Characteristics Twin screw barrels are characterized by […]
Characteristics
Twin screw barrels are characterized by their double screw design. They can work in a variety of applications and are designed for high output with low wear. They can be used for higher filled formulations and regrind materials. They feature the same basic characteristics as a conventional twin screw, with a few important differences. For example, a twin screw barrel features two radii of rotation, and the maximum wear occurs at an angle between 30 degrees and 60 degrees.
In a screw bearing, a single-phase flow will produce a helical pattern, and two-dimensional flow will produce a spiraling motion. However, in a twin screw barrel, two-stage pressure is produced, rather than a continuous flow. In the latter case, the pressure on the upper plate will be less than that on the lower plate. The force from the upper plate will push the screws towards the barrel wall, creating a circumferential flow pattern in the gap.
Materials
The twin screw barrel is the simplest component of a nut-and-bolt assembly. It can be fabricated from different materials, ranging from forged steel to stainless steel. It can also be constructed from corrosion-resistant materials such as CPM-10V powder metallurgical steel. Depending on the specifications, the screw can also have custom-designed nozzles for liquid injection. Other components in the twin screw barrel include the cooling block and the flanges.
A twin screw barrel features a figure-eight cross-section, derived from the machining of two cylindrical bores. The type of twin screw extruder barrel varies according to the screw arrangement and direction of rotation. Three common screw arrangements are shown in Figure 18: corotating twin screw barrel, intermeshing twin screw barrel, and kneading elements. These screw arrangements are characterized by their high rate of intermeshment and a low shear rate.
Functions
In an extruder, a Twin Screw Barrel performs various functions. For example, it can be designed to extrude a compound directly from the feed and side stuff sections. By bypassing the pelletizing operation, a user can optimize the efficiency of the extrusion process by running starved twin screws at high speeds and maintaining zero pressure gradient along the barrel. However, this can lead to a problem with the quality of the compound, since a starved twin screw will die under low pressure, while a slower screw will stabilize the pressure and yield a lower-quality compound.
The mixing rate inside the channel is similar to that of a single-screw extruder. However, the mixing rate in the overflight/tip region is significantly higher. This region is located between the screw tip and the barrel wall, and undergoes 50 times more shear than the channel. The overflight/tip region is also subjected to high shear, and compression of material entering the overflight region increases its mixing rate.
Durability
A durable twin screw barrel is an important feature of a screw production system. Its construction and materials ensure a longer service life and extended durability. In addition, the process of manufacturing it is highly efficient. There are no quality problems and its production is completed within the stipulated deadline.
Depending on the application, twin-screw extruders can be designed with a number of different screw configurations. Depending on the material to be processed, the screw configuration will determine the durability of the barrel. For instance, the twin-screw barrel designed for pelletizing applications is unlikely to withstand the pressure generated by starved feed. The high-speed operation of the twin screws leads to a low pressure gradient in the barrel, while slower-speed screws result in stabilized pressure and a poorer quality compound.
