This document discusses auxiliary equipment used in extrusion processes, including various feeding systems, potential problems that can occur during feeding (e.g., bridging, funneling, selective entrapment), and solutions to address those problems (e.g., using a circular rather than square hopper). It also describes different types of feeding equipment like screw conveyors and belt feeders. Finally, it covers drying equipment like ovens and hopper dryers that are used to reduce the moisture content of hygroscopic polymers before extrusion.

1. Auxiliary Equipment
1. Feeding system

2. The two solid feed systems that rely on gravity are
flood and starve feeding. Both feed systems have a hopper
sitting directly over the throat.
The feed throat section, attached directly to the extruder
barrel, is water jacketed for cooling.
The feed throat should feel warm to the touch but not hot.

3. Flood feeding fills the screw channels completely
and takes advantage of the full length of the
screw. It requires no extra feeding equipment, but
offers reduced process control.

4. Starve feeding requires a feeder but offers greater process
control and tends to reduce melt temperatures.

5. Standard feed throat design for pellets or powder is shown
In geometry A.
Feed throat geometries B and C are more appropriate for
melt-fed extruders. Pellets fed can wedge between the
barrel and screw, causing the screw to deflect.

6. Square feed hoppers usually work well with bulk materials with uniform
pellet size. However, when there is a large variation in particle size and
shape the square feed hopper cause conveying problems. This can
when regrind is added to the virgin material. For this reason it is better
to use a circular hopper with gradual compression

8. Bridging
Bridging is caused by:
• Compressibility of polymer or additives
in the feed hopper
• Non-free flowing formulations
• Material softening in
the feed throat and adhering
to the feed hopper
• Large chunks of material
(particularly regrind)
• Fibers
• Low bulk density materials

9. Funneling
Funneling occurs when material is not free
flowing and sticks to the hopper walls. Center
flow in the hopper leaves build up on the hopper
walls.

10. Selective entrapment
Selective entrapment occurs in mixtures with different
size particles, i.e., pellets and powder, where the particles
can separate. The bottom of the hopper becomes richer in
smaller size particle material, while the top is richer in
large size particle material.

11. Slippage
Large particle size, lubricants, or liquids can cause
slippage in the feed section. Grinding or pulverizing the
particles to a smaller size can eliminate this problem. Slippage
due to either internal or external lubricants coating the barrel
walls is an issue that is extremely difficult to overcome in
normal operation when lubricants or mold releases are being
compounded.
Force feeding only is possible.

12. Cutting
Cutting large pellets or regrind chunks between the
screw flight and feed throat is another feed problem.

13. Ejection
The final feed problem is called ejection;
it is where large rubbery particles can
be ejected or forced out the extruder by
screw rotation.

14. Gravity feeding and forced feeding
If the extruder feed hopper level changes from
almost empty to full, the throughput rate will change
slightly. This is accompanied by a change in the product dimensions.
In a flood-fed extrusion setup, the material level in the feed
hopper needs to be kept at a fairly constant level.

15. Feeding system
Belt feeders are low cost, have high
throughput volumes, and are simply
constructed. The rate of the belt speed
and the size of the gate opening
Determine the throughput.

16. Feeding system.
Extruders typically use either screw, vibratory, or belt
• feeders. Screw conveyors screw with rigid housing
Distance- 60 m
Flow rate -4m3/h

17. The flexible screw conveyor has a helicoid screw that rotates
inside a fixed tube. Screw conveyors typically range in
diameter from 2½ to 8 in., and can be used for horizontal
transports of up to 80 ft, and inclines of up to 60 degrees. An
8-in.-diam screw conveyor can transport 1800 cu ft/hr at 360
RPM at a 45-degree angle.

18. Pressure Conveying Unit
The system requires compressed air at 2-6bar. The
compressed air must be free of oil and water.

19. The level is kept constant by incorporating a vacuum loader,
on top of the feed hopper that adds material to
the feed hopper during operation
.

20. Suction units

22. Individual feeders can be
placed on the extruder,
this is
not the normal mode of
operation for single screw
extrusion
but is quite common for
twin screw extrusion.
In-line blend system

23. In-line blend system

24. Drying
• Certain polymers are hygroscopic in nature,
meaning they absorb moisture from the air
until their equilibrium moisture content is
reached. In general, polar polymer molecules
containing oxygen and/or nitrogen

27. Drying equipment can be divided into
• Ovens
• Hopper dryers

28. Oven drying is generally used for small lots.
It is more efficient to use more trays with less resin
in each tray, as the circulating air does not flow between
pellets.
Oven drying

29. Hopper Dryers
Air exiting the dryer passes through a filter and then is blown
through the heater, where the air is reheated to the desired
drying temperature. The hot air enters the cone-shaped area
at thebottom of the dryer and exits the top.

30. In refrigerant dehumidifying, air flows across a refrigeration
coil, where the air is cooled to its dew point. Moisture
condenses on the coil and drier air goes to the blower and
heater. The dryer shown is an open system; in practice the
system will be designed to re-circulate hot air from the dryer
back through the evaporative and condenser coils to remove
the moisture from the air.