Printing paper for decorative laminates

Example of decorative laminates from Centuryply used for the wardrobe doors.

By T.K. UNNIKRISHNAN

Design papers or decorative papers form one of the most important paper layers in decorative laminates. This is either the top-most layer or the second layer of paper from the top, depending upon the type and grade of the decorative laminate.

The paper, generally used for printing designs, is called base paper. Base papers are loaded with titanium dioxide to give opacity to them. The industry uses two types of printing: roto-gravure printing and silk-screen printing.

Roto-gravure printing is carried out by multi-colour printing machines with three or four printing stations in the same line, assembled one after another. The machine has a paper roll mounting unit along with an unwinding unit fitted with brakes and other controls to ensure proper release of paper while it is being printed.

A tension control unit is also attached to the machine in order to ensure proper and uniform tension on the paper while paper runs on the machine during printing. Improper tension will lead to slackness of paper, resulting in paper snaps or paper brakes.

Photo-etching

Each printing station has a printing cylinder, which is engraved with the type of design desired by the designer. Each printing cylinder is engraved in such a way that the first, second and the third or fourth are properly aligned to each other in the longitudinal and circumferential directions (cylinder registration), so that there is no over-printing of design on the paper.

There is a rubber pressure roll attached parallel and adjacent to the printing cylinder which helps to impart the printing ink from the engraved cylinders to the paper to provide the printed design and impression.

A scraper blade is fitted close to the printing cylinder and in constant touch and contact with the cylinder during printing, which enables the cylinder to have controlled transfer of printing inks on to the paper.

The function of the scraper blade is to scrape off excess printing ink from the cylinders, allowing just the necessary quantity within the engraved cell of the printing cylinder. Any additional ink on the cylinder will otherwise be passed on to the base paper where it is not needed.

There are registration marks on the edge of the cylinders, and during printing, these adjustments are made and corrected. When the registration marks of all cylinders coincide with one another, it can be summarised that all the three or four cylinders are registered to each other and the printing is perfect from this angle.

Colour matching

Fully automatic and computer-based registration technology is available now on all machines, which gives very accurate registration for all printing cylinders. This not only saves time but also avoids generation of paper and printing ink waste, which would take place during manual registration.

There is a moveable tray for printing ink that coats the cylinder(s), which then Transfer it to the paper. Every printing station is fitted with a drying chamber, to prevent wet inks getting mixed up at different printing stages leaving and to avoid wet smudges and patches.

The printing machine is also fitted with a web aligner, an arrangement to ensure that the winding is straight and the paper rolls do not become a cigar-shaped one. Web aligners are fitted with photo-cell arrangement or are monitored with sophisticated electronic controls.

One of the most important areas in the process of printing is colour matching. Each batch of printing should match exactly all other batches in colour and shades. Absence of colour-matched material may not be acceptable to the customers. The speed at which the paper is printed, and the contact time of ink with the paper, are also important factors.

There are many variables involved in this procedure: matching of base paper and printing inks – these are now done using computers as in the case of the paints industry. However, perfect colour matching is not an easy task.

Apart from computerised colour matching arrangements, the human eye, experience and judgment play important roles.

Inks & cylinders

Printing ink preparation also is an important factor to achieve good printing results. Organic, and in certain cases, inorganic pigments are used to make printing inks, along with other additives such as extenders and binders.

There are two types of printing inks: water-based and alcohol-based. Alcohol-based inks are more suitable because these inks dry a lot faster.  The viscosity of ink solution is also an equally important factor: above- or below-optimum viscosity may lead to low or high ink adhesion to the base paper, leading to ill-matched colours and tones.

Roto-gravure type printing cylinders come in different diameters. Wood grain design cylinders and natural marble patterns will naturally have a larger diameter due to the wood grain length or length of marble grains.

Other designs such as stripes, squares, dots, flowers, chips, stones, etc. have cylinders of smaller diameters. These cylinders are made through a process of photo-etching.

These cylinders are either of pure copper, or they are clad with copper plates, where the engraving is done. They are chrome-plated to ensure longer life and to resist corrosion from acidic ink types.

Screen printing

Silk screen printing, commonly known as screen printing, is an age-old technology that has been extensively used by the textile industry. The decorative laminates industry borrowed this from the textile printing by making some modifications and adaptations.

Screen printing is done on long tables, having a length of around 200 feet, which can accommodate more than 20 design papers of 8 feet length. The printing inks are based on starch, which is used as a binder for the pigments.

Unlike roto-gravure printing, the printing ink is now in the form of a thick paste. The screens are made from very fine mesh nylon fabrics, allowing the transfer of inks from the screen to the paper in a limited and required manner.

The selection of nylon fabric of correct mesh size is of paramount importance. These screens are made by a process similar to photo-etching, but the printing process is completely manual.

The paper to be printed is laid on and secured to the printing table. The screen, fitted to a frame, is placed on top of the paper and the ink is poured on the top edge inside the screen. A rubber squeeze fitted on a frame with handles is used for the actual printing operation.

The squeeze is drawn downwards with the ink by applying uniform medium pressure to the transfer the ink from the screen to the paper. This process is continued for the whole table. There are single coloured and multi-coloured printing, up to a maximum of three colours.

It is, therefore, very important to ensure the right thickness and viscosity of the ink, depending upon the mesh size of the nylon fabric. Since the inks used are thick, drying of the inks on the paper is a slow process. Fans or hot air circulators are used to achieve faster drying. Dried paper can now be rolled comfortably and the printing process is continued.

Screen printing tables, fitted with automatic unwinding and winding units, along with tension adjustment devices, are now available in the market. Overhead hot air drying systems with height adjustments, vacuum fitted tables, sophisticated fast drying ink formulations are also available to the industry.

Manufacturing of mural paintings and special effects can be provided through screen printing, while there are limitations in the roto-gravure printing technology for the creation of such special designs and prints.

(To be continued)

–The writer, who passed away recently, was an expert in chemistry in the high-pressure laminates industry, with 55 years’ experience. This is an abridged extract from his new book, ‘Chemistry & Technology of High Pressure Laminates’. This the third part of a series of articles that first appeared in the Sept-Oct and Nov-Dec, 2022, editions of WoodNews. To buy the book, write to ukjay41@yahoo.co.in.

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