How are they made?
Cast film is made by melting down granular resin which is then fed through a narrow slot die. The thickness and width of the stretch film is determined by the size of the slot in the die. After being fed through the die, the film is passed over a chilled roller which cools down the film very quickly. Tension rollers are then used to draw the film to winding and sitting stations.
Blown film production begins in a similar way, with molten resin being forced through a die. However, in blown film production, the die is circular. The molten resin is then blown upwards creating a tube of film which can be many meters high. This way the film cools slowly as it rises before being gathered at the top and drawn to winding and slitting stations by guiding rollers.
What are the main properties of each?
– High transparency
– Consistent thickness
– Quiet unwind
– Higher puncture resistance
– Noisier unwind
– Relatively cloudy and unclear film
– High levels of cling
Which one should you use?
Neither of these types of film are necessarily ‘the best’, however there are certain applications where one might be more suited than the other. For example, the high puncture resistance of blown films means they are well suited to companies wrapping sharp, awkward loads such as companies in the construction materials industry. Cast films, on the other hand, are a good option for companies that require film clarity to scan barcodes through the stretch wrap. When making a decision consider what you will be wrapping (is it a light or heavy load etc.) and whether there are any other requirements for your film (such as unwind noise).
Which is more efficient?
Many tests have shown blown film to have a higher stretch, however it requires more force to stretch meaning optium stretch cannot be achieved on most pallet wrapping machines. This means that is many instances, higher stretch and therefore higher yield can be achieved with cast film. However again there is a ‘but’, the superior strength of blown films means that you may be able to achieve plastic and cost savings by reducing your film gauge.
What are nano films?
A nano film is a cast film that is made up of multiple layers, this greatly improves the strength of the film. Nano films are beginning to replace blown films as they offer the same puncture resistance and durability but the lower production times and cost of cast film.
In conclusion, the differences between blown and cast stretch films are relatively subtle and choosing the wrong type will not be detrimental to your operations but may mean that you are missing out on small efficiency improvements. If you are having trouble choosing the right film then we would be more than happy to take a look at your packaging operations and give your our expert opinion, just get in touch.