Flow Channels of the Twin Screw Extruder
This object is inventively realized in that the ribs of the extrusion sleeve, at the ridge that faces the screw, have a width that is at least one third, in particular at least one half, and preferably approximately 80 to 100% of the width of the flow channels of the twin screw extruder sleeve, and in that there is provided between the ridges of the ribs of the extruder sleeve and the screw a gap of more than 0.5%, in particular of approximately one percent of the diameter of the screw. Advantageous further developments are provided in the dependent claims.
It is particularly expedient that, due to the provision of a transfer gap, a shear flow profile is produced that at the same length of the single screw extruder offers a significantly improved homogenization. Due to the inventively induced elastic or shear flows, there is effected in a defined manner an improved thorough mixing, which makes the temperature level significantly comparable. Colder regions of the mixture can flow into the inventive width gap. The polymer chains that are present there are inclined to slide along one another and to thereby be heated in an efficient manner. Pursuant to the invention, the retention time of the mixture in the sheet production extruder is clearly increased, whereby the length of the retention time can be controlled in a defined manner via the dimensioning of the inventive width gap.
In an advantageous embodiment of the invention, it is provided that the gap be provided with respective inlet or leading inclines that further improve the tendency of the material to flow in. The angle and the precise configuration of the inlet inclines can be adapted over a wide range to requirements.
The transfer is preferably effected using a linear characteristic. It is to be understood that instead of such a characteristic, it is also possible to select a characteristic that deviates from a linear course, for example a characteristic according to which the increase is negative in short regions, in other words, the flow cross-section of the flow channels of the extruders crew increases and in conformity therewith the flow cross-section of the flow channels of the single screw extruder sleeve decreases.
It is particularly expedient that despite the same overall length of the twin screw extruder the thorough mixing is considerably improved. This takes place with surprisingly simple means, whereby tests undertaken in conjunction with the invention have shown that the required power does not differ significantly from state of the art extruders. Due to the shear flow, plainly cold zones of the extruded material are also heated up, so that also with regard to the temperature a homogenization results.