GEAR PUMP USAGE IN POLYMER EXTRUSION
Gear pumps have been successfully used in the extrusion industry for many years. Gear pumps are a kind of positive displacement pumps, which provide stable flow by eliminating the oscillation in the flow. Depending on the operating conditions and the characteristics of the polymer, the fluctuation below 0.5% can be compensated by the gear pump. Figure 2 shows an example of pipe extrusion with high density polyethylene (HDPE) raw material. Thanks to the use of a gear pump, the fluctuation in the flow is perfectly reduced, and less polymer is needed when producing the specified pipe diameter. As a result of this study, 130 kg of raw material gain per day was determined. Given the high raw material prices, this gain cannot be ignored.
Other important function to compensate for fluctuations in the flow is to undertake the pressurization. This makes the job more efficient than the extruder. The extruder-gear pump combination performs pressurization between the inlet and the extruder head with less energy consumption. In addition, the molten polymer is less heated, which is not only a positive effect for temperature-sensitive polymers.
The areas of extrusion application are classified by high viscosity, inlet pressure and differential pressure parameters. Melt pumps are typically designed with max.250 bar differential pressure. The next example is given to illustrate the high demands on the pump material and pump structure.
Extrusion of polystyrene (PS)
at 250 DEG C., the viscosity of the molten polymer is between 1,500-2,500 Pas and a differential pressure of 120 bar (differential pressure -2p = outlet pressure p2 – inlet pressure p1) is formed. With the EXTRU 176-8 (thread diameter and gear length 110 mm), the required torque is transferred to 2,500 kg / hour and the required torque is approximately 3,300 Nm. In this process, half of this torque is transferred from the driving shaft to the other short shaft by means of the small surface area between the gears. Therefore, the use of nitrification (hardened) material steel as the shaft material is the most advantageous for this type of pump. This is the same for the overload bearing bushings during the operation period. In this application, mostly hardened material steel is used. It is also important to recall that pigment and fillers with high abrasive effect are operated with most polymers.
Especially in the field of polymer, it has proven its reliability by using this material combination in shaft and bearings for years. Max. When referring to the permissible pump speed, the peripheral shaft velocity on the threaded ends must be taken into account. Typically, during extrusion, these values are about 0.4-0.5 m / s.
To summarize the advantages of Melt pumps briefly;
It compensates for fluctuation in polymer flow by providing a stable flow.
Increases product quality.
It does this work more efficiently by taking over the pressurization over the extruder. This provides energy gain. It causes capacity increase.
The melt prevents irregular temperature rise in the polymer.
Screw-sleeve reduces maintenance costs by shifting revision times.
In the polymer sector, where costs have been tried to be minimized and quality has come to the forefront, the advantages of the melt pump have been negligible in view of these advantages.