The creation of extruder technology is basically reflected in the development of the screws . For thirty years, creation teams went their different ways in the USA and Europe until, at the beginning of the nineties - also because of increasing globalization - the guidelines of development begun to converge once again. Merging the grooved bush basic principle with barrier screws may be the logical step to optimize extrusion technology [3].
Screw designs and variety criteria As mentioned previously, the choice of a suitable extrusion system (conventional or grooved bush strategy) depends on the particular application. In the end, the look of the screw determines the quantitative and the qualitative properties of the extrudate. Used, unique screw lengths have become established for diverse applications. For applications in extrusion blow molding, for instance, relatively brief extruders (L:D = 20:25) are used, whereas in other applications, such as for example film and pipe extrusion, extruders with longer screws (L:D 30) are generally employed. As a result, what sort of total screw duration is divided up into the "feed and compression" and "melting and homogenizing" sections may differ considerably.
Of all first, for a specific application, a decision must be taken in regards to what proportion of the full total screw length should be reserved for homogenizing the plastificated melt. This dilemma can nowadays only be answered on the basis of experience or following an appraisal of the requirements manufactured on the melt top quality. Even specifying the required melt quality will often cause problems. Complying with an imprecisely identified melt top quality can necessitate not only homogenizing components on the screw (dynamic mixing sections), but also static mixing factors.
The many constructions of homogenizing factors shall 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 focus on such units while taking a wider look at the topic of single-screw extrusion. Fig. 2 displays schematically the basic idea of barrier screws for different lengths of with, extruders and without barrel venting. The concept screw extruders is the same for new extruders as it is usually for the retrofitting of existing devices.
The evaluation of a barrier plastificating section is normally carried out by considering the differences in the pitch and flight depths and the design of the feed section and outlet section of the barrier flights. Both UNITED STATES and European barrier Fig. 2 Basic idea of barrier screws 4 screw advancements have moved in the direction of models which conform, to a very large extent, to the principle of the Lawrence and Dray screw.
The characteristic top 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 desire to being to keep the melt temperatures as low as possible. Today with a solids channel that is not sealed off although barrier screw designs remain, the only way of ensuring whole melting in the barrier plastificating section is by using solids stations with a 'deadend' groove (Fig. 3).
Screw designs and variety criteria As mentioned previously, the choice of a suitable extrusion system (conventional or grooved bush strategy) depends on the particular application. In the end, the look of the screw determines the quantitative and the qualitative properties of the extrudate. Used, unique screw lengths have become established for diverse applications. For applications in extrusion blow molding, for instance, relatively brief extruders (L:D = 20:25) are used, whereas in other applications, such as for example film and pipe extrusion, extruders with longer screws (L:D 30) are generally employed. As a result, what sort of total screw duration is divided up into the "feed and compression" and "melting and homogenizing" sections may differ considerably.
Of all first, for a specific application, a decision must be taken in regards to what proportion of the full total screw length should be reserved for homogenizing the plastificated melt. This dilemma can nowadays only be answered on the basis of experience or following an appraisal of the requirements manufactured on the melt top quality. Even specifying the required melt quality will often cause problems. Complying with an imprecisely identified melt top quality can necessitate not only homogenizing components on the screw (dynamic mixing sections), but also static mixing factors.
The many constructions of homogenizing factors shall 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 focus on such units while taking a wider look at the topic of single-screw extrusion. Fig. 2 displays schematically the basic idea of barrier screws for different lengths of with, extruders and without barrel venting. The concept screw extruders is the same for new extruders as it is usually for the retrofitting of existing devices.
The evaluation of a barrier plastificating section is normally carried out by considering the differences in the pitch and flight depths and the design of the feed section and outlet section of the barrier flights. Both UNITED STATES and European barrier Fig. 2 Basic idea of barrier screws 4 screw advancements have moved in the direction of models which conform, to a very large extent, to the principle of the Lawrence and Dray screw.
The characteristic top 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 desire to being to keep the melt temperatures as low as possible. Today with a solids channel that is not sealed off although barrier screw designs remain, the only way of ensuring whole melting in the barrier plastificating section is by using solids stations with a 'deadend' groove (Fig. 3).