Cast Film Vs Blown Film

The blown film process is similar to inflating a giant balloon.

The cast film process resembles a waterfall flowing downward.

• Process: The molten polymer exits a flat T-die (slit die) and flows downward like a curtain.

• Cooling: The melt comes into immediate contact with a large, rotating chill roll, where it is rapidly cooled and solidified by the low-temperature metal surface.

• Haul-off: The cooled film is trimmed, stretched in the machine direction, and finally wound into rolls.

[Keywords]: T-die; chill-roll quenching; predominantly uniaxial stretching.

• Winner: Cast film

Principle: Cast film is rapidly quenched by chill rolls, which significantly suppresses polymer crystallization and results in fine crystalline structures.

Performance: Excellent transparency, low haze, and high gloss. CPP cast film appears crystal clear.

• Blown film:

Air cooling is relatively slow, allowing more time for crystallization and the formation of larger crystalline structures, which increases light scattering.

Performance: Higher haze and moderate transparency, although advanced water-cooled blown film technologies have improved optical performance in recent years.

• Winner: Blown Film (More Balanced Properties)

Principle: During blown film production, the film is stretched in both the machine direction and the transverse direction.

Performance: Strength is relatively balanced in both directions, with good toughness and high puncture resistance. This is why blown film is widely used for supermarket shopping bags, as it is less likely to tear when carrying heavy loads.

• Cast film (directional weakness)

Principle: Stretching occurs mainly in the machine direction, with minimal transverse orientation.

Performance: The molecular orientation is highly directional. While tensile strength in the machine direction is acceptable, transverse tear resistance is poor. Once a small notch forms, the film tends to tear straight across (similar to adhesive tape).

• Winner: Cast Film

Data: Thickness tolerance can typically be controlled within ±1.5% to 3%.

Advantages: Excellent flatness, making it ideal for high-speed printing and precision coating.

• Blown film

Data: Thickness tolerance is usually around ±5% to 10%, even with rotating dies.

Disadvantages: Uneven thickness may lead to pronounced thickness bands during winding, negatively affecting subsequent lamination quality.

Slight Advantage: Blown Film

Blown film is more suitable for incorporating high-barrier materials such as PA and EVOH through multilayer co-extrusion (e.g., 5-, 7-, or 9-layer structures). The equipment investment is relatively lower, and structural design is more flexible.

• CPP (Cast Polypropylene): Inner heat-sealing layers for food packaging (e.g., biscuit and bread bags), requiring high transparency and good stiffness.

• Stretch film: Used for pallet wrapping, benefiting from the high production speed of cast lines (up to 500 m/min or more).

• Hygiene products: Diaper liners and sanitary product packaging, where softness and comfort (e.g., CPE) are critical.

• Aluminized substrates: Applications with extremely high flatness requirements, where cast film is the preferred choice.

• General packaging: Shopping bags, T-shirt bags, and garbage bags (mainly PE), requiring toughness and low cost.

• Agricultural films: Mulch films and greenhouse films, where blown film can easily achieve ultra-wide widths.

• Heavy-duty packaging: Fertilizer bags and rice bags (FFS), requiring high impact and puncture resistance.

• Heat-shrink films: Utilizing biaxial orientation memory to shrink tightly around products when heated.

• If the product is display-oriented (e.g., flowers, premium textiles, transparent window packaging) and requires high clarity, choose cast film.

• If the product is heavy, rigid, or sharp-edged (e.g., frozen meat, hardware), and puncture resistance is critical, choose blown film.

• High-speed printing or coating: The superior thickness uniformity of cast film significantly improves yield and process stability.

• IBag making: Both films are suitable for simple heat-sealed bags; however, for complex bag structures, the risk of transverse tearing in cast film must be carefully evaluated.

• Cast film: Higher equipment investment and energy consumption, but high output and efficiency, suitable for large-volume production.

• Blown film: Lower equipment threshold and greater flexibility in specifications, suitable for small- to medium-volume, multi-product production.