here is text from the manual...
Printing gradients, gradient mesh objects, color blends, and transparencies
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Files with gradients, gradient mesh objects, or color blends can be difficult for some printers to print smoothly (without discrete bands of color) or at all. There are ways to improve the printed results of gradients, gradient meshes, and color blends on such printers. You can also improve the way that transparencies are printed.
Follow these general guidelines:
Use a blend that changes at least 50% between two or more process color components.
Use shorter blends. The optimum length depends on the colors in your blend, but try to keep blends shorter than 7.5 inches. (See Calculating the maximum blend length based on the color change.)
Use lighter colors or make dark blends short. Banding is most likely to occur between very dark colors and white.
Use an appropriate line screen that retains 256 levels of gray.
When the document contains complex artwork, transparencies, gradients, gradient meshes, or blends, make sure that you have selected the right resolution and Quality/Speed settings in the Document Setup dialog box. For information, see Changing resolution and rasterization settings for complex or transparent artwork.
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