High performance, reliable rubber straining with extruder gear pump systems.

In the past, the requirement for strained, high quality rubber was met by utilizing a conventional extruder to force rubber through an appropriate screen mesh, or strainer. Such simple systems are handicapped by an inherent disadvantage: As the rubber is forced through the screen, a resultant we-screen pressure is produced. This pressure will induce a backflow component to the direction of rubber flow, i.e., there will be a pressure component acting on the compound, inducing it to flow against the direction of extrusion. As the screw speed is increased, this effect will become more and more pronounced. In short, as we attempt to increase throughput by speeding up screw rotation, relative throughput (throughput/revolution) will fall off. Ultimately, the performance of the system will be limited by the high compound temperature resulting from this inefficiency. Consequently, extrusion systems used for straining resulted in very large extruders producing relatively small throughputs.

New extruder gear pump technology provides for a vast improvement over such conventional systems. Gear pumps are positive displacement systems. Backflow is only possible through the very slight clearances, and is thus negligible, as shown in figure 1.

[FIGURE 1 OMITTED]

Gear pumps are far more efficient pressure inducing devices, and thus ideally suited for rubber straining applications. When mounted in front of the screen, the performance of the system will primarily be limited only by the compound temperature increase resulting from the rubber being forced through the screen. The throughput of a gear pump strainer will consequently be far greater than that of a screw strainer.

However, it is not possible to effectively feed a gear pump with strip or sheet rubber. A mechanism is required to consistently feed rubber into the pump. If a cold-feed system is required, this mechanism should also be able to thoroughly plasticize the rubber before it enters the gear pump. Gear pumps may be outstanding pressure inducing/ conveying mechanisms, but are ill suited to the task of plasticizing rubber. Since the gear pump need only be supplied with rubber at relatively low pressure, a screw extruder is the device best matched to a gear pump to achieve regular pump feeding. For cold feed applications, the inclusion of a Transfermix section into the screw extruder guarantees sufficient plasticization of the compound ahead of the gear pump and, in comparison with pin barrel plasticization, can do so within minimal screw length. The VMIAZ cold-feed design, featuring the patented MCTD Transfermix section, is thus said to be the most compact design available.

This integrated system is controlled by a purpose developed PLC slaving screw speed to a pump inlet pressure sensor reading/setpoint. Gear pump entry pressure must be maintained at a specific pressure level to ensure effective filling of the gear teeth. The extruder speed is therefore regulated by the pump entry pressure so that the pump is consistently supplied with material at the required pressure. Furthermore, the MCTD mixing section also enables the manufacturer to easily influence the extrusion process while maintaining a constant throughput.

By raising the pump inlet pressure set point, the work performed on the compound, especially in the MCTD section. will be increased. The pre-screen compound temperature can thus be varied so that the entire straining process can be optimized. Figure 4 is a typical example of the very impressive straining results achieved with such a system.

[FIGURE 4 OMITTED]

Shark high performance extruder gear pump machines are said to offer clear and proven advantages over conventional and competing systems, including reliability, optimal end product quality, high performance at low capital cost, a minimal floor space requirement and easy operation.

Other manufacturers have attempted to adapt plastic-processing gear pumps to the rubber straining application with very poor results. Sharks use purpose designed gear pumps that have a proven track record of reliability and robustness.

By adjusting the pump inlet pressure, and hence prescreen compound temperature, the straining process can be optimized for the highest throughput at the lowest possible extrusion temperature, and hence best possible compound heat history.

Size for size, Shark is said to deliver the highest throughput, so that the production facility can generate a maximum return on its capital investment.

The very compact design of Shark systems provides for easy integration of the machines into today's often crowded production floor.

A simple to operate touch screen panel enables very easy process control and machine operation. Shark systems are also designed for easy maintenance with simple and easy access to the pump inlet area and pre-screen area.

Table 1--specifications of available sizes

Shark           200      150      120       90       70
strainer
type

Max.            Over    6,200    3,300    2,200    1,300
throughput *   7,700    1b./h.   1b./h.   lb./h.   1b./h.
               1b./h.   (2,800   (1,500   (1,000    (600
               (3,500   kg/h.)   kg/h.)   kg/h.)   kg/h.)
               kg/h.)

* Depending on compound

Reference

Olaf Skibba and Oliver Thoma, "The next logical step: Direct extrusion through a gear pump."