Understanding Rollstock: The Foundation of Modern Packaging Processes - Packaging Deep Dive #2

Print layout sketch: The blueprint for your printed image

This is precisely where another important element comes into play: the print layout. Whilst the roll width and repeat pattern define the technical basis, the print layout determines exactly where your design will actually appear on the bag. Put simply, it is the blueprint for your print design.

It shows:

• The effective print area
• Seal areas (which should not be printed on)
• Fold or fin areas
• Safety margins
• Bleed margins
• The exact position of the print register mark

This is particularly crucial for Rollstock. This is because, in this case, it is not a single bag that is printed, but a continuous film web. The design must be precisely aligned with the subsequent bag geometry – including the fin, top and bottom seals.

Understanding roll width: From product to the right web

The machine sets the technical parameters. Within these parameters, decisions must be made regarding the specific layout of the Rollstock and, consequently, the width of the film web. It is precisely at this stage that most uncertainties arise in practice. The roll width is the result of several technical considerations – and always depends on the interplay between the product and the machine.

To ensure the roll width can be accurately defined, several factors are taken into account: Firstly, the front side, i.e. the visible width of your product. Then there is the back side, which consists of two halves of material that are later joined in the middle and sealed. The product depth or height must also be considered. The film must completely enclose your product (on both sides). To ensure the web is not under excessive tension in the machine, an additional material tolerance is factored in. This ‘buffer’ ensures the film runs smoothly over the mould shoulder and does not fit too tightly around the product.

Another key component is the so-called fin – the back seal. This is where the two edges of the material are welded together. Additional width is required for this as well. Finally, the filling machine itself specifies a fixed sealing area. The width of the sealing jaws is defined by the design and must be taken into account during the design phase.

The final roll width is therefore never determined by a single measurement, but always by the sum of all relevant product and machine parameters.

Side note: The fin and why it is so important

When calculating the roll width, one term keeps cropping up: the fin. The fin is the back seal that joins the flat film web to form a closed tube. Put simply: this is where the two edges of the material meet and are welded together.

In most cases, sealing is carried out according to the ‘inside-to-inside’ principle. There is a simple technical reason for this: in composite films, the sealable layer – i.e. the material that melts and bonds under heat – is usually found only on the inside. Both inner edges are therefore raised like a small ‘fin’ and welded together.

This is to be distinguished from the lap seal. Here, the inside is sealed to the outside. However, this only works with materials specifically designed for this purpose, which are sealable on both sides.

The pitch – the stroke length of the filling machine

In addition to the roll width, another factor plays a key role: the repeat length – also known as the print run. The repeat length defines the length of a single printed image or packaging unit on the roll. It therefore determines how far the machine feeds the film forward per cycle.

With flowpacks, the repeat length generally corresponds to the length of the finished package – including the top and bottom seals. With stand-up pouches, the situation is often different. As these are often produced horizontally on FFS machines, the repeat length usually corresponds to the width of the pouch.

The repeat is not only a measure of the format, but is also crucial for synchronising the print image with the machine. The system uses registration marks to identify where cutting and sealing must take place.

The direction of unrolling of the roll goods

Another technical consideration with Rollstock is the direction of unwinding of the film. This determines how the printed web is fed into the filling machine – in other words, the orientation in which the design unwinds from the roll.

This means that the machine always processes the film in a clearly defined direction. If the direction of unwinding is selected incorrectly, the design may later appear upside down, run sideways or be incorrectly positioned.

The following examples show the typical variations in the direction of roll unwinding: