HANS SCHOTT and W. S. KAGHAN Film Division, Olin Mathieson Chemical Corp., New Haven 4, Conn.
Flow Irregularities in the Extrusion of Polyethylene Melts Square-shouldered die inlets and low melt temperatures lead to the irregularities encountered in high speed extrusion of polyethylene
IRREGULARITIES T h e flow pattern a t a die inlet was observed through a heavy-walled glass tube
in the flow of polymer melts have been observed with benchscale equipment. T h e present work was undertaken to study such irregularities in the extrusion of molten polyethylene with commercial scale equipment. Experimental
The polyethylene was Bakelite's DYNH-3 resin, with a melt index of 1.9 and a density of 0.914 at 25' C. A 2-inch Davis-Standard screw extruder was used with cylindrical dies of different inlet angles and one die consisting of a parallel slit with a square-edged entry (Table I). Dies were threaded on the outside and fitted interchangeably into a threaded flange, which was bolted to the flange of a right-angle reducing elbow connected to the extruder. A Helicoid probe chemical gage (American Chain & Cable Co.) was connected to the pressure tap. The melt temperature was measclred at the die exit with a thermometer and near the die inlet with a shielded thermocouple insulated from the threaded holding bolt by a Teflon sleeve (Therm0 Electric Co.). The two values were in close agreement.
i
mounted below the elbow and terminating in square-edged die L or S. A Teflon cover fitted snugly over the conical exterior of the die, making a tight seal with the glass wall. Matching view holes drilled in the die and Teflon cover permitted observation of the flow inside the capillary (Figure 1).
spiraled u p to three quarters of a turn, giving the rod a twisted appearance (knotty extrusion). With the kinks spaced closer and closer, the extrudate finally consisted of a succession of gnarls and constrictions (irregular extrusion). Irregularities from extrusion through the square-shouldered slit consisted of transverse ripples and xvaves. Normal Flow Pattern at D i e inlet
Appearance of Extrudate
When pol>-ethylenemelt was extruded through cylindrical dies with squareedged inlets, irregularities in the flow directly above the die inlet were reflected in the solidified extrudate (2, 4, 5). Regular extrusion produced a smooth, cylindrical rod with a diameter about 50% larger than that of the die channel. iVith gradually increasing throughput rates, the extrudate passed through successive stages of distortion (Figure 2 ) . The surface became dull and rough, the cross section slightly irregular (transition). Surface roughness, even at very low throughput, also occurs at low melt temperatures. Smooth regular ripples turning to close-spaced waves were observed next (wavy extrusion). Increased throughput rates produced uneven, knobby kinks spaced irregularly along an otherwise smooth and even rod. The kinks frequently
411 observations were made with dies L and S mounted in a glass tube. Motion pictures were taken at 64 and 225 frames per second. T h e extruder was alternately fed natural and pigmented resin. This showed that the flow pattern of the melt approaching the die channel took the shape of the frustum of an inverted cone: with an included angle of 30' to 40'. Its lower base was about the size
1 1 " I D. GLASS TUBE
E FF.LU E N 5 CONE"STAGNANT' ZONE"
TEFLON COVER VIEW HOLES
Table 1. Dimensions of Dies Die Inlet Height of Inconical cluded frustrum, entry Caplllaw, Inches Die inches angle, Diameter Length
r-
D G
H J F K L S
Figure 1 . Flow patterns at the square inlet and inside the capillary of the die were observed through a glass tube
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INDUSTRIAL AND ENGINEERING CHEMISTRY
0 17/s2
1 2I/a 0 16/w
0 0
Entry Angle,
O
T
180
180 90 45
3 I/ 1-e-
5/a
3/1e
;6
3/1S
6/S
22 180 90 180 180
3 / 1 ~
=/a
0.082 0.082 3/1c '/lS
0.315 0.300 1/4
1.009
Parallel Slit, Inches Gap Width Length 0.020
11/2
1 '/z
Figure 2. Extrusion through squareedged dies at increasing shear rates produces successive stages of distortion (conditions of Figure 3)
4
f
Figure 3. Flow curves obtained with square-edged dies change slope at the critical shear rate (horizontal bar). Extrudates from dies with gradually tapering inlets were smooth and even over the conditions shown by the flow curves
b
td
0 DIE J 0
0
DIE H DIE S DIE T
Figure 4. All square-edged dies have the same critical shear rate at the same temperature
SLIT
/ 10'1 IO'
I
Ll
SHEAR
10. STRESS,
I0'
DYNE/CMI TEMPERATURE, * C
of the capillary orifice and its height was approximately 2 cm. (Figure 1). The melt directly above the shoulder of the die between the glass wall and the conical effluent zone remained in this volume for an appreciable time. Even half an hour after the feed had been changed from natural to pigmented resin, clear melt was present in parts of this "stagnant" zone. The colored resin gradually spread from the initial, direct flow 40' approach cone to form an outer secondary cone, which was less strongly colored and had a 90' included angle. The melt in the stagnant zone circulated continuously: Particles of charred resin trapped there initially remained in steady motion, descending along the approach cone, moving over the shoulder of the die away from the orifice and u p along the glass wall. Clegg (7) observed the same effect. When the section of the glass tube just above the die was not heated externally, the melt in the stagnant zone cooled off rapidly because it was not continually replaced by fresh, hot melt. As viscosity increased, the circulatory flow slowed to an oscillatory motion. Another technique was to view the glass tube between two Polaroid sheets of perpendicular polarizing directions. Heart-shaped colored bands of stress birefringence fanned out in the melt above the orifice. The farther the bands were from the orifice, the broader they were. The tip of the heart had an approximate included angle of 80'. Except for the portion directly surrounding the entrance cone, the stagnant zone was free of stress birefringence patterns, as was a thin column of resin extending vertically above the orifice in the center of the tube-probably an isoclinic.
die inlet underwent continuous oscillations. Oscillation frequency was the same as that of the waves on the extrudate. At higher throughput rates, the melt above the inlet pulsated. Each pulsation produced a kink in the extrudate. Some strong pulsations were immediately preceded by weak ones. Pulsations were most pronounced at the base of the approach cone, which occasionally was twisted off, causing the tip to snap upward for a moment. This brief recoil was apparent when the glass tube was viewed against a background of vertical lines, or when pigmented resin was extruded after natural resin, so that the colored approach cone was surrounded by a stagnant zone of colorless melt. The twisted off tip of the cone retracted upward momentarily, as colorless melt filled the base of the entrance cone. Immediately thereafter, colored melt from above pushed the colorless melt back and restored the shape of the approach cone. Under those conditions, the straight portions of the extrudate consisted entirely of pigmented resin. Natural resin made u p the convex portions of the kinks and ridges of the spirals. Gnarls produced by very violent pulsations consisted of natural resin, with a colored core. Separation between natural and pigmented portions was sharp. The melt in the stagnant zone underwent a fast turnover once pulsations set in. In extruded rods with a colored center, the colored resin came to the surface on the concave surface of the kinks. Spiraling of the core was considerably more pronounced than twisting of the extrudate as a whole, in agreement with
Flow Anomalies
as a brief distortion of the heart-shaped colored fringes. A colored streak shot out momentarily from the edge of the orifice into the previously dark, stagnant zone. Occasionally the stress birefringence bands expanded just before a
At a 180" entry, as the screw speed was gradually raised and/or the temperature lowered, the extrudate became wavy, and the melt in the approach cone above the
pulsation started. T h e dark channel of melt extending vertically above the orifice also became twisted and distorted during each pulsation. Treatment
,f
Data
For
extrusion through cylindrical the maximum shear and rates of shear at the wall were estimated by PR ' 7
(1)
2L
and
44
? = -
xR~
(2)
For extrusion through the slit, T , the equations were
PH
7 = -
2L
(3)
y = - 6Q
(4)
and *A
where P i s the pressure drop, Q the volumetric flow rate, L the length and R the radius of the cylindrical channel, A the width of the slit, and H the gap. Subscripts c designate critical conditionscorresponding to incipient distortion of the extrudate. Flow curves with the different dies are shown in Figure 3. Discussion
The flow patterns a t a square-edged die inlet indicate that irregularities in the extrudate originate directly above the capillary inlet, and that the melt in the stagnant zone plays an important part; all extrudates obtained from dies with 45" and 22" included inlet angles within experimental conditions were smooth and even. Irregularities were observed chiefly with dies of 180° inlets, even though the effluent melt adopted a natural angle of 45", and melt outside this 45" cone merely circulated. Yet, replacing this stagnant volume of melt by metal-Le., going from a die with a VOL. 51, NO. 7
JULY 1959
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180" angle to one with a 45" or 22" angle-virtually eliminated extrudate irregularities. The highest shear stresses and highest normal stresses probably accumulated in the melt near the rim of the orifice, and shear rupture resulted. This caused some of the stagnant melt to be wedged and dragged into the approach cone of effluent melt, while the flow of the latter was interrupted momentarily. This tearing and wedging disturbance propagated from the initial point to the adjacent melt portions around the rim, causing a pulsation. Because downward
extrusion proceeded while the twitch propagated horizontally around the orifice; it shomed u p as a spiral kink in the extrudate. Because of the elastic properties of the melt, the flow irregularities above the orifice were perpetuated by the shape of the solid extrudate. If shear rupture occurred with the natural approach cone surrounded by metal instead of stagnant resin-in dies with inlet angles of 45" or smallerpulsations and ~vedgingcould not occur: The extrudate would remain even. Clegg ( 7 ) noticed distortions of the colored rore and a rough surface in an
Factors Affecting Flow Irregularity Die Geometry. Within the range of conditions tested (melt temperatures 140" to 260" C., shear stresses to 4 X 106 dynes per sq. c m . shear rates to 3000 sec.-l), irregularities of flow and in the extrudate were observed chiefly with t h e square-edged dies and to a lesser sextent with dies of 90" inlet angles. Dies with included inlet angles of 22" and 45' always produced smooth and straight extrudates. Flow irregularities set in a t lower shear rates and/or higher temperatures for dies with 180" inlets than for those with 90" inlets. Under similar conditions, distortions were more severe for the former. At 150" C., the critical shear rate was 75 set.-* for die D and 805 set.-' for die G. Whereas the i d e t profile was a crucial factor in determining the critical shear rate and severity of flow distortions, capillary dimensions (length, diameter, and length-diameter ratio) had little effect. This agrees with Tordella (5) and Clegg (7) and disagrees with Westover and Maxwell ( 70). Temperature and Shear Rate. Lower melt temperatures and higher throughput rates promote the onset of extrudate distortions and bring on the successive stages. O n cooling the melt or raising the throughput, distortions appeared at a temperature as much as 1 O C. lower or a shear rate as much as 25 set.-' higher than the subsequent return to regular extrudates on heating or lowering the throughput rate. Critical shear rates measured with the square-shouldered dies are plotted as a function of the melt temperature in Figure 4. The points pertaining to the three cylindrical dies and the slit die fell on the same curve. This emphasizes the importance of the inlet profile and the negligible effect of channel shape and
dimensions on extrudate irregularities. The discrepancy with published critical data also shown in Figure 4 is probably due to differences in resin. Plotting the logarithm of the critical shear rate against the reciprocal of the absolute temperature results in a straight line represented by =
8.0 X 109 e
- _15,594 _
(5)
1.987 T
Shear Stress. The critical shear stress measured. with square-edged dies varied somewhat with melt temperature and channel dimensions (Table 11). If the nominal values were corrected for end effects, the critical shear stress might be completely independent of melt temperature and capillary radius, as suggested by Metzner ( 3 ) .
Table 11. Critical Shear Stresses for Square-Edged Dies Depend Somewhat on Temperature and Channel Dimensions Critical Shear Stress, C.
183 187 + 194 158 192 210 170 184 200 221 248 267 180 204 232 246
X 11.6 14.6 14.9 17.9 20.7 14.7 15.3 17.3 20.0 22.4 24.7 7.3 9.5 11.5 12.0
Acknowledgment
The authors thank Leon Marker for helpful discussions, C. A. Strange and Leo Gans for experimental work, and L. P. Faeth for taking high-speed motion pictures. literature Cited
Die
(1) Clegg, P. L., "Rheology of Elastomers," ed. by P. Mason, N. Wookey, pp. 174-89, Pergamon Press, New York, London,
S
(2) Clegg, P. L., Trans. Plastics Inst. 26,
Dyne 1 Sq. Cm. Temp.,
otherwise even filament extruded through a die with a 24' included inlet angle. The tentative explanation of extrudate irregularity as a stress relief mechanism originating in the approach to the die channel agrees in general with Tordella. who termed it melt fracture (5-7, 9), and Clegg ( I , 2). Spencer and Dillon ( 4 ) postulated that extrudate distortion originates at the die exit, where differences in longitudinal contraction of the fully elongated. highly oriented skin and the less oriented and less elongated core of the emerging jet caused it to buckle into spirals. Pulsations of the melt in the approach to the die inlet preceded the appearance of kinks in the extrudate. This places the source of the disturbances near ;he die inlet rather than at the exit. Westover and Maxwell (10) attributed the flow irregularities to Reynolds-type ._ turbulence. This requires volumetric critical flow rate proportional to the viscosity and to the first power of the radius (6). An inverse relationship between critical flow rate and viscosity was found. T h e dwell times of the melt in the extruder were too short to change its viscosity. The isothermal critical shear rate was practically independent of the radius for capillaries with square-edged inlets. This requires critical volumetric flow rate proportional to the third power of the radius, according to Equation 2. Direct comparison of experimental values of the critical flow rate measured with dies D, F, and S at equal temperatures resulted in an exponent of 2.87 f 0.31 for the radius, in agreement with Tordella (8).
S D D D
T T T
T
1956.
151 (1958). (3) Metzner, A. B., IND.ENG.CHEM.50, 1577 (1958). (4) Spencer, R. S., Dillon, R. E., J. Colloid Sci. 4,241 (1949). (5) Toidella, J. P., J . Aflfll. Phys. 27, 454 (1936). (6) Tordella, J. P., Rheol. Acta 1, 216 (1958). (7) Tordella, J. P., SPE J o u r n a l 12, No. 2, 36 (1956). (8) Ibid.,13, No. 8, 36 (1957). (9) Tordella, J. P.,7 r a n s . SOG. Rheol. 1, 203 (1957). (10) Westover, R . F., Maxwell, B.: SPE JoournaZ 13, No. 8, 27 (1957). RECEIVED for review January 19, 1959 ACCEPTEDApril 17, 1959
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