Plastics Decorating - January/February 2008 - (Page 37) • If your experimentation doesn’t reduce the print distortion to an acceptable level, and a custom pad is out of the question, your last resort is to distort the image on the plate to compensate. Often, this is achieved by printing a grid onto the substrate and measuring the distortion of the grid to guide in the alterations that must be made to the original artwork. This will shorten the time taken in test printing, but it won’t eliminate the trial-anderror altogether. This method also leads to ongoing problems since positioning of the part and the pad (relative to the image on the plate) must be absolutely dead-on each time the job is set up to avoid distortion. Hardness The hardness of the pad normally is determined by the amount of silicon oil used when the pad is molded, i.e., the harder the pad, the less silicone oil that was added. Four basic pad hardnesses are standard in the industry and cover most applications. Custom pad hardnesses are available through most pad suppliers. Many pad manufacturers color code the four standard hardnesses by adding pigment to the silicone itself or by coloring the pad base. Not all manufacturers use the same code, but the typical color designations by pad hardness are listed in Figure 3. As a general rule, the harder the pad, the better it will perform. However, a hard pad may be impractical in some applications, such as when using a low-power machine or printing onto a delicate item. Choosing the proper pad hardColor Hardness ness for a job is a matter of Blue 55 Shore (+2) experimentation and experience. Special 55 Shore pads are availPink 50 Shore (+2) able for printing onto abrasive Green 45 Shore (+2) substrates and textured finishes. Two such applications include White 35 Shore (+2) the turn-signal control arms and Yellow 30 Shore (+2) windshield-wiper control arms of automobiles, which are molded Figure 3 in glass-filled nylon. A heavy white ink is required, and the pad must resist the abrasive nature of the substrate. A useful tool for all pad printers is a durometer gauge for determining pad hardness. This simple tool (the same one you would use to measure the Shore hardness of a squeegee) is available through silicone-rubber suppliers and many general service dealers in the screen and pad printing industries. Maintaining accurate pad hardness can be problematic for some pad manufacturers, thereby making the durometer gauge an ideal quality control device for incoming pads. Use the guidelines on the next page for pad hardness when selecting pads: continued on next page Kensol-Franklin www.thethomsongroup.com 37 http://www.thethomsongroup.com
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