunststofftechnik; Düsseldorf 2004, pp Potente, H.; Pohl, T.: Inflow Behaviour in Singlescrew Extruders: Filling Level in Feed Zone Affects Specific Throughput. Kunststoffe plast europe 91 (2001) 6, pp Potente, H.; Pohl, T.: Polymer Pellet Flow out of the Hopper into the First Section of a Single Screw. Intern. Polymer Processing XVII (2002) 1, pp Interne Versuchsergebnisse; Krauss-Maffei Kunststofftechnik; Potente, H.: Auslegung von Schneckenmaschinen- Baureihen; Carl Hanser Verlag, München 1981 THE AUTHORS DR.-ING. HENNING STIEGLITZ, born in 1971, is technical manager at Krauss-Maffei Extrusionstechnik. M.SC.ENG. DIPL. ING. (FH) FLORIAN SCHNEIDER, born in 1972, is project manager in the process engineering development at Krauss-Maffei Extrusionstechnik. 5