A gear pump is a device placed between the extruder and the die, and takes over the function of pumping through the die. It is often called a "melt pump," which is technically correct, but the last zone of an extruder screw is a melt pump too, so it is more precise to call it a gear pump.
Gear pump is a pair of matching gears which form a positive displacement pump. Melt is pushed by the extruder into the pump entry and gets trapped in the chambers between the gears and the housing, carried around and forced out the other end. One gear is driven by an external source, and drives the other gear, and the tooth-to-tooth contact creates a seal between the two gears. The pump shafts are typically lubricated by a small stream of melt diverted from its normal path through the pump.
The pump confers two basic advantages: 1 It evens out irregularities in linear (mass) flow, such as those caused by irregular feed (varying scrap content) and second-zone surging. This may improve thickness control enough to allow a lower aim thickness, thus saving material, and is the basic economic justification for using this device.
2 It takes some load off the extruder, allowing it to run at lower pressures and therefore leads to less frictional heat generation. This may be translated into higher output rate if temperature is the limiting factor (either by cooling limits or degradation).
It is very important that the pump should not run "dry," as this would cause damage to the melt-lubricated shafts. Therefore, the usual way of running the pump is to measure the inlet pressure and vary the screw speed as necessary to ensure the controlled inlet pressure. A typical inlet pressure is around 5.5 MPa (800 psi)), but this can be varied if desired to improve mixing in the extruder (more pressure) or reduce melt temperature (less pressure). This value is commonly called the "suction" pressure, although suction implies one atmosphere or less of pressure differential, and the pressure here is much more.
There are three important pressures which must be known or at least estimated before a pump is specified: one is the pressure drop across the system after the pump - the head resistance; the second is this suction pressure which is regulated by the operator, and the third is the pressure at the screw tip, which is the sum of the controlled suction pressure and the resistance added by the screens and the contamination caught on them. This still may be substantially lower than it would have been without the pump, if the die and passages leading to it are small and therefore offer much resistance. Since the lower pressure at the screw tip can impair mixing, it is common to put a static mixer in the line, after the gear pump. It is placed there so the gear pump takes over the job of pushing through the mixer; it is a more efficient pump than the extruder, so less energy is needed to overcome the pressure drop across the mixer, and lower melt temperature can be achieved.
Screens are typically used to protect the pumps from hard contaminants that could jam in the tight clearances and lubrication channels, in addition to their usual functions of keeping contaminants out of the product, and control of mixing in the last zone of the screw (tighter screens, more pressure, better mixing). The screens could be put after the pump, which would take over that pushing job, too, but then they would lose the protective function, so they are kept in their usual position at the end of the screw.
Gear pumps have been used for a long time in the textile industry to force melts through tiny spinnerets, and also in the manufacture of polymers, to empty reactors and force the molten polymer out of the system, usually through a pelletizer. These uses worked against a relatively low pressure, at 10 MPa (1450 psi) or less. Around 1980 we began to see gear pumps adapted to higher die resistances, which require a heavier pump, notably the sides, which might bow (clam-shell) under high pressure if too thin and thus allow back-flow and compromise the positive-conveying feature of the pump. By now, gear pumps are available to push against pressures as high as 70 MPa (ca. 10,000 psi) and possibly more on special order.
Although the pumps are made of hard and abrasion-resistant grades of steel, they do wear, and their positive-conveying ability is thus reduced. This can be followed without actually taking the pump apart and measuring it - measure actual output and convert to cc/rpm (you need a melt density for this), and compare to pump displacement, which should be provided by the pump maker. If you are getting over 90% of the displacement, that's good, as you will always lose some around the shafts, and a tiny bit always gets around the gears. If the production is between 80 and 90%, watch this value carefully and see how fast it is deteriorating. It may be that the pressure differential is too much for the pump, or the melt is very fluid. If it is below 80%, start thinking about a new pump or at least new gears - that depends on the age of the pump, the pressures, etc. And if the production is below 70% of displacement, it may be better to take the pump out of the line until it can be restored or replaced to give its original advantages.
Be careful not to confuse gear dimensions with displacement, as the numbers (in metrics) may be similar. Pump dimensions are quoted as length across pump x diameter, e.g., a 55 x 55 has gears which are 55 mm across and 55 mm in diameter (they are often the same), but displacement will be in cc/rev, not mm.
Normally, the pump is a pressure booster, with the entry as the low-pressure side. Some pumps can actually run "backward" with the higher pressure at the inlet. This could happen with a low-resistance die and/or a need for high pressure in the extruder. In that case, the pump is acting as a valve. It still can confer the benefits of constant mass flow, but must be able to take the higher inlet pressures and the lubrication channels must be able to run in reverse. Consult the pump maker in advance before trying this.
Gear pumps are useful with vented extruders, to avoid extrusion out the vent if feed is unusually positive or the head resistance unusually high. A typical vented extruder is designed to work against a head resistance of around 17 MPa (2500 psi), and if resistance rises above that, as would normally happen with contamination buildup on the screens, the rear stage will push melt up the vent, and the vent area must be cleared while making unsalable product. The gear pump takes away the resistance of the die (and static mixer), so the line can run longer before the 17 MPa limit is reached.
Gear pumps can be used for most all thermoplastics: nylons, polyolefins, polystyrene, etc. It is not used for PVC due to its degradation characteristics. This may be resolved by formulating with more or different lubricants or more stabilizer, or both, but those things raise material costs and may take away from the economic justification for using the pump in the first place. Pumps have been made with other lubrication systems, but before using one with PVC details about what was the material and speed (as faster = less residence time = less degradation) must be sought, and get a trial if possible with the formulation that would be used with the pump.