The use of robotic technology in the inkjet printing process could change the dynamic of decorating complex industrial components.
By Debbie Thorp
Inkjet printheads each have a defined print width, or swath, so when considering how to print onto a 3D object, the print path for each object has to be defined. The challenge for the OEM or system integrator begins with how to “unwrap” the image and create the swaths in readiness for printing a particular object. Taking a sphere as an example, there are many ways to generate the swath decomposition. In each case, the choice of print path must consider the constraints of the object to be printed, the inkjet printheads, and the capability of the robot. For example, the swath decomposition for a sphere could follow a latitude, a spiral, or a more freeform approach.
The selected print path must then be coordinated with the robot control and its positional capabilities. The precision with which industrial robots can commonly position objects is very good for mechanical processes such as spraying or welding, but is still an order of magnitude less accurate than that required to avoid visible artifacts in inkjet printing. Trade-offs in print quality are rarely acceptable, so software techniques are being developed to compensate for the inaccuracies in robotic registration. If the robotic capabilities are consistent, then they can be measured and compensated. There is also a difference between the accuracy achievable for static poses, when the robot is not moving, and what can be maintained dynamically while the robot is following a chosen print path.
Typically, robotic systems using inkjet will be custom built for specific applications by the system integrator. For every different object, the inkjet printing process needs to consider the geometry of each object, the geometry of every nozzle in the printhead, and the selected print path; and it must compensate for the density effects, optimize the screener for shape, and compensate for the behavior of the datapath.
The drop-laydown strategy is critical. This is the precise coordination of nozzle jetting with positional information of the printhead, all within the constraints, positional accuracy, and repeatability of the robot. Determining when to eject a drop is a challenge, and the key physical characteristics of the printhead (such as the print width, nozzle geometry, number of and distance between nozzle banks, drop size, jet straightness, and drop ejection velocity) have to be carefully considered and coordinated with the relative speed of the object’s movement at the point where the nozzle jets and the drop lands.
Printing the image must take into account all these complexities and also lay down the swaths following the selected print path. Unlike flat surfaces where swaths lie parallel to each other, on complex shapes, the swaths may overlap or meet at an angle to each other. Stitching strategies must be employed to reduce the visibility of this issue. For some objects, the print path may be so complex that a swath may not only require stitching, but also density and nozzle position correction as it twists and turns relative to the surface. This especially applies to shapes with discontinuities in their curvature. In these applications, a whole myriad of issues occurs including density correction, dot gain, and screening artifacts.
These challenges may seem significant, but as we have experienced in many applications already, all the same compelling drivers to implement digital printing technology apply. This is what is driving development and innovation in the field of inkjet and robotics. What we see at tradeshows and conferences is just the smallest sneak peek at what is happening behind closed doors in companies and manufacturing plants worldwide. Although most inkjet robotic systems are being custom built to increase the efficiency of specific manufacturing processes, we can expect more widespread availability as companies such as Heidelberg, EPS, and others continue to invest in direct-to-shape technology and develop systems to make this capability available to printers. As we have seen so many times before, inkjet technology continues to break down technology barriers and open up new possibilities and new markets.
Read more from the October/November 2018 issue.
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