Gravure Roll Coating is an integral method of coating for Adhesive Specialities. It is a process where adhesive with various viscosities (about 1500mPas) can be coated onto substrates up to a maximum speed of 900 m/min. A coating thickness of 1 micron to about 50 microns can be acquired, which makes it a resourceful technique. Gravure Roll Coating consists of a roll which is also referred to as an applicator ball, which is etched to the bottom to control the amount of coating that is delivered. The Gravure is available in numerous dimensions which in turn affects the finishing of the coating.
The Gravure Roll Coating offers several approaches, however, the most two common methods used are Direct Gravure Coating & Offset Gravure Coating.
What is Direct Gravure Coating?
In Direct Gravure, the rolls characteristically contain a steel surface where a particular cell pattern is engraved. The adhesive is introduced to the surface and a blade is used to eliminate any excess adhesive. As the movement of the roll continues, the adhesive is exposed to the web, which is formed between the applicator ball and the rubber covered backing roll. The rotation of both the balls at the same direction and similar speed as the web enables the adhesive to be transferred to the web.
The dimensions of the cell structure etched decide the quantity of coating that is conveyed to the web. Other factors such as rubber thickness, angle, pressure applied, or the speed can also change the coating standards.
What is Offset Gravure Coating?
Offset Gravure Coating is a variant of Direct Gravure Roll Coating. In this process, a transitional roll which is also identified as an ‘Offset’ roll is used for the transfer of the adhesive coating to the web. The intermediate roll is perfectly placed between the gravure roll and the backing roll, which is now portrayed as a rigid surface like steel. The web passes through the nip created by the two rollers. Like the direct gravure roll, the size and shape of the cells determine the quantity of coating that is transferred to the offset roll. However, in this method both the rolls rotate in opposite directions in the same speed. Offset Gravure Coating can control a range of viscosities.
Is There a Difference Between Gravure Coating and Gravure Printing?
There is a difference between Gravure Coating and Gravure Printing. Gravure Printing, also known as Rotogravure, is a printing process, which is specifically constructed to print specific patterns. The quality of this process can be judged by the thickness, range of coating, resolution, and the clarity for gravure printing.
The process of this begins with the ink being applied to the cylinder, which is then passed on to the substrate. Gravure Printing is a method where numerous components are required. Those parts are:
- Gravure Cylinder, the size depends on the product being manufactured.
- Ink Fountain
- Imprint Roller
- Doctor Blade.
The method is commenced by forming a cylinder with the images that need to be printed. The embedding process is formed on the cylinder surface, whereas later the cells will transfer it to the paper after the doctor blade removes the excess ink. The impression roller plays a vital part in this process as it guarantees an even and required amount of ink on the surface. The proportions of the cell should be set cautiously as the ink reacts to different colours on the paper. The larger cells will form dark and solid colours, and the smaller cells will produce dull colours. The substrate with ink goes through a drying process before it reaches for another coat of ink. A rotogravure has an individual unit for a single colour.
This process is extremely beneficial as it has the capability to print on thin films like Polyester, Nylon and PE which is between 10 to 30 micrometers.
Another feature is that it is durable and will not spoil the quality of the image. Rotogravure also assists the manufacturers with economical per unit cost and enables them with a large volume of production.
This technique is being vigorously used in the electric industry for items such as solar cells, which require even coating and high edge definition.
Rotogravure Method à
Gravure Cell Development:
To achieve success with Gravure Coating, the development of cells is highly essential.
Types of cells are divided into two classifications
- Patterns with continuous channels that run at a specific angle on the rolls surface
- Separate patterns that contain individual cells on the surface.
Formation of cells can be done by direct engraving of the cylinder or imprinting it on a cylinder through chemicals or laser.
In direct engraving, a principal shape is embedded on to the roll surface to create a dent, which is done by a tipped stylus onto a roll and every dent formed creates a gravure cell.
Embedding of a cylinder can be done by chemical or the more popular option, by laser. The roll in this case is ready with a thermal spray coating of an ironstone material onto the outward part of the roll, which achieves a levelled surface. A stronger laser is then used to get a favourable pattern.
Direct Engraving is often linked with quadrangular, pyramidal and tri-helical cells and when laser is used it normally gives out a hexagon shape.
Durability and the speed during the process of Gravure Coating makes it favourable for coating PSA on various films such as polyethylene, polypropylene, polyester and also aluminium foil which is used for seals and packaging of various items. Gravure Coating is also specifically used for coating on filmic materials such as surface protection tape which is used is to protect surfaces from dust, scratches, or any damages.
Rotogravure is majorly utilized in the Printing Industry to print newspapers, catalogues, coloured magazine sections. It is most efficient when used for long run printing of black & white or different colour graphics.