Abstract
Further advances in single screw performance can be achieved by combining grooved barrel feeding and barrier melting mechanism. Properly adapted and fitted in an appropriate extruder design, this combination leads to an extrusion system with outstanding output and excellent melt quality for a broad range of resins. The design of this screw concept and the experimental results of extrusion trials with several resins at very high screw speeds are presented in this study.
Introduction
Optimum design for single screw extruders is not a matter of settled technology. New designs and variations of existing designs are constantly being developed.
Advances in extrusion technology are forced by the requirements of plastics processors. There are occa- sions - in particular for single-resin applications - where high performance systems with maximum output or opti- mum mixing quality are called for. In other cases a broad range of resins should be well processed at different tem- peratures and throughput rates without changing screw or barrel.
In the recent past, more rapid product changes and several new resins have led to a growing demand for a versatile high performance extrusion technology with out- standing throughput rates and excellent mixing quality for different resins processed by the same screw.These re- quirements are valid for the design of new extrusion equipment as well as for the modification of existing ones. Scientific workers and machine builders have to answer the challenging question: What is the appropriate design for screw and barrel to come up to these expectations?.
Single Screw Designs
The well known 3-zone-screw with a feed section in a smooth barrel, a compression section and a metering section can be considered as the starting point of the evo-
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lution of screw design [1]. Although not the best in output and mixing, this conventional design is still in use for many applications. To ensure better melt homogeneity mixing sections are usually attached to the metering zone.
At almost the same time - in the early 1960's - in-vented for rubber extrusion by Geyer (Uniroyal) [2] and for plastics by Maillefer [3], the barrier screw introduced a substantially different melting mechanism. A second flight after the feed section forms a second flow channel on the pushing side of the main flight. The second channel is usually closed towards the feed section and always opened towards the screw tip while the main channel diminishes in the direction of flow. Serving as a tight clearance barrier over which the melt must pass, the second flight prevents solids from flowing to the die.
Through the years a wide variety of barrier screw designs has been developed [4]. Most of them have been designed and applied in North America. The "American" barrier section is usually entered from a smooth-bore feed zone and followed by a metering section.Compared to conventional metering screws with mixers, barrier screws are known for better melt quality and better output pump- ing stability at higher throughput rates.
Processing powderedresins such as high-molecular-weight PE-HD on smooth-bore extruders results in unacceptable low throughput rates. Axial or spiral grooves cut in the barrel along the feed section turned out to be an excellent solution to this problem. The basic prin- ciples of grooved barrel extruders were elaborated in Europe in the late 1960's [5]. Due to the improved friction of the solids at the grooved barrel, the throughput rate is dictated by the solids conveyed in the feed zone. This is valid for powdered resins as well as for pellets. The grooved section of the barrel is usually cooled and ther- mally separated from the following heated barrel zones to ensure the solid conveying mechanism. Screws for grooved barrel extruders have a less deep channel in the feed zone. A decompression section after the first three zones which are similar to but shorter thanthe conventional 3-zone- concept provides lower melt temperatures. Mixing sections are essential for a sufficient melt quality.
The main advantages of grooved barrel extruders are the high specific throughput rates and the fact that output is not influenced by the backpressure of the die. They are in widespread use in Europe and they have dis- placed smooth-bore extruders in many applications, in particular in the field of extrusion ofpolyethylenes and polypropylenes.
Further advances in screw performance can be achieved by combining grooved barrel feeding and barrier melting mechanism. Properly adapted, this combination leads to a screw design with maximum output and excellent melt quality for a broad range of resins as is shown for the screw concept presented in this paper.
Barrier Screws for Grooved Barrel Extruders
For a given resin the specific throughput rate of grooved barrel extruders depends on the geometry and the number of grooves as well as on the diameter, the channel depth and the pitch of the feed section of the screw [6]. There is a linear increase of throughput with increasing screw speed up to a certain limit where the formation of a melt film in the grooves destroys the solid conveying mechanism. The classical concept of screw design for grooved barrel extruders generates an extremely high pres- sure of 1000 to 1500 bar - or sometimes even more - at the end of the feed section to force the material through the downstream sections of the screw.
Some severe disadvantages are caused by this pressure and the resulting heavy friction between solid resin particles and the steel of screw and barrel: An amount of 10-20 % of the actual drive power is lost to the cooling system of the grooved barrel section; there is the risk of heavy wear and an overload torque of at least 1.5 times the nominal torque is needed to start-up the extruder with a filled hopper.
A solution to this problems is a "pressure re- lieved" grooved section. This can be achieved by enhanc- ing the conveying rate of the downstream sections e.g. by increasing pitch or/and channel depths [7]. The best con- cept to ensure an excellent melt quality at the same time is the integration of a barrier zone in the screw design (Fig.1).
The barrier section of the presented screw has a considerably larger main pitch than the feed zone. Widths and depths of solids channel and melt channel are adapted to the desired melting progression and conveying charac- teristics. The barrier zone can - depending on operation point and resin - generate a substantial pressure. This re- sults in a pressure at the end of the feed section that is at least the same or even lower than the backpressure of the die.
The homogeneity of the melt is improved by two mixing sections. A dispersive mixing spiral shear element
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tears up melt regions of different viscosity. It is followed by a rhomboid distributive mixing section.Both zones provide a good heat transfer to the wall of the barrel. And, due to their spiral geometry, they are designed for balanced pressure.
Experimental Program
Several extrusion trials were carried out with a prototype screw of 50.0 mm diameter and 28:1 L/D.The screw was fit in a special designed extruder to realize screw speeds far beyond the usual range by using an IEC- standard AC squirrel cage motor controlled by a vector inverter [8]. The grooved barrel section of this machine can be electrically heated or be cooled by water circulating in a closed system with an in-line water/air heat exchanger. The four other barrel zones are electrically heated as well; but only the two downstream sections can be cooled with air blowers removing heat via finned aluminum elements that are fixed to the barrel.
The resins for the experimental analysis were cho-sen to cover a broad range of different properties (Table1): Two similar PE-LDs for blown film and cast film extru- sion (one of them with a high amount of slip agent), a PE- LLD for blown film extrusion, a tough PE-HD used for blow molding applications and a polystyrene for sheet extrusion.
The screw speed for each resin was varied be-tween 10 and 400 r/min, while the flow resistance of the die was not changed.
Experimental Results
The results of the experimental analysis are shown in the performance graphs (Fig.2 to Fig.7). The assign- ment between abbreviations and resins is given in Table 1.
The output rates indicate an almost linear increase with screw speed up to maximum throughputs of 330 kg/h for polystyrene as well as for the lubricated PE-LD 2 at350 r/min and 360 r/min, respectively (Fig.2). Due to the limited power of the drive (55 kW) the desired maximum speed of 400 r/min could not be realized for these materials and the PE-HD.
A better impression of the linear correlation be-tween throughput rates and screw speed can be derived from the calculated specific rates depicted in Fig.3. De- spite the rising pressure at the screw tip (Fig.4), a decrease of the specific rates can hardly be observed for all resins. The differences among the materials are caused by their varying bulk densities and frictional properties [6].For reason of versatility the feed zone of the prototype screw was designed for resins which are known for higher spe- cific rates (e.g.PE-HD,PS,PE-LD with slip agent). A screw for the sole purpose of processing PE-LD or PE-cosity of the melt. There is not an additional negative ef- fect of decreasing throughput as can be observed for smooth-bore extruders.
Conclusions
The experimental data presented in this paper shows that the combination of grooved barrel conveying and barrier melting mechanism can substantially enhance the performance of screws for single screw extruders. Fur- thermore, the improvements in throughput rate and melt temperature control are evident for a broad range of resins.
