Do you really understand the principle of the barrel combination of twin-screw extruders?

Do you really understand the principle of the barrel combination of twin-screw extruders?

The flexibility of twin-screw extruders allows process engineers to configure the extruder to optimize the process and achieve optimal performance. While most engineers recognize the benefits of configuring the screw elements, the barrel section can also be moved to provide the best layout for the process.

In the past, processes such as single-screw extrusion and injection molding typically used fixed screw and barrel configurations. Once the process was designed, it was essentially set in stone, and any changes would require metal cutting, leading to higher potential costs. For example, adding vent holes to a single-screw extruder would require modifying the barrel and manufacturing new screws, each of which could be very expensive.

In contrast, twin-screw extruders are fully configurable. They can be seen as a series of modular units that can be optimized as needed. Segmented barrels and screws offer flexibility not found in other polymer processes, and the correct arrangement of barrel sections with their corresponding screw elements allows for a wide range of process specializations.

Viewing the twin-screw extruder as a series of unit operations, process engineers have the opportunity to address:

• Solid conveying issues;
• Polymer melting issues;
• Mixing of additives with the melt;
• Liquid feeding issues;
• Additive side feeding issues;
• Exhaust (atmospheric and vacuum) issues;
• Water circulation issues;
• Heat transfer issues;
• Chemical reactions under reactive extrusion conditions.

Here, we will discuss the configuration of the barrel and how each barrel is used for various operations.

Barrel Configuration:

Most of us are not enthusiastic about frequently reconfiguring extruder barrels. However, the placement of certain barrel functions can have a profound impact on the performance of twin-screw extruders and the effectiveness of blending operations. Fortunately, twin-screw extruders offer many cost-effective options. Most manufacturers offer segmented twin-screw barrels, which consist of four, five, or six individually heated and cooled sections. Each barrel section can be independently heated and cooled to provide precise barrel temperature control.

Starting with a bare machine consisting only of a motor, gearbox, and frame, we can analyze the extruder process sections that need to be constructed based on the compound being manufactured. For small laboratory and medium-sized production lines, engineers can frequently rearrange barrel sections as needed to optimize the process during development. Obviously, frequent changes are not advisable for large twin-screw extruders, as frequent movement around large and heavy barrels is impractical. Similarly, while screw changes are rare on large production lines, screw configurations for laboratory machines may change daily.

Barrel layouts can be optimized for blending requirements. Once selected, layouts are usually not changed. Process engineers should be aware that changing barrel layouts is possible, and changes can be considered if the required barrel units do not match the preferred process sequence. However, while this is possible, reconfiguring the barrels is still uncommon.

Open Barrel Segments：

The design of some barrel sections offers the unique configurability of twin-screw extruders. When we pair each barrel with the appropriate screw configuration, we will conduct general and more in-depth studies on each type of barrel in these categories for unit operations specific to that extruder.

Each barrel section has an 8-shaped passage through which the screw passes. The open barrels are designed with outward-facing holes to allow for feeding or discharging volatile substances. These open barrel designs can be used for feeding and venting and can be placed at any position in the entire barrel assembly.

Feeding barrel:

Obviously, material must be fed into the extruder to start blending. The feed barrel is an open barrel designed with an opening at the top of the barrel, through which the material is fed.

The most common placement for the feed barrel is at position 1, the first barrel of the process section. Granules and free-flowing particles are metered with feeders to drop directly into the extruder, reaching the screw. Low bulk density powders typically pose challenges, as air is often entrained with the falling powder. This entrapped air impedes the flow of light powder, reducing the ability to feed the powder at the desired rate.

One feeding option is to set up two open barrels in the first two barrel positions of the extruder. In this setup, the powder is fed into barrel 2, allowing the entrained air to be purged from barrel 1. This configuration is known as a rear vented setup. The rear vent provides a pathway for air to escape, allowing the powder to be fed more efficiently.

Once the polymer and additives are fed into the extruder, these solids are conveyed to the melting zone, where the polymer is melted and mixed with the additives. Additives can also be fed downstream of the melting zone using side feeders.

The barrel used for this operation is called the side feed barrel. It features an "8" shaped hole for the extruder screw and a second "8" shaped opening on the side of the barrel, allowing the side feeder to connect directly to the extruder to fill the additives into the molten polymer. Standard open barrels are typically positioned upstream of the side feeder as vent holes, allowing entrained air to escape.  

A more compact side feed barrel with open vent holes is known as a rear vented combination barrel. It features a large rearward-facing atmospheric opening and a side feed port, as well as a replaceable high wear-resistant CPM-10V powder metallurgy steel liner on the primary processing channel and the wear-resistant side feed port. Internal water cooling channels may be optionally provided.

Exhaust barrel:

Open barrel segments can also be used for exhaust; the volatile vapors generated during the blending process must be removed before the polymer passes through the die. 

The most obvious location for the exhaust port is at the end facing away from the extruder. This exhaust port is typically connected to a vacuum pump to ensure that all entrained volatiles in the polymer melt are removed before passing through the die. Residual vapors or gases in the melt can result in poor pellet quality, including foaming and reduced bulk density, which can affect pellet packing performance.

For extruders with at least 10 barrel segments (L/D≥40), my preference is to place the exhaust port on the last two barrel segments upstream of the die head. Often, if the extruder head pressure gets too high, the molten polymer may backflow into the exhaust port. Pressure can fluctuate during compound runs, especially with tight screens. If the viscosity of the polymer melt is low, the polymer can backflow and flow out of the exhaust port. Positioning the exhaust port in the first two barrel segments upstream of the die head essentially eliminates this possibility, making the operation more stable.

If there are high levels of volatiles present, or if diluents are injected to remove unwanted volatiles, if significant amounts of liquid/vapor byproducts are generated, additional exhaust ports can be added along the length of the extruder in the direction of extrusion, including atmospheric exhaust ports