Extruder/gear pump combinations for processing.

Gear pumps are used in the extrusion of rubber compounds as upstream equipment when the product being extruded has to conform to extremely close tolerances and high outputs are required at the same time. At K'98, Berstorff displayed a prototype of a newly developed high-performance gear pump equipped with anti-friction bearings, which for the first time completely eliminated the incidence of wear in the rotor shaft bearings encountered with conventional gear pumps.

The ZP gear pump thus met with great interest at the exhibition and has established a strong position on the market during the last few years. The latest product development is an extruder with an integrated gear pump designed for the processing of rubber. The fact that with this system the pump is driven by the screw may turn out to be the highlight of the development.

Advantages of the gear pump

The main advantage offered by a gear pump as compared to single-screw extruders is its volumetric conveying principle. This results in a higher conveying efficiency and a significant reduction of pressure fluctuations. The relatively high conveying efficiency of a gear pump allows a good pressure build-up at a low stock temperature to be obtained. In production terms, this means that the output can be increased at a given maximum stock temperature while the profiles present an excellent degree of dimensional accuracy.

Gear pump

The core of the Berstorff gear pump is a pair of gear shafts with double helical gearing (figure 1), to prevent axial forces from acting on the bearings of both shafts. This means that the bearings have to absorb radial forces only.

[ILLUSTRATION OMITTED]

A sealing system with return thread is provided at both sides of the double helical gearing. In order to avoid excessively long residence times and the risk of scorching of the compound in the return threads, a defined material flow (approx. 0.5% of the total throughput) is led outside via the return threads. This material is sheared into small pieces between the return thread and the anti-friction bearing and falls into a collecting container. Thus, the compound cannot contaminate the anti-friction bearings. In addition to this, the gear pump can be easily installed and dismantled, which facilitates cleaning.

Applications of the gear pump

The extrusion of profiles, cables and hoses is a typical field of application for the gear pump, as with these tasks the tolerances and the quality of the final product are mainly determined by the extrusion process. In most cases, degassing extruders are used in order to produce pore-free extrudates with continuous vulcanization at atmospheric pressure. The pressure build-up required for profiles with small cross sections is often very high.

In this case, the advantages offered by the operating principle of the gear pump can be fully utilized. As the pressure build-up capacity is up to three times higher than with extruders, the production can be carried out at lower stock temperatures or - in the case of given stock temperatures - with higher outputs.

In addition to this, the risk of compound rising in the degassing dome of the extruder in the event of changed rheological compound properties is eliminated. The screw section arranged downstream from the vacuum zone is simply required to ensure complete filling of the gear pump.

Owing to the higher number of flow volumes, the gear pump dampens the pressure fluctuations caused by the screw pump by a factor of up to 10. The weight and dimension tolerances can thus be considerably improved.

Direct feeding of the gear pump

Feeding the gear pump with an extruder screw has turned out to be an excellent solution, as different raw material shapes, such as strips, puppets or pellets, can be used and a perfect filling of the tooth cavities without any air inclusions is ensured. This task can be carried out by an extremely short extruder (L/D ratio approx. 3 to 5). Such a directly fed gear pump can then be employed for straining tasks, for example. The separate drive units of the feed extruder and the gear pump allow the degree of pre-plastification and thus the specific energy input to be adapted.

Extruder with integrated gear pump

Objective of the development

The main objective pursued with the development of the new extruder/gear pump combination for the rubber industry was to design a small-sized unit with a lower price than conventional extruder/gear pump combinations.

Considering the fact that the use of a gear pump reduces the energy that is mechanically introduced into the compound (i.e., the induced heat of dissipation), additional capacity is available in terms of screw torque. This available screw torque is to be used for driving a gear pump of new design. A separate drive motor, gear unit and control system for the gear pump are thus not necessary. This results in cost advantages over conventional extruder/gear pump combinations.

As a matter of course, it must be possible for the new system to be easily installed and dismantled for cleaning purposes. In ideal circumstances, the efficient self-cleaning effect renders cleaning superfluous, e.g., in the case of a compound change.

Apart from economic objectives, also process-technical goals are to be taken into consideration: The volumetric conveying principle and the resulting advantages, such as a lower temperature increase, are to be maintained.

An improved compound homogeneity is aimed at by dividing the total material flow into more than two partial flows.

Using an even higher number of small tooth cavity volumes will allow pressure fluctuations to be additionally reduced. In view of the poor thermal conductivity of rubber compounds, shorter heat transfer distances ensure at the same time a more efficient temperature control.

Operating principle

The extruder screw is converted from a double-righted to a four-righted design (figure 2) seen in conveying direction and subsequently turns into the actual pump. In this area, the barrel is designed as an internally geared wheel of a planetary gear unit. The resulting large surface offers optimum conditions for heat dissipation.

[ILLUSTRATION OMITTED]

Four planet wheels are arranged on and driven by the screw, i.e., the axles of the rotating planet wheels turn at the same angular velocity as the extruder screw.

The rotating screw conveys the rubber compound into the cavity formed by the screw flight, the internally geared wheel and the planet wheel. At the end of the pump, this cavity is insulated with regard to the pressure zone by means of a pressure plate, which also accommodates the rear bearing of the planetary shafts. The compound is thus not only exposed to pressure and pushed into the tooth cavities of the planets, but also actively pressed into the cavities of the internal gearing by the rotating screw flight. For this reason, no pressure build-up is required for emptying the feed zone of the pump.

The compound volume moves relatively to the rotating screw from the internally geared wheel to the pressure chamber. At the same time, an additional partial material flow coming from the adjacent planet wheel reaches the pressure chamber. When the planetary gearing and the internal gearing intermesh, both partial flows are pressed out together. The pressure chamber is sealed by the intermeshing gearing and the screw flight. Towards the feed side, sealing is ensured by a front pressure plate, which also accommodates the front bearing of the planets.

Flow control

Figure 3 shows a three-dimensional representation of the actual gear pump on the basis of which the flow control shall be illustrated. Two adjacent feed openings are provided, each for 1/4 of the overall material flow. Both material flows are then divided by half, with 50% flowing into the cavities of the planetary gearing and the remaining 50% being pressed into the internal gearing. In the pressure chamber between these adjacent feed openings, the material flow coming from the rotating planet wheel and the flow that was pressed into the internal gearing of the adjacent channel are united. This means that the material flow is not only subdivided into eight partial flows, but that at the same time two different partial flows each are led together. Perfect homogeneity of the extruded compound is thus ensured.

[ILLUSTRATION OMITTED]

Sectional drawing

In order to facilitate installation/dismantling and cleaning, the outside diameters of the pump increase in process direction. It is thus possible to push the extruder screw together with the integrated pump in process direction and thus remove it from the barrel and/or the internal gearing.

Rating

In view of the fact that with the newly developed principle, the extruder screw and the gear pump automatically present an identical speed, the different conveying characteristics can be made use of for the rating. As described above, the extruder is considerably more dependent on the back pressure than the gear pump with its volumetric conveying characteristics (figure 4). The maximum volume throughput of the gear pump is reached at a pressure build-up of zero bar. At this speed, the extruder screw thus has to provide this volume throughput with a pressure that ensures air-free feeding of the pump (e.g., 30 bar).

[ILLUSTRATION OMITTED]

By changing the pressure gradient to be built up downstream from the pump, e.g. by using a die plate with a smaller cross section, the volume throughput of the pump is only slightly reduced due to the minor dependence of the gear pump on the back pressure. This slightly smaller volume flow has to be supplied to the pump by the screw. Due to the considerably stronger dependence of the conveying performance of the screw on the back pressure, this is ensured by an insignificantly higher pump inlet pressure.

Fields of application

The space-saving integrated design of the gear pump allows additional process tasks to be realized on a screw section arranged downstream from the in-line pump (figure 5). It is possible, for instance, to install a pin barrel section or a pin convert section behind the integrated pump. Varying the depth of penetration of the pins, the specific energy input into the stock can thus be adapted to the requirements in terms of extrusion quality. This can be done at an almost constant material flow, as volumetric gear pumps depend on the back pressure only to a limited extent.

[ILLUSTRATION OMITTED]

Conclusion and outlook

High pressure build-up capacity, low stock temperatures and minor pressure fluctuations are the key benefits of this gear pump. They form the basis for a successful implementation of the integrated gear pump concept. The objectives Berstorff pursues with this development are, among others, a more efficient, self-cleaning effect, direct drive of the pump by the screw, division of the material flow into eight partial flows, additional reduction of pressure fluctuations, excellent homogeneity and efficient temperature control. Possible fields of application can be found in the field of profile extrusion or in straining processes.

The gear pump with external bearings and the integrated gear pump, which are characterized by different types of bearing (anti-friction bearing and hydrodynamic bearing), are intended to be marketed simultaneously.