What is the difference between cast and blown film?

Introduction

In plastic film manufacturing, two primary processes dominate the industry: cast film and blown film extrusion. While both methods produce thin plastic films used in countless applications, they differ significantly in their production techniques, equipment requirements used in one process - and final product characteristics. This article explores the fundamental differences between cast and blown film, helping manufacturers, engineers, and product designers make informed decisions about which method best suits their specific needs.

I.Production Process Comparison
 

Cast Film Extrusion
 

The cast film process begins with melted polymer being extruded through a flat die, typically positioned vertically. This molten plastic is then:
 

1. Deposited onto a chilled roller (usually chromium-plated)
2. Quickly cooled to solidify the film
3. Passed through additional rollers for thickness adjustment
4. Wound onto rolls for further processing
 

The rapid cooling in cast film production results in films with excellent clarity and a glossy surface finish. Common materials used include PP (polypropylene), PE (polyethylene), and PET (polyethylene terephthalate).
 

Blown Film Extrusion Using Blown Film Machines
 

Blown film extrusion employs a specialized Blown Film Machine featuring a circular die through which molten plastic is extruded vertically upward. Key steps in the Blown Film Machine process include:

1. Introduction of compressed air to inflate the extruded tube like a balloon
2. Formation of a bubble that stretches the film in both machine and transverse directions
3. Cooling via an external air ring - a critical component of the Blown Film Machine
4. Collapsing of the bubble through nip rollers
5. Winding of the flattened tube

This biaxial stretching in the Blown Film Machine creates films with balanced mechanical properties in all directions. Blown film is commonly made from LDPE, LLDPE, and HDPE.

III.Key Differences Between Cast and Blown Film

 

IV.Applications and Industry Uses
 

Cast Film Applications
 

Cast film's superior clarity and precise thickness control make it ideal for:

Packaging films - for food products requiring high visibility
Metallized films - for decorative and barrier applications
Laminations - where smooth surfaces are critical
Release liners - and adhesive tapes
Photographic films

Blown Film Machine Applications
 

The unique properties of films produced by Blown Film Machines make them suitable for:

Stretch films and shrink wraps
Grocery bags and retail carry bags
Agricultural films (greenhouse covers, mulch films)
Heavy-duty sacks and industrial liners
Medical packaging
 

Many manufacturers prefer Blown Film Machines for these applications due to their ability to produce films with excellent tear resistance and more balanced properties.

V.Advantages and Disadvantages
 

Cast Film Pros and Cons
 

Advantages:

Faster production speeds
Superior optical properties
Better thickness control
Higher barrier properties
Smoother surface for printing
 

Disadvantages:
 

Higher initial equipment costs
Less balanced mechanical properties
Limited width capabilities compared to blown film
 

IV.Blown Film Machine Pros and Cons
 

Advantages of Blown Film Machines:
 

Lower equipment costs compared to cast film lines
Produces films with balanced properties in all directions
Better tear resistance in final product
Ability to produce very wide films
Easier to incorporate multiple layers
More adaptable to different types of materials
 

Disadvantages:

Slower production speeds than cast film
Less precise thickness control
Poorer optical clarity
More variable quality under varying conditions

VI.Technical considerations for selection
 

When choosing between cast film and blown film production, consider the following:
 

1. End-use requirements: optical performance vs. mechanical strength

2. Production volume: High-speed requirements favor cast film

3. Material considerations: Certain polymers process better in blown film machines

4. Cost constraints: Blown film machines typically have lower capital costs

5. Secondary operations: How to print, laminate, or convert the film

6. Film width requirements: Blown film machines can produce wider films

7. Performance balance: Blown film machines offer more isotropic performance

Conclusion
 

The choice between cast film and blown film production depends largely on specific application requirements and production priorities. Cast film excels in areas where optical clarity, high-speed production, and precise thickness are critical, while blown film machines offer the advantages of balanced performance, tear resistance, and cost-effectiveness in many packaging applications. Modern blown film machines with advanced technology continue to close the gap in areas where cast film has traditionally dominated, making the choice between the two processes more nuanced than ever before. Understanding these fundamental differences - especially the capabilities of today's advanced blown film machines - enables manufacturers to select the best production method for their specific needs, ensuring the quality and efficiency of their plastic film products.