Cast Film vs Blow Film: Who is the 'King' in the Film Industry?The Ultimate Battle between Process and Performance

In the vast realm of plastic films, two dominant techniques have long been in competition: the towering blown film and the straightforward cast film.

· Many newcomers to the industry, even seasoned procurement professionals, often find themselves baffled when encountering these two terms:

· “Why can CPP only be cast but not blown?”

· “Why are wrapping films mostly cast films, while shopping bags are all blown films?”

· “Which process yields superior membrane quality?”

Today, we'll dive into these two major processes from a professional perspective—covering their manufacturing principles, performance differences, application fields, and even a troubleshooting guide—to help you identify the perfect match for your needs.

�� Chapter 1: The Great Revelation of Technology-How is it "Born"?

To understand whether a membrane is good or not, one must first examine how it is manufactured. The core difference between these two processes lies in their forming methods and cooling methods.

1. Blown Film: Similar to blowing bubbles

Blow film process is like blowing a huge balloon.

· Process: After the plastic particles are melted, they are extruded from a ring-shaped die to form a tubular structure. Then, compressed air is blown into the tube to expand it into a massive cylindrical bubble (Bubble).

· Cooling: relies on the surrounding air flow (wind ring) for cooling.

Traction: The membrane bubble ascends, passes through the herringbone plate for folding and flattening, and is finally rolled up.

【Keywords】： Ring die, air cooling, and bidirectional stretching (longitudinal traction + transverse blowing).

2. Cast Film: Flowing like a waterfall

· The casting process is more like a waterfall from the sky.

Flow: The plastic melt flows out of a flat T-shaped die (slit die) and falls vertically like a curtain.

Cooling: The material is instantly exposed to a massive rotating chill roll, where its shape is rapidly solidified through rapid cooling by the metal surface.

Traction: The cooled film is trimmed, measured in thickness, and finally rolled up.

【keywords】：The main characteristics of the T-type die head are contact cooling and unidirectional stretching.

�� Chapter 2: Performance Benchmarking – How Significant Are the Differences in Final Products?

Due to differences in "birth environment", the properties of these two membranes are vastly distinct. We conduct a performance comparison (PK) from the following dimensions:

· 1. Optical properties (transparency and gloss)

Principle of Cast Film Superiority: Cast film utilizes quenching rollers for rapid cooling, which effectively inhibits polymer crystallization, resulting in fine crystal spheres with minimal light scattering. Key Performance: Exceptional transparency, low haze, and high gloss. CPP cast film exhibits a crystal-clear appearance.

Blow film principle: The air cooling speed is relatively slow, allowing sufficient time for polymer crystallization, resulting in larger crystal spheres that easily cause light scattering. Manifestations: High haze and average transparency (although current rapid water-cooled blow film technology has improved this, it still falls short of cast film).

· 2. Mechanical strength (tensile and tear resistance)

Blown film  prevails (more balanced) Principle: During the blowing process, the film undergoes stretching both longitudinally (traction) and laterally (inflation). Performance: It exhibits balanced longitudinal and transverse strength, excellent toughness, and strong puncture resistance. This explains why supermarkets use blown film for shopping bags—they are less likely to tear when carrying heavy items.

Cast film (with partiality) Principle: Primarily subjected to longitudinal traction with minimal lateral stretching. Manifestations: Molecular alignment exhibits distinct directional characteristics. The longitudinal tensile strength is acceptable, whereas the transverse tear resistance is suboptimal. A small incision can readily cause linear tearing (refer to transparent tape for reference).

· 3. Uniformity of thickness

· Cast film Winner Data: The thickness tolerance of cast film can be controlled within ±1.5% to 3%. Advantages: Exceptionally smooth surface, ideal for high-speed printing and precision coating.

· Blow-molding Data: The thickness tolerance typically ranges from ±5% to 10% (even with a rotating die head). Disadvantages: Uneven thickness may cause 'buckling' during winding, which affects the quality of subsequent lamination.

· 4. Barrier

· Blown film has a slight edge. It can more easily incorporate high-barrier materials like PA and EVOH through multi-layer co-extrusion (e.g., 5, 7, or 9 layers), with lower equipment costs compared to cast film and greater structural flexibility.

��️ Chapter 3: Who is Dominating the Territory? (Application Field)

Based on performance differences, they occupy different peaks:

�� Home of Cast:

CPP (Cast Polypropylene): A heat-sealable inner layer material for food packaging (e.g., cookie bags, bread bags), requiring high transparency and excellent stiffness.

Stretch film: A packaging film for courier trays, utilizing the high-speed production capacity of the coater (up to 500m/min or more).

Sanitary products: Diaper base films and sanitary pad packaging, with a focus on softness (CPE).

Aluminum-plated substrate: The requirement of flatness is extremely high, and the cast film is the first choice.

Blown home:

1. General packaging: shopping bags, backpack bags, and garbage bags (mainly PE), which should be durable and cost-effective.

2. Agricultural mulch films: mulch films, greenhouse films, and blown films can easily achieve ultra-wide widths (with fold diameters reaching several meters).

3. Repackaging bags: fertilizer bags, rice bags (FFS), requiring extremely high impact and puncture resistance.

4. Thermosetting film: Utilizes the biaxial stretching memory effect to contract and encapsulate products upon heating.

⚠️ Chapter 4: Guide to Model Selection and Pitfall Avoidance

When developing a new packaging, should you choose flow casting or blow molding? Refer to the decision tree below:

· 1. Beauty or Endurance?

For display products like flowers, premium textiles, or transparent window displays requiring high translucency, cast material is the ideal choice.

For heavy or sharp-edged products (e.g., frozen ribs, metal parts) requiring puncture resistance and bag integrity, choose Blown film.

2.See subsequent processing procedures

High-speed printing/coating: When your printing press operates at high speed or requires adhesive coating, the uniform thickness of the cast film will significantly improve your yield.

Bag sealing: For simple heat-seal bags, both methods are acceptable. However, when creating complex bag shapes, consider the risk of the cast film tearing laterally.

3. Cost considerations

· Sluried casting: The equipment investment is large, the energy consumption is high, but the production speed is fast, which is suitable for large-scale orders.

· Blow molding: The equipment threshold is relatively low, with flexible specification changes, making it suitable for multi-variety and small-to-medium batch production.

·  summary

There is no absolute 'better' between cast film and blown film, only 'more suitable'.

The cast film is a gentleman who pursues refinement, smooth, transparent and efficient, but a little fragile.

Blow molding is a tough guy—robust, durable, and versatile, though a bit rough.

By understanding their nature, you can master packaging design with ease—saving money while ensuring practicality!

· References and data sources:

· [1] Giles, H. F., Wagner, J. R., & Mount, E. M. (2013). Extrusion: The Definitive Processing Guide and Handbook. William Andrew.

· [2] Butler, T. I. (2018). Film Extrusion Manual: Process, Materials, Properties. TAPPI Press. （Authoritative Data on Material Properties Comparison Between Blow-molded Film and Cast Film）

· [3] ASTM D882 - Standard Test Method for Tensile Properties of Thin Plastic Sheeting.