Label application troubleshooting tips and guidelines

Information provided by Catchpoint indicates that almost half of pressure-sensitive quality faults resulting in packaging line downtime are related to the laminate converting process, particularly with Glassine or Kraft liners. Percentage figures for these quality faults are:

• Die strike  12.2 percent

• Missing labels  7.8 percent

• Silicone voids in the liner 5.2 percent

• Edge nicks  4.8 percent

• Incorrect winding tension 4.4 percent

• Bursts through substrate 3.9 percent

• Label stuck onto back of liner 3.5 percent

• Matrix waste not removed 3.5 percent

• Inclusions in the liner  2.6 percent

Total 47.9 percent

The percentages above will obviously vary between different label application and packaging production lines, as well as over time. The critical end user question is what effect do these and other issues have on labeling production and on label line downtime?

We now examine each of these problem areas and their effects on the label applicator system.

LABEL CUSTOMER AND LABEL USAGE CONSIDERATIONS

To minimize potential application problems, it is important that the label converter obtains in advance as much relevant information from the label-user customer as possible. In particular the converter will ideally need to know:

• The type of container or product that the label is to be adhered to – glass bottle, plastic bottle, paperboard carton, metal surface, tube, flexible pack, etc. Is it rigid, flexible, squeezable? Any of these can have an impact on the label face material or adhesive to be used

• Is the surface being labeled rough or smooth? Rough surfaces may need a thicker or more aggressive adhesive on the label

• The size and shape of the container – small, large, round, square or rectangular, contoured, indented, curved, etc. Again, important in relation to the label adhesive or face material being used

• If the product to be labeled is curved, what is the curvature of the surface – the greater the curvature, the higher the adhesive tack needed to hold the label in position.

• If the surface being labeled is plastic, what type of plastic? Are there any plasticizers likely to be present? Some adhesives and plastics ‘disagree’ with each other and this can deteriorate certain adhesives and cause labels to lift or fall off

• Is the label to be permanent or removable? Different adhesive requirements

• Where will the labels be stored and for how long before they are applied? What are the storage conditions?

• Are the labels being hand or automatically applied? Containers that have been handled with bare hands might lead to labels failing

• Is the surface to be labeled clean, dry (or wet) and free of debris, oil, cleaning or processing solvents or chemicals? Surface contaminations may lead to adhesion problems

• What are the conditions at the point of application? Is the application environment cold, frosty, humid, damp, variable – which can mean that the label adhesive will not properly adhere to the container or surface being labeled

• Are containers, labels and container contents all roughly at the same temperature at the point of application? If not, this again may give an inconsistent label performance

• What happens to the labeled container or product immediately after the labels have been applied? Are they refrigerated? Are they filled after application? How are they handled and stored? What is the product distribution process? All of these may have an impact on label performance.

PRESSURE-SENSITIVE MATERIAL SPECIFICATION

Pressure-sensitive labels are a sandwich construction consisting of different layers:

• The label face material itself, which may be paper, plastic, foil, or other material

• The pressure-sensitive adhesive layer

• A silicone release coating on a backing paper or film that is used to prevent the adhesive sticking to itself or the face material in the reel and during handling.

In general, the application of pressure-sensitive labels tends to be a simpler operation than that for wet-glue applied labels. Apart from accurate positioning, most of the problems that arise with pressure-sensitive labels are due to factors such as the incorrect matching of the label material to the specific end-use requirement, defective converting operations or bad storage, rather than the label applicator or label application process.

For this reason, some of the more important properties of the adhesives and face materials, as well as the conversion operation, are outlined in the following pages.

Pressure-sensitive adhesives. Adhesives used for the production of pressure-sensitive labels are available in a wide range of types that can be used to label almost any kind of product or surface – dry, wet, frozen, glass, plastic, paper, rough, smooth, absorbent, rigid, flexible, stretchable or squeezable, and in different conditions.

Although there are general purpose adhesives that can be satisfactorily used to label a wide range of products and in various application requirements, it is important that the label converter is able to match the adhesive to the specific end-use application if labeling problems, either during, or post application, are to be minimized or avoided altogether.

That is why some or all of the questions set-out in the ‘Label customer and label usage considerations’ section on the previous page are discussed and agreed between the converter and the user in advance of proceeding with the order.

Pressure-sensitive face materials. A wide range of papers, plastics, metallic foils, metallized and other face materials are used for the production of pressure-sensitive labels. Face material surfaces may be coated or uncoated and, during production may also be printed, varnished, over-laminated, embossed, etc. With the enormous range of face materials, as well as the printing and converting stages, the selection of face material will be based on a compromise between chemical and physical properties, economic considerations and manufacturing possibilities.

It should be recognized that the individual components of a label are not independent in their action. A change in, for example, the thickness or stiffness of the label stock can produce large variations in the properties of the pressure-sensitive layer. It is as well to bear in mind that properties such as peel adhesion or tack are properties of the whole label and not of the adhesion alone.

Factors important to successful dispensing and application of label face materials include the stiffness of the material, or its thinness and flexibility.

These will be discussed later in terms of problems that may occur on the applicator itself.

Pressure-sensitive silicone coating. A problem that may be found with siliconized liners is that of silicone voids – small spots where there is no silicone, which in turn allows the adhesive to bleed into the liner fibres. When this occurs it is a laminator issue which is impossible to detect during label printing and converting. The cause is usually dust during the silicone coating process.

Silicone voids can create a diagonal tear in in the liner at the dispensing beak where the liner is being stressed, causing the applicator to stop. Voids will not break a PET liner, only possibly have an impact on labeling accuracy.

PROBLEMS THAT MAY ARISE FROM THE LABEL CONVERTING PROCESS

Label stock is usually supplied to the converter in the form of reels of pressure-sensitive laminate which will then be printed and die-cut to shape and size in one in-line operation. The printing and the ink drying/curing operations are done on roll-label presses, using standard flexographic, offset, letterpress, screen processes – or combinations of these processes – as well as the latest electrophotographic or inkjet technologies.

Some of the most common converting problems that may arise and impact on the labeling line and label application include:

Die Strike. The most critical operation in terms of successful label application is die-cutting, in which the die must cut cleanly through the label face material and pierce the adhesive layer, but must on no account be clearly marked in the release coating and the liner – particularly with a Glassine or Kraft web – and should certainly not cut into or through the backing release liner.

Die-cutting is an operation that requires fine control which needs the laminate and, in particular, the release liner backing material to be of a constant thickness.