How many factors need to be considered when designing twin screw extruders and barrels?


Shear regions in a twin screw extruder The corotating t […]

Shear regions in a twin screw extruder
The corotating twin screws of a twin screw extruder produce a self-wiping effect, which sequentially discharges a small volume of material. This effect leads to a uniform deformation history for the screw, which is associated with a residence time distribution. Closed barrel regions in a twin screw extruder are generally characterized by a tight RTD while those with open ends are characterized by a wide RTD.

The pressure gradient is higher in the left-handed screw element than in the right-handed screw element for the same flow rate. The difference between the two pressure gradients is about 50%. The pressure gradient in the left-handed screw element is constant at 4Q*/3, while the right-handed screw element increases continuously. This gradient is due to the drag force of the left-handed screw element. This pressure gradient is referred to as the transverse shear rate.

The mixing rate in the channel is similar to that of a single screw extruder. The mixing rate in the overflight/tip region, which is located between the screw tip and barrel wall, is approximately 50 percent higher than the channel. The overflight/tip region undergoes high shear rates, and the lobal pool is at least 50% greater than the channel. Moreover, the mixing rate in this region increases with the compression of material in the overflight region, which results in a strong extensional shear effect.

Design options for a twin screw barrel
Twin screw extruders can be customized to accommodate the unique properties of different materials. They can be designed to promote unit operations such as melting, mixing, devolitalization, pumping, and extrusion. The twin screw extruder has many advantages over single-screw models.

A dual-screw extruder's design provides advantages and disadvantages in different manufacturing environments. The twin screw barrel section is available in several basic configurations, including solid-feed, liquid-feed, and side-feed. The design can incorporate closed or venting barrel zones for high-pressure compounding. It can also be configured to incorporate pumping, mixing, and kneading zones.

Twin-screw extruders can be designed with a range of screw diameters, ranging from 25 to 244 mm. The length-to-diameter ratio of a twin-screw barrel is typically 39-48. This modular design makes it possible to make changes as required.

Upstream feeding and downstream system options
Upstream feeding and downstream system options for twin-screw barrel extruders are based on the material flow characteristics. Upstream feeding requires the material to pass through a feed intake zone. In this zone, the speed and pitch of the screw elements determine the conveying capacity. Downstream feeding is also possible with high filler loadings. Combined with upstream feeding, downstream feeding can increase throughput while reducing barrel wear.

When installing a compounding system, medical manufacturers should consider critical design factors such as the type of material to be processed, the expected run size and the nature of the plant where the equipment is located. Upstream feeding and downstream system options are just as critical as the choice of corotation and counterrotation. Shear intensity is one of the most critical design considerations, and users should carefully consider each option before making a decision.

The twin screw extruder is designed for both direct and indirect extrusion. Direct extrusion offers dispersive mixing benefits because the materials are mixed without high-shear stress. The twin screw design of the extruder is the core of its operation. Its flighted elements provide excellent mixing characteristics. Depending on the material type, the extruder can be pre-mixed with the ingredients in a batch-type mixing system, or volumetrically fed into the main feed port. Upstream feeding can be done manually, or loss-weight feeders can feed multiple streams simultaneously.