INTEGRATION OF INKJET AND ROBOTICS TO ENHANCE MANUFACTURING AUTOMATION

Inkjet technology continues to replace the incumbent analogue techniques in traditional 2D graphics for decorative industries. However, new applications are emerging in the manufacture of products that require decoration or coating over a 3D surface. Although, this comes with some challenges. Industries as varied as automotive to footwear are seeking to make use of the advantages of digital printing. Key drivers include reduction of waste material and customisation of individual products in a serial production line. The cover-all term for this type of application is direct-to-shape (DTS) or direct-to-object (DTO) printing. These applications are part of a trend towards greater manufacturing automation.

Use of inkjet printing in manufacturing automation often involves the integration of a robot arm to handle either the product or the printhead. Using a six-axis robot, allows for the ability to cover the entirety of a complex 3D surface. It also offers additional flexibility to handle parts of varying size (for example, due to manufacturing tolerances) or an entirely different design using the same production cell. For some applications, it is possible to handle the part using the robot and move it underneath the printheads – ‘part-on-end-effector’. However, physical space in the cell and load capacity of the robot, constrains the size of parts that can be practically handled in this way. Imagine trying to inkjet print a car roof by moving the car! For decoration of parts above a certain size, these practical limitations drive integrators towards the ‘printhead-on-end-effector’ approach, which places the printheads and ancillary components onto the robot itself.

An example of the ‘printhead-on-end-effector’ process is in the automotive industry, for application of paint and other coatings onto the car body. Although the necessary spray technology is effective for coating parts, the use of inkjet can increase transfer efficiency and reduce waste by minimising overspray. The high addressability of an inkjet printhead ensures that material is jetted only where it is needed. As well as saving waste, there is less aerosolised material that must be extracted from the air and treated to make it safe. Process costs are further reduced since the need for masking off areas that are to be kept clear or painted a different colour is eliminated.

Another example is applying a functional layer to the surface of a 3D part. In these applications, a high-quality, undistorted application of the functional material to the surface is required. Integrating the printhead and robot arm produces a system that is highly flexible for printing on parts with different geometries, as well as curved surfaces. The combined robotic inkjet system provides significant benefits over incumbent analogue techniques. These would typically employ foils or contact-printing techniques – pad printing or screen printing – which, in some cases, cannot be applied to complex 3D surfaces or may lead to distortion of the design.

Integrating a robot arm and inkjet, using the ‘printhead-on-end-effector’ approach, has some clear advantages in terms of coating large parts. In addition, it offers flexibility to address different part geometries within a single cell. However, this does not come without challenges in the design and engineering of such a system. A printhead mounted on the end-effector of an industrial robot is subject to acceleration in any axis. The orientation of the part may mean that the printhead is rotated vertically – known as skyscraper configuration – and may accelerate and change orientation whilst jetting. The fluid-supply system must regulate to keep a consistent level of meniscus pressure at the printhead. Good jetting is achievable under these conditions and reacts quickly to changes in printhead orientation or acceleration. Typically, for coating applications, a high rate of material deposition is also required, placing further demands on the fluid supply.

There are also challenges for the ink/paint/coating formulator in achieving the necessary material properties. Inkjet is typically capable of jetting fluids with viscosities in the range of 5–30cP with temperature control utilised to bring more viscous fluids within the jettable range. A traditional automotive paint would be in the range 50–500cP. A thicker paint generally produces less sag, which is important where coatings are applied to a non-horizontal surface. Of course, the desired physical and chemical properties of the cured finish must be maintained with any modification to the chemical make-up.

Integrating inkjet technology on an industrial robot is a challenging undertaking that requires a multi-disciplinary approach

When it comes to process design, there are decisions to be made about how to apply the coating/decoration efficiently. If the area to be addressed is wider than the printhead swath, the printhead must pass over the part multiple times to cover the area in a similar manner to a 2D multi-pass printer. The robot should move the printhead over the surface, with the most efficient path, to ensure that the full area is addressed – path planning. The image is then broken down into swaths to be printed on each pass. Alignment and stitching of multiple passes is complex when dealing with a 3D surface, since the paths are not necessarily parallel. Curved areas must also be handled, sometimes including tightly concave sections. It may be necessary to increase deposition to ensure coverage of hard-to-reach corners. Path planning and image processing must be tightly coupled to achieve good coverage. Deformations and distortions due to mapping a 2D image onto a 3D surface, or other process limitations, can be compensated in the image data to produce a high-quality result.

As with any practical inkjet system, there will be less-than-ideal behaviour that can impact the finished result. Even using high-performance industrial robots, with excellent repeatability, the absolute accuracy of trajectory and velocity may deviate from the ideal case. Synchronising the fire pulses – essentially timing of when the printhead jets ink, such that a line is printed in the correct place – to the motion of the robot is a challenging task. Furthermore, there is a possibility of introducing vibrations that cause relative movement between the printhead and substrate. This can lead to banding effects. For simpler geometries, where motion is highly repeatable, it is possible to run open-loop and get acceptable results. For more complex geometries, or situations where uniformity of deposition is important, feedback from the robot can be used to better synchronise movement and jetting. This ensures precise placement of each pixel on the surface.

Drop-placement errors can also result from dimensional tolerances, meaning that the part to be printed does not accurately match the CAD model. A 3D scan of the surface enables corrections to be applied that will avoid distortion/misplacement of a graphical image – as well as gaps in coverage – that could result from these positional errors.

Integrating inkjet technology on an industrial robot is a challenging undertaking that requires a multi-disciplinary approach. Different applications have different requirements and there is no off-the-shelf solution. The material must be carefully formulated to ensure it can be jetted reliably, whilst maintaining desired properties in the finished product. The robot, printhead, data path and fluid control must work seamlessly together for a reliable, accurate system. The software workflow needs to provide necessary tools to process graphics or prepare the coating strategy, to plan the robot motion and prepare the image data.

Despite the challenges, several examples of successful implementation exist today including aerospace, automotive, clothing, packaging and furniture decoration. The trend towards manufacturing automation, smart factories and greater product customisation are likely to drive the need for DTS printing. As a result, the combination of industrial robots and inkjet will become more widespread.

Working closely with all major industrial-inkjet printhead manufacturers, Meteor Inkjet Ltd offers production-ready solutions to print-system builders worldwide.