Two routes to film
Film is an extrusion product – but with two fundamentally different cooling routes. In blown film the melt leaves a ring die, is cooled with an air ring and inflated into a free-standing, biaxially stretched bubble. In cast film the melt leaves a flat die onto a chilled roll and is quenched in seconds.
From this single difference – slow air cooling versus fast quench – almost all property, optics and cost differences follow. Both belong to extrusion but require differently set materials.
Process technology in detail
The blown-film line uses a ring die, air ring (single or, for higher output, dual-lip) and a vertical bubble. Key variables are the blow-up ratio (BUR) – typically 2:1 to 4:1 – and the frost line height, which marks the melt→solid transition and governs gauge uniformity and bubble stability.
The cast-film line uses a wide slot die and chilled rolls. Thickness is set via a point-by-point – often automatically controlled – slot die, allowing very tight tolerances. For high-molecular HDPE there is high-stalk blown film: a long bubble neck (5–9 die diameters) with subsequent rapid expansion gives the high strength of thin HMW-HDPE films.
Output reflects the cooling: blown film reaches typically about 10–20 lb/h per inch of layflat width, cast film about 15–30 lb/h per inch of web width – roughly three times, because the chill roll cools more effectively than the air ring. The bubble is cooled with single- or, for higher output, dual-lip air rings; the correct frost line position governs gauge uniformity and maximum output.
Properties: MD/TD balance and impact
Through biaxial stretching – in the machine direction (MD) by haul-off, in the cross direction (TD) by inflation – the mechanical properties of blown film are more balanced. Tear strength and elongation distribute more evenly across MD and TD, which boosts dart-drop impact.
Cast film is mostly monoaxially (MD) oriented. For linear polyolefins such as HDPE, monoaxial stretching plus neck-in can produce very split-prone ("splitty") film. Blown film is therefore often preferred for balanced strength, while the directional orientation of cast film can even be desirable for tear-line applications.
Optics, density and barrier
Cooling rate controls crystallinity and thus optics and barrier. The very fast quench on the casting roll keeps cast film more amorphous → less haze, more gloss, better clarity. The slower air cooling of the bubble allows more crystallite formation and higher density → higher modulus (stiffer) and lower water-vapour/gas permeability, i.e. a better barrier at the same polymer.
Melt temperature also differs by method: cast film tends to run hotter (about 246 °C) than blown film (about 191 °C), partly due to the lower-viscosity raw materials.
Thickness tolerance, output and cost
The hardest documented process difference is thickness tolerance: cast film reaches typically ±2 %, blown film about ±10 % (reducible to ±5 % with good automatic cross control).
On output cast film leads because chill-roll cooling is more effective than air cooling – line speed at equal throughput is roughly double. In return, blown film has lower capital cost (roughly 14–28 ct per annual pound versus 25–50 ct for cast), because the fast cast line needs elaborate haul-off/winding and effectively an automatic gauge control.
Blown or cast? The decision
The overview contrasts the documented differences:
| Criterion | Blown film | Cast film |
|---|---|---|
| Cooling | air ring on free bubble (slow) | chilled roll / quench (fast) |
| Orientation | biaxial (MD + TD) | mostly monoaxial (MD) |
| Thickness tolerance | ±10 % (±5 % with control) | ±2 % |
| Optics (haze/gloss) | hazier, matter | clear, glossy |
| Barrier / stiffness | higher (more crystallinity) | lower |
| Line speed | reference | about double |
| Investment per annual kg | ~14–28 ct/lb | ~25–50 ct/lb |
| Required melt strength | high | low possible (higher MFR) |
Rule of thumb: blown film when balanced strength, barrier and stiffness matter (heavy-duty, refuse, packaging and barrier films). Cast film when best transparency, tight tolerance and high speed are needed (stretch, hygiene and high-clarity packaging films). For linear polyolefins like HDPE, note that pure cast film can become split-prone.
Materials and their setting
Which grade tolerates which method depends on melt strength:
- LDPE: long-chain branching (LCB) creates strain hardening and thus high melt strength – ideal for bubble stability. Example: Repsol Alcudia LDPE PE063/A (MFR 4 g/10 min, 190 °C/2.16 kg).
- LLDPE: linear, very tough, but low in melt strength; good for downgauging, often blended with LDPE. Examples: Dow DOWLEX 2045G (MI 1.0), ExxonMobil LL 1001 (MI 1.0).
- mLLDPE (metallocene): narrow distribution, best optics and strength, but lower melt strength → reduced BUR (~2.0–2.5) and stronger cooling needed. Example: INEOS Eltex PF6130AA (cast, MFI 3.5).
- HMW-HDPE: for stiff high-stalk blown films, specified via HLMI. Example: ExxonMobil HD 7960.13 (MI 0.60; HLMI 10).
The die gap must suit the material: LLDPE dies usually have 2.0–2.5 mm gaps; a too-narrow LDPE die raises pressure and sharkskin/melt-fracture risk with LLDPE. MFR-setting fundamentals are in Plastics grades by processing method.
Coextrusion, barrier and downgauging
High-grade packaging films are mostly multilayer. In blown film 3, 5, 7, 9 to 11 layers are industrially common; new multi-micro-nano-layer techniques reach 129 to over 1,000 layers in one die head with multiplier elements. Typical barrier architectures use EVOH as a gas-tight core, protected by polyamide (PA) and polyolefin skins – only coextrusion solves this combination.
Downgauging (thinner film at equal function) cuts material use across industries by about 10–30 %; achieved via higher-performance mLLDPE materials, optimised layer distribution and multilayer construction. Which film ultimately fits is decided along property profile, optics, barrier and unit cost – material and request via the materials database.