How does a shrink tunnel work?

The shrink tunnel uses hot air to shrink POF (polyolefin) or PE (polyethylene) shrink films onto a product in order to safely package the product and protect it from external environmental influences.

Packaging process

The first step the product to be packaged has to be placed in a center-folded film. The process can be done in two ways. It can be done manually or fully automatically - this depends on the level of automation of upstream machine. 

Semi-automatic means placing and sealing of the products needs to be done by hand.

Fully automatic means a conveyor is used for infeed the product. Placing and sealing of the product is automatically.

However, the film must completely envelop the product. For shrinking a little extra material at the edges is needed.

In the next step, the still open sides of the film are welded together. This will be done by semi-automatic or fully automatic L-sealers. For smaller quantities a semi-automatic L-sealer is a good choice. Fully automatic side sealers are used for particularly high cycle rates.

Once the product has been wrapped and sealed, it can then be fed into the shrink tunnel for the shrinking process.

Shrink wrapping in the shrink tunnel

• Transport through the shrinking chamber: The shrink-wrapped product passes through the shrinking chamber of the shrink tunnel on the conveyor belt. The shrinking chamber has heaters or heating elements that heat the air to a very high temperature (depending on the type of film). The transport speed can be adjusted and, along with the temperature, has a significant influence on the final packaging.
• Heating the film: As the packaged products pass through the tunnel, the film is quickly heated by the hot air. This heating causes the film to shrink. In the process, the film pulls itself around the product and adapts closely to the shape. The shrinking process is carried out both by heat radiation and by convection of the hot air. 
• Shrinkage of the film: Since POF and PE film have thermoelastic properties, the film contracts when heated and adapts exactly to the product. The shrinking process continues until the film has reached its maximum shrink rate.
• Cooling of the film: After shrinking, the product is removed from the tunnel and must cool down. During cooling, the film solidifies again so that it retains its shrink shape and the product is stably packaged.

Shrink result influencing factors

In addition to the shape of the product to be packaged and the film properties themselves, the shrink tunnel is quite simple in its settings, which affect the shrink result.

The temperature and throughput speed parameters can be used to directly influence the appearance of the packaging. Nevertheless, it is crucial that a suitable film is selected in terms of film thickness and structure (number of layers, multi-layer/cross-linked).

Once the appropriate film has been selected, the temperature and speed of the conveyor belt of the shrink tunnel must be adjusted to the film properties as well as to the size and characteristics (e.g. shape) of the product to ensure even shrinkage.

For which products can a shrink tunnel be suitable?

The process ensures professional and attractive packaging that also protects against dust, moisture,  or damage at a relatively low cost in terms of the machinery required.

The areas of application are very diverse, ranging from frozen pizza to calendars, books, all types of outer packaging such as boxes made of cardboard from the cosmetics industry, to ironing boards, clothes horses, picture frames, interior doors and bulky insulation materials – to name just a few examples.