The material handling of the feedstock is a very critical part of the extrusion process. If the feedstock is not introduced to the feedsection of screw in a smooth and uniform matter, then the likelihood of a stable and consistent output is unlikely. This is the reason why it is very important that if regrind is added to the virgin feedstock, it must be done very consistently and uniformly. It should also be mentioned here that a very important part of the extrusion equipment is the hopper and feedthroat section. If the hopper and feedthroat sections are not designed properly, inconsistent material flow to the screw can take place. For example, if the conical section of the hopper (see figure) does not have the proper transition, the resin will not flow smoothly into the feedthroat of the extruder.

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Feed Section - Solids Conveying ( C = Consistently)
“First you must feed the polymer Consistently”
By
Timothy W. Womer
TWWomer & Associates, LLC
Material Handling
The material handling of the feedstock is a very critical part of the extrusion process. If
the feedstock is not introduced to the feedsection of screw in a smooth and uniform
matter, then the likelihood of a stable and consistent output is unlikely. This is the reason
why it is very important that if regrind is added to the virgin feedstock, it must be done
very consistently and uniformly. It should also be mentioned here that a very important
part of the extrusion equipment is the hopper and feedthroat section. If the hopper and
feedthroat sections are not designed properly, inconsistent material flow to the screw can
take place. For example, if the conical section of the hopper (see figure) does not have
the proper transition, the resin will not flow smoothly into the feedthroat of the extruder.
Many times converts will modify the Original Equipment Manufacturer’s hoppers in
order to increase the residence time in the hopper by making new hoppers similar to the
above figure. By doing this re-engineering, the converter is actually doing themselves a
dis-service and a lot of times causing themselves unknown grief.
The feed section of the screw is where the polymer is introduced to the feedscrew.
Before discussing the mechanical portions of the extruder, it is necessary to mention the
importance of the consistency of the feedstock. Extruders are just like computers in one
main feature, G.I.G.O or “Garbage In, Garbage Out”. The feedstock entering the
extruder must be delivered to the screw consistently and uniformly. The resin cannot
play leapfrog over the channels of the screw and balance itself out. Therefore, the
material handling and feedstock my present the resin to the feedthroat section of the
Conical Section
Correct Incorrect

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extruder precisely. One of the biggest problems in extrusion of polyethylene film, be it
blown or cast, the edge trim, off-spec and start material typically has to be feed back into
the extruder as some form of regrind. Due to the fact that most of this material has been
produced as very thin film, causes this regrind to have a very light bulk density. The bulk
density of this regrind can be as low as 2 – 3 lb/ cu. ft. versus its original pellet bulk
density of ~30 lb/ cu. ft. This lightness causes most difficult not only in material storage
and handling, but also in the area of re-introducing it back into the extruder. Several
different methods have been used over the years, such as densification, re-pelletizing,
fluff/pellet feeders and side arm extruders.
In the case of the smooth bore feedscrew, this is typically the deepest portion of the
screw. Also, it might be noted here that since it is the deepest section and closest to the
drive end of the feedscrew, it is most liable to torsional breakage. Therefore, when
designing the feedscrew it very important to take this into consideration. Sometimes, in
the case of very small screws (2” in diameter and smaller) it might be advisable to have
the screw manufactured from a steel alloy which has a higher yield strength. A Stainless
Steel is a good choice sometimes to help combat this design problem.
In the feed section of the screw, the primary function is to forward the polymer. This is
where solids conveying takes place. The basic theory of solids conveying is that the
polymer must stick to the inside diameter of the barrel or sometimes referred to the barrel
wall and slip on the screw root. If this simple phenomenon does not occur then the resin
can not be transported down the screw channel. In some cases, if the root of the screw is
too hot and the resin melts prematurely onto the root of the screw, then a melt plug is
formed and no material at all is conveyed forward. This phenomenon was first
investigated by Darnell and Mol and presented to the industry in a technical paper that
was presented at the 1960 ANTEC (?). Their theory and approach has been the basis of
many studies over the years. Since then many others have also studied the feeding
mechanism of single stage screws, such as Chung, Kun, Spalding, Campbell and others.
But after many dollars of research with very similar conclusions, one item hasn’t changed
and that is, the resin must stick to the barrel wall and slip on the screw in order for the
resin to be forward.
Sometimes it is possible to enhance the slippage of the resin on the screw root, reduce the
coefficient of friction (COF) between the resin and the screw root. This reduction of the
coefficient of friction can be done by means of either screw root coatings or screw
cooling. As for screw coatings, the most common is chrome plating with other choices
being Poly-ond® or Armolloy®, all of these are surface treatments which can be applied
to the basic metal of the screw and give a better release between the resin and the screw
interface.
Another means of improving the coefficient of friction between the resin and the screw
interface is screw core cooling. (See Figure 3)

3. 3
Internal Screw Cooling
Figure 3
Screw Cooling is also a means by which the coefficient of friction may possibly be
reduced between the root of the screw in the feed section and some resins. By
introducing temperature-controlled water through the core of the screw, heat that has
migrated into the core of the screw via convection can be removed. Typically, for this
application the core of the screw is gundrilled through the first four to six diameters of
the screw. Then once the screw is installed into the extruder, a rotary union with a siphon
tube is installed into the screw core. The important part of this application is to be able to
monitor the amount of water flow or heat removal is taking place. This can be
accomplished by installing an immersion thermometer and flow gauge on the discharge
side of the flow schematic. It is also recommended that the water flow be controlled with
a water valve on the return side of the flow schematic. By placing the valve on the return
side of the flow schematic, this allows the rotary union and screw core always to be filled
with water and no chance of cavitation and therefore flashing to steam. Typically it is
found that by keeping the water flow through the screw core to an exit temperature of
about 100º F to 120º F allows the screw core to remain cool enough to eliminate the
possibility of the resin sticking to the steel surface. Basically, this simple cooling
system gives the operator another temperature cooling zone on the screw which can be
used to improve the feeding mechanism of the screw. It will also make it possible for the
temperature profile of the barrel heater to be increased which is higher than typical
without the chance of causing a melt block in the feed section of the screw. This process
was presented in work that was done by Hyun and Spalding at the 1997 ANTEC (?).