Plastics Decorating - January/February 2008 - (Page 36) SPECIAL FOCUS Understanding the PAD in By Peter Kiddell, PDS Consulting PRINTING For very unusual parts, custom pads sometimes will be made that combine two different profiles, as in the top illustration in Figure 2. These ‘combination’ pads are worth considering, but they can be expensive and are prone to print distortions unless they are very carefully designed. A preferable solution is to Figure 2 - Combination Pads use two separate pads and mount them close together on a single machine. The lower half on Figure 2 shows how the combination pad above could have been designed as two separate pads. Another advantage to using two pads is that if one is damaged, the cost of replacing it is much less than replacing a combination pad. Use the following guidelines when choosing a pad shape for a particular job: • First, try whichever standard pads you think would do the job for the particular part. Do a test print to verify that the proposed print area is imaged accurately. • If the pad shape chosen provides a satisfactory print over just a part of the area, look for similar pad shapes that extend the profile in a way that will cover the entire image. Undersized pads almost always are the cause of distortion at the edges of the image. • If the obvious pads fail, try ones that appear to be unsuitable. Maybe the pad has a sharper angle than would seem to be appropriate or is clearly too large for the image. It still may solve the problem. • Irregular ink pick-up during the test print usually means that air is being trapped between the pad surface and the plate. Watch carefully as the pad is being imaged to be sure that a rolling action is occurring. • Whenever possible, ensure that the point or apex of the pad does not come into contact with the image area of the plate. This tends to thin the ink at that point, causing an inconsistent ink deposit. • If the pad is ‘overstressed’ (that is, too small for the image) or if the image is too close to the edge of the pad, distortion is likely to occur. Always use as little pressure as possible to pick up and print the image. If the machine is running too fast, excessive pad pressure can cause distortion as well as poor ink transfer. One of the most difficult questions to answer about the pad printing process is, “How do I determine what pad to use?” One answer won’t apply for every shop or application. Rather than lay down inflexible rules, this article will provide a methodology for determining the optimum pad characteristics and a framework you can work within to get the best performance from your pads time after time. The key to good pad printing is to reduce and control the variables. Though the function of the pad seems simple - to transfer the image from the printing plate onto the substrate - it is subject to print quality fluctuations if the correct pad type is not chosen or used properly. Five key pad characteristics can affect the quality of the printed image. These include shape, size, hardness, surface finish, and material. Vary any one of these and the print quality will shift. These factors must be taken into account when planning the job and ordering pads from suppliers. Shape and Size Shape is the most important variable in selecting a pad. In order to achieve a satisfactory print, the pad surface must roll onto the plate and the image area of the substrate. The shape of the pad largely determines how well the pad will achieve this rolling action. This makes shape the most important variable in selecting a pad. Most pad suppliers have hundreds of pad shapes in their standard inventory, but the majority are based on three basic shapes: round, rectangular, and bar (see Figure 1). Each of these shapes could have either curved or flat printing surfaces, depending on the nature of the part to be printed. Regardless of what shape the pad has, it must roll onto both the plate and substrate for good printing results. Try to avoid flat-bottomed pads, as they have a tendency to trap air when they come into contact with the plate, hampering ink pick-up. Again, the more rolling action that is achieved, the more ink that will be transferred. Another important variable to consider is pad size relative to image size. In screenprinting, the larger the screen is in relation to the image size, the less distortion that will occur. The same holds true in pad printing. The larger the pad, the less the image is likely to distort. Often, the distance between the plate and the body of the machine (sometimes called the ‘throat’ of the machine) will determine the maximum Figure 1 - Basic Pad Shapes pad size to use. 36
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