7/27/2019 Use of Gear Pumps for Extruders
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Gear pumps play an important role in extrusion system for all
thermoplastics, except PVC
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:
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
ee 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 possiblymore 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 55mm 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
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).
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7/27/2019 Use of Gear Pumps for Extruders
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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 isreached.
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.
(Source: Special Chem)
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