Laminated film packaging: structure and role

Laminated film, several layers of film bonded into one sheet, is the flexible packaging around most snacks and nutrition bars on the shelf. Each layer does one job: the outer layer carries the print and takes the scuffing, the middle layer blocks moisture and oxygen, the inner layer seals the pack shut and touches the food. Knowing those three jobs is enough to read a film supplier's offer, ask the right questions, and understand why one pack of crackers is still crisp while another has gone soft. This piece is for R&D candidates and for packaging suppliers working with Color of Vietnam.

Three layers, three jobs

A film structure is usually written as a chain of abbreviations, for example a polyester outer layer, an aluminium foil or metallised layer in the middle, and a polyethylene inner layer. How to read it:

• The outer layer carries the ink on its inside face, so the print is protected by the film itself and does not rub off. It also gives stiffness, so the pack holds its shape and runs smoothly through the machine.
• The barrier layer keeps water vapour and oxygen out and aroma in. Foil is the best barrier but cannot be seen through. Metallised film is a good barrier at lower cost. Clear film is a weaker barrier but shows the product.
• The seal layer is the innermost one. It melts under heat to close the edges of the pack. It touches the food, so it needs a food contact declaration.

Between the layers sits the lamination adhesive. A good adhesive keeps the layers from separating at the seal and carries no odour into the product.

Two numbers to ask for

Two figures decide whether a film suits a product:

• OTR (oxygen transmission rate): how much oxygen passes through a given area of film in a day. An oily nut bar such as Natural & Healthy needs a low OTR, because oil that meets oxygen goes stale.
• WVTR (water vapour transmission rate): how much moisture passes through. Rice crackers and puffed snacks need a low WVTR, because taking on moisture means losing crunch.

Alongside these come seal strength, the force needed to pull a sealed edge apart, and the coefficient of friction of the film surface, which decides whether the film slides through the packing machine too easily or not easily enough. A serious supplier lists all of these on the technical data sheet, with the conditions under which they were measured.

A good barrier is useless without a good seal

However good the barrier, a leaking seal makes it worthless. Seals leak for three common reasons: the sealing temperature does not suit the film's seal layer, seasoning dust or oil has landed in the seal area, or the machine runs faster than the film can take. So when we trial a new film, we do not trial it at the office. We run it on the actual machine at our partner facilities, at production speed, and check the seal the simplest way there is: squeeze the pack under water and look for bubbles.

The check is repeated after a few weeks in the warehouse, because seals and adhesives can weaken with time and temperature.

An example: choosing film for PLAY Protein Bar

When we chose the film for the PLAY Protein Bar, we sent the supplier a product description before asking for a price: a chewy bar with a slightly tacky surface and oil from nuts, packed as a pillow pack on a horizontal machine, needing an oxygen barrier to hold the nut aroma, printed on an ink ground with yellow type from the brand palette.

The supplier offered two structures: one metallised, one clear so the bar could be seen. We took small trial rolls of both and ran them at the partner facility. The clear film showed the product beautifully, but its OTR was higher, and after storage the nut aroma had faded noticeably compared with the metallised version. The trial also raised a second question: crumbs from the bar were landing in the seal area, so we had to ask about a seal layer that tolerates contamination. We chose the metallised structure, and the product specification gained a line about keeping the seal area clean.

Questions to put to a film supplier

Before accepting a quotation, we ask:

• What structure do you propose, with the thickness of each layer, and why does it suit this product?
• What are the OTR, WVTR, seal strength and coefficient of friction on the data sheet, and under which conditions were they measured?
• Which markets is the seal layer's food contact declaration valid for?
• What is the minimum order per roll and per print design, and the lead time?
• Can you supply small trial rolls to run on the real machine?
• Do the inks and adhesive leave any residual odour, and have they been tested with oily products?

A good answer comes with numbers and measuring conditions. An answer made only of adjectives is a sign to look for another supplier.

If you supply laminated film and can answer those six questions, get in touch. If you are an R&D candidate who wants to understand packaging as well as recipes, the Fresher Product R&D (FMCG) role is open in Hanoi.