The concept of a gear pump is quite simple. Its most basic form consists of two identical gears meshing and rotating within a tightly fitted housing. The housing's interior is shaped like a figure "8," housing the two gears with their outer diameters and sides tightly fitted to the housing. Material from the extruder enters between the two gears through the inlet, filling this space. As the gears rotate, the material moves along the housing and is finally discharged when the two gears mesh.
Technically, a gear pump is also called a positive displacement device. It's like a piston in a cylinder. When one gear enters the fluid space of another gear, because liquid is incompressible, the liquid and the gear cannot occupy the same space simultaneously. Thus, the liquid is mechanically squeezed out. Due to the continuous meshing of the gears, this phenomenon occurs continuously, providing a continuous discharge volume at the pump outlet. The amount discharged per revolution of the pump is the same. As the drive shaft rotates continuously, the pump continuously discharges fluid. The pump's flow rate is directly related to its rotational speed.
In reality, a small amount of fluid is lost within the pump. This is because the fluid is used to lubricate the bearings and gears, and the pump body can never be perfectly flush, so 100% of the fluid cannot be discharged from the outlet. Therefore, a small amount of fluid loss is inevitable, preventing the pump from reaching 100% efficiency. However, the pump can still operate well, achieving 93%–98% efficiency for most extruded materials.
For fluids whose viscosity or density varies during the process, this type of pump is not significantly affected. If there is a damper, such as a filter or restrictor on the outlet side, the pump will force the fluid through them. If this damper changes during operation-that is, if the filter becomes dirty or clogged, or the back pressure of the restrictor increases-the pump will still maintain a constant flow rate until it reaches the mechanical limit of the weakest component in the unit (usually equipped with a torque limiter).
There are limitations to the rotational speed of a pump, primarily depending on the process fluid. If the fluid is an oil, the pump can rotate at very high speeds, but this limitation increases significantly when the fluid is a high-viscosity polymer melt.
It is crucial to push the high-viscosity fluid into the two-tooth space on the suction side. If this space is not filled, the pump cannot deliver the accurate flow rate. Therefore, the PV value (pressure × velocity) is another limiting factor and a process variable. Due to these limitations, gear pump manufacturers offer a range of products with different specifications and displacements (the amount discharged per revolution). These pumps are matched to specific application processes to optimize system capacity and price.
The PEP-II pump features integrated gears and shafts, using a through-hardening process for extended service life. The "D" type bearing incorporates a forced lubrication mechanism, allowing polymer to pass through the bearing surface and return to the pump inlet side, ensuring effective lubrication of the rotating shaft. This feature reduces the possibility of polymer retention and degradation. The precision-machined pump body allows for a precise fit between the "D" type bearing and the gear shaft, ensuring the gear shaft is not misaligned and preventing gear wear. The Parkool seal structure, together with the PTFE lip seal, forms a water-cooled seal. This seal does not actually contact the shaft surface; its sealing principle is based on cooling the polymer to a semi-molten state to form a self-sealing effect. Alternatively, a Rheoseal seal can be used, which has reverse spiral grooves machined on the inner surface of the shaft seal, allowing the polymer to be back-pressurized back to the inlet. For ease of installation, the manufacturer has designed an annular bolt mounting surface to accommodate flange mountings with other equipment, making the manufacture of cylindrical flanges easier.
The PEP-II gear pump comes with heating elements matched to the pump specifications, available as an option for users. This ensures rapid heating and controlled heat distribution. Unlike internal heating methods within the pump body, damage to these elements is limited to a single plate and is not related to the entire pump.
