VCMT rotors improve mixing time, throughput

Rubber & Plastics News February 24, 2003
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Technical
VCMT rotors improve mixing
time, throughput
By Ramaswamy G. Raj, Dennis Groat, David Norman and Richard Jorkasky
GDX Automotive and Kobelco Stewart Bolling Inc.
Executive summary
An EPDM-based compound with high filler content was evaluated using four different rotor designs with variations in
cross-sectional profile and wing configurations. The effect of these rotors on the mixing efficiency, fill factor, single
pass mixing, extruded surface quality and Mooney viscosity and scorch was studied. The results demonstrated that the
six-wing Variable Clearance Mixing Technology rotors gave the fastest cycle times and highest throughput for a single
pass system. In addition, the six-wing VCMT rotors also achieved lower Mooney viscosity and scorch at higher rotor
rpm than the other rotor designs.
TECHNICAL NOTEBOOK
Edited by Harold Herzlich
Four different rotor designs with variation in cross-sectional profiles and wing configurations were evaluated. The
rotors used in this study include two-wing standard, four-wing H Swirl, (patented) six-wing Variable Clearance
Mixing Technology and intermeshing rotors. An EPDM-based compound with high filler content was used. Effects on
mixing efficiency, extruded surface quality, fill factor, single pass mixing and viscosity were studied. The results show
that faster mixing time (single pass system), lower Mooney viscosity (ML) and Mooney scorch were achieved at
higher rpm using the six-wing VCMT rotors as compared to other rotor designs.
Introduction
From a rubber compound mixing standpoint, the main objective is to obtain a high quality mix from an energy efficient
process. It is widely believed that rotor design has the greatest effect on mixing efficiency, while rotor speed is the
single most important factor influencing the productivity of internal mixers.1 Tangential and intermeshing rotors are
the two basic designs in the market for internal mixers. Both use two rotors within a closed chamber to perform the
mixing. Intermesh rotor assemblies are comprised of a forged high tensile steel shaft and a heat shrunk-on, high tensile
rotor casing. Whereas, tangential rotors are comprised of a single casting from high tensile steel with a unique design
to give an even wall thickness for maximum heat transfer. Closed loop cooling allows the large helical projections to
be cooled and the water can be maintained relatively close to the surface of the rotor skin or shell.
In tangential rotor mixers, the shearing takes place between the rotor and the chamber, whereas in the case of
intermeshing rotor mixers, the shearing takes place between the rotors.2 If the peripheral speed of the tips of one rotor
and that of the root is to be considered, the average shear rate of the processing compound is much lower in
intermeshing rotors than in tangential rotors. On the other hand, the greater contact surface between the metal and the
processing batch makes it possible to achieve a greater transfer of the mechanical strength to the batch in intermeshing
rotors. It is believed that for stress-crystallizing, polymer-based compounds (tire tread) the tangential rotors are
preferred. Whereas, the weatherstripping industry prefers the intermeshing rotor type as the polymer of choice for this
industry is non-stress-crystallizing EPDM polymers. Also, the rotor design plays an important role in influencing the
ultimate batch size. The amount of compound that can be mixed in an internal mixer depends on two influential
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factors: total free volume (liquid volume) of mixing chamber and mix viscosity (component dependent). The fill factor
could be varied based on the process method and the viscosity of the rubber compound being mixed.3 The internal
mixing operation can be influenced by the multivariable process parameters such as the initial conditions of the mixing
process, process variables, cure characteristics and the physical properties of the end product.
In the present study, the effect of rotor design on EPDM mixing for development of a class A compound was
investigated. The other objective of the study was the comparison of single pass and two-pass mixing and its effect on
extruded surface quality, cycle time, Mooney viscosity and Mooney scorch. The effect of rotor processing parameters
also was studied to arrive at the best consistency in the manufacturing process. The variables studied include the rotor
speed, ram pressure, ram position, water temperature, batch temperature, batch size and order of ingredients addition.
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Equipment and experimental
The mixer used for this experimental series is a 16-liter intensive mixer located in Kobelco Stewart Bolling Inc.’s
processing and testing facility in Hudson, Ohio. The mixer can achieve rotor speeds from 0-120 rpm and is controlled
by a mixing control and monitoring system that allows runs to be done in either manual or preprogrammed automatic
mode. The mixer’s sides, rotors and drop door were cooled with either 45°F city water or 95°F temperature-controlled
water and the ram pressure was 60 psi.
The rotors employed for this series were the standard two-wing rotors (Fig. 1), four-wing H Swirl rotors (Fig. 2),
patented six-wing VCMT (Fig. 3) and intermeshing rotors (Fig. 4). Samples mixed with the tangential rotors were run
in friction speed (1.15:1 ratio). The samples mixed with the intermeshing rotors were run in even speed (1:1 ratio). All
but two samples were run in automatic mode. When the programmed drop temperature was reached, the material was
dropped into and sheeted out with a twin screw extruder fitted with a two-roll calender.
The different types of rotors, net chamber volumes and batch weights are shown in Table I.
The four-wing H Swirl rotors have a4:1 long wing to short wing ratio as compared to a standard four-wing rotor which
has an ~2:1 long wing to short wing ratio. The H Swirl design allows the mixed material to circulate more smoothly
throughout the chamber, improving additive dispersion. The orientation of the four-wing H Swirl rotors was 90°.
Each long wing tip of the patented sixwing VCMT rotors is separated into three distinct rotor tip to chamber side
clearances and tip widths compared to the constant clearance of the long wing tip of a standard four-wing rotor. The
small tip clearance with narrow tip width reduces the dead area on the mixer side, providing a high shear mixing area
with excellent dispersion. The medium tip clearance with normal tip width is equivalent to a normal tip clearance. The
large tip clearance with wider width allows greater movement of the material, with decreased frictional heat while
increasing material homogeneity. Each of the short wings has a constant tip to side clearance, but one wing has a small
tip clearance, one has a medium tip clearance and the third has a large clearance. The patented sixwing VCMT rotors
were oriented 60° for this comparison.
The intermeshing rotors are designed to be less aggressive than the four-wing H Swirl and the patented six-wing
VCMT rotors, building up temperature at a slower rate while giving excellent dispersion, mixing and output. The
clearance (rotor tip to adjacent rotor body) between the intermeshing rotors is approximately 3.5 mm.
In this study, an EPDM compound formulation with high carbon black loading was used.
The compounds were mixed as both a single pass and a two-pass mix to establish the best mixing procedure available
for a given compound. The mixer control unit provided the following data: mixing steps, charge time, mixing time,
cumulative time, batch temperature, ram position, rotor rpm, gross power and net power. The mix’s power curve is
printed out after the batch has been discharged and a typical curve is shown in Fig. 5.
Samples were taken from different sheets of the same batch and multiple Mooney viscosity and Mooney scorch (ML
and Ts+5) tests were done with the large rotor using a Visc Tech Mooney shearing disk viscometer set at 250°F. The
Mooney results are an average of two tests per sample.
Results and discussion
Single pass mixing was carried out using an upside-down mix procedure. Fig. 6 shows the effect of rotor design on
Mooney viscosity. When using a single pass mix with the rotor speed at 30 rpm, the ML was high for the two-wing
rotor followed by four-wing H and six-wing VCMT rotors. The data show that by using the six-wing VCMT rotor, a
reduction in viscosity of 9.5 percent was observed as compared to the intermeshing rotors. Although intermeshing
rotors improve disperse mixing, the rate of rise in temperature of the batch was lower as compared to the six-wing
VCMT rotor. The power curves of the batches mixed showed that the intermesh mixer temperature was lower by 20°F
for the same length of time. The viscosity is high at the start of the mix cycle. The shear generated by the rotor tip is
high enough to generate a rise in temperature of the mix in the six-wing VCMT rotor. In the case of the two-pass mix,
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the results show that the rotor type has no significant effect on Mooney viscosity. However, it is important to note that
the difference in Mooney viscosity between single pass and two-pass mixing was much lower when the six-wing
VCMT rotor was used.
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Fig. 7 shows the influence of rotor design on Mooney scorch. The two-wing rotor showed the longest Ts+5 value
followed by the six-wing VCMT rotor. The shortest Ts+5 value was observed for the intermeshing rotor, followed by
the four-wing H rotor. The Ts+5 value was lower by 31 percent in the case of intermeshing as compared to the sixwing VCMT rotor. The power curves showed that the six-wing VCMT rotor batch had reached its power peak and was
starting down before it reached the drop temperature. In the case of the intermeshing batch, it did not reach the power
peak before it reached its drop temperature. Consequently, the scorch time was lower in the intermesh batch. The data
also show that the type of rotor had no significant effect on scorch time when the two-pass mix was used.
The total cycle time was longer for the batches mixed with two-wing and intermesh rotors. The batch mixed using the
six-wing VCMT rotor had 59 percent and 28 percent lower mixing time than the intermesh and four-wing H rotors,
respectively. The longer cycle time may be due to the poor cooling efficiency of the rotor (Fig. 8). The rotor speed for
the second batch was increased to 60 rpm and mixed to 180°F, at which time . the rpm was dropped to 40, the cure
package was added and the batch was dropped at 235°F. The mix time for this batch was shorter by 21 percent and 22
percent for six-wing VCMT and intermesh rotors, respectively. The physical appearance of the batches looked good.
The effect of rotor design on fill factor is shown in Fig. 9. The highest fill factor was observed using the six-wing
VCMT rotor followed by four-wing H and twowing rotors. The intermesh rotor had the lowest fill factor of 67 percent
as compared to 78.5 percent for the sixwing VCMT rotor.
The effect of rotor rpm on ML, Mooney scorch and cycle time is shown in Table II for the four-wing H rotor. The
data show that a decrease in ML of 4 percent was observed by increasing the add time and sweep and decreasing the
mix time (4W3). An increase in Ts+5 value was observed for the 4W3 batch. The effect of the mix procedure on
Mooney viscosity and Mooney scorch was studied. For batches 4W4 and 4W6, the initial rotor speed was 30 rpm at
200°F, the rpm was lowered to 20 and the batch was dropped at 245°F. The results show there is no significant
difference in Mooney viscosity and Ts+5 values for upside-down and right side up mix procedures. Batches mixed at
40 rpm and 53 rpm (4W7 and 4W8) using right side up mix procedures also did not show any appreciable change in
Mooney viscosity and Mooney scorch.
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Table III shows the effect of the rotor rpm on ML, Mooney scorch and cycle time for the six-wing VCMT rotor. The
data show that the 6W3 batch, mixed at 25 rpm, had a 9.4-percent increase in ML and a 10.4-percent decrease in Ts+5
as compared to the 6W1 batch mixed at 30 rpm. To decrease the Mooney viscosity, the rotor rpm was increased to 40.
For the 6W5 batch mixed at 40 rpm, the cycle time reduced by 16.65 percent. But there was no significant difference
in Mooney viscosity and scorch time as compared to the 6W1 batch mixed at 30 rpm. Increasing the rpm to 60 (batch
6W7) showed a decrease in ML (4.5 percent) and increase in Ts+5 value (10.2 percent) as compared to the 6W1 batch
mixed at 30 rpm.
The effects of rotor rpm and the influence of cooling water temperature are shown in Table IV. For runs 6W7 and
6W8, the batches were loaded upside down and the mix was started at 60 rpm. At 180°F, the cure package was added,
the rpm was lowered to 40 and the batch was dropped at 235°F. The cooling water temperature was decreased to 45°F
from 95°F to minimize the effect of temperature rise during the mixing process. The results show that for the 6W7
batch, the Mooney viscosity decreased by 5.1 percent and the Ts+5 value increased by 6.6 percent as compared to the
6W1 batch mixed at 30 rpm. In an attempt to lower the Mooney viscosity further, an extra 5 parts of oil was added to
the 6W8 batch and the mix procedure was repeated as in 6W7 batch. As one would expect, the addition of oil helped to
decrease the Mooney viscosity by 14.4 percent and the Ts+5 value increased by 4.4 percent. The mix time also
decreased by 27.1 percent as compared to the 6W1 batch mixed at 30 rpm.
The effect of intermesh rotor rpm on Mooney viscosity and Mooney scorch is shown in Table V. All the ingredients
were added in an upside-down mix procedure at 60 rpm and mixed until the temperature reached 180°F. At that point,
the rpm was lowered to 40, the cure package was added and the batch was dropped at 235°F. The cooling water
temperature also was decreased to 45°F from 95°F. For the IM3 batch, the data show that the Mooney viscosity
decreased by 20 percent and the Ts+5 value increased by 37.5 percent as compared to the batch IM1 mixed at 30 rpm.
Further addition of 5 parts of oil and repeating the mix procedure as in the IM3 batch showed a decrease in Mooney
viscosity by 26.6 percent and an increase in Ts+5 value of 30.3 percent, as compared to the batch IM1 mixed at 30
rpm. The cycle time also decreased by 21.5 percent and 4.6 percent for the batches IM3 and IM4, respectively, as
compared to the IM1 batch mixed at 30 rpm. This indicates that the intermesh rotor mix incorporates the material
equally well, but it disperses the material better than the six-wing VCMT rotor mixer.
The two-pass batches were mixed using a right side up procedure. Table VI shows the effect of rotor design on
Mooney viscosity, Ts+5 and cycle time. In the first pass, the ingredients were added right side up at 40 rpm, mixed to
240°F, the ram was raised for 10 seconds then lowered and the mix was dropped at 300°F. In the second pass, half of
the masterbatch followed by the cure package and the remaining masterbatch were added at 20 rpm and the mix was
dropped at 200°F. The results indicate that there is no significant change in Mooney viscosity with the type of rotor
used. However, the Ts+5 values were higher by 16.2 percent and 13.2 percent for the batches mixed using two-wing
and intermesh rotors, respectively. As observed in single pass systems, the total mixing time was lowest for the batch
mixed with the six-wing VCMT rotor. The data show that the intermesh rotor had the highest total mixing time among
all the rotors compared in this study.
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The physical appearance for the single pass and two-pass mix samples were rated after they were extruded in a
laboratory extruder (one-inch tape). On visual observation, the samples mixed in two passes showed a glassy
appearance than the single pass batches. Among the batches mixed using the six-wing VCMT and intermesh rotors, it
was difficult to see any significant difference in physical appearance. Further investigation on quantitative analysis for
the batches mixed using single pass and two-pass mix procedures, is under way.
Conclusions
The six-wing VCMT rotor design offers faster mixing time and lower Mooney viscosity than the two-wing and four
wing H rotors. Among the batches mixed using six-wing VCMT and intermesh rotors, the batches mixed at higher rpm
using intermesh rotor had lower Mooney viscosity, but the six-wing VCMT mixed batches show a significant increase
in throughput. The upsidedown mix is better than the right side up mix procedure. The six-wing VCMT mixer
incorporates material at a much faster rate than the intermeshing mixer. Further studies, using dense and sponge
compounds, are needed to arrive at any conclusion on the rotor design and its effect on productivity and quality of
mixing in terms of Mooney viscosity, Mooney scorch and physical properties.
References
1. Ken Nekola and Mitch Asada, paper presented at 140th ACS Rubber Division Meeting, American Chemical Society, Oct. 8-11,
1991.
2. Juergen W. Pohl, paper presented at 152nd technical meeting and Rubber Expo ’97, American Chemical Society, Rubber
Division, Oct. 21-24,
1997.
3. Albert McGuinness and S.N. Ghafouri, paper presented at 156th ACS Rubber Division, American Chemical Society, Sept. 2124, 1999.
The authors
Ramaswamy G. Raj is a materials engineer at GDX Automotive International headquarters in Farmington Hills, Mich.
He can be reached by phone at 248-553-5308 or by e-mail at ramaswamy. [email protected].
Dennis Groat is a materials process engineer at GDX Automotive’s Welland, Ontario, plant. He can be reached by
phone at 905-735-5631, Ext. 311, or email [email protected].
David Norman is vice president of sales for Kobelco Stewart Bolling Inc. in Hudson, Ohio. He can be reached by
phone at 330-655-3111, Ext. 117, or email [email protected].
Richard Jorkasky is a laboratory manager for Kobelco Stewart Bolling in Hudson. He can be reached by phone at 330655-3111, Ext. 133, or e-mail [email protected] .
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