experimental studies on efc -...

CHAPTER 4

Experimental Studies on EFC

Chapter 4 Experimental Studies on EFC EFC

University of Pune CSIR-NCL 107

Chapter 4 Experimental Studies on Extrusion

Film Casting (EFC)

4.1 Extrusion Film Casting set-up

EFC experiments were performed using a laboratory scale single-

screw extruder (Model: Haake PolyLab System from ThermoFisher Scientific,

GmbH, Germany). The single-screw extruder (SSE) (Model: Haake Rheomex

252) had a ¾” (or 19.05 mm) diameter standard metering screw with a

length-to-diameter (L/D) ratio of 25. The extruder (as seen in figure 4.1) had

three electric heating zones with pneumatic air cooling for temperature

control. For the present research, the three heating zones of the extruder

were kept at 170, 180, and 190oC, respectively. A coat-hanger cast-film die

(as shown in figure 4.2) of 100 mm fixed width and 0.3-0.6 mm adjustable die

gap was attached to the extruder. For the present research, the die gap was

kept constant at 0.46 mm, which implied a die aspect ratio (AR = die

width/die gap) of 218. The die gap dimensions were confirmed with brass

measurement shims provided by the manufacturer that were inserted into

the die gap and moved across the width to take gap measurement. The cast-

film die was kept at a constant temperature of 190oC by electric cartridge

heaters.

A chill-roll take-up unit of controllable RPM was utilized for drawing

and winding the extruded film. The chill-roll unit consisted of three stainless-

steel crowned rolls of 75 mm diameter with mirror polishing. They were

internally cored for refrigerant fluid circulation for temperature control. The

chill-rolls were maintained at temperatures ranging between 5-10oC for

rapid quenching of the molten extruded film.



Figure 4.1 Schematic of Rheomex 252 SSE.

Majority of the EFC experiments were carried out at a constant

extrusion temperature of 190oC (zone#3 of SSE and the die temperature) and

at a constant screw speed of 20 RPM, which implied a volumetric flow rate of

187 mm3/s and a thickness averaged exit velocity from the die of 4.3 mm/s

(u0). The take-up length (TUL) i.e. the distance between the die exit and the

chill-rolls could be varied as required and for the present research three

different TUL’s were used: long (230 mm), intermediate (90 mm), and short

(10 mm).

For maintaining parallelism between the extruder-die set-up and the

chill-roll take-up unit, a custom frame was designed and built that also had

other attachments serving various purposes such as online stress

birefringence optics, etc. Though not used in this PhD research, the online

stress birefringence set-up is currently being built (as shown in figure 4.5)

and is planned to be used in the near future to obtain stress profiles in the

molten polymer film at various locations. A temporary dark-room was also

set-up around the EFC process set-up which allowed for proper video

imaging of the experiments under controlled illumination for obtaining the

necking and velocity profiles. Figures 4.3, 4.4 and 4.5 outline details of the

EFC experimental set-up.



Figure 4.2 Schematic of ThermoHaake coat-hanger cast film die of 100 mm width.

Figure 4.3 EFC process set-up inside a make-shift dark-room.



The rotational speed of the chill-roll (in RPM) was measured using a

hand-held tachometer (Model: DT2236 from Lutron, Taiwan). This rotational

speed was converted into tangential speed in mm/s using standard

conversion formula containing the circumference of the chill-roll. The

tangential speed was then used for setting the required draw ratio (DR)

defined as the ratio of the tangential chill-roll velocity to the exit velocity at

the die. The EFC process itself consisted of extruding the polymer at a set

screw RPM from the cast-film die; threading the molten film in the gap

between the first and second chill-roll and then through the gap between the

second and the third and final chill-roll before being wound up. In the present

research, the DR was varied from 2 to 17.

Figure 4.4 Close-up of necking of molten polymer film between the die-exit and chill-rolls.



Figure 4.5 Online stress birefringence set-up in EFC.

Two different broad sets of EFC experiments were performed by

changing the way in which a set DR was achieved. In the first set of EFC

experiments (Set#1), the desired DR was achieved by keeping the-velocity of

the polymer melt at the die-exit (or in other words the extruder screw RPM)

as constant and varying the chill-roll velocity. In a second set of EFC

experiments (Set#2), the chill-roll velocity was kept constant while the

velocity of the polymer melt at the die-exit was varied by changing the

extruder screw RPM. As mentioned earlier in the chapter 2 on literature

review, Lamberti and coworkers 143 had observed that film width actually

increased (or necking decreased) on increasing the DR when the DR was

achieved by keeping the chill-roll velocity as constant and varying the

extruder screw RPM. The second set of EFC experiments were done using

two representative PE’s viz. the linear LLDPE 2045G and the LCB-containing

LDPE 170A at a representative TUL of 10 mm. It will be seen later in Chapter



6 that the second set of EFC experiments on the two PE’s did not reproduce

the necking observations of Lamberti and coworkers mentioned earlier in

this paragraph. Therefore, it was decided to concentrate only on the first set

of EFC experiments, i.e. where the DR was adjusted by varying chill-roll speed

keeping the extrusion speed constant and all relevant experimental data was

obtained from this set of experiments. A representative DR of 2 or 4 was

designated as low DR, a DR of 10 was designated as intermediate DR and

finally a DR of 17 was designated as high DR. It was observed for these set of

EFC experiments that the pressure drop (P) across the die as well as the

torque on the extruder screw did not vary with changing DR or TUL.

However, this was not the case for the second set of experiments wherein the

DR was varied by changing the extrusion speed while keeping constant the

chill-roll speed. In these experiments, the P across the die decreased with

increasing DR, as might be expected.

It was observed that for all TUL’s and for all DR’s, the extruded film

remained in melt state and a freeze-line was not visible in the free surface

area between the die-exit and the chill-roll. About three meters of film was

collected at each DR for the given TUL. The solidified film was then carefully

labeled and placed in a marked zip-lock bag for further analysis of width and

thickness profiles. The thickness profile across the width of the solidified film

and the final width of the solidified extruded film were measured off-line

with a digital micrometer (Model: 293-240 with least count of 0.001 mm

from Mitutoyo, Japan). The center and edge thicknesses were measured at a

minimum of five locations along the length of the film for each DR and TUL.

The width of the solidified film for each DR and TUL was also measured at a

minimum of five locations along the length of the film using a standard meter

scale having a least count of 1 mm. The solidified film’s thickness and width

values were averaged and then normalized with respect to the initial

thickness and width values present at the die exit (or in other words with the

die gap and die width).



Table 4.1Processing parameters of SSE during EFC of the PE resins: (a) Set#1: constant extruder screw RPM for all PE resins and (b) Set#2

Constant chill-roll velocity for LDPE and (c) Set#2 Constant chill-roll velocity for LLDPE

(a)

PE Screw RPM

P at end of SSE before die (MPa)

Torque on extruder screw (Nm)

LLDPE 2045G

20 0.18 25

LLDPE PL1840G

20 0.15 28

HDPE DMDH6400

20 0.18 23

LDPE 170A 20 0.15 24

(b)

Screw RPM

Die exit velocity

(V0) (mm/s)

Chill-roll velocity

(VL) (mm/s)

DR = (VL/V0)

P at end of SSE before

die (MPa)

Torque on

extruder screw (Nm)

83 16.6 50 3.01 0.43 63.00 41 8.3 50 6.02 0.30 50.60 27 5.5 50 9.09 0.26 39.30 20 4.15 50 12.05 0.22 33.00 16 3.33 50 15.02 0.19 28.80 13 2.77 50 18.05 0.17 21.00 11 2.5 50 20.00 0.15 18.90

(c)

Screw RPM

Die exit velocity

(V0) (mm/s)

Chill-roll velocity

(VL) (mm/s)

DR = (VL/V0)

P at end of SSE before

die (MPa)

Torque on

extruder screw (Nm)

83 16.6 50 3.01 0.29 46.40 41 8.3 50 6.02 0.21 39.20 27 5.5 50 9.09 0.18 37.50 20 4.15 50 12.05 0.17 32.80 16 3.33 50 15.02 0.16 32.00 13 2.77 50 18.05 0.15 28.70 11 2.5 50 20.00 0.14 28.00



4.2 Image Analysis for Necking and Velocity Profiles

As shown in figure 4.3, a commercial CCD camera (Model: UM201

from Uniq Vision Inc., USA) with a zoom lens (Model: C61215TH of 12 mm

focal length and C-mount from Pentax, Japan) was used to record the necking

and the surface velocity profiles of the film online during the EFC

experiments. The CCD camera was connected to a PC equipped with a frame

grabber card (Model: SV5 from EPIX Inc., USA). The frame grabber card was

driven by a software (Model: XCAP LTD, EPIX Inc., USA) which contained the

image capture utility to record both image sequence (TIFF or JPEG format)

and video (AVI format). Since image processing requires a high amount of

RAM and storage space, the image sequence recording PC had a RAM of 16 GB

which proved adequate for real-time sequence capturing. The PC also

contained a 1 TB partitioned hard disk to store the image sequences. The CCD

camera with the zoom lens was mounted on a tripod (Model CX440 from

Velbon, Japan) and placed below the molten film in the space between the

extruder and the chill-roll take-up unit. The CCD camera was focused on the

bottom surface of the molten polymer film. The CCD camera tilt was adjusted

to maintain reasonable parallelity with the molten film surface. As mentioned

before, a temporary dark room was constructed using standard 1” diameter

PVC pipes on which a thick black cloth was overlaid. All EFC experiments

were conducted inside this dark room. Two fluorescent lamps were placed in

the dark room at pre-determined locations to supply adequate illumination.

The positions of the lamps were determined after a series of trial EFC

experiments that were recorded for image sequences and subsequent

processing. Care was taken to ensure that the image recorded by the CCD

camera was as sharp as possible and filled the entire PC screen. This was

achieved by both adjusting the aperture of the CCD camera and also by

focusing of the zoom lens. It must be mentioned at this point that image

analysis could be performed for the long (230 mm) and intermediate (90

mm) TUL’s only and such analysis was not possible for the short (10 mm)

TUL.



Figure 4.6 Placement of tracer particles on surface of molten PE film.

Surface velocity profiles were determined by tracking the motion of

paper disks (ca. 2 mm diameter) that were placed on the molten film at

predetermined positions along the film width near the die exit (as shown in

figure 4.6 above). If the coordinates (in mm) at the center of the die exit were

marked as (0,0), then the other positions (on which tracer particles were

placed at the die exit) were as follows: lower edge: (0,100); intermediate

edge: (0,83) and middle edge: (0,67). Since PE molten films are transparent,

the CCD camera could easily capture the particle even though it was focused

on the bottom surface of the molten film while the particle was placed on top

surface of the molten film. The CCD camera had a fixed shutter speed of 25

frames-per-second (fps). This was independently verified by recording

images of a marked disk that was located on the shaft of a calibrated stepper

motor and rotated at exactly 25 RPM. Recording the rotation of the disk for

one minute created exactly 1500 images in which the particular mark at the

edge of the disc achieved its original position in every 60 sequential images.

This confirmed that the CCD camera was indeed capturing images at 25 fps.

The sequence of images captured by the CCD camera during film casting were

analyzed using either ImageJ® with a manual edge-detection and automatic

particle detection and tracking modules or by using Matlab® with image

analysis & processing toolbox. The images were corrected for camera tilt.



4.2.1 ImageJ® Analysis

ImageJ®, a public-domain Java-based image processing program

developed at the National Institutes of Health in the USA. ImageJ® is a

program that can be run on any computer running any operating system.

Since ImageJ® has an open architecture; many users have developed their

own Java-based modules that can perform several image processing tasks

including edge-detection and particle tracking velocimetry (PTV). These

plugins are freely available online along with the basic ImageJ® program

(http://rsbweb.nih.gov/ij/).

4.2.1.1 Particle Tracking Velocimetry (PTV) using ImageJ®

For the present research, the SpotTracker2D plugin 144 was utilized

for PTV analysis consisting of particle detection and tracking to obtain the

surface velocity profiles for the given PE films processed using EFC. As

mentioned earlier, image sequences of the motion of particles (from die-exit

to chill-roll) over the surface of the molten polymer film were captured using

the frame grabber card for each DR and TUL. These sequences of images

were labeled sequentially by the XCAP-LTD software and the time interval

between each image was 0.04 seconds (1/25 fps). The PTV process consisted

of the following steps:

1. In the ImageJ® program, a sequence of TIFF images (at a fixed DR and

TUL) was imported.

2. The image sequence was cropped appropriately ensuring that the area

of interest was fully visible.

3. When needed, the image sequence was corrected for camera tilt using

the inbuilt image rotation tools.

4. A scale was then set for measurements wherein a known distance

inside the image sequence was used for calibration. In the present

case, it was the die width or the film width just at the die exit which

was exactly 100 mm. This ensured that a certain length in pixels in the

image corresponded to an exact distance in mm. It was found for the

global set of image sequences in this research that a distance of 1 mm

http://rsbweb.nih.gov/ij/



corresponded with about 2.6 (±0.05) pixels. This conversion was

recorded for each set of image sequences for other DR’s at different

TUL’s.

5. The SpotEnhancingFilter2D tool in the SpotTracker2D plugin was

utilized to process the image sequence in such a way that only the

particle was visible as a bright spot against a dark background.

6. The main SpotTracker2D tool was then used to automatically track the

image-enhanced spot (or centroid) in the given set of image sequence.

This tool also contained several adjustments that included inputting

ranges for maximum displacement of the particle, movement and

center constraint of the particle and finally an intensity factor

correction. These fine adjustments had to be done manually for

accurate particle detection and tracking. It was observed that small

changes in intensity factor and maximum displacement of the particle

significantly affected the particle detection and tracking. Incorrect

values led to the particle being not tracked at all or tracked to a small

distance only and not for the entire TUL.

7. Upon successful particle detection and tracking, the plugin outputted

the x and y coordinates of the particle in each image frame. The

coordinates of the particle along with the associated time led to the

calculation of the velocities in the x and y directions.

8. For centerline velocity profiles, the Vx values were found to be

increasing from the die exit to the chill-roll. The Vy values remained at

zero or very close to zero indicating the particle was indeed moving

along centerline of the film.

9. For particles that were placed at other positions (viz. lower,

intermediate and middle edge locations) shown earlier in figure 4.6,

the velocities in y-direction (or in transverse direction) were finite

and measurable. Both the Vx and Vy values were determined from the

particle coordinates.

10. In this way, a set of Vx and Vy values were determined as a function of

either x or y directions for each PE at each DR and TUL.



4.2.1.2 Edge-detection using ImageJ®

Necking profiles for the extruded PE films were determined using

manual edge-detection in ImageJ® using the following steps:

1. A representative image was opened in the ImageJ® program and the

image was cropped to obtain the required area of interest.

2. The image was then enhanced by adjusting the brightness/contrast

and the sharpness. The edges of the film were clearly visible after

image enhancing

3. As mentioned earlier in the PTV sub-section, a length scale was set by

using the die width as a reference.

4. The point (or multi-point) selection tool was used to trace the edge

manually by clicking on the edge from the point where the film exits

from the die to the point where it goes inside the nip of the chill-rolls.

The tool allowed for outputting of the coordinates of each point

clicked as x and y locations.

5. The x and y coordinates (in pixels) were converted to millimeters and

used as such (for the necking profile)

6. Alternatively, the x and y coordinates could be plotted in a suitable

spreadsheet program (such as MS Excel or Sigma Plot) wherein a

smooth curve could be fitted to the coordinates to obtain a necking

profile.

7. This exercise was repeated for several other images in the same image

sequence (for a particular DR and TUL) but taken at different times to

verify that the necking profiles overlaid on top of each other. This also

meant that the film was not moving (or swaying) laterally over the

chill-roll.

8. The necking profiles so determined experimentally were then used for

subsequent comparisons with numerical predictions.

Figure 4.7 (a through d) show the centerline velocity, necking and

thickness profiles, respectively for selected PE resins. The details about the

DR’s used and the figures in the brackets next to the DR will be explained



later in Chapter 6. The average film thickness was calculated based on the

measured width and velocity and using the conservation of mass principle for

an incompressible melt wherein | ̇. Here, ̇ is

the volumetric flow rate. This equation implicitly assumes no edge-beading. It

is clearly observed in figure 4.7(a & b) that there is lowering of velocity very

close to the die exit (as seen in the inset) which is caused by the extrudate

swell of the PE resins. The extrudate swell phenomenon manifests itself in an

increase in thickness especially near the die exit (as observed in figure

4.7(d)). Figure 4.7(c) displays the necking profile for the linear LLDPE and

HDPE resins at a DR of 6 for the highest TUL of 230 mm. A small area of lower

necking is observed just near the die exit for both these linear PE’s. This

necking profile was observed for the linear PEs but not for the branched

LDPE.



Figure 4.7 Representative plots for experimental (a and b). centerline velocity,(c) necking, and (d). thickness profiles for selected PE resins at

given TUL's and DR's.



4.2.2 Matlab® Analysis for Necking, Velocity and Thickness Profiles

Matlab® software was also used for determining the necking profiles

by edge-detection and the surface velocity profiles by centroid detection and

tracking 145. The images were first corrected for camera tilt, if any. The

symmetry of the film was then confirmed and an edge-detection algorithm

(based on Canny technique) was utilized to determine the necking profile of

the film at different DR’s. Depending upon the intensity of the images an

adaptive thresholding technique was utilized. The edge coordinates were

then fitted with a polynomial to obtain a smoother edge and to remove any

fluctuations. The centreline thickness profile of the molten film was also

calculated from the known flow rate, the measured film width and the

measured centreline surface velocities using the continuity equation under

the assumption that the film thickness does not vary over the film width,

which is consistent with the model used in this research A centroid detection

algorithm was utilized to determine the positions of the particles on the

surface of the film. For consecutive images in a sequence (for a particular DR

at a fixed TUL), the coordinates (x, y) of the particles were obtained and the

velocity was determined in a fashion similar to that used in ImageJ® analysis.

Further details can be found in the work of Banik 145.

Finally, it was observed that the surface velocity, necking, and

thickness profiles determined from the two different image processing

techniques: ImageJ® and Matlab® matched very well.

4.3 Infra-red Thermographic Analysis of EFC

Infra-red (IR) thermographic images of the extruded molten film were

also taken at each draw ratio using a handheld IR camera (Model: i5 from

FLIR Systems Inc., USA). The IR camera had a fixed focus and was hand-held.

All IR images were taken from a fixed position from above the molten

extruded film in such a way that all details could be effectively captured. The

IR images were processed using FLIR’s custom software (QuickReportTM

from FLIR Systems Inc., USA) to obtain a thermographic profile in the image.



In particular, temperature profiles along the centerline of the film were

determined for every draw ratio. The emissivity values in the IR camera and

the analysis software were chosen in such a way that the temperature of a

point on the film centerline at the die exit was equal to the experimentally set

die temperature of 190oC, which was independently verified by a

thermocouple inside the die body. Emissivity values for the molten PE films

used in this research were in the range of 0.8-0.9 indicating that the films

were about 80-90% efficient in radiating thermal energy from their surface.

Figure 4.8 shows a representative IR thermographic image after processing it

by the QuickReportTM software.

Figure 4.8 A representative IR Thermographic image of a PE film.

Figure 4.9 below shows the variation of centerline surface

temperature of a representative PE film as it cools from the die exit to the

chill-roll at a fixed TUL and DR.



Figure 4.9 Measured centerline surface temperature of a representative PE film at a fixed TUL and DR.

In summary, all the experimental techniques used for measurements

in EFC were outlined in detail in this chapter. In the next chapter, CFD

modeling studies in EFC will be discussed.

experimental studies on efc -...

Documents