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Previously, single-screw extruders and injection molding machines typically used fixed screw and barrel structures. Once designed and manufactured, these configurations were largely immutable. For example, adding vent holes to a single-screw extruder required modifying the barrel and manufacturing a new screw, necessitating metal cutting and thus high costs.
In contrast, twin-screw extruders employ modular barrel and screw designs, offering advantages over other plastic processing equipment. This flexibility allows engineers to freely configure the barrel and screw to optimize the process and achieve optimal performance. Most engineers recognize the benefits of configuring threaded elements, but the barrel itself can also be freely configured, providing the best options for production.
For small laboratories and pilot production lines, engineers can frequently rearrange the barrel sections according to mixing requirements for process optimization. If the required barrel units do not match the preferred sequence, changes are considered. Although frequent replacement of barrel layouts is less common and less advisable for large twin-screw extruders, reconfiguration of the barrel is still possible and sometimes necessary in real production scenarios.
Each barrel section has an eight-shaped channel through which the screw axis passes. Open-ended barrel sections have external openings allowing for feeding or venting volatile substances. These open-ended barrel sections can be placed at any position within the barrel assembly for feeding and venting.
1). Feeding
Material must be fed into the extruder to start mixing. Feeding barrels are open-ended and located at the top of the barrel, through which materials are fed. They are most commonly placed in the first section of the process, the first barrel. Particles with good flow properties fall directly from the feed machine into the opening of the extruder’s barrel to reach the screw.
Low bulk density powders present challenges because air can carry falling powder, obstructing its flow and reducing feeding capacity. A solution involves placing two open-ended barrel sections at the front two positions. In this arrangement, powder is fed into barrel 2 while air escapes from barrel 1. This setup, known as a post-venting device, provides a vent channel without blocking the feeding slot, allowing air to escape and improving powder feeding efficiency.
Once polymers and additives are fed into the extruder, they are transported to the melting zone where they are melted and mixed with additives. Additives can also be fed downstream using side feeders, with barrel sections designated for side feeding having an “8” shaped hole in addition to a second “8” shaped opening on the barrel side for direct connection to the extruder for additive filling into the molten polymer. Standard open-ended barrel sections usually serve as vents upstream of the side feeders, allowing air to escape.
A more compact version with open vent holes is called the post-venting combined barrel (see Figure 1). It features an “8” shaped hole to match the side feeder and a small vent at the top of the barrel, facing upstream, for air escape.
Figure 1: This combined barrel features a rear exhaust port and a side feed port.
2). Ventilation
Open-ended barrel sections can also be used for venting; volatile steam generated during mixing must be removed before the polymer passes through the die.。
The most obvious placement for vacuum vents is towards the end of the extruder, typically connected to a vacuum pump to ensure all volatiles in the polymer melt are removed before passing through the die. Residual steam or gas in the melt can degrade particle quality, including bubbling and reduced bulk density, affecting packaging.
For extruders with at least ten barrel sections (L/D ≥ 40), vents are generally placed in the second section upstream of the die. Often, if the head pressure rises too high, molten polymer can reflux into the vent, which is avoided by placing the vent in the third section upstream, ensuring stable production.
2. Closed Barrel Sections
The most common barrel type is the closed barrel (Figure 3), completely encasing the polymer melt on its inner surface, except for a single “8” shaped opening for the screw center.
Figure 3
Once polymers and additives are fully inside the extruder, they are melted and thoroughly mixed. All sides of the closed barrel are temperature controlled (heated and cooled), whereas open-ended barrels have fewer heating and cooling channels.
Lastly, for liquid media, special barrel combinations are used. A common approach involves a liquid injection barrel (see Figure 2), featuring a liquid injection hole on top of a standard closed barrel and a liquid injection needle valve powered by a plunger pump to deliver liquid into the barrel. The delivery rate can be set according to needs.
Figure 2 Liquid feeding combination barrel
3. Assembly Of Extruder Barrels
Typically, extruder barrels are assembled by manufacturers to match the required process configuration. In most mixing systems, extruders start with an open-ended feeding barrel followed by several closed barrels for solid conveying, melting polymer, and mixing melted polymer with additives.
Combined barrels may be placed in the fourth or fifth section for side feeding additives, followed by several closed barrels for further mixing. Vacuum vents are located near the end of the extruder, immediately before the last closed barrel before the die (Figure 4 shows an example assembly).
Figure 4: The barrel configuration of a twin-screw extruder.
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Modular design of barrels has a profound impact on the performance and effectiveness of twin-screw mixing operations. Most manufacturers offer modular twin-screw barrels consisting of ten, eleven, or twelve individual parts, each independently heated and cooled for precise temperature control.
4. Barrel Materials
Barrel materials vary depending on application needs, such as corrosion resistance and wear resistance：
Cylinder α101: Two split alloy cylinders forming an “8” shaped hole liner, meeting basic wear resistance requirements.
Integral alloy liner α101: One integral alloy cylinder, highly wear-resistant.
Nitrocarburized steel 38CrMoAl: High hardness, corrosion-resistant.
HaC alloy cylinder: Superior corrosion resistance, often made as a whole, used for fluoroplastics.
316L stainless steel cylinder: Superior corrosion resistance, rustproof, mainly used in food industry extrusion.
Cr26, Cr12MoV integral sleeve: High-wear resistance alloy material with high cost-effectiveness.
Powder nickel alloy integral sleeve: Extremely wear-resistant hardfacing alloy material with both wear and corrosion resistance at a high cost-effectiveness ratio.
Imported powder metallurgy integral sleeve: Extremely wear-resistant and corrosion-resistant in environments requiring both wear and corrosion resistance.
Nanjing Granuwel Machinery Co., Ltd. has been dedicated to the research and sales of plastic machinery for over a decade, gathering a team of top professionals with outstanding innovation and product development capabilities. The company focuses on the design, manufacture, and optimization of twin-screw extruders and other plastic processing equipment, especially in terms of innovative barrel designs. We provide various materials and configurations of barrels, including open-ended, closed barrels, and specialized liquid injection barrels to meet different process requirements.
Our modular barrel designs enable customers to flexibly configure and adjust based on actual production needs, optimizing processes and improving production efficiency. Additionally, we offer various materials for barrels, such as Cylinder α101, Integral Alloy Liner α101, Nitrocarburized Steel 38CrMoAl, HaC Alloy Cylinder, 316L Stainless Steel Cylinder, Cr26/Cr12MoV Integral Sleeve, and Powder Nickel Alloy Integral Sleeve, to suit different application environments and corrosion and wear resistance requirements.
Nanjing Granuwel Machinery Co., Ltd. aims to improve equipment structure through practical experience and technological innovation, reduce energy consumption, and provide the best products to our customers.