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Technology Notebook: Elongational mixing screws boost extruder performance

January 7, 2006

6 Min Read
Technology Notebook: Elongational mixing screws boost extruder performance

Elongational mixing screws, which have only recently become available to processors, promote elongational flow, which improves the efficiency of mixing, particularly dispersive mixing. Elongational mixing reduces viscous dissipation relative to shear mixing. This results in lower motor load and reduced melt temperatures.

The following detailed operational data identify the performance of a 90-mm mixing screw producing cast film and a 130-mm mixing screw with a special barrier section producing filled polyolefin film. Data produced with the elongational mixing screw are compared to a conventional mixing screw. The ability of elongational mixing screws to disperse gels is a particular advantage in film extrusion.

Mixing is one of the most critical functions of an extruder; it strongly affects the process stability and the extrudate quality. The film experiments demonstrated that elongational mixing also offers effective breakup of agglomerates and droplets.

Elongational mixing has been used for decades in devices such as the two-roll mill in which strong elongational flow is created in the nip region. Elongational mixing also occurs in corotating twinscrew extruders, particularly in kneading disks as the material is accelerated in the wedge-shaped region between the leading flank and the barrel. Recently, elongational mixing designs have been developed for screws used in injection molding machines.

This elongational mixing technology, designated CRD (for Chris Rauwendaal Design), is now used in various extrusion and molding operations. The greatest improvements have been achieved in applications in which mixing and good control of melt temperature are critically important.

Examples of such applications involve foam extrusion and color concentrates. Flow is produced by two methods. One way by which strong elongational flow is produced is by using a slanted pushing flight flank. This creates a wedge-shaped region between the flank and the barrel in which the material is accelerated as it is forced over the crest of the mixing flight. This type of mixing is similar to the lobal mixing in kneading disks of corotating twinscrew extruders.

The other method of producing strong elongational flow is by using tapered slots in the screw flights. The wedge shape of the slots causes the material to accelerate as it flows through the slots. As a result, strong elongational flow is also produced in these slots. The flow through the slots contributes not only to dispersive but also to distributive mixing. As a result, the newer CRD mixers rely more on the flow through the tapered slots than over the mixing flights with tapered flanks.

Performance Data

The following data come from two applications. The first case is a 130-mm extruder used in monolayer film extrusion, the second case is a 90-mm extruder used in three-layer coextruded film.

130-mm grooved-feed extruder?-A 130-mm, CRD mixing screw with a flighted length of 31D was used in film extrusion of LLDPE with 50% calcium carbonate. The screw is designed with a special barrier section that eliminates the plugging problem that is common in conventional barrier screws.

At the end of the screw there is a mixing section. This mixing section is designed with multiple flights with a large helix angle. Tapered slots are machined into the flights to achieve effective dispersive and distributive mixing. Since the flights have a forward, helical orientation, the mixing section has forward-pumping capability. As a result, the mixing section does not have negative effect on the output capability of the screw.

The installed motor power was 200 kW. The original screw was a Maddock mixing screw. The CRD screw achieves higher specific throughput (about 45% higher), lower specific energy consumption (about 25% lower), and lower melt temperature. The difference in melt temperature increases when the screw speed is raised.

At the lower screw speeds, the melt temperature difference is about 4-6 deg C, at higher screw speed the melt temperature difference is about 24-25 deg C. The melt temperatures for the two screws are approximately the same throughput.

The actual melt temperatures for the CRD screw are close (within 5-10 deg C) to the barrel temperature. However, for the Maddock screw the melt temperatures are substantially (12-32 deg C) above the barrel temperature. This indicates that there is substantially more viscous heating in the Maddock screw than in the CRD screw. The quality of the film produced with the CRD screw was judged to be improved vs. film produced by the Maddock screw.

90-mm grooved-feed extruder?-A 90-mm, CRD mixing screw with a flighted length of 30D was used in extrusion of coextruded cast film (three-layer). The line has one 120-mm extruder and two 90-mm extruders. It runs a variety of polyolefins such as LDPE, MDPE, LLDPE, and HDPE, in addition to PP and EVA.

The CRD screw was installed in one of the 90-mm extruders. The extruder runs up to 40% regrind, so the mixing capability of the screw has to be very good.

The original screw was a barrier screw. The barrier screw produced excessive thickness variation in the film and buildup of blackened plastic on the root of the screw. The CRD mixing screw for this application included a fluted CRD mixing section in which the barrier flights have a larger helix angle than the main flights; creating a Z-shaped flight geometry (see Fig. 3).

In the first trials, the use of feed-throat cooling led to high throughput with a melt temperature too low to achieve good film quality. When the screw speed was increased to achieve higher melt temperature, the throughput increased, but the film showed signs of interfacial instability.

The solution was to turn off the water cooling at the feed throat. This reduced the throughput and increased the melt temperature to the level required to make a high-quality film.

With cooling of the grooved feed section the output is about 10-15% higher. However, the high output results in a melt temperature that is too low for the process. Adjusting the cooling of the grooved feed section is a method to control the melt temperature without changing output.

Table 2 shows some of the important process parameters comparing performance data of the CRD mixing screw running a blend of polyethylenes.

The material consisted of a blend of 40% LLDPE, 3.0 melt index (MI); 30% LDPE, 2.0 MI; and 30% HDPE, 7.0 MI. In addition, 15% regrind was added to the material. With the CRD mixing screw, it was possible to run the extrusion line at higher line speed and output than was possible previously.

Also, the film thickness variation was reduced to acceptable levels. The results prompted a decision to replace the screws in extruders A and B as well.

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