Applied technology - from motorsport to machine building
09 September 2016
When a global leader in the industrial inkjet sector was developing a new digital printer for the large-scale wall coverings and fabrics market, it approached KWSP to help it overcome some technical challenges.
It became evident to the in-house engineering team that the print head carrier was going to weigh over 800kgs if designed and manufactured using conventional materials such as steel and aluminium. This was a real concern - having implications for the efficiency, power and overall cost of the project.
Measuring approximately 1m by 2m, the print carriage - which housed over 100 print heads, ink systems, electronics and plumbing - had to be stiff, accelerate within the global motion system and maintain accuracy for the internally mounted print heads of 0.1mm to the print surface. It also had to cope with temperature changes of the factory conditions of up to 25°C. A conventional aluminium structure compromised almost all of these requirements.
It was believed there may be a solution in replacing some of the aluminium components with carbon composite components to save some weight.
Analysing the key project requirements, KWSP focused on three key areas:
• Structural mounting and accurate adjustment for the print heads: A significant consideration was also that the operating environments for the machine can change significantly in factory conditions. A 20° change in temperature can cause thermal expansion of aluminium of more that 1mm over a 2m section. Contractions and expansion on this scale would have a significant impact of the accuracy and repeatability of the print heads.
• Accessibility and serviceability: The ability for maintenance engineers to easily gain access to all parts of the finished machine is vital when it comes to running costs.
• Weight and scale: Trying to design the machine in conventional aluminium plate was a compromise. The structure was heavy and increasingly complex to allow mounting of the print heads. The additional weight had further implications on the motion system with bigger inertias and additional motor sizing required.
Having considered all of these challenges, as well as the cost of manufacture, the KWSP team proposed a radically different construction for the inkjet printer; producing a lightweight carbon fibre monocoque structure instead of machined aluminium.
Designing a structural solution
First, a schematic was developed of the concept in CAD. This involved finite element analysis to predict the properties of the proposed structure and its overall mass. The team then concentrated on the challenge of mounting the print heads – and how they could be adjusted accurately in relation to each other.
One of the challenges was the vertical stiffness of the mounting plate for the print heads. It was a challenge to make the structure stiff enough to maintain the maximum 0.1mm of vertical displacement allowed, without the use of vertical structures.
To solve this, KWSP took inspiration from a suspension bridge and came up with the concept of mounting the heads on a largely unsupported, but structural plate. This had mountings at each end and was held up by vertical suspension elements from a structural overhead beam, according to Stuart Banyard, head of advanced manufacturing at KWSP.
To give maximum accessibility to print heads, KWSP needed to devise a completely new way of supporting them. It did this by moving the structural elements away from the areas that needed access, thus solving the problem.
Having developed the structure in CAD, KWSP then built a proof of concept model to validate the carbon fibre construction and crucially used the prototype to validate the manufacturing capability and head mounting system.
This approach maximised access to the print heads, therefore lowering the cost of maintenance. The use of a moulded carbon fibre structure allowed cable and pipe runs to be integrated into the design, which dispensed with the need for additional brackets and components, maximising integration by design and reducing parts count and cost.
Dynamic engineering solution
While KWSP’s knowledge in lightweight structural engineering delivered a solution to the challenge of weight and accessibility, it also incorporated a closed cooling system, plumbing and wiring ducting. As a result of its carbon composite structure, it had a near zero coefficient of thermal expansion.
During functional development of the integrated industrial digital printing machine, some issues were identified with ventilation and extraction of fumes around the print heads. Using Computational Fluid Dynamics (CFD) KWSP modelled the air flows, the particulate and fume migration, heat rejection and convection flows in the print head carriage. From the data, KWSP were then able to design a fully integrated and retro-fittable fume extraction and ventilation system that did not have an impact on the temperature controls inside the carriage. CFD analysis is more often used in the fields of motorsport, aerospace and high performance sports, areas in which KWSP has previous experience, but demonstrates how technology developed for motorsport technology can be equally useful to solve industrial scale problems.
The area of vapour and overspray management became a key challenge once product development testing started. The printing process often produces tiny breakaway droplets from the large inkjet droplet, known as satellites. These satellites form a very fine mist that can then contaminate the product.
CFD analysis was employed to understand and map the natural flow of air around each print head as it operated at speed. Air flow, heat and convection currents were all considered in a bid to understand which forces were affecting the vapour flows.
Print perfect finish
While the in-depth CFD analysis enabled the delivery of stable and consistent print heads, the adoption of carbon fibre resulted in a lightweight structure that also negated the unwanted impact of thermal expansion. The decision to use an alternative material to aluminium reduced the cost and weight of the machine. While the original structure was approaching a mass of 800kg, the final carbon composite assembly came in at under 140kg.
This project is a prime example of innovative industrial inkjet development. It also demonstrates the tangible benefits of technology transfer. Increasingly referred to as ‘horizontal innovation’, sharing of knowledge and technology between sectors is a significant and untapped source of innovation within UK manufacturing, according to Kieron Salter, Managing Director at KWSP.
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