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Technical Articles
Home >> Technical Articles >>Brief introduction of processing with single screw extruders
Brief introduction of processing with single screw extruders
Time: 2008-12-18
Single screw extruders are a proven and simple way to extrude almost every kind of resin, and are probably the predominant machines in use. But in recent years, multiple-screw machines have made inroads based on their perceived benefits. However, single-screw design is not static. New designs are being developed to meet the increasing demands of processors. These include applications that require a range of polymers to be processed at different temperatures and throughput rates without changing screw or barrel.
Recently, advances in single-screw performance have been a gained by combining grooved-barrel feeding with a barrier melting mechanism. Properly adapted and fitted in an appropriate extruder design, this combination leads to extrusion system with excellent output and melt quality for a range of resins including polyolefin, polystyrenes and engineering thermoplastics (ETP).
Conventional three-zone screws have a feed section in a smooth barrel, a compression section, and metering section at the screw tip. Adequate in output and mixing, they are widely used. Mixing sections can be attached to the metering zone to improve melt homogeneity.
By Contrast, barrier screws employ a melting mechanism that separates molten from solid polymer. An added flight after the feed section forms a second flow channel on the pushing side of the main flight. This second channel is narrowest at the feed section and gets wider towards the screw tip while the main channel width diminishes in the direction of flow. The second flight acts as a tight clearance barrier which allows the melt to pass, but prevents solids from reaching the die.
Many barrier-screw designs have been developed and applied over time, particularly in North America. The America-style barrier sections are usually entered from a smooth-bore feed zone and followed by metering section. In comparison to conventional metering screws with mixers, barrier screws typically produce better melt quality at higher throughput rates.
Processing powdered resins such as high-molecular-weight HDPE on smooth-bore extruders results in unacceptably low through-put rates. In Europe in the late 1990’s, it was found that cutting axial or spiral grooves in the barrel along the feed section provides an excellent solution to this problem by improving the friction of the solids at the grooved barrel. Throughput (metering) is then dictated by the solids conveyed in the feed zone. The grooved barrel section is cooled to ensure solid conveyance. It is also thermally insulated from the adjacent heated barrel zones. Screws for grooved-barrel extruders have a shallower channel in the feed zone. A decompression section after the first three zones- which are similar to but shorter than the conventional 3-zone concept - provides lower melt temperatures. Mixing sections are essential to achieve sufficient melt quality.
The main advantages of grooved-barrel extruders are the high specific throughput rates and the fact that out-put is not influenced by the backpressure of the die. They are widely used in Europe, having displaced smooth-bore extruders in many extrusion applications of PE, PP, and their copolymers.
In the recent past, we combined grooved-barrel feeding with barrier screws for the extrusion of polyolefins. Now we adapt the technology for processing polystyrenes and engineering thermoplastics as well.
The conventional design for grooved-barrel extruders generates very high pressure at the end of the feed section in order to force resin through the downstream sections of the screw. Various problems can arise from the resulting heavy friction between solid resin particles and the steel surfaces of the screw and barrel. These include: the risk of excessive wear; the development of torque overload when extruder startup occurs in conjunction with a filled hopper; and the need for intensive cooling, which does not fully prevent the formation of a melt film in grooves at higher screw speeds.
One solution is to create a pressure-relived grooved feeding section. This can be achieved by increasing the screw pitches and the channel depths in order to lift the conveying rate of the downstream sections. The lower pressure also allows processing the pellets of engineering thermoplastics ranging from very rigid to very soft. Additional benefits are achieved if a barrier zone is integrated into the screw design. This results in both a high melting capacity and an excellent melt quality.
The feeding zone of the optimized barrier screw changes into a decompression section with deeper channel and larger pitch (diag., P. 141). The following barrier zone has a considerably larger main pitch than the feed zones. Widths and depths of the solids channel and the melt channel are adapted to the desired melting progression and conveying characteristics. The barrier zone can – depending on operation point and resin- generate substantial pressure. This results in a pressure at the end of the feed section that is at least the same or even lower than the back pressure of the die.
The homogeneity of the melt is improved by two mixing sections: a depressive-mixing spiral-shear element, which is followed by a rhomboid distributive-mixing section. Both zones provide good heat transfer to the wall of the barrel. And, due to their spiral geometry, they are designed for balanced pressure.
Extrusion trials were carried out with a prototype screw (50-mm dia, 28D) on resins and blends covering a range of properties. The engineering grades included rigid resins with higher melting polymers with lower processing temperatures (TPE, EVA, TPU).
A throttle die with constant flow resistance was used to generate back pressure. Despite the rising pressure at the screw tip, a decrease of the specific rates with rising screw speed could hardly be observed. Maximum output of 330 Kg/h for PS and 320 kg/h for LDPE with slip agent were reached. Screw mixing quality – examined by variations of melt pressure, melt temperature, and visual inspections – was excellent for all resins.
As screw speed is increased, the screw design provides excellent control of the rise in melt temperature. Except for LLDPE at screw speeds above 300 rpm and the PC above 100 rpm, all resins remained within the recommended temperature limits for processing.
When operating at the highest screw speed, the four barrel zones downstream of the cooled feeding zone can be easily cooled or heated to maintain the constant correct melt temperature for the resin. Air blown over finned aluminum elements fixed to the barrel is sufficient to cool downstream zones. The higher temperatures produced in the feeding zone are still moderate due to the reduced friction. This allows cooling the grooved barrel section with water circulating in a closed system incorporating a heat exchanger instead of the conventional connection to a chilled water system.
The data demonstrate that the combination of grooved-barrel conveying and barrier-melting mechanism can enhance the performance and melt temperature control of single-screw extruders. Improvements in through-put rate are between 20% and 30% for polyolefin, a bit higher for PS. This is compared to conventional screws for grooved-barrel extruders. The main advantage, however, is the ability to process engineering reins on the same screw at a high throughput and good melt quality.
Nevertheless, this concept still has limitations. The differences in specific throughput rates are due to the given geometrical system of the feed zone. These are probably explained by the influence of bulk densities and – importantly – frictional behavior of the different resins. Plus, the lower shear-thinning behavior of some resins (LLDPE, PC) has a detrimental effect on melt-temperature control.
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