Introducing a polymer into the extruder
In accordance with the present invention there is provided a single screw extruder comprising a) an elongate barrel having an inlet for introducing a polymer into the extruder and an outlet for discharging extruded polymer from the barrel b) a rotatable screw axially disposed within the barrel for advancing polymer in the extruder from the inlet to the outlet c) one or more additive introduction means for introducing at least one additive at one or more predefined positions along the barrel of the single screw extruder between the inlet and the outlet, wherein immediately upstream of the or each additive introduction means there is provided, on or around the screw, a plug having a plurality of apertures, said plug being adapted such that, in use, the only way for polymer or polymer/additive mixture to move downstream of the plug is by passing through the apertures and that, having passed through the apertures, the polymer or polymer/additive mixture is subjected to a reduction in pressure and is simultaneously mixed, with an additive from the one or more additive introduction means, in a mixing zone section, prior to transportation of the resulting polymer/additive mixture to the outlet.
In use, polymer is preferably fed through the elongate barrel inlet by way of a feeding body having a chamber for optional thermal conditioning of the polymer prior to introduction onto the twin screw extruder and a feeding chamber made for example from case-hardened and/or tempered steel. A feed roller may be utilised to assist in the introduction of the polymer into the extruder. The feed roller may be made of any appropriate material but is preferably made from case-hardened and/or tempered steel and comprises a rotary joint for optional heating/cooling. The feed roller may be provided with a scraper and/or an adjustable blade and polymer may be introduced onto the feed roller by any appropriate means such as a feed hopper.
Preferably there is only one region along the barrel where the one or more additive introduction means is/are situated. In the case of there being greater than one additive introduction means, they are preferably all situated at the same distance along the barrel from the inlet, i. e. in a ring around the barrel feeding into a single mixing zone section within the sheet production extruder barrel. In such cases each additive introduction means can be the same or different. Preferably there is one or two additive introduction means, most preferably there is only one additive introduction means.