Machines

The shrink film packaging process is divided into two stages:

1. Sealing
2. Shrinking.

The necessary conditions for a durable and aesthetic heat seal are:

• a sealing device temperature in the region of 170-190°C (the exception on advanced automatic machines is the longitudinal sealing device allowing a theoretically continuous seal, which requires temperatures in the range of 350-380°C due to its short contact with the film). On simple manual machines with sealing bars in the form of a wire or resistance bar, the temperature is regulated by varying the current. As a general rule, we can assume that the most suitable temperature is the minimum value at which the foil is seamlessly cut.
• adequate pressure on the welding tool and stabilisation of the foil by its covers during sealing (during sealing, there must be no tension on the foil, which would cause it to slip out from under the welding tool during the process)
• sealing time approx. 0.5 -1s

On very high-speed packaging machines in the longitudinal closure system, we can encounter film ionisation, which, in short, means that the film is electrified at the overlap (approx. 1.5-2 cm wide) and a virtually imperceptible seal is achieved. Unfortunately, this system only works on products with a flat surface and is mainly used in high-volume production of CDs or DVDs, books, magazines, etc. Ionisation of cross-linked films is also problematic due to their lower susceptibility to this process.

When shrinking a product, we need to pay particular attention to:

• the right amount of film – it is necessary to have a balloon around the packaged product at the moment of the first blow of hot air. Too much film will not allow it to shrink completely, too little will cause wrinkles on the edges of the pack.
• perforation – we need to ensure that the air can be removed from the pack in a controlled manner. Too much perforation will result in accelerated shrinking and wrinkles on the surface of the pack, too little perforation can lead to tearing or burning of the film.

There are two main ways of perforating

• suitable temperature – usually around 140-160°C. These values should be regarded as purely indicative, as they are highly dependent on the type of film, the product being packed, the length of the tunnel and the air circulation inside, and above all on the speed at which packaging takes place. In some cases, even temperatures above 250°C guarantee the right shrinking conditions.

Easy-opening systems allow easier access to the packaged product. With very good quality packaging, this is not easy. Three types of such systems are offered:

• an additional sticker on the outside of the ionisation seal (e.g. Lindt)
• heat-shrinkable tape applied to the film as it unwinds on the packaging machine
• an additional sticker with a very strong adhesive placed on the pack after shrinking to allow easy removal of the film