Agriculture Reference
In-Depth Information
section of the screw is enough and, if it is for the second and third reasons, then
cooling should be along the screw. For the extrusion of PCRH, extra heat will be
generated due to silica in the rice hulls.
•
Purging of the Extruder Barrel
Some polymers degrade relatively easily. For example, PVC cannot be left in the
barrel at shutdown because it cannot withstand heating on the next start-up.
Degradation or burning of material will take place with decomposition, creating
gases inside the barrel that can cause serious problems. To prevent this happening,
purging materials such as LDPE or polystyrene (PS), which are more heat-stable,
is done to keep the barrel dry at shutdown. At purge time it is advisable to remove
or open the extruder head assembly to allow for easy flow and to avoid mixing
with the resins.
•
Cooling of the Hopper Block
Some extrusion systems use cool water to pass around the hopper throat. This
ensures that the material enters the feed section of the screw freely. Variation
in the temperature of the water source should be avoided and it is best to use a
closed loop water recirculating system to ensure that the melt temperature is not
affected.
•
Guidelines for Extruder Start-up
When we talk of an extruder in terms of an extrusion set-up, the whole system is
involved. Before starting the extruder(s), all the connected equipment should be
started and checked for smooth working and shut down temporarily and restarted
before the extruders commence processing. The start-up time of an extruder is
the most dangerous time of an extrusion process. If material is left in the barrel
from a previous operation, it may decompose and spit out from the die and
accessories. Special care should be taken by an operator as he/she has to come
into direct contact with the machine to fit the die and accessories. The operator
should follow the following basic rules:
o
Review all data and machine settings
o
Ensure all tools required are at hand
o
Wear protective gear
o
Turn power on and set temperature specifics
o
Allow the melt to flow freely until the correct melt temperature is reached
o
Start the extrusion process
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