Misregistration and curling of film labels

With the continuous development of China's economy, consumers have increasingly higher demands for product packaging, and more and more film-based materials are being used in self-adhesive label printing. At the same time, as the use of film materials has increased year by year, the production volume of self-adhesive labels has also gradually increased. Traditional letterpress printing has gradually been phased out due to low production efficiency, and more and more companies are adopting flexographic printing for production. Since film materials have higher requirements for printing compared to paper materials, many problems often occur in the actual production process. In this article, the author shares with readers some common issues and solutions for film materials in the flexographic printing process for reference.

Misregistration

Theoretically, the registration standard for self-adhesive labels is ±0.2mm, but registration errors exceeding ±0.1mm can be noticed with the naked eye. Therefore, in actual production, printing companies require the registration error to be controlled within ±0.1mm. Film materials are easily deformed when used in flexographic printing, making it harder to control registration accuracy during production, often leading to issues with misregistration. Below, we analyze some common causes of misregistration of film materials.

01. UV Lamp Temperature

Currently, mainstream flexographic presses on the market are basically equipped with UV drying systems. Whether using traditional UV lamps or LED-UV lamps, a certain amount of heat is released during operation. If the heat is too high, it will cause film materials to deform, resulting in misregistration. To prevent this phenomenon, most flexographic printing equipment now is equipped with cooling devices on their UV drying systems. Currently, water-cooled devices are the most commonly used, but due to differences in working principles among water-cooling devices on the market, the cooling effect varies greatly. In principle, the surface temperature of film materials before entering and after exiting the UV lamp should not change by more than 2°C. If this range is exceeded, the film materials are likely to deform due to high temperatures.

In addition, the running speed of the press also affects registration. Generally, at the same temperature, the slower the equipment runs, the higher the probability that film materials will experience registration errors due to UV lamp temperature. This is determined by the amount of thermal radiation energy received by the film materials per unit time. In other words, the faster the material passes through the UV lamp area, the smaller the amount of thermal radiation energy it receives per unit time, and the lower the chance of thermal deformation.

In summary, controlling the UV lamp temperature during production is very important. If the UV lamp temperature can be kept within a reasonable range, its impact on the registration of film materials will be minimal.

02. Equipment Tension

It is known that flexographic equipment must maintain stable running tension during production. The more stable the tension, the smaller the registration error. At the same time, the tension cannot be too large; excessive running tension can cause film materials to stretch and deform, resulting in registration errors. Currently, mainstream flexographic presses on the market mostly use servo motors to control tension, mainly because servo motors can detect tension changes in real time during operation and adjust the tension through programming. This ensures stable tension during operation without causing film materials to stretch and deform.

03. Physical Properties of the Material Itself

Different physical properties of film materials lead to different responses to temperature changes. For example, BOPP and PET materials have relatively stable physical properties and are not easily deformed under high temperatures; PE materials are softer and more prone to deformation in high temperatures. Therefore, when printing some softer film materials, the UV lamp temperature must be controlled within standard ranges.

Film Material Curling

Experienced operators know that film materials are prone to curling during printing. This issue significantly affects production efficiency, especially for sheet-cut products. Sometimes, after printing, the material cannot be neatly stacked during cutting due to severe curling, affecting production efficiency. The material is already printed; not continuing would waste it, but continuing makes it difficult to stack neatly, leaving operators in a dilemma. So, what are the reasons behind this phenomenon?

01. Impact of Environmental Humidity

Self-adhesive materials are composite materials, with the face material being film and the backing usually paper. During printing, the film material is in an unfolded state and fully exposed to the air. If the moisture in the air is not balanced with the moisture in the backing paper, the backing paper will either lose or absorb water to balance with the external environment. During this process, the area of the backing paper may change dramatically (lengthening or shrinking), while the face film remains unchanged. This causes the two layers bonded together to have mismatched areas, leading to curling of the material.

Normally, the tension between the backing material and the release liner balances out after die-cutting and waste removal. This is why we often see film materials curl significantly during the printing process but flatten out after die-cutting. Moreover, most film labels on the market today are processed roll-to-roll, and this curling can be fully controlled under the device's tension. Therefore, when using a flexographic printing machine to print film labels, it is best to print and die-cut in one step to minimize intermediate handling. This is also one of the reasons why flexographic printing machines have increasingly comprehensive functions in recent years-raw material goes in on one side and finished products come out on the other, resolving many issues easily.

02

Effect of Heat

The UV lamps in a flexographic printing machine emit a large amount of heat. When film materials pass through the UV lamp, the backing paper dries and loses moisture rapidly. After leaving the UV lamp, the backing paper absorbs moisture quickly due to environmental conditions, which can easily cause the material to curl or deform. Fortunately, UV lamp technology is gradually improving, and LED and other cold UV light sources are progressively solving this issue.

03

Effect of Static Electricity

During the printing process, film materials can generate static electricity due to friction with the equipment. This static electricity makes the materials stick to each other. If the labels are sheet-cut products, the materials may curl during cutting due to static attraction, making it difficult to collect them neatly. In such cases, installing anti-static devices on the printing equipment can be considered. In fact, many new flexographic machines now come equipped with anti-static devices.