The growing use of matal-locenes, highly filled compounds, and engineering resins- all materials notoriously hard on processing machine components has prompted manufacturers of plasticizing screws and barrels to employ new materials, designs, and manufacturing process. The result for processors is longer service life, in some cases by a magnitude of over 1000%, as well as output gains exceeding 20%.

The secretive nature of screw and barrel markets, and their reservations in reveling technical details, make it difficult to pick out significant trends across the industry. Although based on hard science, trial-and-error plays a large part in development of these components. Hussein Zaire, managing director of Stanley Vickers Ltd., 3 2614 Middles borough, England, says, "Screw design is a black art; continuous process development never stops. None of us can afford to say, 'this screw is the best screw.' It is important for our industry to continue to test new designs and new materials to limit screw wear."

Zarei recently acquired a 7-axis CNC milling machine which he says in the only one of its kind in England. It is integrated into the firm's CAD/CAM systems for production of identical twin screws for extruders. Screws from 20 to 200 mm in diameter and 5 m long can be milled. Vickers has also developed its own hard-facing material, SVL 1215, which is welded rather than sprayed onto screws. This helps screws survive repeated refurbishing, he says.

Screws are developed to match the process

Screw designers are increasingly developing custom screws to enhance plasticizing for particular process. Nick King, sales manager at Battenfeld Gloucester, 3 2624 Droitwich, England, says development continues on the patent-pending Ex Cool screws, primarily used by processors of foamed polystyrene and polyethylene. It is a broken-flight single screw able to mix polymer melt with little transfer of shear heat to the melt. The development hinges on the screw's ability to push the melt against the barrel wall, which ensures a homogenous melt temperature. Processors receive a better mixed resin that allows them to use lower screw speeds, thussaving energy. King says some processors are realizing output gains up to 20%. Six European processors, and more in the U. S., are currently using the screws. Production of the screws is entirely in-house.

At Maplan, 3 2634 Schwerin, Germany, screw designers have come up with a breed of barrier screw expressly for extrusion of recyclate. Processors extruding recyclate experience difficulties with the material's viscosity, which leads to lower productivity or poor parts. These screws are designed as replacements and fit inside existing extrusion machinery with no modification to extruder drive or motor. Maplan officials say processors can see output increases up to 180% as well as raw material savings.

At Davis Standard, 3 2644 Pawcautck, CT, patents are pending on a new breed of barrier screw utilizing a new melting mechanism. The new mechanism is said to better transfer mechanical energy to the solid plug of the material in the solid channel, improving melt performance. As the melt film is removed from the solid bed, force is induced into the melt, which improves mixing by causing melt flow to rearrange.

The support processors of wood-polymer compounds, a special extrusion system has been launched. The system's screw uses multiple compression zones to eliminate moisture. Once moisture is extracted, mixing elements ensure homogeneity of the polymer and wood fiber. The screws handle up to 80% wood fiber content. Both barrier screw and wood-polymer extrusion screw will be available early this year.

Jeff Myers, engineering manager at Glycon, 3 2654 Tecumseh, MI, says the Infuser single-screw mixing device incorporates a modular design offering processors increased flexibility. The mixing section can be applied to extrusion screws and reciprocating injection molding screws. Degree of mixing is controlled by the number of elements added to the screw.

The mixing device incorporates a series of floating rings turning freely as the screw rotates. These floating rings are affixed to the screw. These rings and rotors are perforated with holes facing parallel to material flow. The unattached floating rings turn at a slower speed compared to the screw and fixed rotors. As the rotor turns, the holes in the rings line up momentarily, allowing material to move to the next series of rings and rotors. This facilitates a constant radial and axial division and transfer of the melt, which Myers says lead to thorough mixing without excessive shear or increase in pressure drop.

Xaloy, 3 2664 Pulaski, VA, offers processors a Microsoft Excel spreadsheet system which determines when screws and barrels require replacement. Designed by Hartmut Jahnke, vp. of technology, the Screw Performance Monitor tracks the time the screw needs to deliver melt for the next shot (on injection molding machinery) to the front of the screw. By tracking this time, Jahnke says it is possible for a processor to accurately gauge wear on components. He recommends processors order spare screws and/or barrels when recovery time climbs by 10% to 20%.

Earlier this year, Xaloy opened a screw and barrel production plant in Chonburi, Thailand; the firm now manufactures in Europe, North America and Asia. The most recent barrel developed, the X-802, incorporates a tungsten carbide/nickel alloy using a proprietary manufacturing process. Jhanke says this process produces a denser, more uniformly dispersed level of carbide in the nickel alloy matrix compared to order carbide composite barrels. Xaloy also offers ready-to-install screw/barrel sets including screw tip, barrel end cap, collars, and other necessary parts. These greatly reduce installation time for machine manufacturers.

David Peterson, international sales and marketing manager at Entek Extruders, 3 2674 Traverse city, MI, says the firm is developing a high vanadium, cobalt high-speed steel with Rockwell between 68-71. The material can be hot isostatic processed to oval liners or solid barrels. Screws and barrels made using the materials will allow processors to run machines longer without requiring downtime for screw or barrel replacement. Entek also offers to replace screws and barrels on existing extruders with screws having a larger diameter and barrels with greater bore size.

Peterson says the same centerline distance of the great system is maintained. These changes lead to increased free volume and through put, he says, with increases to 50% in some cases. Processors benefit by increasing output at a fraction of the cost of acquiring new processing machinery. In most cases, no change is required for upstream and downstream equipment.