Adhesive bleeding from the edge usually means big trouble, as it makes the labels unusable. Adhesive bleeding can be caused by the following factors:
1 - Correct storage = mild climate and no pressure
Labels should ideally be stored at 50-60% humidity and at a constant room teperature of 18-24 degrees Celcius. In general, it can be assumed that pharmaceutical manufacturers have appropriately equipped storage facilities for packaging and self-adhesive products. However, they often neglect to ensure that the rolls are stored without coming into contact with other objects. Label rolls which are placed on their end on a shelf or packed tightly together will be unusable after a short time. The high pressure causes the adhesive to bleed from the label edges. When a roll is unwound, labels in one particular area will stick to the rear of the superior carrier.
After a while, increased humidity can cause paper labels and carriers to puff. The pressure applied to the labels inside the roll continually increases. This likewise causes the adhesive to bleed out at the edges. The roll sticks together, hardens and cannot be used.
2 - The winding tension must be set correctly
Modern roll winding machines have a web-tension control. Schreiner is an expert on the tensions which should be applied when winding labels made from different materials. Factors which should be taken into account when selecting the tension are the size of the label and above all the diameter of the label roll. The simplest way of preventing adhesive bleeding would be to wind up at the lowest possible tension setting. However, this increases the risk of larger-diameter rolls loosening and unraveling.
3 - Selecting the right adhesive is all-important
There are numerous materials which can be used without difficulty in the manufacture and processing of pharmaceutical labels. These materials are usually combined with solvent, dispersion or emulsion-based adhesives.
Other applications, such as labeling low-energy surfaces, frozen containers or surfaces with silicone residues, call for special adhesives with specific properties. Materials incorporating hotmelt adhesives or larger quantities of other types of adhesive exhibit the requisite properties but are also susceptible to adhesive bleeding. Very thin adhesive paper and films can also be used for this type of application, but they also run the risk of adhesive bleeding.
Sample print runs producing several thousand labels of this kind will only rarely provide any information on the risk of adhesive bleeding as the labels are generally used shortly after being produced. It is only after a longer storage period that the natural flow properties of the adhesives cause them to bleed from underneath the label. In case of thin materials, the label material "floats" on top of the adhesive.
In cases of this kind, it can be advisable to use a masking paper coated on both sides with silicon to prevent the roll from sticking together. Before doing this it is necessary to establish whether the label dispenser is capable of transporting material of this kind.
4 - Label materials can shrink
The fact that plastic films can shrink by up to 1mm after punching can be illustrated very clearly with PE films. In general, the cutting rules are slightly visible on the silicone paper and can be compared with the length of the shrunk label. Shrinkage can occur if the manufacturer has dried the colors printed onto the labels at an excessively high temperature. However, it is more likely to be attributable to the fact that film suppliers laminate soft, thermoplastic films like PE onto the silicon paper at very high tensions. When the labels are subsequently punched out, they yield to the tension and shrink.
Schreiner makes a cross line incision on the material to check the tension prior to every production run. In certain circumstances, it can also be advisable to produce and supply the labels rolled inwards. Adhesive bleeding is particularly likely to occur if the material shrinks but the adhesive has already bonded with the silicon paper and does not shrink. This adhesive film can be seen on the upper and lower edge of the label using a magnifying glass.
5 - Sharp punching tools make for clean cuts
Every label manufacturer with any sense will carry out a check before commencing a print run to ensure that the punching tools are sharp enough to make a clean cut through the adhesive compound. Wear on the punching tool and the need to prevent the silicon paper from being punched mean that the pressure exerted by the punching tool needs to be adjusted frequently.
The adhesive is punched through together with the face material, i.e. the adhesive paper or film. If the punch does not penetrate deeply enough into the material and adhesive or is too blunt, the adhesive is merely squeezed off rather than being cut cleanly through. This produces small veins of adhesive at the edges of the labels, which become visible when the remaining waste skeleton, or the waste material between the labels which is removed after punching, is pulled away. In a matter of a few hours, the labels are unusable.
It can be assumed that all label manufacturers and users will suffer from problems of this kind to a certain extent. Schreiner is currently developing an unwinding device which will make it possible to use rolls which have been stuck together.
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