Although there are many different
pelletizer designs, all pelletizers fall into two broad categories: cold
pelletizing systems and die-face thermal pelleting systems. The main
difference between the two is the timing of the pelletizing process.
Cold pelletizing systems that pelletize the solidified polymer at the
end of the process, such as the pelletizing of a water draw process, and
in the die-face thermal pelletizing system, the molten polymer is
sliced as it emerges from the die Pellets, while the pellets are cooled
downstream.
Two kinds of pelletizing systems have their own advantages and disadvantages, the following will be a brief introduction:
Cold pelletizing system
Cold
pelletizing systems include dies, cooling zones (air or water cooled),
drying zones (if water cooled), and pelletizing chambers. There are two
main types of cold pelletizing systems, namely pellet pelletizing and
strip pelletizing.
a: Pelletizer The molten polymer is extruded
from a mixing apparatus through a belt die or roll mill to a thickness
of polymer sheet. Sheets are solidified and cooled by a distance during
transport, then cut into round or square pellets using a pelletizer in a
chamber.
Sheets Pelletizing is the oldest method of making
pellets and can be used for a wide range of polymers from nylon to
polyvinyl chloride. Advantages: large output. It has been reported that
the accuracy is quite good with a granulation capacity of up to 1843.69
kg / h. This is a cold pelletizing method where the noise emission is
higher than the method of pelletizing from a molten polymer. Solidified
state cutting polymer knife life is short, generate powder often become a
problem. Some polymers can see some "grain chain" phenomenon.
b:
The history of pelletizing pelletizers is almost as long as pelletizing
pelletizers. Including die, cooling section (water bath or blower),
drying section (if water-cooled) and pelletizing knife. The polymer
melted with a machine or gear pump is formed into a strip through a
horizontally mounted die (the modern die is precision machined and
heated evenly to produce a stable strip). After the strip is drained
from the die, it is cooled with a blower or air / vacuum facility or
cooled in a water bath. If water-cooled, the strip passes through a
drying section, blows moisture with forced ventilation, and feeds the
strip to the dicing chamber. Using a pair of fixed knife and rotary
knife shearing action, the strip cut to the exact length required. The
diameter of the pellets is 3.175 mm, 3.175 mm long, with sharp edges and
corners.
c: The traditional method of drawing the strip is to
stretch the strip through the cooling section (most commonly a water
bath), sometimes causing the strip to fall or have inconsistent sizes.
This is most commonly found in polymers with poor melt strength, such as
polypropylene, polyester and nylon. When the material falls, the
material is scrapped, so operators need to pay close attention. If the
strip is drawn inconsistently, downstream pellets need to be sifted.
Other
modes of striping can be closely monitored without the need for an
operator by using a motorized slotted feeder conveyor that supports and
divides the strip from die to pelletizer. This is the size of the screw
conveying more uniform, will not drop and thus scrap less. Some of these
methods give them a throughput of 6803.89 kg / h, compared to about
1814.37 kg / h for drawing into strips because operators can only manage
a limited number of strips.
Strip production line cost is not
high, easy to operate, and easy to clean. This has advantages for
colorant blending because the two batches of different colorant changes
must be thoroughly cleaned. However, the disadvantage of the bar-making
method is that the cooling section takes up space and its length is
determined by the temperature of the polymer.
Die-cutting system
Die-cutting
system has three basic types, namely spray granulator, water (water
ring) granulator and underwater granulator. Although such systems may
have different designs, typical systems include die, cutting chamber,
electric rotary blade, cooling media, and dry granulation if water
cooling is used.
The die is an important part of the die-face
thermal pelletizing system. It is mounted vertically or horizontally and
is usually heated with oil, steam or cartridge or band heaters.
Electric heat is typically used for smaller dies; however, larger dies
are usually heated with steam or oil. Die structure materials have
different materials, but no matter what kind of material or heating
medium, the die orifice diameter must be uniform; have sufficient heat
to maintain the temperature of the polymer throughout the process; The
die face must be tough and smooth - these are necessary to make a
uniform pellet.
When the molten polymer is die-diced, it is cut
into pellets using a pelletizing knife rotating at a high speed. The
typical case is pelletizing or touching or very close to the die face.
After the pellets have been cut, they are thrown off the knife by the
action of centrifugal force and delivered to the cooling medium.
Pelletizing knife size, shape, material and installation can be
different. In some systems, the pelletizing knife has a spring applied
load to automatically adjust the spacing between pelletizing knives and
dies; in some systems, the spacing between the pelletizing knife and the
die must be adjusted manually. Because knife life depends on
knife-to-roll accuracy, abrasiveness of the polymer and operator's
aggressiveness, it is advisable to cut the polymer pellets in a molten
state.
a: Spray granulators are recommended for polymers that are
sensitive to heat and long residence times, such as PVC, TPR and XLPE.
The pelletizing rate is up to 4989.52 kg / h The flow path of the
polymer slab to the pelletizing chamber should be kept as short as
possible with the least amount of heat. As the polymer passes through
the die, the rotation of the die-face rotates it into pellets. After the
pellets were cut, they were thrown away from the rotary knife and
captured for forced circulation of air in a specially designed
pelletizing chamber. The air stream preliminarily quenches the surface
of the pellet and takes it out of the pelletizing chamber to the cooling
zone.
Fluid bed dryers are often used to cool pellets. The
pellets slip down an adjustable bevel while the circulating fans blow
air through the pellets. Adjusting the bevel angle lengthens or shortens
the pellet dwell time in the dryer. Another common cooling method is to
discharge the pellets from the pelletizing chamber into a tank, and
then use a fluid bed dryer or centrifugal dryer to remove moisture.
b:
Water jet granulators, except for polymers with low melt viscosity or
viscosity, suitable for most polymers. Such equipment, also known as
water ring granulator, granulation rate reached 13607.77 kg / h.
The
molten polymer was cut from the hot die into pellets by a rotating
knife that rotates in the mold. This sweet granule system features a
specially designed water jet pelletizing chamber. The water spirals
around due to flow until it flows out of the granule chamber. After the
pellets were cut, they were thrown into a stream of water for initial
quenching. Pellet water slurry into the slurry tank was further cooled,
and then sent to the centrifugal dryer to remove moisture.
c: The
underwater pelletizer is similar to the spray pelletizer and water jet
pelletizer, except that it has a smooth flow of water through the die
face, but in direct contact with the die face. The size of the pelleting
chamber is just enough to allow the pelletizing knife to freely rotate
across the die without limiting the flow of water. Melt polymer from the
die, rotary knife cut pellets, the pellets were tempered water out of
the pellet chamber into the centrifugal dryer. In the dryer, the water
is discharged back to the tank, cooled and recycled; the pellets are
removed by centrifugal desiccator.
Underwater pelletizers require
the use of dies with a uniform heat distribution and special insulation.
Small pelletizing knife with electric heating; large pelletizing need
oil or steam heated die. Conventional process water is heated to maximum
temperature under normal conditions but not enough heat to adversely
affect the free flow of pellets. Underwater pelletizers are used for
most polymers, with some models achieving a pelleting capacity of
22679.62kglh. The advantages of water flowing over the die face when
pelleting a low viscosity or adherent polymer are a major advantage, but
for some polymers such as nylon and some brands of polyester this may
cause the die to freeze . Other advantages include low noise emission
due to pelletizing in a molten state while water plays a noise barrier;
fewer pelletizing replacements compared to cold-cut systems.
Grain Cutting Method Of Granulating Line