Ink and printing methods are important factors for the production of shrink sleeve labels. For finished shrink sleeve labels, the formulation and selection of inks is the key. A full understanding of shrink sleeve label printing methods and inks is necessary to get the best shrink wrap labels.

Shrink wrap labels can be printed using any printing process including, gravure, flexo, digital printing etc. Most shrink sleeve labels are printed on medium- and wide-format rotary presses, but some are printed on narrow-format rotary processes with significant growth, especially UV flexo and water-based flexo. In addition, the production of shrink sleeve labels needs to take the special needs of shrink film labels for inks into account. When the ink is adsorbed onto the film, it can shrink with the film in the oven.

Shrink Sleeve Label Printing Inks Types

At present, inks for shrink sleeve label printing have solvent-based ink, water-based ink, UV ink (such as cationic UV ink, free radical UV ink) and so on.

Eco-friendly Water-based inks

its shrinkage rate is over 40%. It is very elastic and its cost is lower than that of UV inks. The ink surface is less affected by the movement of atoms in the molecules. It is suitable for materials with medium and low shrinkage.

Solvent-based Inks

Its shrinkage rate is more than 40%, and the maximum shrinkage rate can reach 70%~75%. The drying time required for solvent-based inks is less affected by moisture but more affected by the movement of atoms in the chemical molecules. So, it is more suitable for materials with high shrinkage rate.

Cationic UV Inks

Have good adhesion on a wide range of substrates. It has a shrinkage rate of more than 40%. The maximum shrinkage rate can reach 70%~75%, which is more suitable for applications with high shrinkage rate. The ink can be cured in real time and there is less atomic movement in the chemical molecules. However, they are also more expensive than free radical UV inks.

Free Radical UV inks

Real-time curing, shrinkage is generally less than 20%. The adhesion is limited in some substrates. Atomic movement in chemical molecules may occur on the ink surface. It is suitable for materials with medium and low shrinkage.

Influencing Factors of Inks

Just like choosing the shrink film materials, selecting the right inks for shrink sleeve label printing also need to have a understanding about factors that have influences on inks.

Printing Methods

The printability of inks is different for different printing methods. For shrink sleeve label printing, flexographic printing and gravure printing are still the main printing method adopted by most manufacturer, especially for high volume production. Gravure printing generally uses solvent-based inks. In contrast, environmentally friendly water-based inks are mainly used for printing on flexographic printing presses.

Shrink Film Materials

Different printing materials, such as PVC, OPS, OPP, PETG, etc. have different requirements for inks; PVC, PETG and OPS account for 90% to 95% of the market share for shrink labels. Since different shrink films have their own surface characteristics, each film requires a specific oil composition to achieve the right performance.

However, general-purpose inks will have good development since it can help reduce the number of press cleanings, minimize cross-contamination and control ink levels. This type of ink is suitable for a wide range of shrink sleeve label materials.

Some ink manufacturers have already introduced such general-purpose inks. For example, XSYS has introduced Flexocure XSUV free radical curing inks that offer 70% shrinkage and are suitable for a wide range of film materials, including PVC, OPS, OPP, PET and PETG, and can be used for both outside and inside printing.

Shape of Container

Different container shapes have different shrinkage requirements. For example, OPP film or PVC film can be considered for cylindrical containers with low shrinkage requirements, while OPS films with high shrinkage and strength can be considered for containers with curved or streamlined designs. OPS films can be laminated to containers of different shapes, allowing them not only to be beautifully printed, but also to meet the needs of a wide range of novel containers with very different shapes.

Storage and Transportation Condition

What conditions are encountered by containers during the production, transportation, and distribution of products? What coefficient of friction is required? What is the temperature and humidity environment? Are there any special requirements? All these factors should also into take in consideration when choosing inks for shrink sleeve labels. Shrink sleeve labels for beer, beverage, dairy products, for example, are required with good resistance to ice and water.

How to Choose Inks for Shrink Sleeve Labels?

Knowing the above factors, we can choose the right ink in terms of shrinkage, adhesion, and the ability to release residual solvents (ability to dry quickly at lower temperatures).

Shrinkage

This is the primary consideration when selecting shrink sleeve label inks. Shrinkage requirements vary greatly from products. Some may require high shrinkage, about 60%-70%, while some may only require 10%-20% shrinkage.

Adhesion

Contact between ink layers occurs occasionally during printing and winding. If inks with poor adhesion are used, problems can occur with the contact between the ink layers as the product is being stacked. The most common problem is adhesion, where the ink layers stick from one side of the material to the back of the other.

Usually, if the ink doesn’t adhere well, raising the temperature of the drying oven can be used to solve it. However, heat-shrinkable materials are sensitive to temperature and cannot be relied upon to increase temperature to improve ink adhesion.

For now, the inks with the best adhesion are solvent-based inks and cationic UV inks. Water-based ink and free radical UV ink due to its own raw material limitations, adhesion is relatively speaking poor.

Solvent Residue (VOC)

Now the national standard for solvent residue has quite strict standards. In order to make the solvent volatilization faster, some printers adjust the drying temperature higher in the ink drying process, which will lead to pre-shrinkage of the material. Therefore, excessive heat should not be applied to shrink labels in printing. It requires finding a balance between printing conditions and drying conditions.

In the printing of shrink film labels, the heat control of the printing machine is quite important. The press must be able to dissipate and eliminate heat to prevent shrink film from shrinking during the printing process. Some of the newer printing presses are equipped with cooling rollers to take the heat away and reduce the temperature of the film.

Inks Tests

Before choosing a suitable ink for shrink sleeve label, manufacturer must have rigorous test for the compatibility of the heat-shrinkable film and the ink. Conventional test items for inks include adhesion, water resistance, adhesion, rubbing factor, residual solvents, etc. There are also some special tests for resistance, special effects, thermal adhesives, ink migration, etc. Below mainly introduces some special tests.

Resistance Test

To ensure proper application of shrink labels on certain specialty products, such as dairy products, a series of tests are required, including chemical resistance tests, wrinkle tests, friction factor tests, and post-shrink performance tests to ensure that the labels retain their color, appearance, and functionality after shrinkage.

Special Effect Test

For shrink labels with added special effects such as gloss, matte, foil stamping, pearlescent, iridescent, bright metallic, fluorescent, etc. Some of these special effects are best implemented in the lower shrink area of the container.

For any of these special effects, it is important to talk to the supplier to confirm that these effects are applied correctly. For example, a piece of screening plate and a gradient halo image may be required for a high shrinkage area where cold foil is needed. When it shrinks, it shrinks together so it will look better. Otherwise, the foil will squeeze into clumps and become gray, not achieving the desired effect.

Thermal Adhesives

Containers with shrink label sleeves can expand and contract thermally in the oven, the result of which is that the shrink sleeve loosens and the cylindrical bottle spins in the user’s hand. To overcome this problem, some manufacturer will apply a hot melt adhesive to the checkerboard or dotted pattern area of the film, which prevents the shrink sleeve from loosening on the container, helps to hold it in place during the heat shrink process, and locks the sleeve in place to prevent it from shifting position or even falling off.

Ink Migration

If shrink sleeve label is used in the field of beverage or food packaging, it is necessary to pay attention to whether the printed shrink label affects the odor and taste of the product inside the package. There is no problem with glass or metal containers, but plastic containers must use low migration inks.

The rules for ink migration and ink systems are.

• <50HgL (or <SML Swiss List) for substrates used for evaluation and licensing.
• <10HgL for all other substrates.
• If the substrate is toxic, the limit is to be <10PgL (calculated based on daily intake).

The general rules for food packaging and labeling are as follows.

• The responsibility for packaging compliance does not lie solely with individual members of the packaging chain.
• The ultimate responsibility lies with the “people who put the packaging on the market”, but we must all work together.
• There should be no unacceptable variations/adulterations in the quality, odor or taste of food/beverage.
• Carcinogenic, mutagenic, and toxic to reproduction (CMR substances) should not be used.
• Migration of substances evaluated for food contact should be kept below specified limits.