Single-Screw Extruders and Barrier Screws 1

Single-Screw Extruders and Barrier Screws 1 Peter Fischer, Johannes Wortberg 1 Extended version of a paper presented at the VDI conference on "The Single Screw Extruder Basics and System Optimization", published by VDI-Verlag Düsseldorf, 1997 Kunststofftechnik

The development of single-screw plastificating extruders In the USA, extruder development was - and still is - largely characterized by machines with smooth barrels. Further development has tended to concentrate more on the screws than anything else, with so-called 'barrier screws' screws in which the solid material is kept separate from the melt in the melting section at the center of attention. Although the first barrier screw was actually invented in Europe in 1959 by Maillefer, most of the further development work and the practical application of this principle took place in the USA. The first USA patent was not applied for until 1961 by Geyer from Uniroyal [1]. Even today, smooth-bore extruders with barrier screws are superior to grooved barrel extruders for many applications, provided the conveying stability is adequate. This applies in particular to applications in which fluctuating proportions of recycled or regrind material have a disruptive influence on the normal conveying characteristics of the solid material. In such cases, extrusion is likely to be more stable with a smooth-bore extruder. In Europe, the development of extruders with heat-separated grooved bushes in the feed section began at the end of the fifties and beginning of the sixties. Grooves in the barrels to increase barrel friction and assist conveying of the solid material had been tried out long before then. They were, however, not enough to process the newer high-molecular weight HDPEs in powder and grit form. This specifically European phenomenon on the raw materials side has come from the systematic analysis and development of the grooved bush principle. Extruders with grooved bushes were initially operated with the conventionally flighted threesection screws commonly used in Europe. To have better control of the melt temperatures, vented screws were later developed, and, to improve the melt homogeneity, were subsequently equipped with shearing/mixing sections [2]. One problem nevertheless remained: very high pressures at the end of the feed section and, as a result, considerable wear and tear on the screw and barrel. Fig. 1: Development of extruder screws in the USA and Europe a = b = c = d = e = f = g = h = i = k = l = m = 3-zone compression screw Uniroyal screw Maillefer screw compression screw with UCC(Maddock) mixer compression screw with pin mixer barrier screw barrier screw with UCC (Maddock) mixer 5-zone decompression screw with shearing/mixing device compression screw with pin mixer no-compression screw with/ shearing/mixing device barrier screw with shearing/mixing device high-output barrier screw with shearing/mixing section 2

The development of extruder technology is basically reflected in the development of the screws. For thirty years, development teams went their different ways in the USA and Europe until, at the beginning of the nineties also due to increasing globalization the directions of development began to converge again. Combining the grooved bush principle with barrier screws is the logical step to optimize extrusion technology [3]. Screw designs and selection criteria As already mentioned, the choice of a suitable extrusion system (conventional or grooved bush concept) depends on the particular application. After all, the design of the screw determines the quantitative and the qualitative properties of the extrudate. In practice, different screw lengths have become established for different applications. For applications in extrusion blow molding, for example, relatively short extruders (L:D = 20:25) are used, whereas in other applications, such as film and pipe extrusion, extruders with longer screws (L:D 30) are generally employed. As a result, the way the total screw length is divided up into the "feed and compression" and "melting and homogenizing" sections can vary considerably. First of all, for a specific application, a decision has to be taken as to what proportion of the total screw length should be reserved for homogenizing the plastificated melt. This question can nowadays only be answered on the basis of experience or following an appraisal of the demands made on the melt quality. Even specifying the necessary melt quality can sometimes cause problems. Complying with an imprecisely defined melt quality can necessitate not only homogenizing elements on the screw (dynamic mixing sections), but also static mixing elements. The various constructions of homogenizing elements will be dealt with in more detail later. While a wide variety of screw concepts are still in use, current developments are concentrating very much on barrier screws. For this reason, this report will concentrate on such models while taking a wider look at the topic of single-screw extrusion. Fig. 2 shows schematically the basic concept of barrier screws for different lengths of extruders, with and without barrel venting. The concept is the same for new extruders as it is for the retrofitting of existing machines. The evaluation of a barrier plastificating section is generally carried out by looking at the differences in the pitch and flight depths and the design of the feed section and outlet area of the barrier flights. Both North American and European barrier Fig. 2 Basic concept of barrier screws 3

screw developments have moved in the direction of designs which conform, to a very large extent, to the principle of the Dray and Lawrence screw. The characteristic features of these screws are that, through elevations in the respective pitches of the main flight of the screw and the barrier flight, a sufficiently wide channel is created in the solids channel this encourages plastificating and that, through a variable adjustment of the flight depth profiles, the melt temperature curve can also be adjusted, with the aim being to keep the melt temperatures as low as possible. Although barrier screw designs still exist today with a solids channel that is not sealed off, the only way of ensuring complete melting in the barrier plastificating section is to use solids channels with a 'deadend' groove (Fig. 3). The front of the barrier flight at the beginning of the barrier plastificating section can be designed with the melt channel closed at the rear end or with an open melt channel. In this case, even if we assume that unmelted material enters the melt channel, complete plastification is nevertheless ensured by the end of the barrier section because of the long residence time in the melt channel. A detailed description of different barrier screw concepts, including their characteristic features, is given in [4]. For extrusion applications in which relatively high extrusion temperatures are required (e.g. paper coating), the screw geometries must be modified by making the flight depths in the melt-filled sections smaller so that, due to the higher dissipation energy, the target melt temperatures are reached. This could possibly also be done by adapting the feed sections to reduce the specific melt throughputs. Last but not least, the shearing sections used for such applications can - and must - be dimensioned in such a way that the necessary temperature increase is reached. For other applications, for example foam extrusion, exactly the opposite course must be taken to keep the melt temperatures as low as possible after injecting the blowing agent. Here, the best solution is to regard the extrusion system as a highly effective heat exchanger, and to enhance its effectiveness through reduced dissipation in large-dimensioned screw channels and through frequent interface renewal by the screw flights on the inner wall of the barrel. Fig. 3: Transition between feed section and barrier section on a double flighted/paired screw of 150 mm diameter 4

We will now deal with the influences of the raw material and the extruder feed section geometry, which determine the conveying properties of an extruder. Universal screws / highoutput screws For the user, the ideal situation would be to have a screw on which as many different plastics as possible could be processed at high throughput speeds and with good melt homogeneity. Some of the most important requirements are: Processability of mixtures with different sized and different shaped granules High plastificating performance Gentle but complete plastification Good melt homogeneity Controlled melt temperatures Minimal change in the material through degradation or crosslinking High level of versatility: ability to process a broad selection of raw materials with a wide range of throughput rates Low performance-related investment and operating costs In recent years, so-called grooved barrel extruders with barrier screws have proved to be the most suitable systems among single-screw machines. With many grooved barrel extruders, the pressure build-up at the end of the feed section is too high, encouraging wear and tear and impairing the stability of the process. This can be countered by enlarging the pitch or making the screw channel deeper, although this involves the risk of plastification and homogeneity problems. A better solution than a screw with a stepped pitch or channel depth is a barrier screw. At the beginning of the barrier plastificating section, the conveying flight changes to a greater pitch; the beginning of the barrier flight also has a higher pitch. The depth of the channels is adjusted to the desired conveying and melting characteristics. These two measures result in a fairly balanced, low pressure profile, or even in a pressure build-up towards the end of the screw (Fig. 4). When assessing the "universality" of a screw, the homogeneity of the melt plays a dominant role. This is particularly true when processing mixtures of different materials, and also with regrind material, with color masterbatches and with the socalled 'direct extrusion' process (combining compounding and extrusion in one step, "in-line"). pressure [bar] 500 450 400 350 300 250 200 150 100 50 0 PE - HD Hostalen GM 7746 Extruder 50 mm, 28:1 L/D 0 50 100 150 200 250 screw speed [rpm] meltpressure in front of the screw tip meltpressure after grooved feed bush Fig. 4: Meltpressure in front of the screw tip and after grooved feed bush (source: KKM) Barrier screws, too, must be provided with elements for homogenizing after the melting section. Depending on the requirements of the raw materials and the demands made on the product, shearing sections must be used for dispersion (for example for color pigments), and/or distributing mixing elements must be provided for axial and transverse mixing. 5

Fig. 5: Spiral shearing section and faceted mixing section after a barrier section In practice, barrier screws with neutral-pressure, (multiple- )spiral shearing elements and with faceted mixing sections designed to give good flow properties have proved successful, also for direct coloring with a color masterbatch (Fig. 5). With homogenizing elements of this kind, the best way of maintaining full control of the general thermal conditions, and thus of keeping the melt temperature closely under control, is to ensure that good heat transfer by convection to the temperature control system of the extruder barrel is possible, both in the area of the spiral shearing elements and in the area of the faceted mixing sections, through constant renewal of the surfaces and/or interfaces between the moving screw elements and the fixed inner surface of the barrel. A further influence can be exerted on the temperature of the melt by taking additional measures, for example, by fitting a temperature control system on the inside of the screw (e.g. as a closed cooling system), As regards extruders for universal applications - in other words, extruders capable of processing a very wide range of raw materials, including regrind (fluctuating proportions, recycled material etc.) - a decision has to be taken in each individual case whether or not to use the grooved barrel extrusion concept. The decision is not always an easy one to make. If excessively large variations in the raw material properties especially the bulk density, flow properties and friction coefficients are to be expected, it is probable that using the grooved bush concept will lead to excessive fluctuations in melt throughput due to the fact that the output characteristics are governed by the solids conveying in the feed section. In such cases a smooth-bore extrusion system can or must be used. This is particularly true for processing raw materials with a fairly high shredded content and consequently a low bulk density. Another possibility is to precompact the shredded material so that grooved barrel machines can function perfectly. Homogenizing elements Barrier section plastificating Feed section Good dispersive/ distributive mixing effect Heat transfer to the barrel Low pressure loss Effective separation of solids from melt High homogenizing effect Good control of melt temperature Clear pressure build-up Melt throughput geared to homogenizing capacity Low pressure level Low torque Reduced wear and tear Fig. 6: Barrier screw concept with homogenizing elements 6

Fig. 6 summarizes the three sections and shows the special characteristics of a barrier screw with homogenizing elements. For universal application, use can generally be made of screws designed according to the above concept in lengths of between 20 and 30 x D. The feed section consists either of a shallow-flighted feed section with subsequent decompression (grooved bush concept) or a constantly deep-flighted feed section (smooth-bore extruder), followed by the barrier plastificating section and the homogenizing sections.the best solution is first to have dispersively acting mixing elements, and then distributively acting elements. Fig. 7 shows possible and commonly used constructions. In recent years, "static-dynamic" mixer systems with spherical indentations in a rotor and stator have become quite popular. They are marketed under such names as CTM, TMR, STAROMIX and 3-DD [5]. Although they have a good mixing action, some of them have an inadequate selfcleaning system and others have problems with wear and tear. Apart from this, it is essential that the melt is 100 %. Operation with barrier screws With barrier screws, designed according to the principle of Dray and Lawrence screws, the solids channel has been made wider. This provides a larger contact surface area for the material being melted so as to introduce energy via the barrel heating. This means that, with barrier screws, the heating process must be started immediately after the material is fed in. Either a constant temperature program must be set over the length of the barrel, or the temperature must be set so that it actually drops from the feed section to the end of the barrel. The temperature at the end of the barrel is the same as in a conventional screw, in other words it is geared to the melt temperature. In the first heating zone after the grooved bush, it is perfectly in order to work at a temperature which is about 20 30 C higher. In the lower to medium speed range, the temperature in the final barrel section is set at the same level as the melt temperature. The temperature at the end of the barrel is the same as in a conventional screw, in other words it is geared to the melt temperature. In the first heating zone after the grooved bush, it is perfectly in order to work at a temperature which is about 20 30 C higher. In the lower to medium speed range, the temperature in the final barrel section is set at the same level as the melt temperature. Fig. 7: Executions of shearing and mixing elements (source: KTP) With low-viscosity melts, or in cases in which high melt temperatures are required, correspondingly higher settings are recommended. It is also advisable to keep a watch on the relative periods in which the heating and cooling units are switched on (controller output signals), so as to work in the medium range of settings (20... 80 %). As a rule, this will mean that the deviations between target values and actual values are sufficiently small to ensure process stability. 7

and to raise the melting capacity. Fig. 8: Temperature program for barrier screws In practice, it is not difficult to maximum melt throughputs in establish the optimum the specified melt quality. For temperature settings. this purpose, the combination of a grooved barrel extruder and a Because of the special barrier screw with a characteristics of barrier screws, homogenizing section is it has often proved an particularly recommended. With advantage to turn up the heating larger screw diameters, a output in the first and second double-flighted or twin-pair barrel sections, and to increase screw system can be used to the fan or cooling capacity in the improve the conveying two sections at the end. properties in the feed section One example at the upper end of the speed scale involves retrofitting an existing 150 mm 33 D extruder for working with MDPE and LDPE for the sheathing of steel pipes. The objective in this case was quite clearly to achieve maximum possible melt output with high product homogeneity (specified in reference samples) and, at the same time, to keep the melt temperatures as low as possible. In addition, the system had to have outstanding selfcleaning and material changeover characteristics. Practical experience The broad range of application of barrier screws for polyolefins can be seen in Fig. 9. All these materials were successfully processed with the same 50 mm/28 D screw with a twinspiral shearing section and a faceted mixing section on a grooved barrel extruder. Further details are contained in one of the later articles. High-speed extrusion systems are generally characterized by the fact that the system is set up to suit a limited range of raw materials, but so as to achieve Fig. 9: Specific throughput vs. Screw speed (Quelle: KKM) 8

Fig. 10 gives some examples of results obtained with and without a gear pump. Using this concept, the targets were readily achieved, which meant that the installed drive power was almost completely utilized for the given range of raw materials. Further increases in throughput are conceivable over and above these figures. However, this would make it necessary to adapt the drive unit by raising the motor power and proportionally increasing the screw speed. It also becomes increasingly important to take special measures to prevent excessive wear and tear because of the greater influence of the peripheral speeds of the screw. The production of fuel tanks is an impressive example of the direct recycling of production scrap. A problem here is that, with blow molding, comparatively short extruders are used. Depending on the shape of the tank and on other boundary conditions, between 40 and 60 % flash occurs as scrap at the production machine. This is material which was cut off at the top and bottom of the parison and from the pinch-off edges of the blow mould. This material is directly ground and fed back into the machine. Fig. 10: Production data with extruder 150mm/33:1 for steel pipe coating 9

700 215 600 500 Extruder 150 mm / 20 D PE-HD Lupolen 4261A with 50 % regrind 210 Throughput [kg/h] 400 300 200 100 [kg/h] [ C] 205 200 195 Melt temperature [ C] 0 5 10 15 20 25 30 35 40 45 screw speed [rpm] 190 Fig. 11: Production data with extruder 150mm/20:1 for industrial blow moulding Fig. 11 gives the key operating data for a grooved barrel extruder with a diameter of 150 mm/20 D equipped with a barrier shearing/mixing section screw for processing highmolecular weight HDPE grit containing regrind material. One notable application for 'specialty screws' are vented extruders, which are used, for example, in plants producing flat film and sheets. Because such machines are being increasingly combined with melt pumps, the second stage of the screw only needs to convey the melt against the pump pre-pressure (and possibly against the resistance of melt filters), but does not have to overcome the resistance of the connection, possibly a static mixing element, and the die. Consequently, much higher throughputs can be achieved, with plastification and homogenization being carried out in the first stage of polystyrene, polycarbonate and the screw. PMMA. Fig. 12 shows the concept of a vented screw with a barrier plastificating section in stage 1, and a three-zone profile plus faceted mixing section in stage 2. Such vented screws with barrier plastification are being successfully used for e.g. Fig. 12: barrier screw configuration for vented extruder 90mm/30:1 10

H Extruder concepts for different plastics / new high-performance materials For most applications, grooved barrel extruders with a heat separation system between the feed section and the subsequent barrel have become established in Europe. As a rule, the feed bushes are axially grooved and correspond to the construction concept shown in Fig. 13. A good thermal layout - in other words sufficient and uniform heat dissipation in the area of the grooved bush - is of particular importance. For this purpose, the cooling channels and heat transfer resistances must be optimized. In many cases, temperature control units or installations with bypass control etc. are fitted to create constant thermal conditions. An optimized thermal layout is also essential for the rest of the barrel. Even though the target is to generate as little excess heat energy as possible via the screw, it is not possible with high-speed extrusion to dispense with good cooling of the barrel, at least not in some sections. Special companies offer heating/cooling combinations for this purpose, in which a great deal of heat can be dissipated via aluminum or copper elements. Some machine manufacturers also supply customized systems of this kind. When new materials come on to the market, the question continually arises about the most suitable extruder and whether they can be processed on existing machines. This was the case with LLDPE, and it is now happening with the metallocene polyolefins, for example mlldpe. The goal of the chemical industry is where this has not happened already to make it possible to run mpe on existing extruders by modifying the molecular structure. One aspect not being examined here, but nevertheless of considerable importance, is the question of how the material behaves in the dies and after emerging from the die. With blown film extrusion, for example, this would concern the melt elasticity and the bubble stability. A α D N DT B n=d(mm)/5 DT - D N= ca.4mm B=7-8mm A L H=3-3,5mm α =7-8 L=3-3,5D Fig. 13 : Lay-out data for grooved feed sections 11

Fig. 14: Specific throughput vs. Screw speed for grooved feed extruder Ø80 mm/30:1 (source: Reifenhäuser [6]) Basically, it can be said that mpe can also be run on extruders used for processing LLDPE. This applies both to grooved barrel machines and smooth-bore extruders [6]. However, because of the specific material properties, there is a difference in the throughput rates (Fig. 14) which, in turn, leads to differences in melt temperature (Fig. 15) and outputs. On the other hand, this phenomenon is not specific to metallocene. something which has also been encountered with "normal" polyolefins (cf. Fig. 9). One way of countering the poor conveying characteristics of the solid material is to add a lubricant or material containing a lubricant. Another possibility, this time on the machine side, is to reduce the coefficient of friction by cooling the screw, coating the surface etc. One of the most important factors concerning the conveying properties in the feed section is evidently not the free flowing characteristics or the low degree of hardness of the granules. In fact, the large influence of lubricants as can be seen in Fig. 16 indicates that friction on the surface of the screw plays a major role, Fig. 15: Melt temperature vs. Screw speed (source: KKM) 12

EXCEED in Blends Effect of slip in the LDPE blend component on specific output on grooved barrelextruders kg/rpm/h 3.4 3.2 3 2.8 2.6 2.4 2.2 2 Increase: 10 % 15 % 20 % EXCEED 1MI / 0.918D Slip additivated LDPE as blend partner gives 10-20 % higher specific versus no slip in the LDPE + 5 % LDPE no slip 80mm 24 L/D grooved + 5 % LDPE EXCEED 1MI / 0.918D + 5 % LDPE with with slip slip 75mm 25 L/D grooved EXCEED 1MI / 0.918D + 10 % LDPE + 20 % LDPE with slip 80mm 30 L/D grooved + 30 % LDPE Fig 16: Influence of slip agent on specific throughput with mpe (source: EXXON [7]) For engineering plastics (i.e. polyamides, polyesters, polyurethanes or thermoplastic elastomers), use is nowadays predominantly made of smoothbore extruders. This is due not only to "tradition", but also to the predominantly low throughputs involved. Plastics of this kind can, however, also be processed without problem on grooved barrel extruders, as is shown in [8]. For blow molding with PA6, a grooved bush/screw concept similar to the one used for PE has given good results [9]. Grooved feed sections are also being increasingly used in extruders with a barrel venting system. The grooves with a semi-circular, sickle, saw-tooth or rectangular cross-section are either cut in the one-part barrel, or a normal grooved bush concept is used. This means that the construction of vented extruders is currently in a process of change, as was already explained with the barrier vented screws. Direct compounding in the extruder This term is used to explain the combination of compounding and extruding in one step. The process, which aims primarily to cut down costs, is still in its infancy, despite all the efforts being made by machine manufacturers and plastics producers. For processing, use is made primarily of co-rotating twin-screw extruders, which offer far more possibilities in terms of process technology. They can, for example, be used for incorporating fillers and reinforcing agents, for blending different polymers or for simultaneously carrying out reactions (reactive treatment/extrusion). There has, however, been no lack of attempts to also use single-screw extruders for compounding or for the socalled in-line extrusion. Systems of this kind are repeatedly shown and marketed. The possibilities and limitations are obvious. One special kind of in-line extrusion is the blow molding of tubular film from mixed film waste (DSD fraction). After a temporary phase of euphoria, normality has, however, returned. Apart from technical problems with individual components of the plant and the doubts about the product quality, it has been found that the cost structure is also negative over the long term. 13

Despite all the progress being made to adapt extruders and extrusion lines to the requirements of waste processing, it must be said that the production of extruded quality products using recyclate is limited. Not so much because of the machine and processing technologies, but more because of the products and the specified quality. For the time being, in extrusion, the recycling of production scrap will continue to have priority over the processing of post-consumer recyclate. Possible applications and limitations At this point, it should be said that the possible range of applications for grooved barrel extruders with barrier screws is almost "boundless". This is shown elsewhere. Retrofitting to optimize the system Whenever funds for capital expenditures become short, the purchase of new machines tends to be put back or eliminated completely. In such circumstances, optimizing the existing system can be a help. Where there is a need to modify existing extrusion lines to cope with a higher output and/or improved melt quality, a modern screw concept can be adapted to the given circumstances. When making a modification of this kind, it first has to be borne in mind that the machine in question is of a given length (e.g. frequently between 20 and 25 x D), which can not be changed, and that it has an existing drive unit. This sometimes leads to restrictions as far as the attainable specific melt throughput is concerned, due to bottlenecks with the torque of the screw drive unit. The torque results from the installed motor power and the installed gear reduction. In some cases, the gear reduction can be adapted so that a higher drive torque is produced on the screw shank. Since there is a directly proportional relationship between the specific output and the screw drive torque, it is possible in such cases to achieve an increase in the specific melt throughput equal to the increase in torque. Fig. 17 gives an example of a successful retrofit. Fig. 17: Production data with grooved feed extruder 60mm/24:1 after installing a barrier mixing screw (source: Kuhne) 14

Protecting the screw and barrel against wear and tear One important aspect concerned with protection against wear and tear has been discussed already, namely wearreducing screw geometries. Quite astonishing results can be obtained by harmonizing the conveying characteristics and the pressure build-up, and by optimizing the melting process from time to time in conjunction with a multiple flight design. With grooved barrel extruders, for example, through optimized harmonization of the system (adapting the feed section geometry to the downstream sections and vice versa), grooved bushes made of nitride steel can be used instead of hard metal or PM-HIP material (see below), because of the much lower pressures involved. On the other hand, new plastics - in some cases necessitating higher processing temperatures - fillers and reinforcing agents or pigments, higher peripheral velocities of the screw etc. etc. are resulting in higher and higher stresses. They can only be countered by taking special measures to increase the protection against wear and tear, as is state-of-the-art already in injection molding [10]. In the USA, so-called bimetal barrels have been used in extruders for years and years. In Europe, too, instead of the conventional nitride steel barrels, processors are making increasing use of barrels that have been given a centrifuged, wear-resistant and, if needed, corrosion-resistant armored layer. Apart from the fact that this approx. 1.5 2 mm thick coat can be adapted to suit the particular type of stress, it also offers, with its consistent properties, a certain "reserve" of wear and tear, even if the process engineering parameters are not quite right. For small and medium-sized wear-protected screws (up to approx. 50 mm), fully hardened tool steel is used, particularly cold work steel X 155 CrVmO 12.1 (DIN 1.2379). To achieve a (limited) corrosion resistance, frequent use is made of rustproof, acid-resistant 17 % chrome steel X 35 CrMo17 (DIN 1.4122). By ionitriding to increase the surface hardness, however, this material loses some of its corrosion resistance. For very high corrosion protection, it is preferable to choose special materials, e.g. Inconel 625. With larger screws, it is common to armor-plate the screw flights, which are particularly prone to wear and tear. This involves using the tungsten inert gas arc welding or the plasma-powder application (PPA) welding method. The most popular materials for this are nitride steel, 30 CrMoV9 (DIN 1.8519) and 14 CrMoV6.9 (DIN 1.7735), or chrome steel X 35 CrMo17 (DIN 1.4122). Hard alloys such as Stellite 12, Colmonoy 50, Colmonoy 56, Colmonoy 83 etc. are also used for armor-plating. The screw root surface and flanks can also be protected by nitriding, by a hard chrome layer or by armor-plating. Hot isostatic pressed materials (HIP) [10, 11] produced by powder metallurgy are gaining increasing importance. Both "natural hard" and hardenable alloys are used. The materials can be produced either as homogeneous systems or as composite systems, in the latter case, either in conjunction with steel, e.g. as the core with an external hard shell for screws or screw elements, or as a composite of the hard alloy powder with inserted hard substances. The PM-HIP materials have the advantage that a fine, homogeneous and pore-free structure is formed, which is much preferable to the conventionally produced materials. The wear-inhibiting hard phases (usually carbides) are also distributed more finely and evenly in the fine-grain structure, which means that less surface area is open to attack in the matrix. The components can be equipped specifically to cope with the expected stresses. Hardness values of up to 72 HRC can be attained. Fig. 18 shows the overall properties and behavior of PM-HIP materials. 15

With materials examined on a universal disk tribometer, it was found that the relative wear decreases significantly with an increasing proportion of vanadium carbide. exception rather than the rule. The higher the demands made on the plastics and their additives, the more popular the new systems will become, also for these machines. If we look at the market as a whole, solutions in which PM- HIP materials are used in singlescrew extruders are still the 6 5 Rel. Wear 4 3 2 1 0 0 10 20 30 40 Vol.-% VC Rel. Volumetric wear Material Element [Gew.- %] VC [Vol.- %] C Cr V X 220 CrVMoW 20 4 2,2 20 4,1 6,9 X 250 CrVMoW 22 6 2,5 21,6 6 10,3 X 260 CrVMo 26 4 2,6 26 4 6,2 X 270 CrVMoW 17 9 2,7 17 9 15,7 X 310 CrVMoW 15 10 3,1 15,2 10,3 18 X 340 VCrWMo 13 13 3,4 12,8 13,3 23,4 X 350 VCrMoW 13 9 3,5 8,5 13 22,8 X 380 VCrWMo 17 13 3,8 12,5 17 29,7 X 410 VCrWMo 17 14 4,1 14 17 29,5 X 450 VCrWMo 18 13 4,5 13 18 31,1 X 500 VCrWMo 20 13 5 13 19,5 33,4 Fig 18: Relative abrasive wear of PM-HIP materials dependent from VC content (source: Reiloy) 16

References [1] Rauwendaal, C.: Polymer Extrusion. München: Carl Hanser Verlag 1990 [2] Fischer P.: Stand der Einschneckenextruder in Europa. Kunststoffberater (1984) 4, S. 20-23 [3] Fischer, P., Wortberg, J.: Hochleistungs- und Unversal-Schnecken für die Extrusion. Plastics No. One 8/95, S. 21-27 [4] Wortberg, J.: Schneckenkonzepte für die Hochleistungsextrusion - Barriereschnecken. In Einschneckenextruder - Grundlagen und Systemoptimierung. Reihe Kunststofftechnik. Düsseldorf: VDI-Verlag 1991, S. 107-139 [5] Bos, H. L., Meijer H. E. H.: Mischen und Kneten im Einschneckenextruder. In Einschneckenextruder - Grundlagen und Systemoptimierung. Reihe Kunststofftechnik. Düsseldorf: VDI-Verlag 1991, S. 25-58 [6] Schröter, B.: Processing of Metallocene PE (mpe) on Blown Film Lines. METALLOCENES EUROPE 97, S. 353-373 [7] EXCEED POLYETHYLENE, Film Processing Facts. Firmenschrift EXXON CHEMICAL EUROPE, Brüssel 1996 [8] Michels, R., Wortberg, J.: Innovative Entwicklungen bei Einschneckenextrudern. In Der Einschneckenextruder. Reihe Kunststofftechnik. Düsseldorf: VDI-Verlag 1997 [9] Völkel, M.: Eigenschaften und Anwendungen von verstärkten und unverstärkten Polyamiden. In Blasformen 97 - Innovationen und Perspektiven. Reihe Kunststofftechnik. Düsseldorf: VDI-Verlag 1997, S. 217-233 [10] Lülsdorf, P.: 40 Jahre Verschleißschutz für Schnecken und Zylinder in Spritzgießmaschinen. Vortrag auf dem KKM / IKM - Fachkolloquium Spritzgießtechnik, Essen 26.01.96 [11] Deppe, E.: Verschleißschutzmaßnahmen an Schnecken und Zylindern in Extrudern und Spritzgießmaschinen. Vortrag auf dem VDI - Seminar Technischer Oberflächenschutz, Düsseldorf 19. und 20.06.97 17