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Selecting screws for polyolefin processing.

Selecting Screws for Polyolefin Processing

Film processors hav a choice of many different screw designs in meeting their quality requirements. These designs vary from conventional mixing screws to exotic barrier screws with differng barrier section designs. Over the last few years there has been an increase in the use of barrier screws over conventional mixing screws for processing film-grade LDPE and LLDPE in blown and cast film applications. Barrier screws have been claimed to show improved performance over conventional screws, in higher output rates at lower melt temperatures under more stable processing conditions.

The geometries of the different designs are described in this article, and their performances are compared and contrasted. Four screw designs were investigated--a conventional mixing screw and three different barrier screws that represent design concepts widely used in industry.

Conventional Mixing


Of the two general categories of screw designs investigated, the conventional mixing screw is the more widely used throughout industry because of its processing flexibility. Barrier screws are also widely used, but in practice they are usually dedicated to a specific application.

The conventional mixing screw is normally single flighted and square pitched, and its design typically has four distinct sections: feed, transition, metering, and mixing. The feed section is of constant depth and is primarily for solids conveying. The compression or transition section is typically an involute taper that extends from the feed depth to the metering channel depth. The metering section is a channel of constant depth. The mixing section is normally located near the end of the screw and is responsible for either dispersive or distributive mixing.

These screws have been studied extensively and the mechanisms of solids conveying, melting, and pressure generation thoroughly described. The mixing screw used for this trial (Fig. 1) is a high-performance screw of a thermomix design with an Egan mixing section for dispersive mixing and a pineapple mixer for distributive mixing.

barrier Screws

Barrier screws generally consist of feed, barrier, and metering sections with a dispersive mixing section at the end of the metering section. Although the three barrier screws used differ a great deal in overall geometry, they are all based upon the same principle--an additional flight, known as the barrier flight, has been introduced over the transition section of the screw. The two separate channels formed (Fig. 2) are called the solds and melt channels. The solids channel, containing the unmelted polymer, is located on the pushing side of the barrier flight, while the melt channel is located on the trailing edge of the opposite side. The solids channel continually reduces in cross-sectional area over the length of the barrier section while the melt channel correspondingly increases. The barrier flight separating the two channels has a radial clearance larger than the clearance of the main flight. This allows the molten polymer in the solds channel to flow over the barrier flight into the melt channel while the solids are retained, unable to pass over the small clearance. This removal of the melt film helps expose more solds to the extruder barrel surface, thus increasing the rate of melting.

The three barrier designs tested (Fig. 3) achieve reductions in sold channel cross-sectional area and increases in melt cross-sectional area in three different ways. The first screw tested, a Maillefer type design, introduces a barrier flight smoothly over the transition section of the screw. The lead of the barrier flight is greater than the lead of the square-pitched main flight. This geometry forms a solids channel decreasing in cross-sectional area as a function of decreasing channel depth and width. The melt channel, conversely, increases in cross-sectional area as a function of increasing channel width. The barrier flight progresses across the screw channel and ends as the solids channel reduces to nothing and as the melt channel increases to the width of the main channel.

The second barrier screw, a barrier thermo-mix design (BTMI) (Fig. 3), is in a popular category of screws that include the Barr, Willert, and Maxmelt screws. Each of these designs uniquely begins and ends the barrier section of the screw, but all use a similar geometry throughout the barrier section. The barrier flight abruptly starts off the main flight at the beginning of the barrier section at a lead greater than the main flight and travels a short distance until the melt channel reaches the appropriate width. This is approximately one-third the width of the main channel. The barrier flight then remains parallel to the main flight, maintaining a square pitch over the entire length of the barrier section. This forms solids and melt channels of constant width. The cross-sectional areas of the solids and melt channels are varied only as a function of depth. The solids channel decreases in depth as the melt channel depth increases as one travels down the barrier section.

The design of the third barrier screw, a barrier thermo-mix design (BTMII) (Fig. 3), is similar in concept to the second barrier screw design, except that the lead of the screw in the barrier section is increased in order to maintain a wide solids channel. The Dray and Lawrence or Efficient screw is in this category. The lead of the main flight is increased at the beginning of the barrier section as the barrier flight is suddenly started. Once the melt channel reaches the appropriate width, the barrier flight runs parallel to the main flight, maintaining a constant width of solids and melt channels for the remaining length of the barrier section.


Trials were conducted on a fully instrumented, 3.5-in diameter, 32:1 L/D Egan Powerflight extruder under CMR-1000 microprocessor control and data acquisition. The extruder had a 150-hp motor, a top screw speed of 92 RPM, and a strand-type die with an adjustable valve. A 1-MI LLDPE resin was used.

Operating conditions were identical for the four screws. Each screw was run at three speeds, 50, 75, and 92 RPM, with the barrel temperature profile set at 204[deg.]C. Values of the process variables monitored--melt temperature in the center of the melt stream, extruder head pressure, power consumption, and output rate--are given in the Table. The fluctuations over time of the melt temperature and head pressure, shown in the Table, were used to evaluate the stability of each screw design. The melt temperature gradient was determined by measuring the melt temperature with a variable-depth thermocouple at several locations across the melt stream.

Pressure development along the screw and the melting length required to reduce all of the solids to melt were also measured. The data was obtained under steady-state conditions at 92 RPM. Values of the pressure taken along the extruder barrel in the feed, barrier, and metering sections of each screw are shown in Fig. 6. Melting length was determined by suddenly stopping the extruder and immediately applying cooling to the barrels and internally along the length of the screw by screw cooling. After the polymer solidified, the screw was pushed out with a hydraulic jack, then photographed, and the polymer removed for analysis.


The barrier screws achieved higher output rates (Fig. 4) and lower melt temperatures (Fig. 5) than the mixing screw. The barrier screws also required higher torques, i.e., kw/RPM, but achieved higher energy efficiencies, i.e., lb/hr-hp. The Maillefer type screw gave the highest output rates at the lowest melt temperatures, but above 75 RPM gave poor melt quality. This screw design is intended for high outputs and low melt temperatures at low screw RPMs in blown film applications.

The BTMI and BTMII screws showed the lowest melt temperature and pressure fluctuations with time and therefore gave the best process stability. The BTMI and BTMII screws also showed the highest pressure-generation capabilities (Fig. 6) and the shortest melting lengths at 92 RPM. The BTMI had melted the solids after 13-L/D distance down the screw and the BTMII after 14 L/D. The Maillefer type screw at 92 RPM had visible unmelt in the strands. The mixing screw had unmelted solids down to 19 L/D. Therefore, the BTMI and BTMII screws are clearly superior to the mixing screw in melting.


Enhanced melting and superior pressure-generating capabilities have been demonstrated by the BTMI and BTMII screws. Their designs maintain a solids channel of constant width and continually expose the solid bed to the extruder barrel under high pressure, thereby maximizing the melting process. The Maillefer type design constantly reduces the width of the solids channel, causing a reduction in the melting rate. The conventional mixing screw does not have the capability to remove the melt film efficiently from the top of the solids bed. This leads to a reduction in melting rate as the melt film thickens.

The barrier screws also showed the best pressure stability. The barrier solids channel contains the solid bed and keeps it from breaking up, helping to eliminate surging.

Barrier screws offer many process advantages over conventional screws but still have some disadvantages. Unlike conventional screw designs, where computer models have been applied successfully for years to optimize design, barrier screw design still remains an experimental procedure. The screw is normally tailored to a specific melting rate. This reduces the flexibility of processing a wide range of materials on one design. The barrier section must be designed so that the melting rate of the polymer is greater than the rate of reduction of the cross-sectional area of the solids channel. If the melting rate is not matched correctly, instabilities and poor performance may result. Mechanically, the high pressures developed in the barrier section can lead to wear.
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Author:Christiano, John P.
Publication:Plastics Engineering
Date:Jun 1, 1989
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