Imagine the ability to manufacture monolithic structural parts or cornportents that already have a degree of functionality inbuilt, instead of adding it at a later stage in the manufacturing process or at another tier in the supply chain. A group of technologies, known variously as direct writing, solid freeform fabrication, additive fabrication or plain old rapid manufacturing, have the potential to create what some in the field are calling ‘the next industrial revolution’.
Individually-Customised Goods with Integrated Functionalities
Instead of the familiar mass-produced components and products made around the world today, researchers foresee a brave new world of individually-customised goods that have their functionalities integrated with their mechanical structures to form a coherent system. The scope of applications is broad, stretching from nanoscale printing of components or electric circuits to industrial-sized projects.
Direct Writing of Functionality
With recent advances in the capability of layering additive materials, it is now possible to deposit dissimilar materials as discrete or graded structures onto surfaces, providing an opportunity to ‘write on’ functionality. Rapid prototyping and rapid manufacturing techniques are generally characterised as being both incremental and additive. These attributes, combined with the ability to process multiple material and material types enables the direct writing of functionality.
The Additive Process
The additive process has two fundamental stages - the transportation of a dissimilar material to the workpiece surface in some form of pattern or geometry (often as a powder or paste) and its subsequent transformation into a fit for purpose state, often by the application of thermal energy.
By bringing together the fields of rapid prototyping and thick film electronics, manufacturers can bypass the traditional staged assembly process and print, spray on, or build up products in layers. To enable this write on, or direct write functionality, adjacent or layered materials must have variations in physical properties relevant to the particular application. Materials can be varied in their structure, thermodynamic and electrical properties to provide this functionality. It is possible to consider how the variation and patterning of these properties give rise to the functionality of systems in an assembly containing many discrete components together with some of the possible applications that arise from this.
Enhancing Structural Stiffness
The structural stiffness of a component can be enhanced by the additive deposition of material with a higher modulus than the properties of the bulk material. Owing to the volumetric quantities involved, this manifestation of layered additive manufacturing is best envisaged as a blown powder continuous process. Variation would include the composition of a metal alloy or the inclusion of ceramic particulates that would give rise to local metal matrix composites (MMCs).
Improving Surface Toughness
It is easy to harden surface layers by creating metastable microstructures, but there are not quite so many options for local surface toughening. The deposition of hard or wear resistant coatings is comparable with many alternatives for increasing the hardness of the surface, but this is a relatively mundane use of layered additive manufacturing. The use of layering materials with higher fracture energy than the bulk material in areas of higher surface stress would be expected to give a tougher ultimate product by reducing the tendency for fatigue crack initiation.
Improving Thermal Properties
Thermal properties can be tailored to control heat flow paths. Using layered direct writing processes, the type of products that could be created in several components using engineered heat dissipating parts can be made more integrally and with fewer effects from film coefficients. A ‘pad’ of high thermal diffusivity material can be written onto the structure in copper alloy, for example, in any appropriate pattern. A ‘spider’ network similar to the cross-section of conventional finned heat exchangers would dissipate heat more effectively
Building Thermoelectric Devices
Thermoelectric devices of patterned dissimilar materials using the Peltier effect can be created. It is easy to consider a functional device created by an array of thermocouple junctions and enabled by the ability to write down lines of thermocouple materials. Directly written, these can be physically integrated with the structural or systems component where the thermal gradient is either found or required, depending on whether thermoelectric voltage or cooling is demanded. By building up small-scale thick textures, two-and-a-half dimensional patterns can increase the local surface area and give enhanced emission or absorption, depending on the requirement.
Producing Electrically Conductive and Insulating Properties
The writing of conducting and insulating features enables structural surfaces to be used as printed circuit boards. Conventional electronic materials used for processes such as screen-printing and requiting temperature to complete the conversion from precursor to final material (like the transition from silver nitrate paste to pure silver - a common process in screen printed thick film circuitry) may use the energy of a rastered or written laser beam.
Other Electrical Properties
Devices fabricated from polymers may be written to give circuitry analogue or digital signal processing capabilities, particularly in the field of displays. Dielectric materials layered in conjunction with conducting tracks and pads can produce capacitors, while piezoelectric materials, such as PZT, written in patterns can give both sensing and actuation capabilities. Applications for this would include strain gauging or vibration sensing.
Chemical and Biological Functionality
It is also possible to impart chemical and biological functionality using direct writing, but this is a much harder area to define. Chemical and biological applications of direct writing will rely on the influence of passive materials on reaction rates, such as hydroxyapatite for surgical prosthetics, or on the placing of active materials to create direct effects, such as the positioning of live cells of dissimilar types to build up in-situ non-homogeneous tissue.
This article, provides an insight into some of the ‘building block’ technologies that are possible for adding functional value to products using direct writing. The technology to print transistors, capacitors, inductive coils and other semiconductor components has already been finalised, so who knows what could be next?