SAMPE Journal - July/August 2024 - 10

FEATURE / LARGE FORMAT ADDITIVE MANUFACTURING
temperatures, Ph is the maximum head pressure, r
is the material density, and q is the heat of fusion
of the material which was assumed to be zero for
the thermoplastics. By using the results from this
equation, the throughput for each motor Hp value
could be found in kg/hr by multiplying the motor
power by the efficiency of the motor and dividing it
by the specific energy. The resulting throughputs for
typical LFAM materials can be seen in Figure 5. This
figure shows that in order to have enough power in
the motor to handle the torques generated by the
high temperature thermoplastics like ULTEM, the
extruder would require at least a 0.60 kW motor to
achieve the 4.5 kg/hr requirement while for CF-ABS
it would only require a 0.25 kW motor. This motor
sizing does not take into account the speed of the
motor as the speed required for a given throughput
is dependent on the screw dimensions themselves
and not the motor power.
[1]
Screw and Barrel Sizing
To correctly size the screw for the new extruder,
current injection molding screws and calculations
were analyzed. The different dimensions of the
screw include the number of flights, the barrel
diameter, screw diameter, flight width, flight length,
channel width, depth of feed zone, and the screw
lead. Figure 6 shows the values used to calculate
the throughput in the feed zone. By putting the
values of the designed screw into Equation 2 the
maximum throughput of the designed screw can
be validated[1]
. The main dimension used to define
the screw size is the barrel diameter or Db which in
this case was 22 mm.
[2]
This throughput equation also is dependent on
the material type with ro
of the material and hF
representing the density
representing the conveying
efficiency between the material and the barrel. For
validating the screw sizing, the density of a typical
CF-ABS material was used, which is 1140 kg/m3
.
For this material the conveying efficiency was
not known but the minimum efficiency required
to reach 4.5 kg/hr throughput was found to be
0.26. The range of efficiencies for typical polymer
smooth
barrel extruders falls between
0.170.45
so 0.26 is on the low end but is a reasonable
assumption for CF-ABS material. This validates
that the 22 mm screw will be the proper size for this
extruder to reach at least 4.5 kg/hr.
Gear Pump Sizing
A typical cross section of a gear pump is shown in
Figure 7. The dimensions for the gear pump can be
sized using standard fluid gear pump calculations
but the efficiency of the pump will be lower due to
Figure 5. Motor power vs. plastic throughput (kg/hr).
the higher viscosity of the polymer when compared
to hydraulic fluid or water. These sizing equations
use the addendum radius, the dedendum radius,
and the width of the gears to find the volume of
material moved by the gears per revolution. The
volume can then be multiplied by the gear pump's
speed (in RPM) and the material's density to find
the gear pump's throughput for a specific material.
In this case, 22 mm outer diameter gears were used
to match the screw diameter. This led to a desired
throughput of at least 4.5 kg/hr being reached at
100 RPM, which is within the speed range of the
motor that was chosen.
Figure 6. Feed section screw dimensions[1]
10 | SAMPE JOURNAL | JULY AUGUST 2024
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SAMPE Journal - July/August 2024

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