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Optimizing extrusion with an adjustable dam.



In three different tests,, raising the adjustable dam increased stability and rate Of melting.

"The Controller" is an invention that permits, by means of its ad"justable dam, micrometer adjustment of internal shear devices at strategic locations in an extruder screw configuration. The device allows for optimization of internal pressure profiles.

Controlling internal pressure with a movable internal shear device downstream of the solids bed breakup defines all extrusion conditions. The optimization of internal pressure controls rate, as well as variations of pressure and melt temperature. In vented operations, vent flow can also be controlled.


Many attempts have been made to control internal pressure; the first probably involved increasing or decreasing the density of screen packs. This procedure could control the head pressure, but because the metering section designs were normally more than capable of overcoming head pressure, the results were increases in melt temperature and consumption of power. Normally, increasing the screen density would cause the screens to clog. The breakup of the solids bed would still occur, allowing the unmelt to reach the screen pack.

Similar results were achieved with adjustable down stream valves: The increase in pressure could vary the breakup of the solids bed but could not eliminate it.

The next device was the Ring Valved Extruder." Its design, shown in Fig. 1, permitted axial adjustment of the screw when the extruder was stopped. On the screw, which moved mechanically, was a male ring (1) at the end of the first stage (2). Between the sections of the two-piece barrel (3) was a female insert (4). The design allowed for first-stage adjustment of pressure, but it was expensive and required stopping the machine. Because of the ring valve's location at the end of the first metering section, it refined the melt primarily through shear stress when the majority of the melting was complete. It also could do little to control the breakup of the solids bed.

Maillefer was probably the first to address the problem of solids bed breakup. His solution was to place an intermediate flight across the channel, thereby retaining the solids bed upstream of this barrier. The design, shown in Fig. 2, is widely used today. The problem lies in locating the barrier and determining its clearance. If conditions change with bulk density, viscosity, or zone temperatures, the position and clearance of the barrier cannot compensate for the changes, because the barrier is fixed. The result can be excessive melt temperatures and, in some cases, instability.

The examples shown in Figs. 1 and 2 appear in many other designs in all types of extrusion, single and twin screw.


In smooth-bore single-screw extrusion, the optimum internal pressure is a maximum point on a curve Fig. 3), the initial slope of which consists of points within the strength of the melt film (to convey) or the feed section design (to generate pressure). Within this slope, rate of melting increases as pressure increases. Up to the maximum optimum pressure, rate either stays the same or increases. Quantitative results vary with the specific screw design, but the general trend does not.

If the pressure climbs beyond either the strength limits of the melt film or the pressure-building capabilities of the upstream sections, the rate decreases. Instability will also occur in some cases, depending on the screw design. The decline in rate shows an increase in melt temperature.

An increase in pressure beyond the maximum efficiency point can have many causes. To ensure peak performance, a Screw design must be able to compensate dynamically for the increase.

Every screw design has only one set of optimum conditions, even if the design is perfect. If changes are made in rpm, bulk density, viscosity, barrel settings, or head pressure, inefficiencies or instabilities may occur.


In today's extrusion, stability has become increasingly important. When a selected screw design has not exhibited the desired stability, processors have selected downstream devices or melt pumps to cope with its deficiencies. An adjustable dam can help achieve the desired stability by eliminating deficiencies and optimizing existing screw designs.

Certain screw designs of grooved feed cylinder-equipped extruders that run viscous materials are capable of developing pressure through the increase in frictional coefficients at the barrel wall. However, the ability to control this pressure has not yet been achieved. Therefore, downstream pressure can be less than the upstream capabilities-resulting in lower than optimum rates. Whereas excessive uncontrolled internal pressures can cause excessive screw wear, controlled internal pressure can optimize screw performance and minimize unnecessary screw wear.

The adjustable dam can be utilized in virtually any process. Of particular interest are processes in which different additives are added to the resin, or in which different viscosities and types of resins must be processed on the same extruder. Optimization of the particular screw configuration can be achieved with each resin.

It is never desirable to change screws in any extruder; in many applications, it is very costly to do so. Therefore, changing screws for different resins is not desirable. The test data show examples of an extruder's running three different resins successfully with the same design.

While the ability to run wide ranges of viscosity without changing screws is important, the ability to optimize any screw design while operating the extruder is of even greater importance, for it permits control of the extrusion operation at various rpm. For example, it is not unusual for an extruder to run well at a given rpm. But if the given rpm is increased only slightly, melt quality suffers, as the particular design loses the ability to control the complete melting. A raised adjustable dam can increase the shear stress applied to the material that crosses over it and thus decrease the temperature gradients. This would, perhaps, allow for higher rpm and an increased rate.

Only one optimum pressure exists for any screw design. A screw design that is running at peak efficiency may not exist. As conditions vary, and if screws are not properly adjustable, peak efficiency may not be recognized even if it is achieved.


The adjustable dam mechanism, shown in Fig. 4, can be mechanically or hydraulically actuated. The device consists of a movable internal shear device or devices, a tapered sled, a drive shaft, a thrust housing, a housing, and an axial movement device.

The drive shaft rotates with the screw, allowing for adjustment of the device during the extrusion operation. Placement of the device(s) is limited only by the imagination of the designer. With computer control, the movement can be controlled through a closed loop system that is actuated by either temperature or pressure.

In a barrier type of design, the adjustable dam would normally be located at the end of the primary channel; in a conventional design, at the end of the transition section. Other possible locations include movable barriers in dispersive mixing devices, and shear devices in twin-screw extruders.


The test data refer to three extruders of different sizes-two are production machines, the third is a laboratory extruder. All three extruders are equipped with an adjustable dam, which is shown in Fig. 5.

All three examples illustrate a particular phenomenon that to the author's knowledge has never been observed before: As the dam is raised, or as the restriction to the flow path of the material is increased, the rate increases.

The first example is described in Table 1. It is a 153.4-mm extruder, L/D ratio 32:1, processing ABS sheet. The feedstock is 50% virgin ABS and 50% ABS regrind. The machine is a two-stage vented extruder with 323 kw 400 HP). Table I shows the conditions that were observed. As the dam was first raised from a full-down position to a clearance of 1.1 mm, the rate increased significantly: from 951 kg/hr to 1144 kg/hr at the same rpm. The amps increased from 326 to 414, whereas the melt temperature increased only from 251.7 degrees C to 253.3 degrees C. Further raising the dam to 2.2 mm yielded an increase in rate to 1182 kg/hr. It is believed that after the first increase in dam height, the head pressure declined because of an increase in melt quality or a reduction in temperature gradients (screen packs were used). Upon the second increase in dam height, the pressure increased.

The second example, shown in Table 2, is a 220-mm extruder with an L/D ratio of 32:1. It processes 6-Ml polypropylene with 22% film scrap, running biaxially oriented polypropylene 22.8 micrometers thick.

The conditions shown in Table 2, Run 1, were observed after the extruder was leveled out. The adjustable dam was then increased from zero to 3.3 mm, and the conditions shown in Table 2, Run 2, were observed. The amps had increased, stability had improved, and melt temperature had not increased. The conditions described in Table 2, Run 3, resulted from a further increase in the height of the dam to 6.6 mm.

At this position, the amps were stable. A fixed infrared pyrometer measured, for 10 min, a melt temperature of 238 degreesC to 239 degrees C (total traverse and inline variation). Runs 1, 2, and 3 had been raised above desired levels (it had been desirable to limit amps below 700), which, in these cases, were adiabatic so as to lower the rate. At the lower settings, 22.7 kg/hr/ rpm were observed.

The third example is an 88.9-mm extruder with a drive of 110 kw, a maximum rpm of 95, and an L/D ratio of 30:1. The extruder is equipped with a computer that incorporates continuous logging of data relative to four pressure transducers Fig. 5), barrel die and melt temperatures, rpm, and amp load. Three different materials were tested for production of multilayer cast film on a single "Concept" design screw.

Table 3 shows the run conditions for the three materials. The first material is 6.5-Ml polypropylene (Runs 1 and 2), the second is a nylon 6 (Runs 3-6), and the third is a 6.0-Ml HDPE (Runs 7-9).

Figure 6, which shows the results of the nylon test, indicates that the amps were not stable when the dam was in the full-down position (Run 3). When the rpm were increased to 60 (Run 4), stability improved. Forty-seven minutes into the run, the dam was raised to 2.2 mm. The amps initially increased and amp stability decreased. At 62 min, the dam was raised further-the result was an increase in amps, rate, and amp stability.

The rpm and amps Fig. 7) and the pressures Fig. 8) of the HDPE test show the effectiveness of the dam. Initially, the extruder was stable at 30 rpm. When the rpm were increased to 60, the rate stayed nearly constant but the stability decreased. At 23 min into the run, the dam was raised to 2.2 mm: The rate and amps increased, melt temperature decreased, and stability improved substantially.

Time limits, imposed because of the amounts of materials available for testing purposes, prevented complete optimization of the three materials. Further adjustment of the two production machines (the first and second examples) produced the additional increases in production and stability that are possible with longer runs. In all designs that have been tested to date, an increase in dam height (downstream resistance) resulted in increased rate and increased stability. The same results would not hold true in all screw designs, because pressure building capability varies with the use of different designs and different materials.

In the third example, times were also limited by the amount of available material. To find the optimum dam height, longer runs would be required. In Runs I and 2, time was not a factor; further tests were performed over a period of time to determine the conditions at various dam heights. In Run 2, a slight lowering of the dam further improved the results. In Run 1, a further increase in height optimized the stability.


An adjustable shear device that is installed in the proper location can quickly assist in the evaluation of a screw design. The adjustable dam can optimize a particular design. If the design does not lend itself to improvement after the dam has been installed, the increased restriction to the flow path provided by the dam will indicate the inadequacies of the design.

This dam permits reexamination of requirements for costly downstream devices. It also permits the testing of further improvements in screw designs without the need for costly screw changes.
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Title Annotation:adjustment of internal shear devices
Author:Dray, Robert F.
Publication:Plastics Engineering
Date:Dec 1, 1990
Previous Article:Machinery and controls.
Next Article:Vented barrel injection molding of PC and a PC/ABS blend.

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