The introduction of additives
The compression screw section is positioned between the mixing screw geometry section and the outlet and is designed to compress the additive mixture to remove air pockets or the like prior to exit from the barrel typically through a twin screw extruder head. Alternatively the outer edge of the plug is sized such that no polymer may pass between the outer edge of the plug and the inner barrel wall. In the latter case the plug is an annular flight on the screw body and the apertures are formed in the annular flight such that polymer or a polymer/additive mixture upstream of the plug may pass through the apertures in the plug and into the mixing zone. In a further alternative the apertures may be a combination of the above two options, however the first of the above options is preferred for ease of cleaning and the decreased likelihood of blockages in the apertures.
When the apertures in the plug are formed entirely in the body of the plug, each aperture may have any shape but preferably has a circular cross-section with a diameter which is smaller than the width of the annular flight. Preferably there is a plurality of equally spaced apertures around the annular flight of the plug, these may be of any appropriate shape but preferably have the appearance of teeth projecting outwardly from the plug such that polymer would only be allowed through the plug by passing through gaps formed between adjacent teeth and the inner wall of the single screw sheet production extruder barrel. The additive may be introduced into the barrel of the single screw extruder through a single outlet i. e. directly or may be directed from the end of the side arm extruder into a substantially cylindrical channel formed in or using a casing around the main barrel of the single screw extruder which has a plurality of holes connecting the channel to the mixing zone section such that additive may be introduced into the mixing zone section from a plurality of sites to enhance mixing capabilities between polymer and additive, in either case there being a significant pressure reduction when entering the mixing zone section.
Theoretically the volume of a substance travelling along a screw in a single screw extruder is the actual volume which can determined by simple mathematics, however it is to be appreciated that extruder systems operate under relatively high pressures which typically result in slippage in the vicinity of the barrel wall, in turn resulting in a reduction in screw output which reduces as pressure increases and thereby a reduction in apparent"reduced volume"within the extruder increases, the term reduced volume in the above equation relates to such a value.