Methoda of extruding material
Extruders are used to force materials, usually plastic materials, through dies and are particularly useful when it is required to work or mix the material prior to extrusion.Sheet production extruder typically comprise a barrel within which rotates a close fitting screw. The screw is made up of screw elements, some of which are helical screw threads to move material through the barrel. Material is introduced into the barrel toward one end, moved along the barrel by the action of the screw and is forced out of the barrel through a nozzle or die at the other end. The rotating screw mixes and works the material in the barrel and compresses it to force it through the die or nozzle. The degree of mixing and work to which the material is subjected, the speed of movement of the material through the barrel and thus the residence time in the barrel and the pressure developed in the barrel can be controlled by the pitch of the screw thread elements, the speed of rotation of the screw and the rate of introduction of material into the barrel.
Extruded material may be treated after extrusion; for example, the material may be dried of water or other liquid. It would be advantageous were it possible to begin this treatment in the single screw extruder. It is known to draw a vacuum on the extruder barrel, for example to flash evaporate liquids within the extruder. A vacuum is applied to the extruder barrel through a port provided in the barrel wall. However, some materials are temperature sensitive so that a sufficiently low boiling point of the liquid to be removed can only be reached at very low pressure. The extruder and its contents can be cooled, but this is inefficient and may still be inadequate to preserve the extruded contents.
According to the invention there is provided twin screw extruder comprising: a barrel and a screw mounted in the barrel for rotation, the screw comprising screw elements and the barrel comprising: an inlet toward one end for material to be extruded and an outlet for the material toward the other end; and a fluid inlet and a fluid outlet in the barrel wall, the fluid inlet and the fluid outlet being separated along the barrel wall in the longitudinal direction thereof and the screw elements being such that material in the barrel is moved more quickly along the portion of the barrel between the fluid inlet and the fluid outlet than upstream of the said portion, one of the fluid inlet and the fluid outlet being connected to a pump so that there is a pressure difference between the fluid inlet and the fluid outlet. Preferably, the fluid outlet is closer to the material outlet than is the fluid inlet. The extruder may be a twin screw extruder.
Also according to the invention there is provided a method of extruding material comprising forcing material through an extruder barrel from a material inlet to a material outlet and additionally flowing a fluid through the extruder barrel from a fluid inlet to a fluid outlet. The present invention provides a method and apparatus which allows drying, that is, partial or total removal of water or other liquid, or other treatment of material to take place in an extruder barrel without the need to subject the material to elevated temperature.