Extrusion, Sheet

This guide describes sheet extrusion of Fortiflex

Introduction
This guide describes sheet extrusion of Fortiflex
®
HDPE. The information results from studies at the BP
Solvay Polyethylene North America research laboratories, and practical experience from many commercial
operations. It is intended as an aid to BP Solvay Polyethylene customers. Because extrusion conditions vary
broadly, this information represents suggestions. Readers should carefully determine appropriate actions for
specific situations. Before undertaking any operations, readers should consult material safety data sheets,
operations manuals and equipment safety guides.
The first section presents various components of an extrusion line. It suggests guidelines for equipment
factors and processing. The section covers:
q
material handling
q
screw design
q
polishing and pull rolls
q
drying
q
static mixers
q
shearing and sheet handling
q
extrusion
q
die design
q
gauge monitoring and control
The second section is a troubleshooting guide, which lists sheet defects with potential "causes" and appropriate
suggested actions.
Sheet Extrusion Process
General comments
In an extrusion line, pellets melt in the extruder, form into the desired shape in the die, size and cool on
polishing rolls, and are cut and stacked (or coiled on cores). Each step is vital to finished sheet quality. Each
equipment component must be selected and sized with final sheet quality in mind. Sheet quality is defined
by quantitative measure of gauge control (both across the sheet and in the machine direction), surface
characteristics, color uniformity and physical properties.
This brochure will touch on each major component of an extrusion line with specific comments about process-
ing Fortiflex
®
HDPE. The brochure does not examine construction materials, thermal controllers, drive units,
screw cooling, etc. in detail.
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Raw Materials
Warehousing
No special techniques are required beyond keeping materials in a dry, covered area. Cartons, bulk trucks,
or railcars are common containers, depending on the material-handling system used. Bags are available,
but not common due to extra handling required and a greater possibility of contamination.
Material handling
Conventional plastic equipment adequately handles Fortiflex
®
HDPE pellets. These include:
q
vacuum loaders
q
pneumatic-venturi loaders
q
screw conveyors.
Regrind
Most extrusion operations must utilize regrind as part of the material mixture. Extrusion and thermoforming
trim scrap provides material for regrind. Scrap can be ground and fed back to the extruder with virgin pellets.
Take care to keep regrind free from contamination. Use a constant ratio of regrind with virgin pellets to main-
tain consistent feed and melting characteristics in the extruder. Use only compatible materials. High levels of
regrind may require changes in extrusion conditions to maintain desired output rates, depending on the
regrind’s particle size.
A mixture of particles, ranging from fine dust to large irregular chunks, constitutes regrind. Regrind with a high
percentage of fines (dust-like particles) can contribute to feed problems. It can also affect appearance of the
sheet with buildup on the cooling rolls. A fines removal system or a modification of grinding equipment could
be required if this problem is serious.
Colorants
Color Fortiflex
®
HDPE with any conventional coloring system. If using liquid colorants, the carrier must be
compatible with virgin material. If using color concentrates, the polymer base should be similar to the virgin
material, preferably higher in melt index. Check so-called "universal colorants" for compatibility, particularly in
white or colors where Iow pigment loadings result in high concentrations of the carrier. Incompatible materials
can result in deterioration of sheet properties due to delamination or color streaking.
When changing colorants or colorant loading, consider shrinkage characteristics of the sheet product. Certain
colorants could behave as a nucleating agent, altering polymer morphology.
Drying & Sheet Extrusion
Drying
Fortiflex
®
HDPE resin is not hygroscopic and produces acceptable sheet without drying when good storage
techniques are used,
Extrusion
Equipment factors
Major extrusion equipment suppliers operate in the U.S. and other countries. This bulletin does not outline
advantages or special features of various extruders, but the following factors should be considered in selecting
an extruder:
q
output
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thrust bearing design
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costs (initial, maintenance)
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design of drive
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type of heating and cooling
q
available horsepower
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barrel construction
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ease of cleaning
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screw design.
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Sheet size and desired output guide selection of the basic extruder size. Table 1 summarizes the range of
outputs typical of extruders in current use.
Table 1 also includes a range of drive motors supplied with extruders. The horsepower of motors typically
supplied has more than doubled in the past 10 years as higher outputs are achieved. Fortiflex
®
HDPE resin
requires 0.12 to 0.17 hp/Ib/hr. The drive units generally come with variable speed control. Both AC/DC drives
and magnetic eddy current clutch drives are used, as well as mechanical types. If possible, protect drive units
from surges in the main power supply.
Stock temperature control is very important. Temperature control of the extruder barrel is usually separated
into several sections or zones. Electrical heaters are most common. Many new extrusion lines come with
microprocessor control systems which provide exceptional dependability and reproducibility. Water cooling
and air cooling are both common, but water cooling is preferred. Since frictional heat (generated by conveying
resin down the screw) provides most of the heat required to melt the polymer, heaters primarily help maintain
a uniform barrel temperature.
Barrel heaters can help change feed patterns in the rear zones or alter conveying properties in the feed section
to overcome minor screw design problems or material differences. Most extruders have a melt temperature
indicator. Since surrounding barrel conditions affect melt temperature thermocouples, indicated temperature
may only be approximate. Some designs, however, place melt thermocouple in the melt stream just down-
stream from the screw for improved monitoring. Recent developments in fiber optics have made available melt
temperature indicators that read actual melt temperature with infrared technology. Since this is one of the most
important processing parameters, choose a measurement option that delivers accurate information. Record
data comparing extruder performance with observed process changes. In addition, occasional pyrometer
measurements of the stock temperature validate melt temperature indicator performance.
Table 1
Typical output rates and horsepower requirements for various extruder sizes
Machine size*
output (#/hr.)
Horsepower (hp)
2 1/2
100-250
25-100
3 1/2
300-800
50-150
4 1/2
700-1500
100-250
6
1200-2000
200-350
*screw diameter =inches
Sheet Extrusion
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Many extruders have a vent to use with multistage screws. Removing volatiles through the vent improves
sheet quality. Vented extruders are not generally necessary for processing Fortiflex
®
HDPE, although this
material can run on either two-stage or single-stage extruders. Since an existing barrel cannot be drilled
economically to add a vent, many new units are purchased with a vent, then plugged if not needed.
The breaker plate and screen pack are located in line with the extruder just after the screw tip. Use the screen
pack to eliminate contamination which might damage the die or affect sheet quality. Also use screens to
increase residence time and mixing in the extruder. The screen pack is usually a combination of screens from
20 mesh to 100 mesh. Screens are normally changed with a hydraulic screen changer which does not require
stopping the line.
Pressure gauges are also commonly found in the screen changer area. They measure melt pressure
between screw tip and screen pack. Gauges easily detect surging and warn when a screen pack is plugging
up, Excessive back pressure from a plugged screen results in high melt temperatures and/or reduced rates.
A gate valve may be located after the screen pack to adjust back pressure. This helps proper polymer melting.
Processing factors
Barrel settings shown in Table 2 are typical for a single-stage extruder. Actual settings depend on the grade
of HDPE to be extruded, extrusion line design and desired size of sheet. The important criteria are melt
temperature and uniformity exiting through the die. While this is primarily a function of the extrusion rate and
screw design, barrel sets can affect melt temperature, particularly if melt temperature is Iow. Take care to keep
the heaters and controllers in good repair to avoid hot or cold sections in the barrel. Actual melt temperature
should be checked with a pyrometer, either needle or optical (IR) type. The drive unit must maintain uniform
speed under varying load conditions. Check periodically by timing screw revolutions. Uniform output is critical
to maintaining uniform gauge control in the machine direction.
Table 2
Typical Extruder Barrel Temperatures for Fortiflex
®
HDPE resin
Rear......................................................... 375-425°F
Middle......................................................400-450°F
Front........................................................400-475°F
Melt temperature.................................... 400-475°F
Sheet Extrusion
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Sheet Extrusion
Screw design
The screw is the extruder’s heart. Producing quality sheet at an optimum rate relates directly to the screw
design. The screw design depends on several factors related to polymer melt rheology, desired output rates,
target melt temperature and desired sheet quality. Consider each factor in designing the screw.
One-stage or two-stage extruders are common for processing thermoplastics, The two-stage (devolatilizing
or vented) extruder uses a single screw which has the characteristics of two screws on a common shaft. This
technique allows volatiles to escape to the atmosphere through the vent or decompression section, Two-stage
screws are not normally used for processing HDPE.
A single-stage barrier-type screw design is typically used for extruding HDPE. All designs of barrier screws
ideally separate the mass of un-melted pellets from the polymer melt pool. This ensures that melt feed exiting
the screw is completely melted and homogenized. Using a screw mixing device, such as a Maddock mixing
section, furthers mixing. Do not overheat the polymer.
Proper screw design cannot be overstressed. Consider all necessary parameters, such as target melt tem-
perature, desired output and rheology of the polymer. Ignoring these factors can result in a design not optimal
for the application, leading to feed, temperature or output instabilities. Discuss design with equipment suppliers
and resin suppliers before committing to a screw design.
Before discussing design specifics, review these definitions. The 2-1/2 inch screw depicted above will be used
for an example. "D" stands for barrel diameter, which in this case is 2-1/2 inches. Screws are usually defined
in terms of L/D where "L" is the length. For this 24/1 L/D, "L" would be 24 x 2-1/2 or 60 inches. The range of
typical L/Ds are 24/1 to 36/1. Recent trends are toward longer L/D machines since output is proportionate to
L/D and, therefore, can be increased with longer screws. Additional horsepower may be required.
Static mixers
Extruder manufacturers constantly design and build extrusion lines for higher output. Most extrusion opera-
tions involve processing regrind with virgin pellets and/or adding color. Uniform distribution of feed components
helps achieve acceptable sheet properties, but it becomes more difficult as output increases. Head pressure
and stock temperature must be held constant to main-rain consistent sheet gauge.
Manufacturers are equipping more extruders with static mixers to overcome the problems mentioned above.
Static mixers, similar to the one shown, contain a series of passive elements placed in the flow channel, These
elements cause material to subdivide and recombine, increasing melt homogeneity. There are no moving parts,
and only a small increase in screw energy is needed to overcome resistance from mechanical baffles.
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Installing a static mixer increases the effective L/D of the extruder and usually raises melt temperature/head
pressure at the extruder end. However, the result is a more homogeneous melt presented to the die and a
more stable extrusion process with less output variation. For more information on static mixers, contact an
equipment supplier.
One note of caution: when using static mixers—allow sufficient heat-up time when starting an extruder,
especially if polyethylene remains in the mixer after shutdown. To prevent excessive pressures and extruder
damage, keep the extruder rpm Iow until material flows from the die.
Die design
Equipment factors
Sheet quality requirements strongly influence die selection. Fortiflex
®
HDPE works with dies that have either
a constant diameter or tapered manifold. BP Solvay Polyethylene recommends running Fortiflex
®
HDPE resins
on streamlined manifold dies ("coat hanger" flexible lip types).
Choose die land length for HDPE based on expected sheet thickness. Generally, shorter land lengths are
suitable for thinner gauge sheet and longer lengths for thicker sheet. The longer land increases the internal
die pressure to allow better control of the melt distribution. It also improves sheet surface quality.
Sheet thickness control and die versatility dictate die complexity. A restrictor bar is normally required, although
some newer dies designed for specific applications and materials use only a flexible lip and have no restrictor
bar. For optimum versatility, both features are found in most dies.
The die should be equipped with heaters that control die temperature in several zones across the die width.
Heaters allow better control of both melt temperature and flow through the die. They also help distribute the
polymer to the ends of the die. Extremely thick sheet (>0.500 inch) may require separate controls for top and
bottom sections on each zone. End caps should also have separate temperature controllers.
All interior die surfaces should be chrome-plated to minimize flow resistance and improve sheet surface
characteristics. Some new polymeric die coating materials offer improved material throughput and sheet
quality by smoothing die surfaces at a microscopic level.
Deckle bars or dams adjust sheet width to minimize scrap and regrind. Using them brings some disadvan-
tages. Material can "hang up" in the die behind the deckles and degrade. These bars disrupt the streamlined
material flow through the die, especially in computer-designed flow channel dies. Pressure/material buildup
behind the bars usually results in heavy sheet edges. They prevent good roll contact in the center of the sheet
with the polishing rolls and, in turn, lead to poor sheet finish. Consider eliminating deckles, running thin edges
and trimming edges.
Sheet Extrusion
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Processing factors
The die functions primarily to control the extrudate shape. The extruder must deliver melted material to the die
at constant rate, temperature and pressure. Routinely measure these variables. Advances in control systems
allow for the automatic monitoring and trending of this information.
The die lip-gap setting is generally equivalent to the desired sheet gauge. Heavy gauge sheet (> 0.125 inch)
may require a setting below desired sheet thickness for optimum gauge control. Settings should be uniform
across the entire die width.
Die temperature is set approximately equal to stock temperature. The intent is to keep melt temperature
constant. Die end zones may be set 10 to 20°F hotter than the center zones to help polymer distribution.
Fortiflex
®
HDPE is normally run at 400 to 450°F die settings, except for Fortiflex
®
HMW HDPE which is
generally run at 425 to 480°F.
Adjusting the restrictor bar balances the melt profile exiting the die. Normal restrictor bar settings bow it
slightly in the middle, restricting flow and forcing it out to the ends. Once properly set. this profile for a given
polymer should remain constant. Need for frequent changes may signify problems elsewhere.
Polishing & pull rolls
Equipment factors
Polishing rolls cool the sheet, correct for minor gauge variations and impart the desired surface to the sheet.
The pull rolls, usually rubber, take the sheet off the final cooling roll and feed it to the shear or winder.
Sheet surface quality relates directly to quality of the cooling roll surface. Therefore, highly polished chrome-
plated rolls help assure good gloss. If a surface texture is desired on the sheet, the roll must be embossed
with the required pattern. In all cases, rolls must be perfectly round and revolve with equal peripheral speed,
although it may be desirable to have an independent means for adjusting top roll speed to account for shrink-
age. A means for setting the roll gap using positive stops and precise control of the roll pressure are important
factors. The volume of water passing through the rolls is a major consideration for cooling and temperature
control. This rate usually ranges between 40 and 60 gpm.
Treated water in a closed loop minimizes scale buildup in cooling passages. Evaluate the interior condition
by monitoring coolant temperature between inlet and outlet sides of the rolls. Cooling capacity of the roll stand
must be sized to the extrusion rate of the extruder. Stock temperature, sheet thickness and linear rate are
important in optimizing roll diameter. Having too large a roll is seldom a problem since roll temperatures can be
elevated.
While an "up" roll stack is illustrated below, "down"’ stacks also work. When embossing the sheet, it is best to
use an up stack with the embossing roll in the middle. This configuration helps to obtain the detail of the roll
pattern because melt temperature is highest at the die exit. In addition, the embossed surface is up and won’t
Sheet Extrusion
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be scratched as the sheet pulls across the takeoff and toward the shear. An "up" stack also works better
for thick sheet (0.4 inch to 0.5 inch) since sheet can "fall" down to the pull rolls rather than being pulled up.
Pull rolls keep the sheet moving and feed it to the shear or winder. They do no cooling or shaping. The pull
roll speed is usually tied to the cooling roll speed, so dual adjustments are generally not needed. The unit
should have a pull roll differential speed control. Adjust pull rolls to keep slight tension in the sheet to account
for shrinkage.
Processing factors
Rolls should be as close to the die as possible to cool the sheet and avoid surface oxidation. A small,
uniform bank (generally smaller than a pencil) should feed the roll nip where the melt enters the roll stack.
This ensures complete gap filling and a consistent supply of material across the roll face.
Roll diameters for HDPE are usually 18 inches or more. Increased rate (lbs/hr), requires increased cooling
capacity, requiring larger cooling rolls. Thus it becomes necessary to estimate throughput requirements before
selecting roll sizes.
Table 3 lists suggested roll temperatures for cooling Fortiflex
®
HDPE resins on an "up" stack system.
With "down" stack systems, Solvay Polymers recommends a moderate to high top roll pressure to ensure
adequate roll contact and good polishing. With "up" stack systems, a moderate to high bottom roll pressure
best ensures embossed pattern detail. In both cases, one tries to transfer surface appearance of rolls, whether
polished or embossed, to sheet. Proper roll temperatures and pressures enhance pattern transfer.
Table 3
Up stack - Embossed sheet Fortiflex
®
HDPE
Bottom roll............................................................150-170°F
Middle roll (embossing roll).................................200-250°F
Top roll..................................................................180-220°F
Sheet Extrusion
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Sheet Extrusion
Shearing & sheet handling
Design shears to cut sheet uniformly and prevent chatter marks. Thicknesses up to 0.300 inch are usually
sheared with no problems. If gauge is greater, consider a heavy-duty shear.
Stack cut sheet manually or with a commercial stacking unit. Then remove and store pallets of stacked sheet.
Deionized air will reduce dust pick-up from static charges generated during processing. Wrap stacked sheet
in PE film to reduce surface moisture and keep the sheet clean. Moisture absorption is not normally a problem
with Fortifiex
®
HDPE resin since the material is not hygroscopic. Still, this packaging helps protect sheet during
shipping and handling.
Sheet thickness (gauge) monitoring & control
Historically, gauge control has been
+
3 to 5 percent, based on manual control of the extrusion line. When
material distribution problems occurred on thermoforming, increasing sheet gauge 5 to 10 percent generally
relieved the problem. Many systems that automatically measure and in some cases, control gauge, are
available. Most will hold gauge to one percent. Others have shown variation of less than 0.5 percent across
the sheet, depending on sheet gauge and sheet extrusion equipment quality. Tighter gauge control results in
fewer thermoforming problems and substantial material savings, usually justifying expenses for the system.
In some cases, better control permits gauge reduction. BP Solvay Polyethylene recommends automated
measurement for operations involving ‘long extrusion runs, minimum number of materials and few thicknesses.
The amount of processing information available from these systems should improve screw design and overall
processing. For more information on gauge-control systems, consult an equipment manufacturer.
Summary
This bulletin documents and recommends processing parameters for extruding high quality Fortiflex
®
HDPE
sheet. Commercially, these resins have been extruded on equipment up to 8 inches in diameter and to 36/1 L/D.
Quality sheet at rates in excess of 2,000 lb/hr has been produced with proper equipment selection and
attention to key processing variables. Sheet extruded under guidelines established in this guide should
provide typical physical properties associated with each Fortiflex
®
HDPE resin.
The following table lists typical conditions needed to extrude Fortiflex
®
HDPE. The premise is 1,000 lb/hr
of 0.125 inch extruded sheet. Linear rate is 5 ft/min with 18 inch diameter rolls. Higher outputs and heavier
gauges may require larger diameter cooling rolls.
Readers should carefully determine appropriate actions for specific situations.
Table 4
Summary of extrusion conditions in Fortiflex
®
HDPE
Extrusion - barrel sets
Roll temperatures
Rear .................................375-425°F
Top ................... 150-170°F
Middle ..............................400-450°F
Middle ...............200-250°F
Front ................................400-475°F
Bottom...............180-220°F
Die temperature................425-480°F
Melt temperature..............425-475°F
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Troubleshooting Guide
Surging represents an unsteady state extrusion operation. The
screw fails to deliver a constant material flow. Gross thickness and/
or width changes in sheet in the machine direction indicate surging.
Alternating periods of clean and fogged polishing rolls, which
correspond to thick and thin sheet, also indicate surging. Gross fluctuations in the extruder drive ammeter or
melt pressure usually accompany this phenomenon.
Making frequent material changes and/or using only one "general" or "commercial" screw for all materials can
make surging extrusion’s most common problem.
Causes
Starving screw
This is a condition (more predominant on multiple-
stage screws) in which the forward screw flights are
not full of melt. Usually, this can be corrected by
increasing the melting and conveying of material.
Insufficient back pressure
Without back pressure on the screw, the forward flights
will not be full of material. The screw won’t deliver
material constantly as it pumps against the die.
Non-uniform feedstock temperatures
High stock temperature
Too high a stock may make melt viscosity too Iow for
adequate processing control.
Poor screw design
Suggested Actions
q
Decrease the rear barrel zones. This
moves melt zone forward and increases
material throughput.
q
Check for material blockage. Check for
pellet movement at throat.
q
Close gate valve (if used) to increase back
pressure.
q
Increase screen pack.
q
Close die manifold and/or restrictor bar.
q
Lower heats in gate and/or screen
changer.
q
Check regrind level. Ensure that the
regrind level is consistent through the run.
q
Install static mixer.
q
Reduce barrel temperature profile.
q
Reduce screw speed.
q
In some cases or for some materials,
change screws or re-cut an existing one.
This is the opposite of a problem created by a surging extruder. If the
screw melts and pumps excess material, it comes out the vent.
Causes
Output of screw first stage higher than second stage
can handle.
Suggested Actions
q
Reduce output of first stage by increasing
temperature set point of first and/or
second extruder barrel zone.
q
Increase output of second stage by
opening gate valve or increasing front
barrel zone temperature set point and die
temperature profile or raising restrictor bar.
q
Redesign screw.
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IN THE SHEET SURFACE
(Also called pits, depressions, pock marks, buckshot and craters)
Causes
Air entrapment
Creates dimples which appear on the sheet surface or
in thermoforming. Pellets sintering in the screw closing
off the air-venting outlet may cause entrapment. Air,
collected in pockets, causes voids in the hot sheet.
They contract during sheet cooling and leave depres-
sions. They become more prominent when drawn out
by vacuum forming.
Gross moisture
Defects look like craters caused by a blown-out
surface. General sheet roughening, lines and "chicken
tracks" may appear with the defects. Crackling as the
melt exits the die due to steam pressure-pocket
release may also appear.
(Generally not a problem with PE).
Suggested Actions
q
Reduce rear barrel temperatures to
retard sintering.
q
Reduce screw speed to increase time
to vent air.
q
If using regrind feedstock, vary particle
size (controls bulk factor).
q
If used, check vent for plugging or
vacuum.
q
Investigate screw design.
q
Increase drying temperature.
q
Check function of hopper dryer, dew point,
etc.
q
Check vent vacuum system if applicable.
Volatiles, pits
Blowout of gas pockets through sheet surface can
result in fine dimples or craters. They are usually
considerably smaller than air pockets and may not
appear until vacuum forming.
Contamination (infrequent) Certain types of
contamination will cause gas generation in the extruder
and result in dimples similar to those caused by air
entrapment.
Dust (shallow-dished depressions)
Examine the sheet closely to determine if dust particles
are embedded in the center of these depressions.

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