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The Effect of the Feed Section Groove Taper Angle on the Performance of a Single-Screw Extruder.

A new mechanism is described that allows adjustment of the groove geometry in grooved feed extruders. This mechanism enables efficient, continuous and independent change of the groove geometry during the extrusion process. The patented solution of the activated grooved feed section enables one to change the number of grooves, taper angle and, connected with it, groove depth. The paper contains the graphical presentation of the selected results of experimental studies of autothermal extrusion of a medium density polyethylene in an extruder with the grooved feed section in which the groove taper angle, and thus groove depth, was changed during the extrusion process. The influence of changing the groove taper angle in the range from 0 to 5.236 x [10.sup.-2] rad and screw speeds ranging from 177 to 279 rev/min on extruder output was studied. The energy efficiency of the extruder was studied as well.

INTRODUCTION

For an effective movement of polymer in a screw extruder, the friction between the polymer and the inner barrel surface must be larger than the friction between the polymer and the screw surface. This was first demonstrated in 1941 by Decker [1] and confirmed by Darnell and Mol in 1956 [2]. An effective way of increasing friction on the barrel surface is by machining grooves into the internal barrel surface. In a grooved feed extruder, a separate feed housing is used that contains grooves along the internal surface of the barrel. A smooth barrel section follows the grooved barrel.

Since the end of the 1950s, extensive research on grooved feed extruders has been carried out [3-7]. An in-depth analysis was presented in 1968 by Fuchs [8], and an attempt to explain the conveying behavior was proposed by Schneider [9]. This analysis was further developed by Menges and Hegele [10, 11], while an attempt at an analytical description was undertaken by Menges [12]. An analysis of operating conditions and optimization of grooved feed section of an extruder was provided by Potente [13, 14] and Diakun [15]. In the 1970s, Langecker and co-workers [16, 17] developed the concept of helical grooves, which was further developed and modified in 1979 by GrunschloB [18].

For practical reasons, standard axial and helical grooves are usually made [19, 20] by machining grooves into a sleeve, which is then fitted in a feed housing. Scientific research and experiments have shown that the groove geometry has a strong influence on the extrusion process [8, 21, 22].

Geometrical elements of the grooved feed section are changed by modifying the grooved sleeve. This is labor-intensive, time-consuming, and, as a result, expensive. An independent change of specific geometrical elements of the grooved feed section during the extruding process is not possible with this method. That is why it is often conventionally called the "passively grooved feed section" of the extruder.

In the case of autothermal extrusion, the extrusion process is controlled mainly by adjusting the screw speed, which inevitably causes the change of all parameters of the process and the necessity to adapt the individual component elements of the extruder. That is why in the extrusion process there should be at least two independent factors whose change can influence the process, for example, polymer flow rate or screw speed. One of these factors can be an operating condition and the other a geometrical parameter [23].

There is extensive literature available on passively grooved feed sections, both theoretical and experimental. This research deals with the analysis of polymer movement [24], the influence of constructional elements of the grooved feed section on the characteristics of the process [25, 26], as well as optimization problems [27] and, to a lesser extent, energy problems [28]. Extruders with grooved feed sections have been used so far mainly for extruding polyolefins [29] and polymer recycling [30, 31]. There is also some information about grooved barrels in twin-screw systems [32].

Active control of the grooved feed section is achieved by varying the groove geometry during the extrusion process. This issue has been discussed in literature but only few patents, mainly American [30, 33-36]. describe the principles of design solutions of such systems. It seems that these solutions did not find practical applications because of their complexity and cost.

New design solutions for active control of the grooved feed section of the extruder have been developed. One design was selected and used to construct an extruder with active control of the grooved feed section. For experimental studies of this section, a model and then a prototype of an autothermal extruder with an adjustable grooved feed section was manufactured. The design achieves a relatively fast and simple change of the number of grooves and a continuous adjustment of groove taper angle during the extrusion process. When the taper angle changes the groove depth changes with It. This solution was patented [37]. Results of experimental studies of an active grooved feed section of an autothermal extruder are presented below.

EXPERIMENTAL

Material

Medium density polyethylene, tradenamed ME 2421 and supplied by Borealis Company, with a pellet size of about 3 mm was used. Antioxidant, light stabilizer and pigment of a yellow color were added. Basic properties of processed polyethylene according to the resin supplier are presented in Table 1.

Apparatus

Experimental studies of an adjustable grooved feed section were carried out on a 25-mm extruder (Fig. 1) equipped with an adjustable grooved feed section. The adjustable grooved feed section was developed at the Technical University of Lublin [37] In cooperation with the Institute of Polymer Processing "Metalchem" in Torun. The extruder screw has a length-to-diameter ratio of L/D 20:1; the screw design is shown in Fig. 2.

The screw is considered to be suitable for autothermal extrusion of medium density polyethylene. The end of the screw is equipped with three elements for intensive shearing and mixing. The power to turn the screw is provided by a 3.75 kW electric motor, a belt transmission, a gear reducer, and slip coupling. The screw speed can be adjusted continuously from 0 to 360 rev/min.

The design details of the adjustable grooved feed section are shown in Fig. 3.

Pivot elements [1] are connected to the extruder barrel [2] and inserts [3] are connected to these pivot elements. The other ends of the inserts can be moved by rotation of adjustment bolts [6]. The inner surface [5] of the inserts has the same curvature of the barrel internal diameter [8]. When the inserts are in their "home" position the bore of the feed housing is smooth. However, when the inserts are adjusted radially outward they form longitudinal grooves in the feed housing. The length of these grooves is 94 mm and the width 8 mm. The depth of the grooves reduces from a maximum at the adjustment points to zero at the pivot points.

For the adjustment and change of the number of grooves and taper angle, a mechanism with an adjustment bolt, flange and electrical stepping motor was used. The desired insert taper angle is achieved by turning the adjustment bolt. Clockwise rotation causes a radial movement of the insert outward, which results in the increase of the taper angle of the grooves in the barrel and the increase of their depth. This mechanism allows variation of the taper angle of each groove from 0 to 3 degrees in relation to the barrel axis. This corresponds to a change of maximum groove depth from 0 to almost 5 mm. At the front edge of the feed opening the groove depth can vary from 0 to almost 3.5 mm. The adjustment bolt is turned by an electric stepping motor one full turn of the adjustment bolt corresponds to 400 steps (1 step corresponds to 1.571 x [10.sup.-2] rad).

The extruder was equipped with a strand die with four circular holes with 3-mm diameter.

Between the barrel housing and barrel itself, five electrical ring heaters were placed of the power 300 W each for preliminary heating of the plasticating system.

During actual operation of the extruder, the barrel heaters were turned off to obtain autothermal conditions.

The feed housing and grooved feed section were cooled externally with two cooling fans each with a capacity of 200 [m.sup.3]/hr.

Experimental Factors

In the experimental study the following main parameters were studied: mass of extrudate measured length [m.sub.w], extrusion time of extrudate measured length [T.sup.w], time of energy measurement supplied to the extruder [T.sup.e], energy supplied to the extruder [E.sup.c], screw torque M and temperature of the extrudate when leaving extruder head die [t.sup.w]

Secondary parameters studied were: polymer mass flow rate G, polymer specific enthalpy increase [delta]i, thermal power conveyed by polymer [Q.sub.N] power supplied to the extruder [Q.sub.C] power supplied to the plasticating system [Q.sub.p], energy efficiency of the extruder [[kappa].sub.W] (energy efficiency of the extrusion process and energy efficiency of drive motor and belt and gear transmission), energy efficiency of the extrusion process [[kappa].sub.P] itself, specific energy consumption of the extruder [E.sub.JC] and specific energy consumption of the plasticating system [E.sub.JP].

The variables adjusted in the experiments were: the taper angle of the grooves [alpha] (distributed symmetrically in relation to the barrel axis) and the frequency of screw rotation [nu]. As the screw speed changes the shear rate of the polymer melt in the metering section of the screw, changes according to the equation in Table 2. The taper angle was changed stepwise: 0 rad (0[degrees]0'); 1.309 X [10.sup.-2] rad (0[degrees]45'); 2.618 X [10.sup.-2] rad (1[degrees]30'); 3.927 X [10.sub.-2] rad (2[degrees]15'); 5.236 X [10.sup.-2] rad (3[degrees]00'). causing the change of groove maximum depth respectively by 0; 1.23; 2.46; 3.68 and 4.93 mm.

The number of active grooves was kept constant in the experiments; three active grooves were used. The distribution of grooves in relation to the feed opening and screw rotation directions is shown In Fig. 4.

Groove number I was not adjusted in the experiments because it was found that feed variation occurred when the depth of this particular groove was increased even a very small amount. It is possible that a change in feed opening geometry, as suggested by Upmeier and Schreiber [38] and Rauwendaal and Sikora [39], may improve feeding and eliminate the problem encountered with the first groove.

During planning and then conducting research of the extrusion process, an attempt was made to estimate and establish the accuracy of received results. The results of measurements of the given studied factor constitute the average of five measurements. Particular measurement values differed slightly or were Identical; in connection with this the indication of other estimators was given up.

RESULTS AND DISCUSSION

The change in flow rate with taper angle is shown in Fig. 5. The change in flow rate with screw speed is shown in Fig. 6.

When the taper angle is increased there is an initial slight reduction in flow rate followed by a steady increase in flow rate. When the groove taper angle equals 0.026 rad [approximate] 1[degrees]30' flow rate equals the flow rate in the extruder without the grooved section.

Further increase of groove taper angle causes an increase in polymer flow rate. At the small groove taper angle, a slight decrease of value G was observed, which may be connected with smaller polymer friction against the barrel surface. Only after exceeding a certain value of groove taper angle, the polymer flow rate begins to increase because polymer friction at the wall increases. At the various screw speeds, an increase of groove taper angle from zero to 0.052 rad [approximate] 3[degrees]0' causes a significant increase in the flow rate G, namely the increase of 6.1% at the lowest screw speed and 7.6% at the highest screw speed. The increase of screw speed from 3.5 [s.sup.-1] to 5 [s.sup.-1], that is, by 43%, causes the increase of mass flow rate by over 32% at the barrel without grooves and by over 36% at the barrel with grooves at the largest taper angle.

The extruder energy efficiency at the very small groove taper angle (from 0.014 rad [approximate] 0[degrees]48') decreases independently of screw speed, while at the bigger groove taper angle, energy efficiency increases (Fig. 7 and Fig. 8). The energy efficiency of the extruder increases with screw speed. The energy efficiency of the extruder without grooves is higher than the energy efficiency of the extruder with grooves with a groove taper angle of 0.013 rad [approximate] 0[degrees]45'. This confirms the thesis that the energy efficiency of the extruder with a grooved feed section is lower than of the same extruder without grooves. However, a further increase of groove taper angle increases the energy efficiency of the extruder. With an increase in groove taper angle from zero to 0.052 rad [approximate] 3[degrees]0', the energy efficiency of the extruder initially decreases by almost 3.1% and then increases by over 8% at the lowest screw speed. At the highest screw speed, the energy efficiency decreases initially by abo ut 5.9% and then increases by about 10.5%. The biggest increase in energy efficiency with screw speed was over 10% and was obtained at the largest groove taper angle; it is only a little larger than in the extruder without grooves.

Energy efficiency of the extrusion process (Fig. 9 and Fig. 10) is always much higher than energy efficiency of the machine but the changes occur in a similar fashion. An increase in groove taper angle from zero to 0.052 rad [approximate] 3[degrees]0' causes a reduction in energy efficiency of the extrusion process by 2.8%. The initial reduction in energy efficiency is smaller than the subsequent increase in energy efficiency. An increase of screw speed from 3.5 [s.sup.-1] to 5 [s.sup.-1] (43%) causes an increase in energy efficiency by about 9% when the groove depth is zero.

The relationship between the specific energy consumption and groove taper angle correlates with the energy efficiency. The initial increase in groove taper angle -- Fig. 11 -- causes an increase in specific energy consumption of the extruder by 4.8% at the lowest screw speed and 6.3% at the highest screw speed. A subsequent increase in groove taper angle reduces the specific energy by 6.1% at the largest taper angle and lowest screw speed and 7.4% at the highest screw speed.

With the increase of screw speed from 3.5 [s.sup.-1] to 5 [s.sup.-1] (43%) and a still larger groove taper angle, slightly larger percentage decreases of specific energy consumption of the extruder can be observed. Thus, for example, for [alpha] = 0.013 rad [approximate] 0[degrees]45', the decrease equals over 4.9% while for [alpha] = 0.052 rad [approximate] 3[degrees]0' it is 6.3%. For the extruder without grooves, the reduction in specific energy consumption is about 5.8% -- Fig 12.

With the increase of screw speed, the values of the specific energy consumption [E.sub.JC] are larger than values [E.sub.JP] by about 40%. The characteristics of specific energy consumption of the plasticating system are similar to specific energy consumption of the extruder, but all phenomena described here occur less intensively -- Fig. 13 and Fig. 14. The smallest values of specific energy consumption of the plasticating system are for the largest groove taper angle and highest screw speed.

It is worth mentioning that at a lower screw speed and a smaller groove taper angle, there is a slight decrease of polymer enthalpy resulting from decreasing polymer flow rate in the plasticating system. With increasing screw speed and taper angle, the polymer enthalpy and friction increase, and therefore heat generation increases as well. This method of heating is more energy efficient than heating by external heaters. With increasing screw speed, the energy added to the extruder increases as well, but the increase is less than the increase in throughput. As a result, the energy efficiency increases and the specific energy consumption reduces.

The change of polymer flow rate in the plasticating system of the extruder is most probably more influenced by the change of screw speed than taper angle of the grooves. However, a temperature increase of the processed polymer is probably caused more by the increase of taper angle of the grooves than by the increase of screw speed.

CONCLUSIONS

On the basis of experimental studies of an active grooved feed section of the extruder during autothermal extrusion of medium density polyethylene, the following conclusions can be drawn.

The relationships between the groove taper angle and the polymer mass flow rate, the energy efficiency of the extruder and the specific energy consumption are nonmonotonic. At small taper angles the values reduce while at larger angle the values increase significantly.

The relationships between the screw speed and the polymer mass flow rate, the energy efficiency of the extruder and the specific energy consumption are always monotonic, either increasing or decreasing.

The best results were obtained with the largest taper angle used in the experiments, three degrees. The effect of larger taper angles on the extrusion process will be the subject of future research.

The significance of the influence of the groove taper angle and groove depth as well as screw speed on the extrusion process depends also on the design of the whole plasticating system, especially the screw, which has to be adapted, mainly to the increased polymer flow rate forced In the grooved feed section of the extruder. This adaptation requires separate research.

The grooved feed section of the extruder should be intensively cooled. This can Increase the polymer flow rate but it may reduce the energy efficiency. In this study the feed housing was cooled with forced air. This air cooling may not have be sufficient to obtain optimal results. This will be studied in more detail in the future.

The control of thermal, rheological, kinematical and other processes cannot be achieved solely by adjustment of screw speed. By adjustment of the geometry of the grooved barrel section, a greater level of process control can be achieved in extrusion.

The application of extruders with adjustable grooved feed sections can contribute to more widespread application of both autothermal and conventional extrusion.

ACKNOWLEDGMENT

This work is supported by a grant No 7 T08E 062 20 from State Committee for Scientific Research.

NOMENCLATURE

D screw diameter, m.

[D.sub.r] core diameter of the screw, m.

[E.sub.C] energy supplied to the extruder, kJ.

[E.sub.JC] specific energy consumption of the extruder, J/g.

[E.sub.JP] specific energy consumption of the plasticating system, J/g.

G polymer mass flow rate, kg/s.

[h.sub.III] depth of the screw channel in the metering section, m.

i polymer specific enthalpy, J/kg.

M screw torque, N * m.

[m.sub.w] mass of extrudate measured length, kg.

[T.sub.e] time of energy measurement supplied to the extruder, s.

[T.sub.w] extrusion time of extrudate measured length, s.

[t.sub.w] temperature of the extrudate when leaving extruder head die, [degrees]C.

[[].sub.C] power supplied to the extruder, W.

[[].sub.N] thermal power conveyed by polymer, W.

[[].sub.P] power supplied to the plasticating system, W.

[V.sub.o] circumferential velocity of the screw, m/s.

[alpha] taper angle of the grooves, rad.

[[alpha].sub.r] helix angle at the screw flight, rad.

[[gamma].sub.R] shear rate in the metering section of the screw, [s.sub.-1].

[k.sub.P] energy efficiency of the extrusion process, %.

[k.sub.W] energy efficiency of the extruder, %.

v frequency of screw rotation, rev/s.

w angular velocity, rad/s.

[[]REPRESENTS GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII]

REFERENCES

(1.) H. Decker, Die Spritzmaschine, Paul Troester Machinenfabrik Hannover (1941).

(2.) W. H. Darnell and E. A. J. Mol, SPE Journal, 12, 20 (1956).

(3.) W. W. Bode, SPE ANTEC Tech. Papers, 37, 153 (1991).

(4.) P. Fischer, Plastverarbeiter, 30, 117 (1979).

(5.) R. Hegele, Plastverarbeiter, 23, 678 (1972).

(6.) H. Potente, Kunststoffe, 80, 80 (1990).

(7.) J. Sikora, Polimery, 43, 548 (1998).

(8.) G. Fuchs, Plastverarbeiter, 19, 765 (1968) and 20, 237 (1969) and 21, 235 (1970).

(9.) K. Schneider, Kunststoffe, 59, 757 (1969).

(10.) G. Menges, W. Predohl, and R. Hegele, Plastverarbeiter, 20, 79 and 188 (1969).

(11.) G. Menges and R. Hegele, plastverarbeiter, 23, 332 (1972).

(12.) G. Menges. Einfurungin die Kunststoffverarbaitung, Carl Hanser Verlag, Munich (1979).

(13.) H. Potente, Kunststoffe, 75, 439 (1985).

(14.) H. Potente, Kunststoffe, 78, 355 (1988).

(15.) J. Diakun, Podstawy uaktywnienia strefy zasilania w konstrukcji wyt aczarki-slimakowej. Wydawnictwo Wyzszej Szko-y Inzynierskiej w Koszalinie, Koszalin (1991).

(16.) E. Langecker, G. Langecker, and W. Fillman, Plastverarbeiter, 28, 531 (1977).

(17.) G. Langecker, European Patent (filed Jun. 22, 1982) A2 0 069 271 (1984).

(18.) E. GrunschloB, SPE ANTEC Tech. Papers, 25, 160 (1979).

(19.) H. Helmy, Plast. Eng., 39, 43 (1983).

(20.) J. Sikora, Przetworstwo Tworzyw, 3/41, 65 (1998).

(21.) G. Menges, W. Feistkorn, and G. Flschbach, Kunststoffe, 74, 695 (1984).

(22.) C. Rauwendaal, Extrusion, Carl Hanser Verlag, Munich (1998).

(23.) J. Sikora, Studium autotermicznosci procesu-wyt aczania i strefy rowkowanej wyt-aczarki. Wydawnictwa Uczelniane Polietchniki Lubelskiej, Lublin (2000).

(24.) E. GrunschloB, Kunststoffe, 83, 309 (1993).

(25.) H. Potente, Kunststoffe, 71, 474 (1981).

(26.) E. GrunschloB, Kunststoffe, 75, 850 (1985).

(27.) G. Menges, A. Mayer, W. Laugwitz, and E. Baur, Plastverarbeiter, 35, 97 (1984).

(28.) H. Potente and A. Fornefeld, Plastverarbeiter, 38, 96 (1987).

(29.) J. Ogando, Plast. Technol., 40, 11(1994).

(30.) H. Bacher and H. Schulz, U.S. Patent (filed Oct. 16, 1997) 5,783,225 (1998).

(31.) J. Stasiek, Polimery, 42, 14 (1997).

(32.) W. Lang and G. Stockmaier, Plastverarbeiter, 31, 573 (1980).

(33.) G. Detlef, German Patent (flied Jul. 19, 1982) 3,226,918 (1984).

(34.) P. Meyer, U.S. Patent (filed May 2, 1983) 4,462,692 (1984).

(35.) H. Peiffer and H. Eberhardt, U.S. Patent (filed Aug. 15, 1985) 4,678,339 (1987).

(36.) C. Rauwendaal, U.S. Patent (filed Apr. 25. 1997) 5,909,958 (1999)

(37.) R. Sikora, J. Sikora, and J. Diakun, Polish Patent (filed Jun. 17, 1994) 174,623 (1998).

(38.) H. Upmeier and T. Schreiber, U.S. Patent (filed Jul. 22, 1983) 4,494,877 (1985).

(39.) C. Rauwendaal and R. Sikora, SPE ANTEC Tech. Papers, 45, 200 (1999).

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Table 1

Some Properties of Processed Medium Density Polyethylene ME 2421.


Property Value

Density in temperature
23[degrees]C, kg/[m.sup.3] 941
Mass flow rate index
[MFR.sub.(190;5)], g/10min 0.9
Yield point at tension, MPa [greater than
 or equal to]19
Coefficient of linear
expansion, [degrees][C.sup.-1] 2.4 X [10.sup.-4]
Coefficient of thermal
conductivity, W/(m - K) 0.36
Elongation at break, % [greater than
 or equal to]600
Softening point according [greater than
to Vicat, [degrees]C or equal to]119
Table 2

Relationship Between Screw
Speed and Polymer Shearing Rate.


 Polymer shearing
 rate (at the wall) in
 Frequency the metering
 Screw speed of screw section of the screw
 Angular Circumferential rotation [[gamma.sub.R],
No [omega], rad/s [V.sub.o], m/s v, rev/s [s.sup.-1]

1 18.53 0.23 2.95 37.53
2 22.05 0.27 3.51 44.66
3 25.94 0.32 4.13 52.57
4 29.21 0.36 4.65 59.16











 Basic
No dependencies

1 [omega] = 2 [pi] v
2 [[nu].sub.0] = Dw/2
3
4 [[gamma].sub.R] =
 [D.sub.rIII] [omega] COS
 [[alpha].sub.r]/
 [2h.sub.III]



Note: Index III includes the metering section in the plasticating system
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Author:SIKORA, JANUSZ W.
Publication:Polymer Engineering and Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Sep 1, 2001
Words:4012
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