Views:8 Author:Site Editor Publish Time: 2017-08-21 Origin:Site
The screw extruders are the most important in the pharmaceutical industry so the screw extruders are used more widely than others. The twin screw extruder is one kind of them. By reading this article, you can know more about the twin screw extruder.
The heart of the twin Screw extruder is its screws and barrels, referred to as the process section. Screws and barrels are often manufactured of hard enable stainless steels. Nickel based alloys are specified for corrosive process environments. The metallurgies selected must not be additive, reactive, or absorptive with the materials being processed to be used in a pharmaceutical class environment. Cleaning and storage protocols must be adhered to when handling twin Screw extruder parts.
In a co-rotating, intermeshing twin Screw extruder, the screws are termed “self-wiping”. The surface velocities of the screws in the intermesh region are in opposing directions, which results in the materials being “wiped” and forced to follow a pattern down the length of the Screw.
There are seemingly an infinite number of screw variations possible. There are, however, only three basic types of screw elements: flighted elements, mixing elements, and zoning elements. Flighted elements forward material past barrel ports, through mixers and out of the extruder to pressurize the die. Zoning elements isolate two unit operations. Screw designs can be made shear-intensive or passive, based upon the elements used in the design. Mixing elements can be dispersive and/or distributive, or a combination thereof.
The kneader is the most prevalent mixing element used in a twin Screw extruder. The wider a kneader is the more dispersive it becomes as extensional and planar shear effects occur as materials are forced up and over the land. Narrower kneaders in the twin screw extruder are more distributive in nature that force high melt division rates with significantly less extensional and planar shear effects. Distributive mixing elements can be particularly useful for mixing heat and shear-sensitive materials. Kneading elements can be arranged with a forward pitch, neutral, or reverse pitch. High liquid phase mixing generally benefits from specialty distributive elements that prevent “pooling”of the liquids in the twin Screw extruder process section .