Efficiency improvements in compounding extruders
Twin screw extruders
For twin screw compounding extruders, an important goal is to optimize screw design in order to transfer energy to mixing most efficiently. Different screw element designs can be used depending on the material processed. For example, when introducing a liquid into the melt, a toothed element is more efficient at mixing very low and very high viscosities than a kneading block, points out Dr. Andersen.
High speed twin screw extruders (TSE) allow higher throughput on smaller extruders through a combination of shaft design and improved materials of construction that allow higher torque and higher speeds. For example, NFM’s new, high-speed TSE can run up to 1500 rpm, compared to 600-900 rpm for older technology, says Butch Noll, national sales manager for extruder manufacturer NFM Welding Engineers, Inc. Because this design generates high shear rates, it is not intended for heat or shear sensitive resins, but is useful for highly filled compounds or colour concentrates, for example. NFM’s TEM Series is being used to produce pelletized concentrates with high fill levels, such as 80-90% calcium carbonate. While these high fill levels had generally been produced in two-part processes such as a continuous mixer and a single screw extruder, high speed TSE can produce these formulations at similar outputs but greater energy efficiency than using two machines. One of NFM’s customers, who was able to replace larger extruders with smaller, high-speed extruders, even chose to reduce the overall size of their building, allowing them to save in utility costs, notes Mr. Noll.
The Leistritz MAXX high speed twin screw extruder machine, introduced in 2004, has a larger outside diameter/inside diameter ratio (Do/Di) of 1.66/1 compared to traditional screws with a Do/Di of about 1.55/1, which increases the free volume by about 30%. The MAXX design uses a patent-pending asymmetrical splined shaft to maintain the same shaft torque despite the smaller diameter shaft, allowing higher throughputs. An asymmetrical geometry transmits power more efficiently into the screw elements by eliminating radial force and increasing tangential force, explains Charlie Martin, president of American Leistritz. To handle higher volumes and output rates, barrel cooling efficiency was increased by adding cooling bores and increasing coolant flow rate. The increased volume is particularly beneficial for feeding low bulk density materials, notes the company.
For twin screw compounding extruders, an important goal is to optimize screw design in order to transfer energy to mixing most efficiently. Different screw element designs can be used depending on the material processed. For example, when introducing a liquid into the melt, a toothed element is more efficient at mixing very low and very high viscosities than a kneading block, points out Dr. Andersen.
High speed twin screw extruders (TSE) allow higher throughput on smaller extruders through a combination of shaft design and improved materials of construction that allow higher torque and higher speeds. For example, NFM’s new, high-speed TSE can run up to 1500 rpm, compared to 600-900 rpm for older technology, says Butch Noll, national sales manager for extruder manufacturer NFM Welding Engineers, Inc. Because this design generates high shear rates, it is not intended for heat or shear sensitive resins, but is useful for highly filled compounds or colour concentrates, for example. NFM’s TEM Series is being used to produce pelletized concentrates with high fill levels, such as 80-90% calcium carbonate. While these high fill levels had generally been produced in two-part processes such as a continuous mixer and a single screw extruder, high speed TSE can produce these formulations at similar outputs but greater energy efficiency than using two machines. One of NFM’s customers, who was able to replace larger extruders with smaller, high-speed extruders, even chose to reduce the overall size of their building, allowing them to save in utility costs, notes Mr. Noll.
The Leistritz MAXX high speed twin screw extruder machine, introduced in 2004, has a larger outside diameter/inside diameter ratio (Do/Di) of 1.66/1 compared to traditional screws with a Do/Di of about 1.55/1, which increases the free volume by about 30%. The MAXX design uses a patent-pending asymmetrical splined shaft to maintain the same shaft torque despite the smaller diameter shaft, allowing higher throughputs. An asymmetrical geometry transmits power more efficiently into the screw elements by eliminating radial force and increasing tangential force, explains Charlie Martin, president of American Leistritz. To handle higher volumes and output rates, barrel cooling efficiency was increased by adding cooling bores and increasing coolant flow rate. The increased volume is particularly beneficial for feeding low bulk density materials, notes the company.
