film converting

Film Converting

Polyethylene Film Training Program

Introduction

Approximately 70% of all low density polyethylene (LDPE + LLDPE) is made into filmApproximately 30% of HDPE is made into film After conversion from pellets, most films require some sort of process to convert them into usable goods– Printing– Sealing– Slitting– (Re)winding– Treating

Many converting techniques are used together to produce usable goods

Film Slitting and WindingCommon technique used for

– Collecting film into a large roll for later use– Rewinding rolls to minimize defects (gauge bands, trapped air, non-uniform

centering)– Portion large rolls into smaller lengths– Slit wide rolls into narrower widths

Types of winders– Center winder

• Most commonly used for polymer films• Web tension controlled by motor driven rollers• Better for faster line speeds

– Surface winder• More commonly used for paper and paperboard• Good for large diameter and heavier rolls

Center Winder

Surface Winder

Film Slitting

Common types of slitting– Razor

• Disposable razors used to cut film• Can be exposed or aligned in grooved rollers• Most common method for slitting

– Shear• Similar mechanism to scissors, using circular knives• Blades need to be sharp with correct pressure and depth of

male blade into female blade– Score

• Circular blade contacts hardened metal roll• Cuts by crushing web against roll• More common in paper slitting

Razor Slitting

Score Slitting

Shear Slitting (Principle)

Heat Sealing - Bar Sealing

Bar sealing is most widely used methodNeed to have uniform pressure across seal so seal bars must have minimal deflection through length of sealFor air-tight and liquid-type seals, best result obtained by using flat faced bar opposed by resilient surface (e.g. silicone rubber)Edges should be rounded to prevent puncturing of packaging materials

Heat Sealing - Impulse Sealing

Impulse sealing has similar equipment to bar sealingEach jaw is covered with resilient surfaceNichrome ribbon laid over resilient surface Electric current passes through Nichrome ribbon for heatIn contrast to bar sealing, in impulse sealing, seal is cool before jaws are opened

– Good for low hot tack strength or sticky materials

Heat Sealing - Band Sealing

Band sealing often used to fabricate pouchesContinuous method for sealingAs film passes through unit, films is softened, compressed, sealed and cooledPouch mouth must be flattened before entering unit to avoid wrinkling and poor seals

Heat Sealing - Other Methods

Other heat sealing methods commonly used with polyethylene– Wire or knife sealing

• Hot wire or knife seals and cuts film– Ultrasonic

• Tooling hammers or rubs materials together at high frequency, generating heat for sealing

– Pneumatic• Heated film is sealed to another surface by air pressure

– Radiant• Infrared radiation sealing without pressure

Bag Making - Sealing

Twin Seal Method

- Most often used for “T-shirt” grocery sacks- Supplies heat to both to and bottom of film and makes two separate and

independent seals during each cycle- Knife located between seal heads is heated or unheated


Slit Seal Method

- Usually done to convert a single large tube of film in to smaller tubes- Seal two or more layers of film together with heated knife, hot air, laser,

or combination of methods- Seal done in machine direction of film

Side view Top View


Sideweld Seal

- Most common sealing method- Seal made with heated knife or blade which cuts, severs, and seals two layers

of film when the knife is depressed into a soft rubber back-up roll


Bottom Seal Method(Top heat only)

- Used for liners, trash bags, and many bags supplied on rolls- Seals produced at bottom of bag from tube stock- Heat bar presses film against a Teflon® coated rubber pad- Used for thicker film gauges than sideweld seal- Can be combined with slit sealing


Bottom Seal Method(Top and bottom heat)

- Used for liners, trash bags, and many bags supplied on rolls- Seals produced at bottom of bag from tube stock- Heat applied to both sides of film- Used for thicker film gauges than sideweld seal- Can be combined with slit sealing

(some) Types of Plastics Bags

(some) Types of Plastic Bags

(some) Types of Plastic Bags

Bag type pictures taken from RutanPoly web site.

Bag Variations - Patch Handle Bags

Picture taken from Chun Yip Plastics literature.

Bag Variations - Sine Wave Bag

• Bags are made 2 up withwave forming the cut betweenthe two bags

Gusseting

Gusseting is the process of placing a crease or fold in a bag to allow for expansion when filledGussets can be on bottom or sides of bag, or bothA gusseted bag lays flat when empty but expands to have greater volume than ungusseted bagGood for holding bulky or odd-shaped articles

Fully Gusseted Bag(sides and bottom)

Bottom Gusset Bag

Bag Making

Important film requirements for efficient bag making– Low COF

• Film must pass through converting equipment without dragging– Stiffness

• Film must have some “body” to prevent excess drooping or sagging during conversion

– Sealability• Film must seal at relatively low temperatures and at high rates

– Openability• Films must have low blocking for ease of conversion and customer

end-use– Printability

• Must not have excessive additives (e.g. slip) to prevent ink adhesion– Overall appearance

• Bag must have good seals, clean cuts, no ragged edges, acceptable gloss

Film Treating

What is Treating?

Treating is a technique by which the surface tension of PE and PP films is increased.

Film Treating

Why Do We Treat Films?

To improve:PrintingCoatingGluing (Laminating)MetallizingHeat sealing to paper

Corona Treatment of Plastic Film

Film Treating

Types of film treating commonly used for polyethylene– Corona discharge

• Film moves between an electrically ground roller and an electrode maintained at high voltage

• Air between surfaces is ionized and continuous arc discharge is generated at film surface which cleans, oxidizes and activates film surface

• Most common in polyethylene films– Gas-plasma

• Batch process using ionized gases• Oxygen cleans and oxidizes surface similar to corona

discharge– Flame

• Film passes by flames which oxidize surface and burns off surface contaminants

Corona Treating

Keys to Good Corona Treating

• Treater bar configuration and condition• Smooth, clean and undamaged hypalon roll• Excellent main nip condition• Proper voltage setting• Uniform and correct air gap between treater bar and hypalon roll• Uniform and correct film tension at treater station

Corona Treating

Treat Test Methods

• Tape adhesion (diaper films)

• Ink pick-off (printed film)

• Inclined plane

• Contact angle

• Wetting tension (dyne solution)

Wetting Tension Test

• Small amount of liquid on cotton tipped applicator

• Cover one square inch area

• Continuous liquid film should hold for two seconds

Corona Treating - Problems SolvingLight or Under Treat

• Dyne solution breaks uniformly over one square inch test area at a minimum specified treat level

• ResultsPoor ink adhesion (ink pick-off, ink flaking)Poor coating or glue adhesion (e.g., Sealed Air)Poor heat seal to paper

• CausesInadequate treater powerCocked or bowed treater barSlip and other additives (time between extrusion & testing critical)Improper test procedureContaminated, aged or improper dyne solution

• SolutionsIncrease treater powerStraighten treater barChange resin formulationModify test procedure or use ink pick-off testObtain fresh dyne solution

Corona Treating - Problem Solving

Heavy or Over Treat

• Dyne solution holds at maximum specified treat level• Results

Blocking (external)Poor film to film seal strength (T/T gusset)Heavy oxidized layer on film surface (poor ink & glue adhesion)Burned odorAccelerated oxidation (rotting)

• CausesToo much treater powerWide treater/hypalon air gapImproper test procedure (timing)Contaminated, aged or improper dyne solution

• SolutionsSet air gap (ideal distance is 1/16” with bar hot)Cut back on treater powerRead dyne solution break-up at 2 secondsObtain fresh dye solution

Corona Treating - Problem SolvingStreaky or Skip Treat

• Dyne solution holds over wide area but breaks in narrow strips• Results

Some ink pick-offChanneling in laminatesUsually no severe consequences

• CausesWrinkle in film caused by:

Improper film tensionOut of round hypalon or idler rollerExcessive treater power

Excessive gap between (segmented) treater bar segments• Solutions

Eliminate wrinkle in film by:Adjusting film tensionRepairing rollersReducing treater power or adjusting air gap

Repair treater bar• Do not report this problem as “Light or under treat!”


Backside Treating

• Scattered areas or streaks hold treat at higher than 32 dynes• Results

Film blocking on rollInk pick-offCoated rolls adhere into solid “log”Severe staticWeak seams (low seam impact)

• Difference between low slip and high slip film backside treat!Low slip or no slip film must be “clean” at 33 dynes (SA film)High slip film must be “clean” at 35 dynes (folded EX-700, etc.)


Backside Treat Causes and Solutions

Blown PE Sheeting Cause Solution• Damaged main nip (cuts in rubber

roll)Repeat patternDefinite shape

• Grind rollDo not cut into rolls in case ofwrap-up

• Trapped air bubble (P-13 oscillatingnip)

Random patternDroplet shape

• Check main nip for air pocketsbetween nip and treat station

• Static discharge at web separationRandom PatternLightening bolt (crow feet) shape

• Use static eliminator at point of webseparation


Folded PE and Cast PP/PE Cause Solution• Dam aged , pock-m arked hypalon roll

Repeat patternTreat level proportional to treatersetting

• C lean or replace hypalon roll

• W rinklesRandom , long streakAggravated by high treater powersettingIm proper web tensionO ut-of-round hypalonTreated side has corresponding weakstreak

• Adjust tensionRepair rollersAdjust treater air gapReduce treater power

• Splash over (edge)Film treated over folded edgeW eak seam im pactB locking tendency

• Do not treat 1/4" from edge of foldedconstructions

Backside Treat Causes and Solutions (cont’d)


Backside Treat Patterns


Backside Treat Patterns (Cont’d)

PrintingMany polyethylene film applications require some form of printing - from simple time/date marking to elaborate multicolor graphicsLocation of printing can vary

– Surface printing - simple, printing exposed, could be rubbed or scratched off package; can affect film COF and gloss

– “Reverse printing” - the process of printing a reverse image onto a film surface and then laminating that surface onto a substrate; done to protect graphics (printing not exposed on outer surface) and give better gloss; common in applications such as snack food bags

Printing classification– Line - image is engraved as it is to appear on surface– Process - image built from dots from colors-black, yellow, magenta, cyan

Types of printing commonly used in polyethylene films– Relief and relief/flexography– Intaglio– Planographic/lithography– Ink-jet

Printing - Relief

Relief printing, often called letterpress printingImages (or lettering) are raised above nonprinting surfaceInk applied by ink rollers which only touch raised surfacesInk is applied by pressing raised surfaces against film while it passes over an impression cylinder causing the image to be transferred

Printing - Relief/Flexography

Flexography is a form of relief printingUses raised elastomeric plates and very low viscosity and rapid drying inks that are both solvent and water reducibleRubber fountain roll supplies ink to steel inking (anilox) rollPrinting plates are attached to plate cylinder and contact the anilox rollSubstrate passes between plat and impression roll to transfer image

Printing - Intaglio

Intaglio printing is also called gravure or rotogravure printingOnly process where there are no plates attached to a cylinderPrinting comes from recessed, machined areas in cylinderRecessed cells are filled as roll rotates through ink wellDoctor blade removes excess inkElastomeric impression roll presses material against gravure cylinder and ink is transferred by capillary action

Printing - Planographic/LithographyPlanographic printing is also called lithography or offset lithographyImage and nonimage areas are on same plane of smooth metal plateInk is fed from ink tray and water is simultaneously fed from water panThe plate accepts ink and repels water in image areas and accepts water and repels ink in nonimage areasImage transferred intermediate (blanket) cylinder then to web

Printing - Ink-jet

Ink-jet printing requires no contact between an inked image and webJets spray electrically charged drops onto substratePrinter electrostatically directs drops to form charactersCan project droplets of ink into various recessesRelatively low costInks are available that adhere to nonabsorbent surfaces

Laminating

Many flexible packaging materials are products of lamination processMaterials are laminated to optimize physical and barrier propertiesBonding variables

– Time (slower process, more contact time, better bond)

– Pressure (greater pressure, better bond)

– Temperature (must be optimized to provide best bond)

Laminating

Types of laminating– Wet-bond

• Liquid adhesive applied to substrate that is then immediately combined with second substrate

• Usually low pressure, no/low heat– Dry-bond

• Solvent-based adhesive applied to substrate• Solvent evaporated in dryer, substrate then combined with

second substrate using heat and pressure– Thermal or pressure

• Requires thermoplastics adhesive, dry-film adhesive, low melting point film

• Done using heat and high pressures– Extrusion

• Coating of molten polymer extruded between layers

Form/Fill/Seal - Vertical

Vertical form/fill/seal (VFFS) produces a flexible bag from a flat sheet of filmFilm is guided around filling tubeLongitudinal seal formedTube advanced downwardsPouch filledTop seal formed (also bottom seal for next bag)Filled bag cut from tube

Vertical Form Fill Seal - Forming Cone


Package Types from Vertical Form/Fill/Seal

- Pillow-style bags with conventional seals at top and bottom- Longitudinal seal can be either lap seal or fin seal


Package Types from Vertical Form/Fill/Seal

- (right) Gusseted bag with tucks on both sides to maintainrectangular bag shape, bottom can also be block shaped- (left) Three or four-sided seal package similar to thosemade by horizontal form/fill/seal

Form/Fill/Seal - Horizontal

Horizontal form/fill/seal (HFFS) also produces a flexible package from a sheet of filmDepending on pouch contents, can either form pouch and fill (dry goods) or form pouch around product (cheese)Operations can intermittent or continuousPouch styles include

– 3- or 4-side fin seal– Single or double gusset– Pillow pouch, lap or fin seal

Form/Fill/Seal

Form/fill/seal are high speed operations– Vertical up to 150 packages per minute– Horizontal up to 1200 packages per minute

Typical applications– Vertical - dry goods, powders, liquids, frozen foods– Horizontal - bakery, cheese, meat, dry goods, frozen foods

Film product requirements– Flexibility (ability to wrap around filling tube)– Low COF (to eliminate dragging in process)– Sealability (need excellent hot tack and heat seal strength)– Stiffness (prevent tube sagging when filled)– Low shrinkage (prevent product deformation during heat seal)

Metallized FilmsMetallizing is the process of applying a thin coat of metal (usually aluminum) onto a substrateMetal is melted vaporized and applied under a vacuumAmount of metal deposited controlled by:

– Temperature (hotter = more metal)

– Speed (slower = more metal)– Number of plating stations

(more = more metal)Done primarily for improved barrier properties

A. Payout mandrelB. Cooled drumC. Take-up mandrelD, Aperture and shutter assemblyE. Wire feed systemF. SourceG. Viewing ports

Metallized Films

- Common materials coated are biaxially oriented PP (BOPP),oriented polyethylene terephthalate (OPET), polyvinyl chloride(PVC), and polyethylene

- Many films have reverse-printing on outer layer to lock ingraphics and make then more durable

Summary

Common converting techniques used for polyethylene films are:– Slitting– (re)winding– Heat sealing– Bag making– Printing– Laminating– Form/fill/seal– Metallizing– Film treating

Many converting techniques are used together, e.g. treating and printing

film converting

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