A polymer feed, usually in the form of pellets,
drops from the hopper through
the feed throat into the rotating screw. This
feeding function occurs by gravity in most cases
for single-screw extruders. Some feeds, such as sticky
powders or recycled film flakes with a large surface to volume ratio, tend to bridge
inside the hopper and do not drop freely from the
hopper into the screw by gravity. Such
non-free flowing feeds require a forced feeding device. A short conical screw
installed inside the hopper, called
a “crammer feeder”, is widely used for non-free flowing feeds. Single-screw
extruders do not require starved-feeding, and they usually operate with a full
hopper in a flood-feeding mode.
A metered feeding
device, such as a volumetric feeder or a loss-in-
weight feeder, is used to control
the feeding rate and to run the screw in a starved
feeding mode in special
situations. Many polymers react with oxygen at high temperatures during extrusion, causing undesirable
oxidation, degradation, or crosslinking of the molecules. Purging of the feed
at the feed throat by an inert gas like nitrogen may be necessary, especially when the screw
is run in a starved
feeding mode.
The feed throat is directly attached
to the heated barrel, and it becomes
hot. Feed materials with a low melting point stick
to the wall of the feed throat, reducing the feeding rate or completely blocking
the feed stream in the worst case. The feed throat must be cooled
by water to avoid such feed sticking
problem.
A feed stream is often
made of several component materials. Even if the component materials are well blended/mixed coming into the hopper,
they could segregate inside the hopper. Two
different materials with the same shape but different densities, or with
the same density but different shapes, readily separate upon flow. “Flow segregation” of the feed materials inside
the hopper is a common
problem in extrusion.
Because an extruder is
a continuous pump without back mixing capability,
the first condition for a successful extrusion
process is to provide a consistent feeding
rate into the screw from the hopper, in terms of both a constant composition and a constant
weight. Extrusion problems often arise from an inconsistent feeding rate.
The feeding
rate of a polymer feed
is determined essentially by the physical
characteristics of the feed, such as size and shape, and their
distribution, controlling the bulk density (the weight divided by the apparent
volume), and the internal friction between the feed constituents. The feeding
rate also depends on the inherent properties of the polymer (the solid density, the external friction on the
metal surface), the hopper design, and the feed throat design. The external friction
of the feed on the hopper wall mainly depends
on the inherent
properties of the polymer and the roughness of the hopper wall. A tiny amount
of lubricant or additive, especially if it is coated on the surface of the
feed, can drastically alter both the internal friction and the external
friction.
Because a polymer
feed, in pellet, powder, or flake
form, becomes interlocked in the hopper, almost
supporting its own weight, the pressure at the bottom of the hopper is very low and the feeding rate is usually
independent of the amount of feed in the hopper.
The driving
force in flood-feeding is gravity.
The opposing forces
are the centrifugal force exerted by screw rotation and the back-flow
of air flowing out of the screw into the hopper
through the feed throat. Feed materials contain 30–70% air by volume, and the
air is squeezed out of the feed as the feed is compacted into a solid bed along
the screw. Continuous flow paths
from the solid bed back to the hopper are necessary for the back-flow of the air. If the flow paths are blocked, the air
is entrapped in the melt. Feed forms with a large surface area per unit volume,
such as powders and film regrinds, are prone to the air entrapment problem.
Unfortunately,
no mathematical model is available to simulate the feeding function at present.
Development of a feeding model will greatly improve the computer simulation of
extruder performance.
Preferred conditions
for the feed material to exhibit good feeding characteristics are:
• Small pellet
size in comparison to the screw channel area
• High
bulk density
• Small
surface area to volume ratio
• Low
internal friction between the pellets
• Low
external friction on the hopper surface
• High melting point
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