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.

New to the U.S. market is a 32D twin-screw extruder for PVC pipe that also is adaptable to PVC profiles. The 32D machine in North America contains bimetallic barrels and screws and a modular horsepower motor. Outputs of the 32D compounding extruder go as high as 3,400 pounds per hour, said Artie Riegger, regional sales manager for the extrusion division. North American customers have asked for bimetallic parts because they run higher calcium carbonate levels than in Europe, Riegger said in an interview at his firm's booth.

“We offer complete systems for PVC foam, fencing, decking and sheet,” Riegger said.

KraussMaffei Berstorff also displayed a single-screw extruder with an in-line color analyzer. It is designed to make polyethylene pipe up to 1,600 millimeters in diameter.

The color analyzer could replace old-fashioned ways of drawing samples from the pellets, said Martin Mack, vice president of research and development for the extrusion division of the Florence, Ky., company.

“It detects process upsets at the melt,” Mack noted.

Color adjustments are made at the feeder stage after detecting variations in real time. Work on the system started at the last NPE and since then some installations have been sold in North America and more are in trials.

Twin screw extruders were first used in the 1950s for manufacturing of thermoplastics. Only since the 1970s have these machines been used for feed and food products. Twin screw extruders (TSE) can be classified by their mechanical way of working into co-rotating (both screws turn same direction) and counter rotating (one screw turns right and the other turns left) and the screw configuration into intermeshing self wiping and non-intermeshing. A TSE compared to a single screw extruder has a much wider operating range in terms of use of moisture, internal fat, mechanical energy input and product size.

Extruded aquatic feeds have several qualities. Looking at the physical properties, with TSE the shape of the final pellet can be adapted to the animal’s mouth and calibration of shape and size is easy. A TSE can influence the texture of the pellet varying in hardness, brittleness, cohesiveness, etceteras. Density can also be influenced, resulting in sinking and floating properties adjusted to the fish’s eating behaviour. Finally storage and handling abilities can be influenced regarding fines content and moisture content amongst others.

The biological properties of aquatic feeds are much easier to match with the animal’s needs when a TSE is used. Recipes can be adapted to physiological needs and depending on species and age. Availability of nutrients can be improved when using TSE and extrusion has a significant effect on protein. This protein can originate from plant sources such as soybeans, legumes, glutens and cereal grains. These have good functional properties, relatively low cost, but amino acid profiles may sometimes be lacking some essentials. Furthermore glutens and cereal grains are excellent binders and expand well. Proteins from animal sources such as meat, fish, blood and gelatine have poor functional properties unless they are fresh or spray dried. Costs are usually higher compared to plant proteins, but amino acid profiles are good. Proteins cannot be processed at a too high temperature (>150°C) since this will damage the protein and make it indigestible for the animal.