Global Graphics, the British software developer based in Cambridge, has developed a set of new software screens for use with inkjet printers that can be applied to any print industry workflow, including existing printers as well as those in development.
These Advanced Inkjet Screens are designed to smooth out imperfections in inkjet printing that are caused by slight changes in the size, shape or position of each drop, which can create visible artefacts.
Tom Mooney, product manager for these screens, explains: “By measuring the characteristics of the drop generation and interaction with the substrate, it is possible to fine-tune the halftone dot shapes and placement within the screening definition to mitigate unwanted variables. The result is print that is visually largely error-free, because the optimized screen compensates for and masks the effect of errors in the print process.”
Martin Bailey, Global Graphics’ chief technology officer, points out that software solutions such as this can save press developers a lot of time and money in not having to tweak the hardware and ink formulations. Global Graphics has been offering a software development service to press vendors since 2016. Bailey says: “We have been doing that with a huge number of customers so we have a lot of experience across multiple heads, substrates and different applications.” As a result of this work the team found that the choice of media played a big role and was able to divide the problems into two main types of substrates, each with a specific screening solution, Pearl and Mirror.
Pearl is an advanced dispersed (FM) screen, optimized for natural images on a more or less absorbent substrate. It is targeted especially at addressing chaining and streaking artefacts that are caused by drops coalescing along the substrate surface. Bailey says that this can be caused by various factors such as issues with head geometry, nozzle patterns and timing, or stitching of multiple heads if people havn’t quite tweaked the wave form correctly. Mooney adds: “It’s particularly bad at high speed so people then run the press more slowly, which reduces the speed they can do. But this kind of problem is quite amenable to screening.”
The Mirror screen, on the other hand, is for non-absorbent and poorly-wetting surfaces such as tin cans and flexible packaging as well as areas of dense metallic ink. These prints can suffer from a mottle effect that looks like orange peel. The problem appears to be triggered by ink shrinkage during the cure and is especially noticeable in areas with reasonably high total area coverage. UV inks are particularly prone to mottle effects, especially when they are being pinned between the stations but this problem can also show up with water-based inks. Bailey says that sometimes vendors try to fix this problem with inter-station pining, noting: “This reduces tonal mottle but increases colour mottle.” He says that another option is to reduce the amount of ink, which does counter tonal mottle effects but can lead to more apparent noise or graininess in images.
The Mirror screen uses a microstructure to counter this. It’s also useful when dense inks such as metallics are used, or where the print should not interfere with the smoothness of the reflection from a shiny substrate.
Bailey says that between them these two screens will be suitable for roughly 80 percent of applications and that Global Graphics will continue to offer a bespoke service for the rest. The solution works for both binary and greyscale printheads, regardless of ink type. Bailey says that it’s faster to integrate it into a Harlequin RIP but that it is often deployed alongside RIPs from other vendors.
Most of the printers that Global Graphics has worked on so far are jetting at 600dpi though some have used higher resolution up to 1200 dpi. Mooney says: “The quality issues aren’t different, it’s just on a smaller scale but you still get streaks and mottling.” He adds: “The bigger problem is the amount of data; 1200 dpi gives you four times the amount of data. But the press manufacturers don’t want to beef up the computing power so we have tried to make this as efficient as possible.”
However, Global Graphics does have a distinct problem. The company wants to shout about its successes to attract new customers and continue to grow, but most of its vendor customers see things like the Advanced Screening as the secret sauce that will help them sell more kit, and would prefer to keep the secret to themselves. Consequently it’s not clear which printers are actually using these screens so there’s no way to evaluate how effective they are in the wild.