How to create a metallic polychromatic image for digital printing?
Metallic ink needs to be integrated as a 5th channel in the image. Mask the relevant parts of your picture and create spot color channels from that. Choose a CMYK + spot color setting in the RIP. In simple images, use the color replacement function of the RIP.
Find the best metallic effect by setting the ink limit of the spot color channels to 30-70%. To achieve the best metallic result a reduction of metallic ink volume is necessary. Generally this corresponds to around 50% of the normal CMYK volume.
Attention should be paid to ink volumes to avoid drying problems when metallic is added to CMYK prints. Metallic ink has a strong coverage, whereas CMYK is transparent. Best results are achieved by printing CMYK on top of metallic. The result depends on the order of the print heads and the printer driver options. Start with a standard mode, find the right ink limit and then optimize by trying uni or under print modes.