Printing Process

Litho Printing

Offset printing is a commonly used printing technique. The "Offsetting" process is the transferral of an image from a plate to a rubber blanket and finally to the printed surface. When the "offset printing" technique is used in combination with the lithographic process it must be noted that both these processes rely on the property that water will not mix with oil. The image is created by applying a "tusche" (greasy substance) to the plate. A planographic or flat surface image carrier then obtains ink from the ink rollers while the non-printing areas on the other hand attract a water-based film (called "fountain solution"), ensuring the non-printing areas remain ink-free. The modern "web" process feeds a large reel of paper through a press machine in several lengths, typically several metres at a time, which allows for continuous printing as the paper is fed through.

Two versions of the offset process came in two versions, namely by Robert Barclay of England in 1875 for printing on tin and then by Washington Rubel of the United States in 1904 for printing on paper.

Flexography (Flexo) Printing

Unlike Offset Printing Flexo printing uses a flexible relief plate. Flexo is essentially a modern version of letterpress which produced numerous copies by repeated direct impression of an inked raised surface on continuous paper or sheets. Flexo printing can be used for printing on almost any type of substrate. These include plastic, metallic films, cellophane and paper. This technique is commonly used for printing on the non-porous substrates which are required for various types of food packaging. This method is also well suited for printing large areas in solid colours.

Dry Offset

The "Dry offset" printing process combines the letterpress and offset printing processes together. The special plate in this process is not dampened as opposed to the dampening of the plate in the "Wet offset" lithography printing process. The special plate prints directly onto the blanket of an offset press, the blanket then offsets the image directly onto the paper.

Silkscreen

Various terms are used for what is essentially the same technique. The name "Silkscreen" or "Screen" is an indication of the material used for this method of printing, although they are one and the same process. Silk was the material of choice prior to polyester mesh that is used now.

Although various synthetic threads are used including the generally used polyester, there are however special threads available namely nylon and stainless steel. All these materials are usable in the "Silkscreening" process. The mesh is also available in different mesh sizes which will ultimately determine the outcome and look of the finished product.

In "Screen" printing a mesh is used to transfer the image to a substrate. A blocking stencil is used to make areas that should not be printed impermeable to ink. Ink is applied to the mesh with either a blade or a squeegee to fill the open apertures. On the reverse stroke the ink will make contact with the substrate momentarily along a line of contact. The ink is pulled from the mesh once the blade has passed and the mesh springs back into place, wetting the substrate.

Stencils are also used with this method of printing by imposing a design on a mesh of polyester or any other fine mesh and coating the blank areas with an impermeable substance. The same process will be followed as for the "screen" printing process to complete the job. This method is also known as silk-screen, screen, serigraphy, and serigraph printing. However only one colour is printed at a time. Several screens may be used to produce designs requiring more than one colour.

Gravure Printing

Gravure Printing employs a very different method of printing to the previous methods mentioned before. The process consists of the an ink duct, a printing (gravure / engraved) cylinder, a doctor blade as well as a rubber pressure cylinder. One colour is used at a time and needs to be completely dried before another colour is applied.

A "Rotogravure" printing press typically uses Cyan, Magenta, Yellow and Key which is the terminology used in the printing industry for Black (CMYK). Each colour is housed in a separate unit, the number of colour units needed is depended on the colours needed to produce the final product.

As the printing cylinder rotates, it runs through an ink duct which coats the cylinder in ink. The doctor blade removes the ink from the non printable areas as it surfaces leaving behind the necessary ink for the printable areas. The ink that will be used to print is housed in the recessed cells of the gravure cylinder until transferred to the substrate.

Above the printing cylinder is the pressure cylinder which forms a nip (impression zone) with the printing cylinder. The substrate is fed through the nip where the design is transferred to the substrate. Maximum ink coverage is achieved with the pressure applied by the pressure cylinder to the gravure.

Gravure

While the press is in operation, the engraved cylinder is partially immersed in the ink tray, filling the recessed cells. As the cylinder rotates, it draws excess ink onto its surface and into the cells. Acting as a squeegee, the doctor blade scrapes the cylinder before it makes contact with the paper, removing the excess ink from the non-printing (non-recessed) areas and leaving in the cells the right amount of ink required. The position of the blade relative to the nip is normally variable.

Next, the substrate gets sandwiched between the impression roller and the gravure cylinder: this is where the ink gets transferred from the recessed cells to the web. The purpose of the impression roller is to apply force, ensuring that the entire substrate is brought into contact with the gravure cylinder, which in turn ensures even and maximum coverage of the ink. Once in contact with the substrate, the ink's surface tension pulls (part of) the ink out of the cell and transfers it to the substrate.

Then the inked substrate goes through a dryer because it must be completely dry before going through the next color unit and accepting another coat of ink. A rotogravure printing press has one printing unit for each color, typically CMYK or cyan, magenta, yellow and key (printing terminology for black), but the number of units varies depending on what colors are required to produce the final image.

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