Abstract
Single-screw extrusion at high screw speeds is established nowadays since it allows for a high mass throughput at a comparatively small extruder size. Compared to conventional extrusion at low screw speeds, a considerable non-linearity in mass throughput appears by exceeding a certain threshold screw speed. In this study, the solid conveying behavior of different plastic granules with varying geometries was investigated in a smooth, a helically and an axially grooved solid conveying zone for screw speeds up to 1350 rpm. These experimental findings are compared to classical analytical predictions in the literature. It is found for the first time that both the shape and size of the plastic granules play a decisive role in determining the threshold screw speed at which a non-linear mass throughput is observed. It is shown that small and spherical granules exhibit a later onset of non-linear throughput compared to larger lenticular and cylindrical shaped granules. Moreover, it is revealed that the mass throughput equalizes for an axially and a helically grooved solid conveying zone at high screw speeds. This is contrary to the low screw speed range where the axially grooved barrel results in a significantly higher throughput than the helically grooved barrel. Thus, the maximum throughput at high screw speeds is limited by the granule stream provided by the hopper opening and is no longer governed by the groove angle.
Highlights
In Europe, the development of extruders with a grooved feed zone began around the same time
The mass throughput for each conducted experiment is predicted by using the linear approach of Equation (1)
The free channel cross-sectional area of the respective extrusion barrel is calculated by Equation (3) using the geometry values given in bTaarbrleel.2O. bTvhieocuosrlyre,sbpootnhdgirnogovfreede bchararnenlselpcorsossess-sseacntieonnlaalrgaeredafirsee44c3ro.8s3s-msemct2iofnoar lthareesamcoomot-h pbaarrerdel,to58t4h.1e4smmmoo2tfhorbtahrerehl.elTichaellygrgoroovoevegdeobmareretrl iaens dw5e9r7e.8p3rmevmio2ufsolry thcheoasxeinalltyo gerxohoivbeidt nbeaarrrleyl.thOe bsavmioeusflrye,ebcorothssg-sreocotivoendalbaarreraelws hpiocshsetshsusanonelnyladrigffeedrsfbreyeacrroousnsd-se2c%tiobneatwl aereena tchoemhpelairceadllytoatnhdetshmeoaoxtihalblyargrreol.oTvehde gbraorroevle. geometries were previously chosen to exhibit nearDlyettheremsainminegfrtehee csroolisds-csoecntvioenyianlgaraenagwlehtoichcatlhcuulsaotentlhyedaifxfiearlscboynvaeroyuinngdv2e%locbiteytwuseue-n atlhlye hreeqliuciarlelys iannfdortmheataixoinalalybogurot othveedsybsatrermels. governing friction coefficients
Summary
Single-screw extrusion is one of the most important processes in plastics industry. It allows for the mass production of semi-finished products such as foils, plates, tubes and other rather simple profiles [1]. Classical single-screw extruders consist of a three-zone screw and a smooth barrel. A continuous pressure build-up over the process length is characteristic for smooth barrel extruders. In grooved barrel extruders the significant pressure build-up already occurs in the feed zone [1]. Attempts to combine barrier screws with grooved feed zones were carried out around 1980 [3,4]. Since these systems have been continuously further developed in terms of increasing the specific throughput and by increasing the screw speeds [5–8]
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