Flexo printing and lamination of nonwovens for hygienic purposes

Hygiene products, in the designs of which non-woven materials are used (diapers, feminine hygiene items, etc.), must be waterproof, breathable and able to effectively absorb liquids in order to effectively perform their functions. Along with the functional aspects, great importance (especially at the psychological level) is attached to the aesthetic appearance of the diaper: products with printed images are certainly more attractive.

A hygienic product with a seal is not only more saleable, but also increases brand awareness in the case of using company symbols, with which the consumer can immediately associate the advantages of a certain product and its superiority over competitors' products. Thus, flexographic printing and lamination of nonwovens are of great importance for hygienic products.

Flexographic printing

Flexography is a rotary printing method used in many industries, such as food packaging, which involves transferring quick-drying inks from flexible printing plates (called cliches) to the desired base.

In the process of printing on non-woven materials, from which such hygienic products as diapers and feminine hygiene items are subsequently made, it is necessary to take into account various details:

Substrate

The seal is applied to a polymer film (usually low-density polyethylene or LDPE, and breathable films are used in diapers), which is then connected to the non-woven material by lamination. These materials require special attention when setting up a flexo printing machine and selecting inks to ensure effective adhesion of the pigment to the substrate.

Ink type

In the process under consideration, we can use water-based inks (preferred and more commonly used in this type of printing), solvent-based and UV-curable inks. The choice is made depending on the substrate used, the end use of the product and local regulations prohibiting the use of paints that are considered potentially harmful to consumer health and the environment.

Technical equipment of the line

Flexographic printer must be adapted to different production needs: from those that require constant and stable operation of the machine for long periods of time (large runs and a small number of adjustments), to those that require more adjustments and, consequently, shorter periods of uninterrupted operation (as, for example, in the case of many orders for small batches).

The process of lamination of nonwoven fabric

In order to adapt nonwoven fabric for a variety of purposes, the lamination process is often used. This process involves joining several materials using adhesives, elevated temperature and pressure, or high-frequency vibrations in order to combine the properties of a material (for example, non-woven fabric) with the properties of another material or polymer film.

Thus, depending on the process used, it is possible to obtain breathable and porous non-woven materials that can simultaneously retain their waterproof properties and be soft, light and printable.

Various types of lamination

To date, the most common lamination processes are adhesive, thermal, ultrasonic lamination, as well as coating by polymer extrusion through a flat-gel die or nozzles.

Adhesive lamination can be performed in various ways:

• hot glue;
• with cold glue;
• dry adhesive;
• with an aqueous solution of glue.

The first method is the most frequently used, because it allows you to connect substrates with a variety of characteristics very precisely and at high speeds, while maintaining the properties of the starting materials (breathable properties, flexibility, etc.), and also allows you to use various methods of applying glue.

In thermal lamination, layers of material that should have the same or similar melting point are joined together by pressure (acting in the gap between the two shafts) and high temperature.

As the name implies, ultrasonic lamination uses high-frequency vibration for local heat generation, which leads to fusion and subsequent bonding of thermoplastic fibers. In addition to being the most environmentally sustainable method, since it does not require the use of chemicals or adhesives, it allows you to laminate up to seven layers in one step and combine materials with different characteristics, obtaining especially homogeneous composites in which the structure and properties of individual materials are not violated.

Finally, the polymer extrusion coating process involves applying a thin polymer coating to one or both sides of one or more layers of nonwoven fabric. The result is a water-resistant, printable and antistatic material with a lower cost of raw materials used and better structural stability of the final product.

How to properly perform flexographic printing and subsequent lamination

If you want to print on a polymer film for subsequent connection to a non-woven substrate by lamination, in addition to ensuring high print quality of the desired print, it is important to set and maintain the print repetition length of the reproduced template.

Changing the length of the print repeat, i.e. the distance between two identical points on two adjacent template images, going beyond the specified tolerance, will actually lead to the rejection of non-conforming rolls with obvious losses (even more significant if these rolls undergo the next lamination process). For this reason, the following factors become fundamental:

• tension control: apply the correct tension values depending on the specific substrate and processing stage before, during and after the printing process (including the winding and lamination stage, which may change the previously obtained print repetition length);
• coronary treatment of the printed surface;
• structural stability of the printing machine: the result of an accurate balance between all parts of the machine, both mechanical and electronic;
• air temperature: at the stage of drying the ink, it is necessary to set an air temperature that will achieve the desired result without damaging the printed base under tension;
• elastic cliche wear and aging management;
• management of the phenomenon of elongation of the printing step on the outer coils of the roll due to the elasticity of non-woven material, etc.

After printing and moving to the lamination stage, it is necessary to make sure that the selected bonding system will not damage the materials being joined, and the tension values of the above materials will be adequate in relation to each other in order to avoid the formation of defects on the target composite fabric.

Thanks to extensive knowledge in the field of hygiene and the problems of processing nonwovens, which you will definitely encounter when receiving a high-quality final product, A.Celli, Italy provides its customers with the means to achieve the desired results.

Flexographic printing machine of the IRIDIUM&reg series;:

• Allows you to achieve optimal control and control of the print repeat length throughout the entire production process thanks to solutions such as an interactive system for adjusting the print repeat length and automatic positioning of the slitting sections relative to the Slittomatic® printer.
• Ensures correct tension throughout the printing process thanks to dynamic tension control of the thinnest materials.
• Significantly reduces the amount of waste at the setup stage.
• Ensures efficient drying of water-based ink regardless of the working speed of printing (600 m/min and above).

As for lamination, it is possible to distinguish a number of ideal characteristics that a lamination line should have, namely:

• Reduction of roll change time
• Robust and reliable construction
• Multiple control points and quick setup
• The unwinders are equipped with double stations that allow changing rolls during machine operation (non-stop).
• Centralized control panel with business intelligence and data collection capabilities
• Automatic edge control system of the connected webs

All of the above can be found in A.Celli F-LINE® complex lamination lines designed to perform high-quality adhesive, thermal and ultrasonic lamination. The laminator, equipped with unwinding and laminating stations, as well as a slitting section, is capable of laminating two or more layers with a maximum web width of 3600 mm at a maximum operating speed of 400/800 m/min.