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The silicone rubber transfer pad, developed in the 1960s, propelled pad printing to prominence as the pre-eminent industrial printing process. Let’s consider what a transfer pad is and how it works. The transfer pad is available in a variety of shapes, sizes and hardnesses. The shape, size and hardness of the pad work in combination to effect the appearance of the printed image.
There are three principal groups of printing pad shapes. There are the "round pads", then there are long narrow pads called "bar pads", and then there are other shapes such as rectangles called "loaf pads". For each printing pad group there are three separate size categories: small, medium, and large pads. Custom-shaped pads can also be specially engineered to meet special application requirements.
Printing Pad Shape
The transfer pad is able to pull the image out of the printing plate due to the ink tack that is created as the cliché is doctored clean. As the pad compresses onto the plate a rolling effect occurs. This rolling action pushes air away from the image. Greater angles of descent enable less air entrapment, resulting in the elimination of pinholes. Conversely, shallow angles of descent prompt air entrapment and will result in the creation of pinholes. The same principle applies to the action of the pad compressing and then releasing the image onto the part.
Printing Pad Size
Sizing a transfer pad to a specific application requires that a sufficient mass of silicone be present to avoid image distortion. As the pad compresses onto the printing plate and part, the print surface of the pad is absorbed into the pad body. The pad body acts as a support mechanism for the pad’s print surface. If the print surface is not adequately supported, it will deform, resulting in image distortion. It may seem that the simple solution is to oversize all transfer pads.
Printing Pad Durometer
Transfer pad durometer, or hardness, plays a key role in print quality. Generally, the harder the transfer pad, the better the print quality. A harder pad, when compressed, will maintain its shape much longer than a pad of the same design with a softer durometer. This retention of structure results in a better rolling effect and therefore is less likely to trap air.
Printing Pad Selection
The first step in choosing the right transfer pad begins with identifying the desired image size. Most transfer pad catalogs include image size capability with the individual pad illustrations. The next consideration will be the part’s surface. Specifically, you will need to know whether the surface is flat, curved, smooth or textured. On a flat and smooth surface, a standard durometer pad may be utilized. If the surface is flat but textured, a harder durometer pad is required. A pad with a harder durometer will do a better job of forcing itself into the bottom of the texture while retaining its basic shape. A softer pad will deform by expanding at the sides and will bridge the texture peaks at the print surface. This will leave a void at the bottom of the texture. If a pad with a harder durometer is unavailable, a pad that utilizes a steep angle will perform the same function.
A part that exhibits a smooth surface, but contains a radius, typically requires the use of a softer durometer. The softer silicone will better conform to the part’s surface. The radius of the part will act to increase the pad’s angle of descent, reducing the potential for pinholes.
Custom Printing Pads
Custom pads are available from most manufacturers and are designed for a specific application. Unique custom designs can be manufactured to allow printing with a variety of special medical and electronic parts.
The pad printing inks that are used with these pad printing pads are specifically designed for the pad printing process.
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