Troubleshooting the pad-printing process requires the identification and control of associated variables. The tips presented here will help you avoid some of the persistent problems you encounter on press.
By Julian Joffe
Printing larger images
One of the challenges pad printing en-counters is transferring a quality image when printing a large, open area. As the cup passes over the open area, the ring tends to scoop out some of the ink from the etched area, resulting in less ink being picked up and transferred (Figure 6). The print will then appear faint in those areas. Several steps can prevent scooping and improve print quality.
1. Select the correct line screen and halftone pattern. The halftones in the etched area help prevent the ink from being removed as the inkcup passes over the plate. Increasing the dot size and arranging dots in cross patterns prevents much of the ink from being removed. Most pad printers choose either a 300-line/in. 90% or 200-line/in. 80% screen when creating the cliche plates. This allows for a deeper etch with the peaks still supporting the cup.
2. Align the registration of the artwork. Ensuring that the shorter side of the image faces the operator is a technique that works with some graphics. For example, when printing an image that is rectangular, register the image so that the ceramic ring travels in the direction of the longer side.
3. When possible, add lines or text to break up the open area to help prevent scooping. An open circle often presents problems.
Using excessive pressure during ink pick-up from the cliche and downward force of the pad on the object is a common mistake. Decrease ink pick-up and print pressure to the minimum force needed to pick up the complete image and transfer it completely. This prevents overstretching the pad and smudging/squishing out ink during pick-up or printing.
Check the pad after its pick-up stroke to see if it picked up the complete image off the cliche. Many times the cliche stroke is too short and the extremities of the image are incomplete. Use a mirror to see if the image is complete. In the case of rough substrates, you may need to reconsider the rule of minimum force, as the down stroke on the print side may have to be excessive to improve print quality. The down stroke on the cliche should be as conservative as necessary to pick up the complete image. Center the image on the pad—that also helps to eliminate the need for excessive pressure in the down stroke.
If your ink is drying too fast, could you speed up the machine? Know what your equipment is capable of in terms of programming and use those features. Simply adding a pause before the print phase often helps to cure image-transfer problems caused by a deeply etched cliche or ink that is a bit thin. Whatever you do, be sure to document press settings.
The single most expensive mistake pad printers make is not thinking about problems and the possible solutions. The second biggest mistake is not asking for help when needed. Both of these can quickly become quite costly.
Julian Joffe is CEO of East Dorset, VT-based Pad Print Machinery of Vermont.
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