Direct extrusion through a gear pump - the next logical step in rubber extrusion. (Cover Story).

The benefits of extrusion through a gear pump, i.e., very effective pressure build-up and constant throughput with the thereby associated dimensional accuracy of the extrudate, are well known to the plastics/thermoplastics field where gear pumps have long since been effectively utilized. Only in recent years, however, have these applications been extended to the rubber industry, with development activities intensifying, in particular in the last three years. Initial difficulties were experienced in actual implementation in the rubber extrusion process mainly due to vulnerability of the gear pump and the high viscosity of the compounds. Consequently, many applications required straining prior to the gear pump to prevent contaminants, such as nails or wear from the mixer, for example, from entering the gear pump and damaging the rotors. Furthermore, throughput limitations due to gear pump sizes have in the past limited the application to, for example tire extrusion lines. An extruder/gear pump combination system with the necessary robustness and throughput ranges specifically for new tire extrusion applications has now been developed and successfully tested. These tests have verified that a shortened 120 mm Transfermix extruder and 110 mm gear pump combination delivers throughputs ranging between 1,800-2,200 kg/h for standard SBR compounds and between 1,000-1,600 kg/h for highly viscous NR compounds (with Mooney values of ML (1+4) > 100), which is comparable to the throughput of conventional single screw 150 mm extruders.

Extrusion objectives

One of the principal objectives of extrusion is the effective plasticization of a given compound for the production of profiles with dimensional stability, smooth surface characteristics and exact die geometries. Additionally, this is to be achieved at highest possible throughputs at temperatures below the scorching point of the extrudate. To achieve this the processing operations performed by extruders are of extreme importance. These can be divided into the following main areas of feeding and transporting the stock; homogeneously heating and plasticizing it to the required processing temperature; and subsequently creating the necessary pressure to process the compound through the die.

The Transfermix extruder, a single screw extruder, is an effective mixing, homogenizing and plastifying system for rubber extrusion for a wide range of rubber compounds. This can be attributed to its unique barrel and screw configuration. Extruders of this type have a geometry which, by means of reducing and increasing flight depths in screw and barrel, transports the compound out of the rotating screw into the stationary barrel having concentric opposite-handed threads and back into the pressure build-up section of the screw (ref. 1). The receding flights of the screw interact with the increasing channels of the barrel in such a manner so that the extrudate is partially forced out of the screw into the barrel and hence sheared normal to the flow already transferred into the barrel (ref. 2). This configuration enables extremely effective mixing of the compound in a very short section (approximately 2D) of the extruder compared to conventional single screw extruders, allowing for a very compact overall L/D ratio of between 10-12D.

Limitations of the extrusion process

However, due to the tangential feeding of the extrudate in strip or sheet form, the extrusion process is subjected to pulsation and pressure fluctuations that are largely difficult to resolve and often inadequately compensated for in the extrusion system. The use of feed rolls is a good example of additional features to the extrusion system aimed at minimizing pulsation by, in this case, ensuring more constant feed conditions. In part, the pulsation can be attributed to the drag and pressure flow properties within extruders that are essential for conveyance and consequently the resulting pressure build-up. The flow properties are strongly dependent on the feed rate. Feed variations and surging of the extruder result in pressure fluctuations at the extrusion head which invariably lead to a reduction in extrusion quality and increased scrap.

Besides being influenced by the rheological properties of the compound, the performance of the extruder is thus very much dependent on the die pressures needed. The additional energy input required for pressure build-up leads to an increase in the temperature of the compound. As the die pressure increases, the throughput decreases, thus reducing the flow of material through the extruder. This in turn increases the residence time of the compound. Hence for higher die pressures there is an increased possibility of scorching of the compound due to the increased absorption of heat energy. The maximum throughput attainable for any given extruder size is in this manner often limited by the maximum allowable processing temperature of the compound (ref. 3).

Extrusion through a gear pump

By combining the highly effective plasticizing properties of the Transfermix screw and barrel configuration with the volumetric feeding properties of a gear pump, many of these undesirable effects can be eliminated. In this configuration, schematically depicted in figure 1, the excellent plasticizing properties of the Transfermix extruder are used to effectively masticate the compound and feed the gear pump, while the pump is used to generate the high die pressures without generating excessive temperatures. In this configuration, the extruder and gear pump mutually complement each other. While the Transfermix screw performs an excellent plasticization job, it lacks in conveying efficiency. Vice versa, the gear pump is a highly efficient conveyor of material; however, it lacks in terms of plasticization capability.

[FIGURE 1 OMITTED]

By using the gear pump for pressure build-up, less demand is placed on the extruder which leads to an increase in throughput for comparative extruder sizes. The extruder is used only to achieve effective mastication and to ensure sufficient feed to the gear pump. Consequently, the feed to the gear pump and therefore the throughput of the entire system is increased as less work energy is wasted for pressure build-up. One of the major benefits of extrusion through a gear pump is the "freeing" of the extruder, and test results have verified that throughput figures of the combination of extruder/gear pump are comparable to throughput figures of a single screw extruder one size up (ref. 4). In other words, a 120 mm extruder gear pump combination will deliver throughputs comparable to a stand-alone 150 mm extruder. Furthermore, this throughput is linear, as it is directly related to the pump speed, thereby enabling better control and predictability.

A further significant advantage is that the gear pump, due to its volumetric feeding properties, simultaneously compensates for and reduces pressure fluctuations in the system by a factor of about 10. A constant and reproducible throughput in turn ensures a constant head pressure, enabling far superior extrusion uniformity in comparison to current extrusion technology capabilities. This is particularly significant when taking the change over effect of the feed strip into consideration, where a palette change, for example, clearly results in significant weight scale variations for periods of up to more than a minute (ref. 5). Due to the chamber conveyance of gear pumps whereby small volumes of material are displaced with minimal friction exerted on the compound, the conveying efficiency is significantly improved (lying between 90-98%) so that a marked reduction in temperature in the pressure build-up zone is achieved. The risk of scorching the compound is consequently reduced.

Based on the volumetric flow principle of gear pumps, the energetic contribution to the pumping efficiency constitutes up to 40%, depending on the pump design, while for single screw extruders this lies between 15-25% (refs. 6-8). The amount of dissipative energy brought into the compound by the gear pump is thus decidedly less than in the extruder, which is why the gear pump is used for pressure build-up, while the extruder is far more effectively used to perform the plasticization. Consequently, at equal back pressure, the increase in temperature caused by pressure build-up is far less in gear pumps, enabling operation of the system at lower temperatures. This, in turn, also enables increased throughput, as extrusion speeds are comparatively higher before reaching temperature limitations.

Principal operation

The principal operation of the gear pump is schematically depicted in figure 2. The material flow delivered by the gear pump is dependent on the chosen rotor speeds and the displacement volume per revolution. In order to ensure the uniformity and dimensional accuracy of the throughput, a constant delivery of homogeneous plasticized material is vital to ensure that the gear teeth cavities are completely filled at the pump inlet, which is why an extruder is used to feed the gear pump. In this manner, equisized volumes of material are transported along the gear pump housing and displaced into the outlet by the meshing of mating gear teeth. Given the complete filling of the gear teeth cavities at the inlet, pressure fluctuation are hereby greatly reduced.

[FIGURE 2 OMITTED]

By driving the gear pump and extruder separately, a high degree of flexibility is attained. In this configuration, the extruder is driven as a slave to the gear pump. The extruder screw speed is controlled by the chosen inlet pressure set point [P.sub.in] (figure 1) via a proportional-integral-differential (PID) controller which regulates the control loop. When changing the throughput of the system by adjusting the gear speed of the pump, the extruder speed is automatically regulated to deliver the set inlet pressure [P.sub.in] and the corresponding mass flow to the gear pump. The gear pump is thus run independent of the extruder, and the configuration can be optimized for various die pressure requirements.

The mastication of the compound takes place prior to entry into the gear pump and is dependent on the layout of the Transfermix zone and on the chosen processing parameters. The reduction in compound viscosity caused by the increase in temperature results in improved flow properties, which in turn facilitate the complete filling of the gear teeth cavities at the inlet of the pump. Without sufficient mastication of the compound, the pump cannot be effectively filled, thereby nullifying the features that make it so attractive. Complete fill of the gear pump cavities is therefore of paramount importance to ensure uniform head pressures that are free of pressure fluctuations or pulsation.

Experience has shown that the required minimum inlet pressures necessary for the A-Z system to assure 100% filling of the gear teeth cavities range between 10-30 bar. This is a comparatively small minimum back pressure the extruder has to work against when considering typical die pressures of between 50-200 bar in conventional single screw extrusion. With the extruder no longer needing to overcome such high die pressures, but nevertheless having this capability, it is thus evident that the extruder/gear pump combination is a highly flexible system that better enables the mixing effect, i.e., the degree of homogenization of the compound to be controlled. This is particularly useful for the processing of more difficult compounds like highly viscous natural rubber compounds.

Test results

Figure 3 illustrates the behavior of the system when the gear pump speed is maintained constant and the inlet pressure [P.sub.in] increased. From the results it can be clearly seen how the PID controller steps up the extruder screw speed to provide the set inlet pressures. All results were obtained during trials run with customers. In this particular trial, the initial gear pump inlet pressure was set at 35 bar, with a constant gear speed of 5 rpm. During two consecutive adjustments, the inlet pressure was increased first to 55 bar and subsequently to 75 bar. As the set points are adjusted, it can clearly be seen how the extruder screw rpm increase from 13 rpm to 18 rpm and again to 23 rpm to deliver the required pressure set points. Due to the fact that the output of the system remains constant (the gear pump speed remains at 5 rpm) and therefore the residence time of the compound, and assuming that the extruder is always completely full, this implies an increase in the shear rate of the compound. The plasticization process of the compound can thus be influenced. The gradual increase in inlet temperature [T.sub.in] of the compound (figure 1) as illustrated by the middle line in figure 3 verifies this.

[FIGURE 3 OMITTED]

This can be further substantiated by consideration of a theoretical divisional factor K that describes the number of theoretically possible divisions of material making up a defined volume per kg of compound in the Transfermix extruder (ref. 9). The divisional factor K is derived from the extruder throughput m, screw speed n and a Transfermix specific constant C, in turn derived from the number of flow channels in the particular screw and barrel configuration of the Transfermix zone as per the equation:

K = C * n/m

C = machine constant

n = screw speed (rpm)

m = throughput (kg/h)

The factor K defines the theoretical number of partial elements or partial volumes a kg of compound situated in the Transfermix zone is divided into by shear activity when flowing through the zone. By increasing the screw speed n while maintaining a constant throughput m, it can thus clearly be seen that the divisional factor increases, and therefore also the shear rate, thereby leading to improved mixing/homogenization in the extruder.

In conventional single screw extruders without gear pumps, this control of the mixing effect is not possible. An increase in screw speed invariably leads to an increase in pressure and throughput. The increase in throughput in turn negatively impacts upon the mastication properties of the extruder due to a reduction in the compound residence time. So much so that often sufficient homogenization is only realized by way of throttling, which again reduces the throughput.

The combination machine, however, is not bound to these limitations and enables its users far greater flexibility for process optimization. The extrusion through a gear pump ensures that the performance of the extruder is no longer limited by the needed die pressures. The minimal inlet pressure setting for any specific rubber compound is determined with the sole requirement to ensure complete filling of the gear teeth cavities at the pump inlet, regardless of the die pressures. In comparison to a single screw extruder without gear pump, this differs in that the maximum pressure build-up to be delivered by the extruder is independent of the operating die pressures and can be set to optimize the homogenization while maintaining acceptable extrusion temperatures.

A further significant advantage is the fact that the throughput is directly proportional to the gear pump speed, and thus linear. This, again, is due to the volumetric flow principle of the gear pump, whereby the conveying efficiency is not subject to the conventional drag flow properties governing the performance of single screw extruders.

Throughputs are therefore far more predictable. Assuming homogeneous conditions for the entire system, the theoretical throughput can easily be calculated using the displacement volume of the gear pump, the density of the compound, the gear pump speed and the pump efficiency coefficient (ref. 8). Figure 4 depicts the throughput of different rubber compounds ranging from synthetic rubber stock to highly viscous natural rubbers, run at varying gear pump speeds, in comparison to the theoretical throughput value illustrated by the solid line.

[FIGURE 4 OMITTED]

The results clearly show that the conveying efficiency is largely independent of the compound characteristics, which is further verified by comparison of theoretically calculated outputs to those obtained. As long as the material has been sufficiently plasticized and the gear teeth cavities completely filled, the throughput behavior of the pump remains linear. Under these conditions, the efficiency is primarily dependent/determined by the chosen clearance tolerances of the gear teeth and housing, as well as the rotor play (bearing play).

Furthermore, changes in head pressures due to adjustments of die dimension, for example, have a minimal effect on the throughput. Figure 5 illustrates the measured throughput obtained at three constant machine set-points after a reduction of the die geometry from 200 x 5 mm to 50 x 5 mm, illustrated in the relevant pressure ranges between 120 to 200 bar. The corresponding reduction in throughput at the various set-points ranges between 2% and 4.4% at a pressure increase of up to 45 bar. Taking the absolute pressures into consideration under which the tests were run, it is evident that the extruder/gear pump combination exhibits far superior pressure-throughput, as well as temperature-throughput properties, in comparison to a single screw extruder without gear pump. During the prototype testing phase, where the machine was run under real production conditions, comparative throughput figures for 150 mm single screw extruders (ref. 5) were achieved by the 120 mm extruder/gear pump combination.

[FIGURE 5 OMITTED]

Mechanical advantages and features

The decrease in compound viscosity due to the increase in temperature in the Transfermix zone results in a lower energy input requirement for the gear pump. The resultant forces in the gear pump should therefore in turn be considerably lower than during cold feeding. This clearly results in a reduction in mechanical wear. As has been the experience in the extrusion of thermoplastics, the use of a gear pump can significantly prolong the life of the extruder screw to almost double (ref. 10), as it is no longer subjected to extreme back-pressures and associated wear.

Additionally, the use of a feed role to dampen feed fluctuations is no longer required, due to the attenuating effect of the gear pump. In the event of processing difficult compounds, the gear pump may act as a conventional throttle as used in standard Transfermix extruders. Unlike conventional throttling, however, there are no losses in throughputs, and the system exhibits much improved temperature control. For processing an even greater variety of compounds, a pin zone prior to the Transfermix zone has also been added. An optional screen is situated after the gear pump for potential straining applications. The positioning of the screen is a notable difference to many existing gear pump configurations which aim to protect the pump from foreign metallic objects by placing a screen in front of the gear pump due to sensitivity of the gear teeth. The A-Z design has been conceptualized for robustness so much so that in the presence of customers, single pallet nails (3.1 x 70 mm) have been processed through the pump without notable adverse effects.

An additional significant advantage is the compactness of the machine, especially in view of typical space limitations in piggyback applications. Particularly attractive is the compactness of the gear pump, especially in terms of its height. Accordingly, applications in duplex, triplex or quadruplex systems are highly feasible. Furthermore, the gear pump is mounted on linear guide rails and easily separated from the extruder, allowing quick access to facilitate cleaning, maintenance and inspection. Figure 6 illustrates this ease of access. Specifications for the extruder/gear pump are given in table 1.

[FIGURE 6 OMITTED]

Conclusion

With today's ever increasing demands on manufacturing processes which require increased dimensional accuracy, improved productivity and process flexibility, the advantages of extrusion through a gear pump appear to point to the indispensable inclusion of gear pumps in the next generation of extrusion equipment. While certain limitations in single screw direct extrusion (such as dimensional stability) can only be inadequately compensated for, and others (such as throughput improvements) can only be resolved by moving one extruder size up, inclusion of a gear pump, either as retrofit or included in new equipment, offers great potential advantages. These include:

* Profile extrusion with dimensional stability and thus reduction in scrap: The gear pump reduces pressure fluctuations while building up the required die pressures, and thus decouples the system from feed fluctuations or surging of the extruder. Process parameters are also precisely reproducible and far greater predictability enables increased product quality.

* Increased throughput and thus improved productivity: Due to the fact that the gear pump is used to generate the needed die pressures, less demand is placed on the extruder, which is used only to provide the mastication work and feed the gear pump. This means more effective plasticization by the extruder without additional energy being needed for pressure build-up, and thus allowing improved feed. Due to the chamber conveyance of the gear pump, a marked reduction in temperature in the pressure build-up process is achieved, thereby reducing the risk of scorch. In turn, this means lower extrusion temperatures, allowing higher speeds to be run, and thus more throughput to be achieved. Trials run on a 120 mm extruder/gear pump combination have verified throughput figures attained by a 150 mm single screw cold feed extruder.

* Greater process flexibility: The throughput of the system is dependent only upon the gear pump speed, and therefore linear. The system regulates the extruder to deliver the required inlet pressure to the gear pump. This pressure can be varied, and accordingly the extruder screw speeds can be changed without affecting the throughput. This allows a higher degree of flexibility as tougher compounds can be subjected to increased mixing by the extruder. Essentially, the gear pump acts as a conventional throttle, however, without the associated loss in throughput.

* Mechanical advantages: The designed system is extremely compact and robust. Applications in duplex, triplex or quadruplex extrusion are thus highly feasible. Sensitivity to contaminants such as pallet nails has also been tested. Although such contamination is clearly to be avoided where possible, the design of the gear pump is such that straining prior to the gear pump is not necessary due to the robustness.

* Commercial advantages: The greatest commercial significance of the system is clearly the improvement in process quality at a reduced product scrap rate. This is achieved by a system that, in terms of capital investment, lies within a cost bracket equal to or less than the costs for a standard 150 mm extruder. The fact that a smaller extruder/gear pump combination achieves the same throughputs and line speeds of its next size up extruder counterparts is of major commercial significance. Additionally, due to the fact that the gear pump is a much more efficient and effective device for pressure build-up, and due to the fact that the extruder no longer is required to overcome the die pressures, an overall reduction in power consumption is also achieved.

Table 1 - technical specifications

Parameter                 MCTD                 Gear pump

Diameter                  120 mm                 110 mm
Maximum throughput        1,800 - 2,200 kg/h   SBR compounds
                          1,200 - 1,600 kg/h   NR compounds
Maximum outlet pressure                          300 bar
Overall dimensions        Length:              3,060 mm
                          Width:               1,840 mm
                          Height *:            2,033 mm

* Height includes the feed hopper

References

(1.) Fischer, F.W., A-Z Formen- und Maschinenbau GmbH, "MCTD extruder techology - The concentric double screw extruder in application to modern tire tread compounds."

(2.) Fischer, F.W. and Hohl, M.W., A-Z Formen- und Maschinenbau GmbH, "MCTD extruders: From theory to practice," Rubber World, July 2000.

(3.) Nijman, G., KGK 53, (2000) 438.

(4.) Nijman, G., KGK 53, (2000) 484.

(5.) Nijman, G., "Technologie der extrusion von kautschukmischungen," paper presented at workshop at the DIK, December 1999.

(6.) Uphus, R., "Integrated gear pump for single screw extruder, "paper presented at a workshop at the DIK (2000).

(7.) Fischer, F.W., "New developments in extrusion technology, "paper presented at a workshop at the DIK (2000).

(8.) Schuler, W., Doctoral Thesis, University of Wales Swansea (1996).

(9.) Meiertoberens, U., Michaeli, W., I.K.V. University of Aachen, "Untersuchung eines Konzepts zum fertigmischen von kautschukmischungen auf basis des neuen Transfermix," April 1994, Vorschungsvorhaben Nr. 8,877.

(10.) Brunner, D., Eprotec Polymer Processing Technology AG, "Gear pumps for processing efficiency."

by Olaf Skibba and Oliver Thoma, A-Z Formen- und Maschinenbau
GmbH