This document summarizes an experimental investigation into melt pump performance using three different resins. Key findings include: 1. Melt pump efficiency and output decreased as discharge pressure increased, and efficiency was higher for stiffer resins which experienced less backflow. Stiffer HDPE had the highest efficiency while softer PS had the lowest. 2. Higher discharge pressures and pump speeds increased melt temperature. PP had the highest melt temperatures, followed by HDPE then PS. 3. Stiffer resins and higher pressures led to higher motor amperage requirements. Pump sizing should account for resin properties and system pressure differentials.

1. AN EXPERIMENTAL INVESTIGATION INTO MELT PUMP PERFORMANCE
Walter S. Smith
Luke A. Miller
Timothy W. Womer
Xaloy Corporation, New Castle, PA
Abstract
Melt pump performance and efficiencies will vary
according to the viscosity of the resin being pumped, and
the discharge pressure that the melt pump will need to
overcome. Resin melt temperature differences and power
requirements of the pump, will vary according to the
resin, and conditions that the pump will operate under.
This paper will explore the processing differences in (3)
resins on gear pump performance at (4) different pump
speeds, at three different discharge pressures. Discharge
pressures on the pump will be varied keeping the suction
pressure constant, thus increasing the change in pressure
across the pump. Melt temperature, and pump efficiency.
melt pump motor amperage, and total output, (kg/hr) will
then be measured and recorded.
Introduction
Melt pumps are used in extrusion to increase the stability
of the extruder output, in order to get a more uniform
product out of a die. Melt Pumps are also used to
overcome high die pressures, as in blown film, to isolate
the screw from these high pressures for a more stable
screw output. A good example is a vented, or
devolatilizing, screw application where the head pressure
of the extruder cannot be very high as measured at the
screw tip, or the screw will not be able to overcome the
high head pressure, and the resin will bleed out of the vent
hole in the barrel. The extruder is typically “slaved”, or
controlled off the drive of the melt pump; and the extruder
will follow any change in speed made to the gear pump
control.
The melt pump that was used in this study had a high
volumetric output compared to the extruder output,
causing the data to be taken at lower melt pump speeds
compared to an actual production setup in the field.
Equipment
The extruder used for this study was a 90mm (3.5”) x
24:1 L/D NRM Extruder with five-barrel water-cooled
temperature zones. It is equipped with a 112 kW (150
HP) DC motor. The maximum screw speed is 129 rpm.
Figure 1 shows the extruder with (11) melt pressure
transducers located every 2 L/D down the axial length of the
barrel. The standard two-piece extruder configuration
consisted of a separate water-cooled feedblock with a
flanged extrusion barrel bolted on the downstream end of
the feed block. The feed block section then was cooled with
water to prevent any melt bridging due to excessive heat
buildup in the feed port area.
A Xaloy EH-35 slide plate Screen Changer was used,
which was loaded with a breaker plate and a 20/40/60/20
screen pack for each resin trial.
The Melt Pump that was used was a Xaloy MHDP –
300/200, with a 18.64 kW (25 HP) AC motor, and a GPAC
melt pump control system. The volumetric output of the
melt pump was 186.1 cc/revolution. The approximate
relation for the volumetric output of a gear pump is as
follows:
( ) LqFWPDODQ −×−= 22
2
π
Where: Q= Volumetric Output
FW= Face Width
OD= Outside Diameter of Gear
PD= Pitch Diameter of Gear
qL= Leakage Flow
The leakage in the gear pump occurs as follows:
1. Backflow over gear lands
2. Backflow past sides of gear teeth
3. Any material allowed to flow thru bearings
4. Backflow past meshing gear teeth
The gear pump system was equipped with (3) heat zones,
(1) for the pump itself and (2) heat zones for the upstream
and downstream adapters.
The upstream adapter connects the screen changer to the
gear pump. The downstream adapter connects the gear

2. pump to the melt valve. Figure 2 shows the melt valve
crossection assembly. This was used to vary the
downstream pressure of the melt pump for each of the
resin trials used in this study. Figure 3 shows the entire
component configuration downstream of the barrel.
A low shear barrier screw with mixer was used for all
testing. This screw was specifically designed as a general
purpose extrusion screw.
A Fluke Data Acquisition System was used to acquire all
data from the process. It will be referred to as NetDAQ.
Resins Tested
Three virgin resins were used for this study were:
• .45 MFR HDPE – Exxon Mobil HD 7845.30
• .65 MFR PP – Chevron Phillips Marlex BF03A
• 1.5 MFR PS – Dow Styron 685D
Experimental Procedure
Each of the three resins were extruded on the same NRM
90mm x 24:1 L/D extruder–screen changer–melt pump–
melt valve, configuration for three one minute samples at
four different melt pump speeds, at three different melt
pump discharge pressures. The suction pressure of the
melt pump was set at 69 bar for all trial runs in this study.
The discharge pressure of the melt pump was manually
varied via the melt valve; to 138 bar, 207 bar, and 276
bar. There were a total of thirty-six trial runs in this study.
The approximate relation for the volumetric output of a
gear pump is as follows:
For each test, the barrel and screw were completely
cleaned. The extruder and downstream components were
pre-heated to processing temperature for one hour before
the testing started. Steady thermal conditions were then
assumed to prevail throughout each of the thirty-six trial
runs.
Chart 1 shows the processing set temperatures for each of
the three resins that were processed in this study. Please
note that a “hump” type barrel temperature profile was
used in each of the resin trial runs in conjunction with the
low shear barrier screw design. This type barrel
temperature profile does enable this barrier screw design to
process these resins more efficiently.
Each test included melt pump speeds of 4, 7, 10, and 13
rpm. Melt temperature was checked at each screw rpm using
a hand held IR gun, and a pre-heated melt probe at the
discharge of the melt valve. Three one-minute output
samples were taken, and averaged at each set melt pump
rpm to calculate output rate. The screw speed and melt
pump motor amps; inlet and outlet pressures to the melt
pump, were all monitored and recorded at one-second
intervals on the NetDAQ.
The data were then extracted from the NetDAQ and
compiled with a spreadsheet program.
Presentation of Data and Results
The results for the .45 MFR HDPE pump trials were as
follows: The melt pump output decreased, as the outlet
pressure from the pump was increased, by the adjustment of
the melt valve that was downstream of the melt pump. See
Figure 4, for the outputs in kg/hr across all four pump
speeds at the three discharge pressures for HDPE. At a 138
bar outlet pressure, the pump averaged a 97.2% of
calculated theoretical output across all four pump speeds. At
a 207 bar outlet pressure, the melt pump averaged a 94% of
calculated theoretical output across all four pump speeds. At
a 276 bar outlet pressure, the melt pump averaged 87% of
calculated theoretical output across all the four pump
speeds.
The discharge HDPE melt temperature was, as expected;
greater at higher pump speeds and at higher set outlet
pressure across all four pump speeds. See Figure 5, for the
HDPE melt temperatures at the four pump speeds, and three
set outlet pressures. The average measured HDPE melt
temperature for the 138 bar outlet pressure trials was 228
°C. The average melt temperature for the 207 bar outlet
pressure trials was 232°C. The average melt temperature for
the 276 bar outlet pressure trials was 235°C.
The pump motor amps were higher at all three outlet
pressures and pump speeds for the .45 MFR HDPE, than the
.65 MFR PP, and the 1.5 MFR PS trials, as shown in Figure
10. The calculated pump efficiency was also greater with
the stiffer HDPE resin trials, than the PP and PS resin trials,
as shown in Figure 11.
The results for the .65 MFR PP pump trials were as
follows: The melt pump output decreased, as the outlet
pressure from the pump was increased, by the adjustment of
the melt valve that was downstream of the melt pump. See
Figure 6, for the outputs in kg/hr across all four pump
speeds at the three discharge pressures for PP. At a 138 bar

3. outlet pressure, the pump averaged a 95% of calculated
theoretical output across all four pump speeds. At a 207
bar outlet pressure, the melt pump averaged a 91% of
calculated theoretical output across all four pump speeds.
At a 276 bar outlet pressure, the melt pump averaged 78%
of calculated theoretical output across all the four pump
speeds.
The discharge PP melt temperature was, as expected;
greater at higher pump speeds and at higher set outlet
pressure across all four pump speeds. See Figure 7, for the
PP melt temperatures at the four pump speeds, and three
set outlet pressures. The average measured PP melt
temperature for the 138 bar outlet pressure trials was 232
°C. The average melt temperature for the 207 bar outlet
pressure trials was 234°C. The average melt temperature
for the 276 bar outlet pressure trials was 235°C.
The pump motor amps were on average higher, at outlet
pressures and pump speeds for the .65 MFR PP, than the
1.5 MFR PS trials; but lower than the .45 MFR HDPE
resin trials, as shown in Figure 10. The calculated pump
efficiency was also greater with the stiffer PP resin than
the PS resin trials, but lower than the HDPE trials, as
shown in Figure 11.
The results for the 1.5 MFR PS pump trials were as
follows: The melt pump output decreased, as the outlet
pressure from the pump was increased, by the adjustment
of the melt valve that was downstream of the melt pump.
See Figure 8, for the outputs in kg/hr across all four pump
speeds at the three discharge pressures for PS. At a 138
bar outlet pressure, the pump averaged a 94% of
calculated theoretical output across all four pump speeds.
At a 207 bar outlet pressure, the melt pump averaged a
83% of calculated theoretical output across all four pump
speeds. At a 276 bar outlet pressure, the melt pump
averaged 70% of calculated theoretical output across all
the four pump speeds.
The discharge PS melt temperature was, as expected;
greater at higher pump speeds and at higher set outlet
pressure across all four pump speeds. See Figure 9, for the
PS melt temperatures at the four pump speeds, and three
set outlet pressures. The average measured PS melt
temperature for the 138 bar outlet pressure trials was 226
°C. The average melt temperature for the 207 bar outlet
pressure trials was 231°C. The average melt temperature
for the 276 bar outlet pressure trials was 232°C.
The pump motor amps were lower at all three outlet
pressures and pump speeds for the 1.5 MFR PS, than the
.45 MFR HDPE trials and the .65 MFR PP resin trials, as
shown in Figure 10. The calculated pump efficiency was
also lower for the 1.5 MFR PS resin than with the stiffer
PP and HDPE resin trials, as shown in Figure 11.
Discussion of Data and Results
The melt pump efficiency depended on both the stiffness of
the resin and the pressure differential, from the inlet side of
the pump to the outlet side of the pump. The stiffer the
resin, the higher pumping efficiency of the pump was
observed. The lower the pressure differential from the inlet
side of the pump to the outlet side of the pump, the higher
pumping efficiency of the pump was observed. It can be
concluded that at the higher pressure differentials from inlet
to the outlet side of the pump, the more “backflow” of resin
to the lower pressure inlet side of the pump occurs, resulting
in a lower pumping efficiency of the pump. It can also be
concluded that the stiffer the resin, the less pressure
sensitive the pump is to a higher pressure differential and
this also resulted higher pump efficiencies.
It was also observed that melt pump efficiency was lower at
the lower pump speeds than at the higher pump speeds at the
same pressure differential running the same resin. It can be
concluded that resin pressure flow back towards the inlet
side of the pump, was greater part of the overall pumping
capacity at lower pump speeds.
It was also observed that the higher the pressure differential
across the pump, the higher the measured melt temperatures
at the discharge of the melt pump were recorded. The .65
MFR PP had the highest measured melt temperatures, then
the .45 MFR HDPE, and lastly the 1.5 MFR PS.
It was also observed that the higher the pump speed, the
higher the measured resin melt temperatures were obtained.
Higher melt pump motor amperages were observed for the
stiffer resins, and at the higher pressure differentials.
Conclusions
1. Melts pumps should be properly sized for the
extruder and output requirement of the proposed
application. The pump should not be required to
run at excessive speeds resulting in high melt
temperatures, resulting in excess downstream
cooling requirements.
2. Resin rheological “stiffness”, should be taken into
account when designing the clearances on a melt
pump system, to assured high pump efficiencies,
along with adequate pump lubrication.
3. Melt pumps motors should be sized with the
highest pressure differential for the application,
along with the resin “stiffness”, in order to insure
adequate power availability.

4. References
1. C. Rauwendaal, Polymer Extrusion, Hanser
Publishers, NY, 1986
2. Z. Tadmor and I. Klein, Engineering Principles
of Plasticating Extrusion, Reinhold, NY, 1970.
3. C. Chung, Extrusion of Polymers, Hanser
Publishers, NY, 2000.
4. F. Henson, Plastics Extrusion Technology,
Hanser Publishers, NY, 1997.
5. S.Fox, Mechanical Design and Process
Considerations for Polymer Gear Pumps in
Extrusion, Normag Corporation.
Keywords
Volumetric output, melt pump, devolatilizing, NETDAQ Rate of HDPE
0
20
40
60
80
100
120
140
4 7 10 13
Pump Speed (RPM)
KG/HR 138 Bar 207 Bar 276 Bar Theo. Rate
Melt Temperature of HDPE
100
120
140
160
180
200
220
240
260
4 7 10 13
Pump Speed (RPM)
Temperature(°C)
138 Bar 207 Bar 276 Bar
Figure 1-90mm x 24:1 NRM Extruder
Figure 2- Melt Valve
Figure 5- HDPE Melt Temperature
Figure 4- HDPE Output
Figure 3- Down Stream
Equipment
Screen Changer Melt Pump Melt Valve

5. Rate of PP
0
20
40
60
80
100
120
140
4 7 10 13
Pump Speed (RPM)
KG/HR
138 Bar 207 Bar 276 Bar Theo. Rate
Melt Temperature of PP
100
120
140
160
180
200
220
240
260
4 7 10 13
Pump Speed (RPM)
Temperature(°C)
138 Bar 207 Bar 276 Bar
Rate of PS
0
20
40
60
80
100
120
140
4 7 10 13
Pump Speed (RPM)
KG/HR
138 Bar 207 Bar 276 Bar Theo. Rate
Melt Temperature of PS
100
120
140
160
180
200
220
240
260
4 7 10 13
Pump Speed (RPM)
Temperature(°C)
138 Bar 207 Bar 276 Bar
Average Pump Amps Vs.
Pressure
15.0
15.5
16.0
16.5
17.0
17.5
18.0
18.5
19.0
19.5
138 207 276
Pressure (Bar)
Amps
HDPE PP PS
Pump Efficiency
40%
50%
60%
70%
80%
90%
100%
138 207 276
Pressure (Bar)
(%)
HDPE PP PS
Resin BZ1 BZ2 BZ3 BZ4 BZ5 S/C AD MP AD MV
HDPE 190 232 227 221 210 204 204 204 204 204
PP 204 232 227 221 216 210 210 210 210 210
PS 190 218 213 207 204 204 204 204 204 204
Figure 11- Pump Efficiency
Figure 7- PP Melt Temperature
Figure 8- PS Output
Figure 6- PP Output Figure 9- PS Melt Temperature
Figure 10- Motor Amps
Chart 1-Processing Temperatures (°C)