Selecting Pad Printing Silicon Rubber

The pad receives the motif from the cliche, transfers the ink film to the substrate and deposits it there. The pad must be constructed so that it is pliable, but guarantees transfer of the motif without smudges or blurring.

Printing pads are made of silicone rubber and produced as stamps in a wide array of shapes, hardnesses and qualities.

SHAPE

Selecting Pad Printing Silicon Rubberimage shadow Selecting Pad Printing Silicon Rubber

All standard pad shapes have one common characteristic: the printing surface is convex and angled from the sides to the print area in the center. This convexity is a factor influencing the ink transfer, whereby the lateral angles provide the stability necessary to obtain smudge free prints. A semi-circular, tapered pad is the ideal shape. It can be rolled to all sides equally well, thus preventing the formation of air bubbles. This rolling motion allows the ink to be easily transferred from the cliche to the piece to be printed. The above described shape, however, does not lend itself to all objects or print motifs. Square or rectangular pads are sometimes necessary. When square pads are used, it is still important for the central area to be tapered in order to insure that the pad rolls well. The results produced by rectangular (bar-shaped) pads are generally less desirable, as the pads themselves can only be rolled laterally in two directions.

SIZE

Selecting Pad Printing Silicon Rubberimage shadow Selecting Pad Printing Silicon Rubber

In order to guarantee sharp, smudge free print, the pad selected should be as large as possible. The less the pad is deformed, the sharper the printed motif. The pad must be larger than the actual motif to be printed, particularly with “problem prints” , where corners are to be reproduced at an exact angle. The disadvantage of the large pad volume is that a very large pad requires a large press, and, such a large pad is more subject to the vibration caused by the movement of the press than is one of smaller mass. Besides this, the price is considerably higher, as the main factor in pricing pads is the weight of the material.

HARDNESS

Pads are generally available in varying grades of hardness ranging from 2 to 18 Shore A. However, special grades of hardness from 0 Shore A to over 40 Shore A can be utilized. Here, the rule is: the higher the number, the harder the pad. The hardness has a major influence on the quality of the printed motif and the life expectancy. A hard pad can reproduce print well, and has a greater life expectancy due to its physical stability. In many cases, this hardness cannot be exploited, as the pad would damage the material to be printed. In the same way, softer pads must be used for very curved surfaces, as they can adapt to such surfaces better than very hard pads. Selection of the grade of hardness does, of course, depend on the force of the press which is utilized. Many presses are, accordingly, pushed to their limits by large, hard pads.

QUALITY

In the case of silicone rubber, there are basically two different systems: crosslinking by polycondensation and crosslinking by polyaddition. Physical properties, such as tear resistance or resistance to swelling in contact with solvents, are better in polyaddition crosslinking materials than in polycondensation systems. Obtaining the raw material is more expensive, however. The smoothness of the surface is a decisive factor in the quality of the print. The smallest impurities or air bubbles caused by defects in manufacturing result in unclear print. In the beginning, new pads tend not to pick the ink up from the cliche as well. This problem can be solved by making a few prints on paper or by a short cleaning operation using alcohol. If cleaned with aggressive agents, such as thinner, the pad immediately takes the ink from the cliche, but does not transfer it to the piece to be printed quite as well. Once a pad is “broken in”, it is recommendable to dab its surface with adhesive tape to remove any dust particles.

LIFE EXPECTANCY

Approximately 50,000 to 1,00,000 prints can normally be produced with one pad. This, however, depends on the quality of print required and the type of inks used. The life expectancy is longer using single-component inks and shorter using two-component inks. The size, shape and hardness of the pad as well as the shape of the material printed also has an influence on the life expectancy. In the case of 4 very sharp corners or extreme curvatures on an object to be printed, the pad can be physically damaged after as few as 1,000 to 5,000 prints. The life expectancy of a pad can be extended considerably by careful cleaning and treatment with silicone oil.

PAD PREPARATION FOR DAY-TODAY PAD PRINTING

To select a suitable pad, the positive film can be pressed onto the pad using a stable glass plate. This is a simple method to show how strongly the pad must be squeezed. To prepare the pad in the press for the motif, it is recommendable to breathe shortly onto the cliche or clean it with alcohol in order for the impression left on it by the pad to be seen. In this way, the position of the pad can also be corrected at the same time.
Should the pad be too large, it can be cut to size with a sharp knife. This saves having to produce an expensive, custom made pad. When cutting, care must be taken to maintain the approximate lateral angles of inclination in order not to reduce the stability and, thereby, the print quality too greatly.
If a number of different, physically separated motifs are printed, several pads can be mounted onto one single wooden panel. A better result can be achieved in this way than by using one very large pad.
The position of the pad can cause considerable smudging of the image on very curved parts. This can be decreased or eliminated by several X-Y shifts of the pad in the machine.
Here, it is also important for the pad to be pressed onto the cliche and the substrate with only minimal stroke force. Pressing too hard can lead to smudging, greater wear and tear as well as poor ink transfer.