process conditions and procedures to optimize …
TRANSCRIPT
PROCESS CONDITIONS AND PROCEDURES TO
OPTIMIZE QUALITY AND EFFICIENCY AND
REDUCE WASTE IN EXTRUSION
Presented by:
Thomas Bezigian
PLC Technologies Film & Extrusion Consulting
Cicero, New York, USA
Agenda
• Goals
• Introduction
• Review Case Studies / Define Problem
• Review Causes
• Present Solutions
Goals of This Talk
• Reduce start-up time, downtime and waste
• Improve quality, productivity and efficiency
• Pass this knowledge to the next generation
Introduction
• This presentation is a summary of 40+years of
observation
• Single screw extruders
• Varying extruder lengths and diameters
• Flat film & sheet dies as well as blown film dies
• Varying die widths and diameters
• Unmodified olefinic materials, at various temperatures
• Same issues and solutions all over the world
The Real World in Extrusion
• At some point, R&D projects come to completion
• Manufacturing must make the product to specification,
and make it efficiently and with minimal waste
• Manufacturing managers strive to reduce waste, but
sometimes in the wrong places
• Reducing RPM or stopping of the extruder screw during
web-breaks and between jobs is a key area of concern
with regard to polymer degradation
The Real World in Extrusion
• In all melt processes, the polymer is exposed to a
combination of
• Temperature and Shear for a period of Time
• Screw design, process conditions, output,
equipment design, etc, determine TTS
• The ideal combination of TTS yield an ideal
extruded product
• Significant deviation from ideal conditions will yield
less than ideal results
Problem Statement – A Less-Than-Perfect Melt Curtain
Streaks, gels, carbon & voids on start-
up, persisting for days or longer Streaks in the
melt curtain
originate in the
die.
Gauge bands are defects, which in turn cause dull streaks,
crazing, packaging issues, customer complaints, etc.
mLLDPE coated onto a metalized
OPP film with many gauge bands
250 microns of a PVDF/Acrylic blend
coated onto a 100-micron Mylar film
Gauge bands / die streaks
Carbon buildup in the die lands cause streaks in the
melt curtain and gauge bands in the film
• Die streaks are due to
buildup of degraded polymer
(carbon) on the die lands
• Polymers degrade under the
influence of excessive time,
temperature, and/or shear
• Excessive idle time at 0 or
low screw speeds are the
cause
• Shutting down the extruder HOT, that is,
without a proper “cool down” procedure is
a contributing factor to carbon formation in
the die land area of the die.
Carbon build up in the die land area is universal – Seen in
blown film, cast film & sheet, and extrusion coating/laminating
190-200°C
Gels are areas of ultra-
high molecular weight
polymer that cannot be re-
melted, with viscoelastic
properties different than
undegraded polymer, and
thus draw down differently,
and can cause pinholes &
other defects
Gels & carbon are
commonly seen on start-
up & resin changes
Problem Statement
Gels in the melt
curtain generally
originate upstream
of the die
Voids – visible, not visible, and not there
Obvious voids
Micro-voids, resulting in a hazy
appearance to the melt curtain,
quite often accompanied by
excessive smokeDesired state – crystal clear melt,
free of voids, gels, carbon and
streaks
Indicative of improper shutdown
conditions and procedures
Indicative of excessive melt
temperature
Perfect – Crystal clear melt, free of
voids, streaks, gels or carbon
Voids do not possess
barrier properties, strength
properties, heat seal
strength, hot tack, etc.
Voids – No Voids
Gels/Carbon = End Stage Degradation Products
• When gels and carbon are observed, other properties
have already been compromised, including:
• Optical properties
• Haze, gloss, color, surface texture
• Organoleptic properties
• Strength properties
• Tensile/elongation
• Heat seal/hot tack
The general causes of voids, smoke, streaks,
carbon, die lines and thus reduced efficiency and
increased waste are:
• Polymer Degradation, due to:
• Exposure to excessive residence Time
• Exposure to Temperature
• Exposure to excessive Shear
• Especially in the presence of Oxygen** This Includes adsorbed
oxygen on the pellet surface
Compare Extrusion Coating & Blown Film Conditions
Clear pellets are virgin ethylene-propylene copolymer and
white pellets are highly stabilized E-P pellets, heated for
various times at 250ºC
Extrusion Coating Blown Film
320-330ºC 190ºC
•Quite often, extrusion coating
lines are shut down at 320ºC
and blown film lines just shut
down at 190ºC
•Time at elevated temperature
are key factors in polymer
degradation
From the Arrhenius equation we know that
Degradation occurs 28 faster (256x) at 230°C vs 150°C
Degradation occurs 218 faster (262,144x) at 330°C vs 150°C
The Presence of Oxygen Increases Degradation
• There is indeed oxygen in the extruder
• Adsorbed oxygen on the pellet surface
• Entrapped oxygen in the extruder due to improper
solids bed development – extremely important
• Zone 1 temp determines COF between pellet and barrel,
which determines solids conveying angle, which is
critical to creating a solid bed, free of voids, before melting
begins
Proven by Bob Gregory ~50 years ago
COF of pellets to steel, max at DSC + ~ 15°C
Photo Credit: Spirex
Chan Chung Freeze-Pull Experiment Photos
• Solids Conveying 1-4 D (loose pellets only)
• Incomplete compaction (voids in solid bed)
indicates presence of oxygen
• Melting begins at D5
• Melt pool begins at 6D
• Melt pool grows downstream
• Voids are clearly seen downstream
• Pressure at the end of the compression
section can be 400 bar (~4 MPa/6000 psi)
• 400 bar, 320°C, O2, time, shear = reactor
• Maximize solids conveying/compaction
The Solutions
• Key focus areas are:
• Start-up conditions and procedures
• Operating conditions and procedures
• Shut down conditions and procedures
• Stabilizing the melt with anti-oxidant on shutdown
DISCLAIMER:
The following recommendations are general guidelines that are
applicable to most extrusion lines using modern equipment,
processing low density polyethylene (LDPE). Your equipment
or materials may require special conditions not covered
here. Be sure to consult with your resin supplier, machine
supplier or technical experts to ensure that these conditions are
applicable to your equipment and materials. Improper start-up
procedures have been known to damage equipment and/or
cause explosions.
Generalized Start-Up Conditions for PE resins
Step Z 1 Z 2 Z 3 Z 4 Z 5 Z 6 Die, etc
Cold start, filled barrel 100° 100° 100° 100° 100° 100° 100°
Wait until all zones are up to temperature; power on die 1st if needed
Increase temps, wait 125° 125° 125° 125° 125° 125° 125°
Increase temps, wait 125° 150° 150° 150° 150° 150° 150°
Slowly rotate the screw and closely observe motor load and pressure
If all is normal, set zones 3 and forward to desired setpoints and ↑ to 30 RPM
Cold start, empty barrel (example only)
125° 150° 200° 250° 290° 320° 320°
(typical values) Heat up machine to set points, RPM to 50, go
Start-Up Conditions/Procedure
• In general, heat all zones to 100ºC until equilibrium is reached
• Then ↑ all zones to 125ºC (15º above the DSC melting point)
• Let soak, ensure equilibrium, then increase Z2 forward by 25ºC
• Let soak, ensure equilibrium, then rotate the screw slowly (1-2
RPM) and closely observe pressure and motor load
• DO NOT LEAVE THE CONTROL PANEL AT THIS TIME
• IF PRESSURE OR MOTOR LOAD IS EXCESSIVE, STOP
SCREW IMMEDIATELY
• If pressure and motor load are not excessive, increase RPM to
30 and observe. Continue if OK, stop if necessary.
Start-Up Conditions / Procedure
• Exact times and temperatures will be learned
empirically on your particular system - size of
machine, screw design, components, pipes, heater
size (W/m2 and W/kg), etc.
• Minimize exposure of the resin to time and
temperature during start-up, that is…
• MINIMUM RESIDENCE TIME ON START-UP
Normal / Ideal Operating Conditions
Variable Value
Residence Time 6-8 minutes under normal conditions
This typically means an average RPM of 80-100 and a minimum or 30 RPM (never zero)
Temperature For LDPE 190-320°C (depending on process)
Shear Overall average is 200-700 sec-1, but can be much higher in the radial flight clearance or in some adapter valves
Operating Conditions / Procedures
• Typically, a minimum of 30 RPM is recommended
during purging and running (100 RPM is better)
• It is OK to reduce RPM when coming online, or for
short periods of time (5 minutes max)
• Slow RPMs = long residence time (> 3 hours) =
excessive thermal energy input = degradation
• Shim die if necessary; disassemble when required
Shutdown Conditions / Procedures
• This is basically the reverse of starting up
• Reduce barrel temperatures and maintain ~30-50
RPM to cool the downstream equipment (screen
changer, pipes, coex block, die)
• E.g… If purged at 3-5 RPM to save resin on shutdown,
there is insufficient cooling from the melt, and the die
will remain hot for hours, resulting in more polymer
degradation vs 30 RPM (10X res. time)
Typical Shut Down Conditions for PE resins
Step Z 1 Z 2 Z 3 Z 4 Z 5 Z 6 Die, etc
Reduce temperature 125 150 150 150 150 150 150
Maintain ~30 RPM until melt temp ~ 150° +/- 10°C - to cool the dieOutput at 30 RPM cools the die, etc. Higher heats, slower degrades
Introduce 10,000 ppm antioxidant of choice to feed, purge 20-25 min
Reduce screw speed to ~3 RPM, introduce drool stick (for extrusion coating)
Turn off extruder
The combination of low temperature, antioxidant and a drool stickallows for a defect free melt curtain on start-up
Summary
• Common, universal symptoms and causes of polymer
degradation were given
• These solutions include:
• Proper start-up, operating and shutdown conditions
• Proper start-up, operating and shutdown procedures
• Use of an antioxidant purge on shutdown
• It was shown that these conditions and procedures will
improve overall productivity, quality and efficiency, as well
as reduce downtime and waste
Summary
• These defects cause waste and off-quality product. This
will become a real issue in the “new economy” of
sustainability.
• The use of ceramic coatings (AlOx, SiOx) to achieve
barrier will not tolerate streaks, as they will cause crazing
of the ceramic barrier layer)
• If you can’t do it right the first time, when will you find time
to do it right the second time?
Thank You
for your attention
Presented by:
Thomas Bezigian
PLC Technologies Film & Extrusion Consulting
+1 315 382 3241