Introduction to Fabrication Techniques

Polyethylene Film Training Program

Introduction

Polyethylene is shipped to customers in pelleted, granular, or ground (powder) formSome fabrication technique must be used by a resin converter to transform the resin into a usable productExtrusion is the most common technique used to convert resin into film, sheet, molded articlesSelection of an appropriate extruder screw is critical to optimal extrusion and product performanceThe two main types of film production are blown film and cast film extrusionThe main types of molding processes are blow molding, injection molding, and rotomolding

Polymer Fabrication

PELLETS FORMCOOL

&SOLIDIFY

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Extruders Dies, molds Heat removalequipment

Parts,rolls,etc.

Melting pt.,Viscosity,Friction

Viscosity,Melt strength,Equip. design

Cooling rate,Crystalization

Attributes

• The conversion of polymer into useful products

MELT&

FLOW

Tools

ImportantPolymer

Parameters

Extrusion

Most common tool used for conversion of polyethylene pellets into film is the extruderRole of the extruder– Melt polymer– Mix/homogenize– Pressurization– Devolatilization

Types– Single screw– Twin screw

Sizes of commercial lines– 1.5 to 8 inches– 18 to 36 L/D (length of barrel/barrel diameter)– 50 to more than 3000 pounds per hour output

Extrusion - General Schematic

- Barrel heating done with electric heaters- Barrel cooling with air or water

Extrusion - Screws

ScrewDiameter

Pitch FlightWidth

HelixAngle

FeedDepth

MeasuringDepth

Constant Depth Tapered Constant DepthFeedZone

CompressionZone

MeteringZone

Role of screw sectionsFeed Zone - convey resin down heated barrel at consistent rate, soften pellets Compression Zone - melting of pellets with heat and shear, air forced back into

feed sectionMetering Zone - homogenization of melt, mixing (if needed), even flow of polymer

through die

Extrusion - Screw Designs

Conventional Screw

Conventional Screw with Maddock Mixing Section

Barrier Screw

Barrier Screw with Maddock Mixing Section

BarrierFlightunder-cut

Main Flight

Extrusion - Barrier Screws

Barrier screws have been available since the 1960sBarrier screw principles

– At the beginning of the barrier section, a barrier flight is introduced into the screw channel

– Clearance between barrier flight and barrel is greater than clearance between main flight and barrel

– Barrier clearance allows polymer melt to flow over barrier, but is too small for pellets

Separates melt from unmelted pelletsProvides more uniform polymer melt

Extrusion - Screws

Feed zone– Ideally resin sticks to barrel and is pushed forward by screw flights– Pellet conveying affected by density, bulk density, compressibility,

and pellet to pellet frictionCompression zone– Melting and compacting of resin– Heat from barrel heaters and shear

Metering zone– Provides back pressure needed for homogenization of melt– Temperature and composition of melt needs to be homogenized

before melt enters die to provide better product performance– Can contain mixing elements for better homogenization

Compression ratio is defined asDepth of Feed Zone Channel Depth of Metering Zone Channel

Extrusion - Screw Mixing Sections

Maddock Mixing SectionLongitudinal spines, all material has toflow over barrier flight, loss of extruderoutput, very common design

Egan Mixing SectionHelical splines, all material flowsover barrier flight, no loss in extruder output, reduced possibilityof resin stagnation

Polymer Flow

Polymer Flow

Extrusion - Screw Mixing Sections

Dray Mixing SectionOutlet channel is open at start ofmixing section, not all materialis forced over barrier flight resultingin nonuniform shear history in melt

Blister RingSmooth cylindrical screw with smallradial clearance, can causesignificant reduction in output

Extrusion - Screw Mixing Sections

- Resin flow in Maddock mixing section- All material has to flow over barrier flight yielding dispersive mixing- Can reduce output of extruder- Has stagnation zones which can allow polymer

degradation

Extrusion - Screws

Performance measures for screw selection– Rate (lb/hr/rpm)– Power consumption (hp-hr/lb)– Melt uniformity– Temperature uniformity– Melt flow stability

Screws must be designed for the resin and intended use– Need to consider

• Molecular weight• Shear thinning behavior• Target output rate• Target output temperature

– Screw should be able to extrude polymer at desired output rate and temperature without overheating the polymer

Blown Film Extrusion

- General schematic of blown film process

Blown Film Extrusion

Polymer melt is extruded through an annular die to form a tubeThe tube is “nipped” by a set of motor driven rollers forming a “bubble”The tube is pulled away from the die by the nips, causing the tube to become thinnerAs the tube becomes thinner, internal air pressure causes the bubble to expandAs the bubble progresses from the die, it is cooled by external air flow provided by an air ringThe nip rolls collapse the bubble into two sheets joined at the edgesFilm is collected on rolls by a motor driven winder

Blown Film Extrusion

Glossary of commonly used terms in blown film extrusion:– Die - used to form initial bubble shape, needs to be described by die

diameter and die gap– Mandrel - (a.k.a. die pin) removable cylindrical piece of steel around

which the polymer melt flows, can be spiraled for more even distribution of melt

– Die gap - distance between mandrel and outer die wall (examples for LDPE 0.020 to 0.045 inch, LLDPE 0.060 to 0.120 inch)

– Frostline height (FLH) - region where bulk crystallization occurs in bubble, transition from liquid to solid, usually marked by transition from clear melt to slightly hazy film, measured as height above die

– Blow-up ratio (BUR) - ratio of final bubble diameter to die diameter– Layflat width - width of collapsed bubble– Air ring - used to provide external air flow for cooling, can have single

air channel, or two or more air channels

Blown Film ExtrusionBubble Size Calculations

Machine Direction

(MD)

TransverseDirection

(TD)

Layflat Width = ½ x bubble circumference

BubbleDiameter

Die Diameter

NipRolls

Layflat Width

Layflat Width = (die diameter x blow up ratio x π) / 2

Blow up ratio = bubble diameter / die diameter

Blow up ratio = 0.637 x (layflat width / die diameter)

Blown Film Extrusion - LDPE vs LLDPE

LDPE ExtrusionScrews with compression ratios of 2.5 to 4.5 with 3.5 typicalBarrier screws becoming more commonExtruder L/D from 18:1 to 32:1, with 24:1 most typicalDie gaps 0.020 to 0.045 inchMelt temperatures 350 to 400°FDual lip air ring becoming more common, single lip air ring can be used at lower ratesAmbient cooling air acceptableRatio of die gap to film gauge 10:1 to 30:1

LLDPE ExtrusionScrews with compression ratios 2.5 to 3.0 common (LLDPE viscosity less shear sensitive than LDPE)Barrier screw preferredExtruder L/D 24:1 to 36:1Die gaps 0.060 to 0.120 inchMelt temperatures 400 to 460°FDual lip air ring necessaryChilled cooling air neededRatio of die gap to film gauge 20:1 to 120:1

Blown Film Extrusion - Cooling

Single Lip, Venturi Air RingOlder technology

Formerly found on LDPE linesDual lip more common (higher rates)

Dual Lip, Venturi Air RingNewer technology

Needed for efficient LLDPE extrusionAdjustable to vary air flow

Blown Film Extrusion - Cooling

Internal Bubble Cooling (IBC)Exchanges otherwise stagnant internal bubble air with fresh chilled airProvides significantly increased bubble cooling rateAllows for significantly higher extrusion ratesUnit must be designed to direct air flow at bubble surface without causing instabilitiesControl system regulates air flow to maintain preselected layflat width

Blown Film Extrusion - HMW-HDPEMost LDPE and LLDPE films are produced with relatively low FLHHMW-HDPE requires a very high stalk to allow molecular relaxationDie gaps 0.030 to 0.050 inchOptimum bubble configuration

– 3 to 4.5 BUR – Neck height 6 to 9 die

diametersNew technology dual lip are rings give higher rates, better gauge controlGrooved barrel preferred to control feed, pressures and temperatures

Cast Film Extrusion

- Cast film extrusion schematic

Primary Chill Roll

Plate Out Roll Annealing Roll(optional)

To Winder

Secondary Chill Roll

Vacuum Box

Die

Air Knife

Cast Film Extrusion

Polymer melt extruded from flat slit dieFilm cooling done by extruding onto a chilled casting roll (mostcommon) or into waterCasting roll temperature is commonly 50 to 90°FCasting roll finish can be gloss or matteFilm edges are “pinned” to casting roll with air and/or electrostatic unitsAir knife can force film onto casting roll for more uniform coolingVacuum box (behind die) sucks film onto casting rollCasting roll may also have engraved patterns for embossingMelt flow to ends of die causes “edge beads” (thick edges) to form which are trimmed away prior to winding

Blown Film vs Cast Film Extrusion (LLDPE)

Blown FilmMelt temperatures 400 to 460°Commercial die sizes 3 to 100 inches diameterDie gaps 0.060 to 0.120 inchLine speeds up to 800 feet/minute, 250 to 350 feet/minute commonGauge uniformity ± 5 to 25%Greater control over molecular orientation in film

– Temperature– BUR– Die gap (in some cases)

Cast FilmMelt temperatures 450 to 575°Commercial die sizes 60 to 240 inches wideDie gaps 0.020 to 0.035 inchLine speeds up to 2000 feet/minute, 1000 feet/minute or greater commonGauge uniformity ± 2 to 15%Less control over molecular orientation in film, minimal transverse direction orientation

– Temperature– Die gap (in some cases)

Coextrusion

- Coextrusion is the process of simultaneously extruding a product with 2 or more distinct layers- Multiple extruders feed resin to an adapter section that channels the flows into separate layers- Can be used in both blown film and cast film extrusion- Also used in some molding applications (e.g. blow molding)

Extrusion Coating Process

- General schematic of extrusion coating process

Extrusion Coating Process

By appearance, extrusion coating is very similar to cast film extrusionMolten resin is fed through a flat/slot die and is extruded onto a substrate– Typical substrates - paper, paperboard, OPP, polyester, foil

Polyethylene acts as grease barrier, liquid barrier, seal layerExtrusion temperatures (~ 600°F) are usually higher than seen in cast film extrusion - done to promote oxidation and adhesion to substrateTypical applications– Food packaging (dry and liquid)– Board packaging (detergents, microwavable trays)– Metallized balloons– Tarpaulins

Film OrientationFilms can be oriented after extrusion for a variety of applications

– Increase shrinkage– Improve barrier properties– Improve clarity

Orientation usually induced at some temperature below polymer melting pointTypes of orientation

– Mono-directional, MD only, usually done with heated rolls running at different speeds, 2X to 6X MDO common

– Bi-directional, MD and TD• Double bubble process - form tube, then inflate again to larger BUR at

temperature below melting point• Sequential MD and TD - TD done with tenter frame

Materials commonly oriented– Polypropylene– Poly(ethylene terephthalate)– Polyamide– Polyethylene

MD Orientation ProcessPreheat

RollsStretching

RollsAnnealing

Rolls

- Schematic of machine direction orientation (MDO) process- Film is heated by Preheat Rolls then stretched- Annealing Rolls allow for some relaxation to prevent

prevent film waviness- MDO commonly quoted as (value)X where “value” is the

level of orientation induced (see above)

1 2

A ft/min A+ΔA ft/min

A+ΔAA = (answer)XMDO =

TD Orientation Oven

GripPaths

Film

- Top view of typical transverse direction orientation (TDO) tenter frame- Film grips/clamps connected as continuous looping chain- Film enters (left) and is gripped at the edges- Film passes through heated ovens and is stretched in TD- Film released after exiting oven and wound on core

Film GripDrive

Molding Processes

Injection molding– Molten resin injected into a mold with 1 or more cavities, mold is cooled, part is ejected,

process repeats– Short cycle times (seconds), dependent on part size– Need low viscosity for resin flow, MI ~ 10 dg/min and higher

Blow molding– Molten resin extruded as tube, single cavity mold closes, tube is inflated to fit mold,

mold cooled, part is ejected, process repeats– Short cycle times (seconds), dependent on part size– Need high molecular weight for parison strength, broad molecular weight distribution to

minimize die swell, MI < 0.1 to 1 dg/min

Rotomolding– Granular or ground resin placed in mold, rotated in three dimensions while heated,

mold cooled, part removed, process repeats– Longer cycle times (minutes), dependent on part size– Need relative low viscosity for flow in mold, MI ~ 2 to 10 dg/min

Injection Molding

Mold ready to be filled

Mold filled

Part ejected

- Schematic of injection molding process

Injection Molding Examples

Blow Molding(1) (2) (3) (4) (5) (6)

(1) Complete extrusion of parison(2) Close mold(3) Inflation of parison(4) Part cooling(5) Part ejected(6) Mold open, ready for next parison

Blow Molding Examples

Rotomolding

Shuttle Machine Carousel Machine

Door DoorOven

Mold Mold

Station 1 Station 2

CoolingLoading and Unloading

CoolingLoading and Unloading

Molds CoolingStation

Oven

Load-UnloadStation

- Two different rotomolding processes- (Left) two molds share an oven- (Right) work can be done on three articles in the process simultaneously

Rotomolding Examples

Wire and Cable Coating

FormingDie

CoatedWire

Melt fromExtruder

BareWire

Clearance~ 0.002 in

GuiderTip

- Detailed schematic of pressure type of wire coating die

Nonwoven Fabrics - Fabrication

•Continuous fibers•Fibers leaving spinerette aredrawn mechanically or pneumatically

Spunbond Melt Blown

•Discontinuous filaments•Much smaller diameter than spunbond•Pressurized air used to force melt

through die

- Nonwovens are webs or sheets of randomly oriented fibers that arebonded together by mechanical, chemical or thermal methods

- Mechanical strength is derived by bonds

Nonwoven Fabrics - Bonding

Point Bonding

•Preheated, consolidated web passedthrough nip rolls, one with raised surface

•High pressure (20-45 ksi)•10-25% fusion at points of contact

Needlepunch

•Simplest, and least expensive method•Continuous filament web subjected tobarbed needles

•Fibers become interlaced•Not as well bonded as thermal bonds

Nonwoven Fabric Examples

Summary

Extrusion is the primary tool used to transform polymer pellets into usable goodsExtruder screw design is critical in ensuring uniform melt flow through a die and is selected based upon the resin extrudedBlown film extrusion is the most common technique used to make filmFilm extrusion parameters are very different for different polyethylene families - LDPE, LLDPE, HDPECoextrusion is the process of simultaneously extruding differentpolymers into discrete layers of a productMolded parts are made by injection molding, blow molding, and rotational moldingNonwoven fabrics are made using spunbond or melt blown fibers