Plastics machinery series
Blown film line
Extrude through an annular die, inflate a bubble, collapse and wind
Blown film lines make mono-layer and co-extruded (3, 5 and 7-layer) PE film for bags, shrink film, agricultural film and packaging. This page opens the line unit by unit, from the dosing units to the winder, with BUR, frost line and layflat explained, how to size the line and what to inspect in maintenance.
Marketing Department, Advanced Chem Solutions · 22 Oct 2026 · 15 min read
Machine overview
Melt from one or more extruders enters an annular die and leaves as a thin tube. Air blown inside inflates the tube into a bubble while the air ring blows cooling air around the outside. The bubble cools and solidifies at the frost line. The collapsing frame then flattens it, and the primary nip rolls seal the air inside the bubble and pull the film up. The film travels down through the corona treater and slitters to the winder.
Four numbers matter: blow-up ratio (BUR) = bubble diameter ÷ die diameter · frost line height = distance from the die to where the film solidifies · draw-down ratio (DDR) = die gap ÷ (film thickness × BUR), a measure of how much the film is stretched in the machine direction · layflat width = width of the collapsed bubble = π × die diameter × BUR ÷ 2.
- Dosing / blending unit — one per extruder; weighs the base resins, blend partners and masterbatch for that layer.
- Extruder — usually grooved-feed for HDPE and LLDPE; co-extrusion lines have 3, 5 or 7 extruders.
- Screen changer — filters dirt and unmelted material before the die.
- Spiral mandrel annular die — distributes the melt evenly around the circumference; some designs rotate or oscillate.
- Air ring — blows cooling air around the bubble; governs bubble stability and gauge around the circumference.
- Internal bubble cooling (IBC) — exchanges cool air inside the bubble, with a sensor controlling bubble size.
- Bubble cage / calibration basket — steadies the bubble and holds its diameter.
- Collapsing frame (tent) — flattens the bubble before the nip; includes gusseting boards for side-gusseted film.
- Primary nip rolls and oscillating haul-off — seal the air in the bubble and pull the film; the oscillating unit spreads gauge bands so they don't stack in the roll.
- Thickness gauge — measures around the bubble or on the flat film and feeds automatic air-ring control.
- Corona treater — raises film surface energy before printing or laminating.
- Winder — contact, gap or turret type, with slitting.
Unit by unit
Each table gives the part's job, what to check on the quotation or at acceptance, and what you see on the film when it wears or fails. Use it to compare quotations or chase problems on the floor.
1. Dosing units and extruders
Most PE film lines use grooved-feed extruders: the feed section has a water-cooled bushing with axial grooves. The pellets grip the grooves and are pushed forward at high pressure right from the feed zone, so output per rpm is stable and depends little on die pressure. This suits viscous HDPE and LLDPE, but torque and wear in the feed section are higher than with a smooth bore. Film screws are typically barrier designs followed by a mixing section, with lengths commonly around L/D 24:1 to 30:1 depending on the model and resin (typical reference range — confirm with the machine maker and the TDS).
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Dosing unit (gravimetric / loss-in-weight) | Doses several resins and MB per layer to the recipe; some systems control weight per metre (g/m) together with line speed | Number of components, resolution, capacity, link to line control | Colour or slip level drifts, MB use above recipe, average gauge drifts |
| Grooved feed section | Builds pressure from the feed zone for stable output | Groove geometry, cooling water circuit, thermal separation from the barrel | Output loss, worn grooves, pellets melting and bridging at the throat when cooling is short |
| Barrier screw + mixing section | Separates melt from solid bed, then homogenizes | Screw design for your resins, material and hardfacing | More gels and unmelts, higher melt temperature, less output per rpm |
| Barrel and heaters | Zone temperature control | Number of zones, cooling, thermocouple positions | Zones that won't hold, unstable melt temperature |
| Motor and gearbox | Turns the screw at high torque | Maximum torque (critical with grooved feed), thrust bearing | Noise, vibration, hot oil, unstable current |
2. Filtration and the annular die
Modern film dies use a spiral mandrel: the melt is split into several ports, each running as a helix around the mandrel. The spiral channels get shallower so melt leaks over the lands and blends, giving even distribution around the circumference without the weld lines of older spider dies. Co-extrusion dies stack one spiral system per layer. Commonly quoted die gaps: LDPE about 0.6–1.0 mm · HDPE about 1.0–1.5 mm · LLDPE wider, about 1.5–2.5 mm, to reduce shear and melt fracture (typical reference range — confirm with the machine maker and the TDS).
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Screen changer | Filters dirt, gels and unmelts | Filter area, change without stopping, seals | More gels and fish-eyes, pressure spikes, leaks |
| Spiral mandrel | Distributes melt around the circumference | Die diameter, number of layers, design for your resins and output range, flow-channel finish | Gauge bands that won't adjust out; dead spots causing gels and black specks |
| Die lip and die gap | Set the wall thickness of the melt tube before blowing | Interchangeable gap, material and plating, centering system | Die build-up, die lines, uneven gauge around the circumference |
| Die heaters | Control die temperature around the circumference and per layer | Number of zones, circumferential uniformity | One side hotter, film thick on one side |
| Rotating / oscillating die | Turns the die slowly so gauge bands spread across the roll | Rotary seal, drive, leaks | Leaking seals, degraded material at the seal; new lines mostly use an oscillating haul-off instead |
3. Bubble cooling: air ring, IBC and bubble cage
Cooling capacity limits the output of almost every film line. A single-lip air ring is simple. A dual-lip ring keeps the air attached to the bubble surface longer, so it removes more heat and the bubble is steadier. An automatic air ring divides the circumference into segments, each adjusting air temperature or flow according to the gauge. IBC continuously blows cool air into the bubble and extracts warm air, cooling the film from both sides; a sensor (often ultrasonic) measures bubble size and balances supply and exhaust to hold the layflat constant.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Air ring (single / dual-lip) | Blows cooling air around the outside; controls bubble shape and frost line | Lip design, size matched to the die, lip adjustment range, circumferential air uniformity | Dented or dirty lips, uneven air, leaning or fluttering bubble, gauge bands |
| Automatic air ring (segmented auto-profile) | Adjusts air segment by segment to correct gauge | Number of segments, response speed, mapping to gauge positions | Some segments not responding, profile hunting |
| Blower, filter and chiller | Supply a constant volume of air at a constant temperature | Air volume, pressure, air temperature control, filters | Clogged filters reduce air, output drops, dust gets into the film |
| IBC and bubble size sensor | Cools inside the bubble and controls bubble diameter automatically | Supply and exhaust capacity, closed-loop control, sensor type | Layflat drifts, bubble breathes in and out, dirty sensor reads wrong |
| Bubble cage / calibration basket | Steadies the bubble above the frost line | Adjustable diameter and height; rollers or air cushions that don't scratch | Scratches on the film, stiff rollers, bubble flutter |
4. Top of the tower: collapsing frame, nip rolls, haul-off, gauge
The collapsing frame flattens the round bubble gradually; if the angle or friction is unbalanced, the film wrinkles. The primary nip pairs a steel roll with a rubber-covered roll; it both seals the air in the bubble and pulls the film, and nip speed relative to melt speed at the die sets the machine-direction draw. Most current lines use an oscillating haul-off that turns the nip assembly back and forth around the bubble axis, so gauge bands don't stack in one place in the roll. The roll winds flat, but the film itself is no more uniform.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Collapsing frame (tent) | Flattens the bubble before the nip | Slat type (wood, rollers or air cushion), adjustable angle, length | Wrinkles, scratches, crooked fold |
| Gusseting boards | Push the bubble sides in to form side gussets | Gusset depth range, symmetry | Unequal gussets, wrinkles at the fold |
| Primary nip rolls | Seal the air in the bubble and pull the film | Face width, rubber cover and hardness, nip force, parallelism | Air leaks and the bubble shrinks, cut or hardened rubber, wrinkles at the nip |
| Oscillating haul-off | Turns the pulling unit back and forth to spread gauge bands in the roll | Rotation angle, speed, turning bars that don't scratch | Wrinkles or tracking problems at the turning bars, scratches |
| Thickness gauge | Measures thickness around the bubble or on the flat film and feeds automatic air-ring or die control | Principle (capacitive contact on the bubble, or beta / X-ray / IR), resolution, position mapping | Drift, disagreement with a micrometer, corrections in the wrong place |
5. Downstream: corona, slitting, winder
Untreated PE has a surface energy of about 31 mN/m (dyne/cm); printing and laminating commonly need about 38–42 mN/m (typical reference range — confirm with the machine maker and the TDS). Corona applies a high-voltage discharge to the film surface. The treatment level decays in storage, and slip or antistatic blooming to the surface can lower it, so measure again before printing.
| Part | Job | What to check on the spec / at acceptance | Symptom when worn or failing |
|---|---|---|---|
| Corona treater | Raises surface energy so inks and adhesives bond | Power, width, electrode type and roll covering, one- or two-side treatment, ozone extraction | Low or uneven dyne across the width, backside treatment, burnt roll covering |
| Slitting | Trims edges or splits the film into several rolls | Razor or rotary knives, positioning system | Ragged edges, plastic dust, width off spec |
| Contact / gap winder | Contact: a lay-on roll presses air out of the roll · gap: a small gap reduces pressure on mark-sensitive film | Tension and contact-pressure range, top speed, maximum roll diameter | Rolls wound too tight and blocking, too loose and telescoping, air trapped in the roll |
| Turret and automatic changeover | Cuts and starts a new roll without stopping the line | Changeover speed, changeover success rate | Missed transfers, film wrapping the rolls, scrap at every change |
Line types
| Type | Suits | Advantages | Limits |
|---|---|---|---|
| Mono-layer | General bags, shrink film, basic agricultural film | Fewer machines, easy changeovers | Additives through the full thickness; inside and outside surfaces cannot differ |
| 3-layer (A/B/C) | Packaging film, films needing different seal and print surfaces | Slip and antiblock only in the skins; regrind or PCR in the core | Layer ratio and compatibility between layers must be controlled |
| 5 or 7-layer (barrier) | Gas-barrier film with EVOH or PA and tie layers | Barrier properties with thin layers of costly materials | Complex die and control, harder to recycle, long changeovers |
| Low-stalk (pocket) for LDPE / LLDPE | General film | Low frost line, fast cooling | Not suited to HDPE, which needs balanced orientation |
| High-stalk for HDPE | Thin HDPE film such as carrier and shopping bags | Balanced orientation, tough film | Tall tower needed; bubble sensitive to draughts |
| Single-lip / dual-lip / dual-lip + IBC | According to the output needed | Dual-lip and IBC add cooling capacity and stability | More complex; sensors and filters need care |
| Upward, or downward water-quenched | Upward: general PE · downward: high-clarity film or some barrier films | Downward quench cools very fast, giving small crystals and clear film | Downward lines are specialist machines with different control |
How to size the line
Start from the film you will sell, work back to the die, then to the extruders and cooling.
- Define the product Layflat or slit width, thickness range, number and ratio of layers, resins and monthly volume.
- Die diameter from layflat and BUR layflat = π × Ddie × BUR ÷ 2 → Ddie = 2 × layflat ÷ (π × BUR). Example: 600 mm layflat at BUR 2.5 → 2 × 600 ÷ (3.14 × 2.5) ≈ 153 mm. Check that this die also makes your narrowest and widest layflats within a sensible BUR range.
- BUR range by resin LDPE and LLDPE typically about 2–3 (wider range about 1.5–4) · HDPE high-stalk typically about 3–5, with a neck about 6–8 die diameters long.
- Estimate output from die circumference Machine makers often quote specific output in kg/h per mm of die circumference. A broad reference range is about 0.1–0.5 kg/h·mm: the low end for a single-lip ring without IBC, the high end for dual-lip with IBC. Example: a 150 mm die has a circumference of ≈ 471 mm; at 0.3 kg/h·mm → about 140 kg/h.
- Work out line speed m/min = kg/h ÷ (60 × 2 × layflat (m) × thickness (m) × density (kg/m³)). Example: 140 kg/h, 0.6 m layflat, 40 µm film, density 920 → 140 ÷ (60 × 2 × 0.6 × 0.00004 × 920) ≈ 53 m/min. Compare with the maximum speed of the nip and winder.
- Check die gap and DDR DDR = die gap ÷ (film thickness × BUR). Example: 2.0 mm gap, 0.04 mm film, BUR 2.5 → DDR = 20. The thinner the film, the higher the DDR and the stronger the machine-direction orientation.
- Split extruders by layer A 3-layer 1:2:1 structure → each skin extruder 25 %, the core 50 % of total output. Every extruder must run steadily at the lowest output of the job with the thinnest layer.
- Tower and winder Tower height must suit HDPE high-stalk and the cooling length; nip and winder width must take the maximum layflat; the maximum roll diameter must match what your customers use.
The examples above are arithmetic to show the method and use typical reference ranges — confirm with the machine maker and the TDS.
Maintenance checkpoints
A film line has three specific hazards: the nip rolls at the top of the tower and at the winder, which can draw in a hand; the corona treater, which runs at high voltage and produces ozone; and work at height on the tower.
| Frequency | Checkpoint | What to look at | If neglected |
|---|---|---|---|
| Every shift | Guards, emergency stops and interlocks at the nip rolls, winder and corona covers | Test that they really stop; opening the corona cover really cuts the high voltage | Hands drawn into the nip, electric shock |
| Every shift | Die lip and air-ring lips | Build-up, dents, dust on the lips | Die lines, gauge bands, build-up breaking off as specks |
| Every shift | Bubble stability and frost line height | Flutter, leaning bubble, frost line moving, layflat drifting | Gauge variation, wrinkles, width out of spec |
| Every shift | Corona and ozone extraction | Dyne level with test pens; extraction fan running | Poor ink adhesion; ozone building up in the work area |
| Every shift | Screen pressure and grooved-feed cooling water | Pressure trend, throat water temperature | Screen rupture, pellets bridging at the throat, output loss |
| Weekly | Air-ring and IBC blower filters | Cleanliness, pressure drop across the filter | Less air, output loss, dust in the film |
| Weekly | Nip-roll rubber and bubble-cage rollers | Cuts, hardened rubber, rollers turning freely | Air leaking from the bubble, wrinkles or scratches |
| Weekly | Dosing units and slitter knives | Load-cell check, knife sharpness | Wrong MB dosing, ragged edges |
| Monthly | Corona electrodes and roll covering | Deposits, burns, punctures in the covering | Uneven treatment, covering burnt through |
| Monthly | IBC sensors and thickness gauge | Clean; standardize against a micrometer | Layflat and profile controlled to a wrong reading |
| Monthly | Winder | Bearings, tension sensors, air shafts | Uneven roll hardness, telescoping |
| Yearly | Die | Strip, clean and polish spirals and lips; check for corrosion | Permanent gauge bands; gels and black specks from dead spots |
| Yearly | Screw, barrel, grooved-feed bushing and gearbox | Clearance, groove depth, oil and vibration | Output loss, high melt temperature, gearbox damage |
- Never thread film or clear a wrap at the nip rolls or winder while the rolls are turning.
- Never open the corona cover or touch the electrode before the power is off and the charge has dissipated.
- Never work on the tower without fall protection, and never stand beneath it where objects could fall.
- Never work on heaters, the die, blowers or the winder without lock-out/tag-out.
Masterbatch and additives on this line
Masterbatch enters at each extruder's dosing unit. In multilayer film, put additives only in the layer that needs them and calculate let-down against that layer: antiblock in both skins, for example, and slip in the surface that must slide. Amide slip agents migrate across layers, so allow for this when designing the structure and setting the corona. Barrier screws and mixing sections help dispersion, but an MB whose carrier doesn't melt-blend with the base resin can still show up as gels or streaks.
- Slip and antiblock Slip lowers friction; antiblock (such as silica or talc) creates micro-roughness on the surface so film layers don't stick. See film blocking and slip.
- Processing aid (PPA) Removes sharkskin and reduces die build-up. The most widely used PPAs are fluoropolymer-based; they coat the die wall and need some conditioning time before they work. Antiblock and some pigments adsorb PPA and reduce its effect. With regulatory attention on PFAS substances in many markets, PFAS-free PPA alternatives are also available — they must be trialled on your own formulation and die.
- Antistatic For component packaging film or film that attracts dust; migrating types need time after extrusion. See antistatic masterbatch.
- UV for agricultural film Greenhouse and mulch films use HALS and UV absorbers. Agrochemicals containing sulphur or chlorine can deactivate some HALS, so the grade must match the use. See UV masterbatch.
- Anti-fog For greenhouse film and food wrap: condensation spreads as a thin layer instead of droplets. It migrates to the surface, so it interacts with slip and corona.
- Colour White MB for opaque bags, black MB for mulch film; well-dispersed carbon black also protects against UV.
To be straight about it: gauge bands from an off-centre die or a dirty air ring, wrinkles from the collapsing frame or a non-parallel nip, and gels from dead spots in the die or a worn screw are machine problems that additives cannot fix. When comparing formulations, count cost per good roll, not price per kilogram — try your own numbers in the cost-in-use calculator. If you want help separating a formulation problem from a machine problem, the ACS team can look at your data.
Related process articles
BUR and frost line from the process side, and the parameters to control, are covered in Extrusion. Common film defects: gels and fish-eyes · sharkskin (melt fracture)
FAQ
Blown film line FAQ
The film has thick and thin bands along its length. Where do I fix it?
Work from cheapest to most expensive: clean the air-ring lips and check that air leaves evenly all round; check die temperatures around the circumference; check die centering and look for build-up at the lip; then check the thickness gauge and the automatic control. An oscillating haul-off keeps gauge bands from stacking in the roll so the roll winds flat, but it does not make the film itself any more uniform.
What makes the bubble unstable, flutter, or the frost line move up and down?
Common causes are unsteady air-ring air (clogged filters, fluctuating air temperature), unbalanced IBC supply and exhaust, draughts from doors or plant fans, melt temperature too high, or a resin with low melt strength. Change one variable at a time: start by removing draughts and checking filters, then adjust melt temperature and the bubble-cage position.
What should I check when buying a used blown film line?
Check the die (scratches or corrosion in the spirals, the lip, the centering system), the grooved-feed bushing and screws (groove wear, clearance), the air-ring lips (dents), the nip-roll rubber and the winder (tension control, changeover). Ask for a run with the resin and thickness you will actually produce, measure gauge around the circumference and layflat continuously for at least an hour, and confirm the tower is tall enough for your jobs.
How do I change colour or formulation quickly on a film line?
Sequence jobs from light to dark, run down the hoppers and dosing units before the change, and use a purging compound suited to PE and the temperature. Spiral dies clean out faster than older designs but still have places where colour lingers. If the old formulation contained PPA, the die surface keeps a PPA layer for a while; if the new one has high antiblock, it may take time for the PPA effect to settle. Always measure COF and dyne on the first roll after a change.
Caution
General knowledge for understanding only. Numbers are typical reference ranges. Selection, installation, operation and maintenance must follow the machine maker's manual and instructions and your plant's safety rules.
