Spirex Corporation

Contact: Thomas Kegley

PO Box 9130
8469 Southern Blvd
Youngstown, OH 44512
U.S.

Phone: 330-726-4000
Fax: 330-726-9437

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Products Screws Pulsar Pulsar II MeltPro Z-Mixer ELCee V-Mixer GP Screws Combo Screw Designs MeltPro with Z-Mixer MeltPro with Pulsar MeltPro with V-Mixer MeltPro with Pulsar II SPI Guidelines Barrels Single Feed Housings SPI Guidelines Front End Components Valves Auto-Shut FTR Sliding Ring F-LOC Poly-Check End Caps, Nozzles, Tips Eliminator Nozzle Tip Screws The functions of a plasticating screw are: feeding, melting, mixing, and pumping. The feeding process is the conveying or axial forwarding portion. Solid pellets or powders are pushed forward when the screw turns if there is drag on the barrel wall and slipping and sliding on the screw root. This forwarding action is called plug flow. There are many variables that affect this process. Some examples are coefficient of friction of the resin, pellet geometry, screw feed depth, root finish, feed length, barrel finish and screw, resin and barrel temperature in the feeding zone. The melting process is explained graphically by viewing the melt model. As resin is conveyed forward it picks up heat from the barrel wall and forms a melt film. The melt film is collected when the screw turns and forms a melt pool in the channel. As more melting occurs, the melt pool gets larger and the solids bed gets smaller. Eventually, the solids bed breaks up and is dispersed into the melt pool. Heat transfer is very difficult here and the remaining solids are hard to melt. Unless complete melting occurs, efficient mixing will never happen. Mixing, then, can be in the distributive or dispersive form. Distributive Mixing: * Requires a substantial shear strain with a continuous reorientation of the material * Typically a low shear melt homogenizing process characterized by a series of disruptions to the flow of the polymer melt stream. These directional changes separate and rejoin varying bits of polymer melt tending to cause a uniform blending of the material. * The objection in a distributive mixing section is to mix the fluid by reorienting the flow many times while minimizing the shear stress. (Good for both shear and temperature sensitive materials.) * Analogy: An ocean wave that crashes onto a rocky shoreline. As the wave hits the rocks, energy is absorbed by the rocks and the wave action is reversed, causing a tremendous stirring action of the water, i.e., the more directional changes, the better the distributive mixing. Disperse Mixing: * Mixing of a melted, semi-solid polymer, i.e. the materials yield point has not yet been completely exceeded. * Dispersive mixing is typically associated with a shearing of the polymer to eliminate gels and unmelt and to give added homogenization or blending to the polymer melt. * The objective with this type of mixer is to break down the particle size of the filler and to evenly distribute it throughout the fluid. * Analogy: Buttering a piece of toast with cold butter simulates the type of shear that may be found in a typical dispersive mixer. The more force exerted by the knife on the butter, the faster the butter melts, i.e., the higher the shear rate, the better the dispersive mixing. There are certain rules that must be followed when designing mixers. The general rules of mixing are: * The mixing section should have a minimal pressure drop, preferably by having a forward pumping capability. * There should be no hang-up or dead spots in the mixing section. * The mixing device should completely wipe the barrel surface each revolution. * The mixing device should be easy to clean, preferably by using normal purging techniques. * The device should be reasonably priced. Pumping efficiency can be calculated by using the drag flow equation. Barrels Through our merger with Bimetalix, a leading manufacturer of single barrels, we have strengthened our ability to provide plasticating systems solutions to optimize your plasticating process. We have a complete selection of metallurgy from bimetallic inlays and tool steels to match your specific application.     © Copyright 2002    Spirex Corporation    All Rights Reserved    (330) 726-4000