With inkjet printing, the image is defined digitally, meaning that software is at the heart of the printing process. Not only that, but good software implementation is key to making digital printing technology accessible to new applications and markets. Integration of all printer requirements under a dedicated printer manager can be a useful way to achieve a comprehensive solution that is also straightforward to use.
Image creation – digits to ink
In any printing process, the creation of the image is fundamental. So how does printing software turn a digitally-defined image into ink on the substrate? Any printing software first accepts the image in a standard image format such as TIFF or PDF. ‘Lossy’ compressed formats like JPEG are not advised as they can introduce unattractive artefacts into the print. The image path software then has several functions to perform. If the image input is in a vector format such as PDF or EPS, the image needs to be rendered into a raster, bitmap or contone (continuous tone) image (raster image processing or RIP) at the correct resolution for the printer. Next the colour space needs to be converted into that used by the printer, which may be CMYK (cyan, magenta, yellow and black) process colours or an extended process colour set including colours such as orange, red or light cyan/magenta. Additional spot colours may be added where needed, for example for printing of brand-specific colours which must be reproduced exactly. The colour conversion includes the application of a colour profile which attempts to ensure that the colours seen in the final printed image are those expected by the designer. Linearisation is also required as when printed, twice as much ink does not always produce twice the colour. The next process is screening, which attempts to reproduce the large number of grey levels present in a typical digital image using printheads that have either limited or no greyscale capability, by producing patterns of dots that appear as shades when viewed from far enough away, termed a halftone image. Screening algorithms have to be used with care as they can introduce visible artefacts into some designs, and most software offers alternative algorithms to use in these cases. Finally image splitting breaks the image into the appropriate image segments or swathes for each printhead, a process which depends on the configuration of the printer (number of printheads, scanning or fixed array, etc). The image data is then sent to the printhead drive electronics.
One big advantage of digital printing is the ability to make each instance of a print different using variable data printing (VDP). For example a label can be created by combining image elements, some of which may be static and others varied. Applications for variable data include mailing address labels, identity cards, batch and date coding and various kinds of personalisation. The variable elements can be defined in a number of ways, including generation using an algorithm (for a product code, manufacturing date or barcode) or by reading from a database (for all kinds of elements including text and images). Variable data processing can either be done offline, prior to printing, with the result being a series of static images that are sent to the printer in succession, or on-the-fly, with each label created by the print software and sent for printing in one process. Real time variable data printing has the advantage that the images are created at the same time as they are printed, ensuring that the correct data is added to the print, but this calls for fast hardware and the correct software implementation. A simple, yet powerful way of defining labels and using them is essential to the successful deployment of VDP in a production application.
System control – the ‘digital front end’
As well as realising the image in a form that the printer can use, the other vital function of digital printing software is to manage all the other components of the printer so that the image can be printed successfully. Ink must be delivered to the printheads at the right temperature, pressure and flow rate. Curing and drying stations must be turned on and off and set to the correct power. Substrate and/or printhead motion must be controlled so that the ink is delivered to the correct position on the item to be printed. In addition, printhead maintenance needs to be performed either automatically at scheduled times or manually on instruction from the operator, in order to ensure the system works reliably. These system functions are often controlled by a programmable logic controller (PLC) in industrial systems, and all of these subsystems need to be coordinated so that the defined image can be printed time after time on a production line, day after day. The simplest and most user-friendly way to achieve this is to have a central printer services manager interface that integrates all of the control functions in a logical, user-friendly and attractive manner.
When designing and building a digital printing system, a key consideration is how much of the work to do in-house and how much to outsource; this applies to software development as well as hardware design. Third party RIP software is widely available with extensive image processing functionality, but often the original equipment manufacturer (OEM) is left to design its own printer control software, leaving the image processing as a separate software entity. Alternatively, an integrated digital front end and image path software package can be obtained and configured, potentially leading to a more satisfying and cost-effective solution.