The twin screws are designed to be intermeshing or non-intermeshing.
In the case of the intermeshing twin screws, the distance between the two screw axes is shorter than the sum of the outer radius of the two screws. Depending on the degree of intermeshing, the intermeshing type is further divided into fully intermeshing and partially intermeshing types. As the picture shows in the fully intermeshing type, there’s no gap between the screw flight and the roots of the other screw while in the partially intermeshing type, there’s the clearance between the screw flight and the roots of the other screw.
The fully intermeshing type The partially intermeshing type In the case of the non-intermeshing twin screws, the distance between the two screw axes is equal to or larger than the sum of the outer radius of the two screws.According to the screw rotation direction, the twin screws can be divided into the corotating type and the counter-rotating type.
The corotating type The counter-rotating typeThere are 2-types of counterrotating twin screw: the intermeshing type and the non-intermeshing type.
According to whether the screw axis is parallel or not, the twin screws can be divided into the conical twin screw and the parallel twin screw, where the parallel twin screws are divided into fully intermeshing type, partly intermeshing type, non-engaged type, etc. In the case of parallel screws, the feed end and the die end have the same diameter.
The diameter of the screws in the conical twin-screw extruder is larger at the feed end and smaller at the die end.
The parallel twin-screw The conical twin-screwBoth screws and barrels are designed using the “building block” principle. The screw configuration and barrel setup can be optimally adjusted according to different material properties and process requirements. The “building block” design fits the extruder in various applications. In addition, the special design allows the replacement of a single component, extending the lifetime of the whole extruder and greatly saving the repair cost.
Due to the opposite speed of the two screws rotating in the same direction at the intermeshing area, one screw works to pull the material into this area, while the other screw pushes the material out of this area. The result is that the material is pushed forward in an “∞” shape as it moves from one screw to the other.
Due to the small gap in the intermeshing area, the threads and screw grooves at the intermeshing region moves in the opposite direction, resulting in a high shear rate and a good self-cleaning effect, i.e. it can scrape off any accumulated material adhering to the screw, greatly reducing the residence time of the material. Therefore, the intrtmeshing type co-rotating twin-screw extruders are mainly used for mixing and granulation. The twin-screw extruders for polymer reaction extrusion are mostly fully intermeshing twin-screw extruders among which the intermeshing co-rotating parallel twin-screw extruders is the most common chioce.
The intermeshing corotating twin-screw extruders has low-speed and high speed types. The low-speed co-rotating twin-screw extruders are used for profiles while the high-speed co-rotating twin-screw extruders are used for special compounding and reactions.
In an intermeshing counterrotating extruder, the two screws are symmetrical. Due to the different rotation direction, the path of material spiral forward on one screw is blocked by the screw flight of the other screw. Thus intermeshing counterrotating extrudersit cannot form “∞”-shaped material movement.
In the solids conveying section, the material is conveyed forward in the form of an approximate closed “C” shaped chamber. However, the design leaves a certain clearance between the outer diameter of one screw and the root diameter of the other screw so that the material can pass through.
Intermeshing counterrotating extruders peforms welll in conveying and extruding. The extrusion volume in the same diameter is about 1 times higher than that of the co-rotating extruder in the same diameter. Besides, shorter residence time of the material, less heat, fully dispersed materials and homogeneous temperature are achieved.
When passing through the radial clearance of the two screws, the material is subjected to strong shear, mixing and compact forces which increase the plasticizing rate of the material. The desired compression ratio can be achieved by changing the gaps between the screw flights. The intermeshing counter-rotating extruders are often used in process tasks like extruding and granulating.
This extruder with two parallel conveys material by friction and viscosity.
The non-intermeshing screws have a relatively large gap in between, resulting in a large leakage flow. Hence, the non-intermeshing type has fewer applications than the intermeshing type.
Compared to single screw, the non-intermeshing screws increase material exchange between screws but reduce positive displacement flow.They are suitable for mixing, venting, de-volatilizing and chemical reaction.
The non-intermeshing twin-screw extruder is mainly used in the preparation of reactive extrusion, melt or emulsion production of polymers, coloring, glass fiber reinforcement and hot melt adhesives, and is usually used when the melt content is up to 50% and 4500-6750 kg of material needs to be dried per hour.
Conical screws decrease in size as you approach the tips. Usually, the two screws rotate in opposite directions.
In a conical twin-screw extruder, the volume of the C-shaped channel formed by the two screws and the barrel gradually becomes smaller from the feeding section to the discharge section as the diameter of the screws becomes smaller. Powder materials in big bulk will be introduced into the feeding zone and conveyed by the turning motion. As the screws become smaller, the materials are compacted and melted. At the discharge zone, the lower screw circumferential speed due to the smaller diameter of the screws results in a lower shear rate which means less temp rise.
Conical twin-screw extruders are suitable for processing heat-sensitive materials, so mainly used for processing PVC powder directly into products. For lower output extrusion production, conical twin-screw extruders with small end diameters are generally used, as conical twin-screw extruders are considered an economical choice due to their power- and cost-saving features.