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Developments in mixing and screw design improve performance |
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Recent developments in mixing technology and screw design offer performance improvements for the single screw compounding extruder. Several new mixing devices have been developed recently that offer the possibility of significant performance improvement of single screw extruders.
CRD mixing devices take advantage of strong elongational flow to improve both distributive and dispersive mixing. These mixers use tapered slots in the flights and slanted pushing flight flanks to create strong elongational flow As per Chris Rauwendaal, some advantages of elongational mixing include:
* Better dispersive and distributive mixing than conventional mixers
* Lower melt temperatures (less chance of degradation)
* Enhanced dispersion of gels
* Practically no pressure drop due to forward motion of melt
Lower pressure and melt temperature fluctuation results to better output stability, dimensional control and product appearance. The greatest benefits are obtained in applications where mixing is very critical such as foam extrusion, highly filled compounds and colour concentrates).
The main benefit of barrier screws is in the final stages of melting. Barrier geometry if used in this part of the extruder will enhance melting performance while at the same time minimizing the chance of plugging. The cost can be lowered due to easy manufacturing of the screw.
Another feature that can improve the performance of a barrier screw is the design of the barrier flight. A conventional barrier screw has a flat barrier flight crest. The flow in the barrier clearance is therefore a shearing type flow. Shearing type mixing has several disadvantages:
* High power consumption and motor load.
* High level of viscous dissipation and high melt temperatures.
* High melt temperatures reduce the viscosity and the stresses within the melt.
* Lower stresses in the melt will reduce the effectiveness of dispersive mixing.
* Particles will rotate in shear flow; this reduces the ability to disperse agglom�erates and droplets.
The CRD barrier section adds a slanted section to the crest of the barrier flight creating a wedge-shaped region. The elongational flow generated in this wedge-shaped region allows more effective mixing because it reduces power consumption, viscous dissipation, and yet increases the stresses in the melt. Further the dispersive mixing action is more effective than in shear flow because there is no rotation in pure elongational flow. Gels can be dispersed in elongational flow while it is not possible in shear flow.
Compounding machines that use barrel pins intermeshing with the extruder screw provide very efficient distributive mixing. The high cost of these machines has limited commercial applications. REE, Inc. has developed a floating sleeve mixing device that achieves highly effective distributive mixing. This vortex intermeshing pin (VIP) mixer can be used in existing extruders without modification of the barrel. The low cost of the VIP mixer allows it to be used in many applications. In injection moulding the VIP mixer can be incorporated into the non-return valve (NRV). Injection moulders can significantly improve the mixing capability of their injection moulding machines simply by replacing the standard NRV with a VIP NRV.
Sulzer Chemtech from Switzerland has the license of the VIP mixing technology.
VIP mixer achieves a full intermeshing action over the full circumference of the screw. In a twin screw extruder the intermeshing action occurs only in a small fraction of the extruder. VIP mixer in a single screw extruder therefore achieves more effective distributive mixing than a similar device in a twin screw extruder. It can also be incorporated into a non-return valve (NRV) in injection moulding applications. A VIP-NRV can significantly improve the mixing capability of injection moulding machines without additional length of the machine. It only requires exchanging the conventional non-return valve for a VIP-NRV. It provides a simple and inexpensive method of improving mixing in injection moulding. Injection moulding has therefore become a significant market for VIP mixing devices.
Foam extrusion has forced many companies to look for ways to increase the cooling capacity of the secondary extruder on tandem foam extrusion lines. REE has developed a new screw geometry that redistributes the CO2 gas in melt which in turn increases heat transfer significantly. The redistribution is achieved by splitting the melt in the screw channel by introducing a flight shift. Hot material moves into the centre of the channel to the screw and barrel surface while, at the same time, compelling the cooler material to the centre of the channel. The HHT screw has achieved significant increases in cooling capacity in actual operations.
The HHT screw also improves distributive mixing, in particular axial mixing, and narrows the residence time distribution. This reduces the chance of degradation. Axial mixing is critical in extrusion of thick walled products. In pipe extrusion wall thickness variation is to a large extent caused by melt temperature variation along the length of the pipe. The result is often seen as �waviness� of the internal diameter of the pipe.
The melt temperature variation that causes dimensional variation is to a large extent short-term variation occurring over time periods ranging from milliseconds to seconds. This type of short-term melt temperature variation is difficult to measure with conventional melt temperature sensors because the response time is usually much too long. One of the best methods of measuring short-term melt temperature variation is infrared melt temperature measurement.
Reducing melt temperature variation by axial mixing devices can reduce wall thickness variation significantly. This allows a reduction in polymer usage and this can result in significant savings in high volume operations. REE has recently developed static mixing devices specifically designed to improve axial mixing. This axial static mixer (ASM) achieves axial mixing with mixer geometry. Most static mixers are designed to improve cross sectional mixing. In many cases, however, the axial mixing is more critical to the dimensional variation of the extruded product than the cross sectional mixing.
The cross section of the ASM device is divided into multiple sub channels of varying size and shape. By designing the sub channel with significant difference in flow resistance significant changes in axial velocity can be achieved.
Presently tests are being conducted with the ASM to quantify the reduction in dimensional variation that can be achieved with this mixing device.
Today it is possible to achieve distributive and dispersive mixing on single screw extruders (SSE) similar to that can be achieved on twin screw extruders (TSE). REE has cooperated with Rollepaal B.V. to develop a SSCE to be used in direct extrusion of highly filled polyolefins and other polymers. This SSCE uses the CRD, VIP, and HHT mixing technology developed by REE.
This compounding extruder can run high filler levels (50-60%) and generate high discharge pressures. It is well suited for direct extrusion applications where the machine can extrude pipe, sheet, or profile without the need for a melt pump. The SSCE uses multiple ports along the extruder for introducing of fillers and additives and for the removal of volatiles.
A compounding SSE offers a number of significant advantages over a TSE:
• Lower purchase cost
• Lower operating cost
• Better pressure generating capability
• Direct extrusion possible without a gear pump
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