Industry representatives offer their perspective on pad printing onto textures.
By Robert Chadwick, Comdec Inc./Ruco Printing Products & John Kaverman, Service Tectonics, Inc. (2002-01-30)
How can I prevent voids in images when pad printing onto textured surfaces?
Chadwick: To avoid pinholes and other image flaws when printing onto textured surfaces, it's important to treat the causes of the problem, not merely the symptoms. Pinholes in a texture are a direct result of one of two things: air entrapment in the texture itself or "bridging" of the pad and ink film across the texture during printing.
Air entrapment occurs when the pad contacting the surface "scals" or blocks the escape of air from the texture. The air holds the ink away from the substrate and prevents the ink from forming a bond. You can predict the extent of this problem by taking a close look at the texture itself. Textures are constructed in one of two ways: They either comprise raised features on a flat field (positive texture) or are made up of depressions on a flat field (negative texture). From a distance, both types of textures can appear identical. Only close inspection will show the difference.
A positive texture cannot trap air because the air can escape in many ways around the various features of the etched image. This type of texture is not much more difficult to print than a flat surface. A negative texture, on the other hand, can sometimes be very difficult to print because when the top of the texture is sealed by the pad during compression, the air cannot escape. The ink over the depressed areas of the texture never makes contact with the substrate surface, often resulting in pinholes.
The only remedy for this condition is to use a hard and steep-sided printing pad. The harder pad will force its way into the depression while the steep angle allows air to escape ahead of the pad's compression. The trick is to get the air out before its escape route is sealed off.
The location of the pad relative to the image is also important. The point of the pad must lie outside the image area, or the risk of air entrapment increases. You may need to experiment with pad location and size to accommodate your images.
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