The screw of the single screw extruder
When the screw of the single screw extruder in accordance with the present invention is situated in the extruder barrel, the transportational screw section is preferably situated between the inlet and the plug and is utilised to transport polymer along the barrel to the plug. It comprises a flight of any appropriate geometry used in the transportation of silicone rubber through a transportational silicone rubber extruder.
The mixing zone section is situated immediately adjacent to the side of the plug furthest away or downstream of the inlet and as such polymer passing through apertures in the plug immediately enter the mixing zone section when exiting the aperture through which it has passed. The fact that the plug is an obstruction in the way of the polymer travelling along the sheet production extruder barrel means that as polymer moves closer to the plug it is under an increasing pressure. However once polymer passes through one of the apertures because there is no further obstruction it enters the mixing zone section at a significantly reduced pressure compared to polymer at the side of the plug nearest the inlet.
Each side arm extruder has an introduction means for introducing additive into the mixing zone. This may take the form of an injection port for transfer of additive into the single screw extruder, which in effect is an obstruction in the pathway of the additive travelling along the side arm extruder barrel and as such as the additive moves closer to the plug it is under an increasing pressure. As only small volumes of additive reaching the injection port, at any moment in time, may pass through the injection port a pressure drop also occurs when additive enters the mixing zone and it is believed that it is the effect of this pressure reduction in relation to both the additive and polymer which enhances their inter- mixing in the mixing zone.
The mixing zone section is deemed to end where flights used for the mixing screw geometry section begin on the screw downstream of the mixing zone. Polymer/additive mixture issuing from the mixing zone section, although already relatively well mixed, is subsequently subjected to further mixing using the mixing screw geometry section to thoroughly mix the additive into the polymer so that a homogeneous polymer /additive mixture is obtained prior to entering the compression section. The flight (s) of the mixing screw geometry section is/are preferably in the form of a double flight with varying flight pitches, most preferably where one flight has a smaller diameter than the other to enable back flow and thereby promoting additional mixing as the polymer/additive mixture progresses along the twin screw extruder barrel towards the outlet.