Which is best, cast or blown foil?

Louise Smyth

From packaging or textile membranes to functional automotive components, products made directly or indirectly of films figure largely in our lives. Here Heidi Fraefel from Emerell compares two different manufacturing techniques.

Bubble or chill roll

In blown film extrusion, plastic melt is run through extruders ending with air rings that blow air into the mass to create and subsequently wrap a bubble. Before wrapping, the bubble is flattened to create a lay-flat tube of film and blades are used to cut it into two single-layer films. In chill roll extrusion, the plastic melt is poured onto a tempered (in most cases cooled) revolving roll before being transported further. The film is wrapped after both edges have been trimmed off.

It’s subsequently hard to tell which technology was used to produce a film. Our experience is that films made by blown film extrusion are more likely to crop up in the packaging industry, while those produced by chill roll extrusion tend to be used for technical and graphical applications. One reason for this could be that blown film extrusion can be used to simultaneously extrude up to 13 layers from different plastics, which allows a wide range of functions (with barrier effect, mechanical properties) to be combined in one film. This variety is currently not possible with chill roll extrusion.

Technology dictated by material

The technology is selected primarily on the basis of the customer’s requirements, and particularly the choice of materials. Various factors have to be taken into account. For example, the flow properties of the material are crucial. One characteristic often used in practice is the so-called MFR (melt flow rate) which indicates the amount of polymer that will flow out of a nozzle per unit of time under defined conditions. Blown film extrusion tends to be superior to chill roll technology for high MFRs, while chill roll extrusion is more likely to be chosen for lower MFRs. Other factors influencing the decision include the size of the order and lot, the basis weight of the film and other physical properties specified in the order. 

When bubble can be too heavy

By its nature, blown film extrusion is subject to limitations imposed by the weight of the film. Emerell can use blown film extrusion to form films with a basis weight of up to around 300 grams per square metre. At higher basis weights, the bubble gets too heavy, in which case the film has to be produced by chill roll extrusion, which can produce films with a basis weight of up to around 1,000 grams per square metre. The desired maximum film breadth is also an important criterion. Its blown film extrusion equipment can produce widths of up to 2,650 millimetres (in individual cases up to 3,350 millimetres), while its chill roll extrusion equipment can produce films of up to 2,150 millimetres in width.

Differences resulting from blown film extrusion include elongation in machine direction (MD) and cross direction (CD), and slower cooling by comparison with chill roll extrusion. In chill roll extrusion there is only slight elongation in MD, and the more rapid and intense cooling of the plastic melt results in differences in mechanical and optical properties (elasticity and transparency) compared with chemically identical blown films. The differences depend to a large degree on the material used. For example, films using semi-crystalline polyolefin polymers display differences in terms of their stretch and shrink properties when under thermal and mechanical stress that can be attributed to the manufacturing process.

 

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