Wednesday, 26 October 2011

WIDFP? - Pad Printing

What is pad printing?
A printing process that can transfer a 2D image onto a 3D object.

Transfer pad printing is considered an indirect gravure printing process since, like gravure printing, it employs an etched plate. But the plate does not come in direct contact with the substrate. Instead, the image is transferred to the surface by means of the pad. The benefits associated with pad printing are numerous, but it is most commonly recognised for its ability to print on 3D surfaces. Common products that illustrate this feature include golfballs, syringe barrels, windshield wiper knobs, taillight lenses, consumer electronics and many more items. (link)

Pad printers designed for production, business, or home are easy to learn, operate, and achieve perfectly registered repeat prints on almost any surface, article, or product, using fast drying inks - 10-20 seconds drying time. Printers are supplied with magnetic printing plate holder, blades, pads, inks, basic accessories and full instruction material. (link)

Pad printing is an indirect intaglio process where the etched image on a flat plate is tilted with ink by a 'spatula' or wiper, the excess ink is wiped off by a doctor blade, leaving ink only in the etched image. This ink is then picked up by a silicone rubber pad and transferred to the product surface. The etched image can vary from .0005" to .0015", by exposure of a film positive to the sensitised surface of a Nylopolymer or steel plate. The pad is basically a silicone rubber compound, moulded to geometrically determined shapes. In order to transfer images with the minimum peripheral distortion onto flat or curved surfaces. Being elastic the pad shapes itself to the product surface. The pad shape usually relates to the image size and product shape. (link)

Basic elements of pad printing?
The transfer pad printing process consists of four main elements: pad, cliché, ink and machine. Below is a brief introduction of the elements followed by a detailed discussion of each.
  • Silicone Transfer Pad - It was the introduction of the transfer pad in the late 1960's that accelerated pad printing to its current status. The transfer pad, constructed of silicone rubber, is the key ingredient that enables printing on 3D surfaces. Available in a variety of shapes and hardness (durometer), it is the job of the pad to pick up the ink image out of the cliché plate, act as a carrier, and then transfer the image to the part. It is the unique nature of the silicone rubber that allows the pick up and release process to occur.
  • Cliché Plate - The second key element that we will look at is the printing plate or cliché. The cliché is manufactured through a special photo-etching process and is available in an assortment of sizes and materials. The most commonly used steel cliché has a life expectancy in excess of one million cycles. Other temporary cliché materials can be used for shorter production runs and can even be manufactured at the user's facility. The choice between using steel or temporary clichés is based on volume and print quality considerations.
  • Pad Printing Ink - Pad printing ink, the third key element, includes a wide range of various inks, all designed specifically for the pad printing process. Due to the nature of this process, most clichés are etched to a depth of approximately one thousandth of an inch (.001"). With such an extremely shallow etched depth, the ink deposited within this space must be highly pigmented to obtain the necessary opacity. In addition, thinners are moved with the ink to control viscosity and to facilitate ink 'tackiness', a critical factor in the image transfer process.
  • Pad Printing Machine - The pad printing machine represents the fourth key element. Many machine designs exist but there are three basic configurations: the conventional open inkwell design, the rotary gravure process, and the sealed ink cup system. As with the other key elements, the variations exist within each of these categories. (link)

Pad Printing Inks?
It is common for those unfamiliar with the pad printing process to refer to pad printing inks as 'pains'. The truth is that these inks have very little in common with paint or even inks, including screen printing inks. The inks that we know and use today are designed specifically for the pad printing process. Although a full line of inks is available for printing on different substrates, all pad printing inks share two common traits: they are solvent-based and contain a special pigment density. (link)

Generally, pad printing inks have been especially developed and adapted for quick application and drying times, and in many cases are immediately stackable. About 75% of the products shown here, except glass, have been printed with a single re-usable ink type. These quick drying inks are easy to work with, and are very resistant to abrasion and chemicals. Two component inks are used where there is a need for high wear resistance and durability. These inks take several hours to cure, but the process can be speeded up by heat treatment after printing. Glass Inks, for example only require heat treatment in a standard domestic oven to effect a full dishwasher proof cure. Repeat registration is so accurate that several duplicate prints can achieve an effect very similar to the raised letter or 'embossed' effect created on quality business cards by more expensive and difficult thermographic print processes. (link)

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