film converting
Post on 03-Oct-2021
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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
Bag Making - Sealing
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
Bag Making - Sealing
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
Bag Making - Sealing
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
Bag Making - 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
(some) Types of Plastic 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!”
Corona Treating - Problem Solving
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.)
Corona Treating - Problem Solving
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
Corona Treating - Problem Solving
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)
Corona Treating - Problem Solving
Backside Treat Patterns
Corona Treating - Problem Solving
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
Form/Fill/Seal - Vertical
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
Form/Fill/Seal - Vertical
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
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