the part. When compared to other AM methods, this process can be performed at lower temperature, temperatures ranging from 22oC to 204oC are possible; the temperature is normally 150oC (). Moreover, it uses a machining process to shape the contour of the final object precisely (). The main problem of this technique is voids that occur along the interfaces between layers because of foil surface roughness, insufficient or excessive welding energy, and positioning inaccuracies of foil placement (). This defect affects the strength of built parts (). Also, this technique can use only metal materials and it is inefficient in material usage due to the subtractive process. Furthermore, the system can fabricate multiple material objects but with only vertically oriented interfaces (). Fig. 11. Schematic system of UC (Left) and samples of material bonding (Right) a) Ni/Ag foils on Al 3003 -H14 substrate; b) 2Ag/Cu/Ni on Al 3003-H14 substrate; c) Ni/Cu foils welded on Al 3003-H14 substrate and d) Al 6061/Ni/Al 6061/Cu/Al 6061/Ag/Al 6061 on Al 6061-T6 substrate () The sheet lamination processes uses a relatively low processing temperature that is suitable for electronic encapsulation and embedded electronic system applications because it does not provide the protection of the embedded components. The challenge is the process parameter that can achieve a defect-free bonding layer at low temperature. Delamination between
Hybrid & DW LCVD High-resolution process, metals and semiconductors low-deposition rate, gas might be involved Between and within layers Medium Poor by point Aerosol Jet Printing High resolution, wide range of materials, ability of writing in 3D space, ideal for deposition of biological inks, noncontact, easy material handling Solvent involved, shield gas may blow off powders when integrated with powder bed systems Between and within layers High Good Line by line SDM Wide range of materials Require machining process during fabrication, feature size is limited by cutting tool Between and within layers Low Fair Line by line DPP+binder printing or laser sintering Wide range of materials, possible complex materials and geometry control Limited to good flowability powders, overflow problem, low compaction density of discharged powders Between and within layers High Good Point by point Several research centres and institutes have been developing MMAM technologies to allow more materials to be used in a single process. Most of the MMAM systems discussed in this paper are still being investigated as research projects. There is still much effort to be made to incorporate multiple materials into AM commercially. In this section, some of the challenges that need to be faced so that MMAM becomes more widespread will be discussed. The general improvements and technological barriers that need to be considered in MMAM processes are as follows: Contamination: Contamination in the material feeding system when changing to a different material is an important in most MMAM systems. This makes the processes imperfect and the reuse of materials difficult. Bonding: Bonding of dissimilar materials has often been a challenging issue even in conventional manufacturing techniques. In the same way, achieving a good bond between dissimilar layers in MMAM is a matter that needs to be taken into consideration. Data processing: Most AM systems currently take STL files as standard input which is a surface approximation only and there is no information of the material content. A demand for a powerful CAD system for multiple material pre-processing has appeared so that the operator can be able to identify the type of materials from region to region in a part.
Fig. 20. Incorporating dry powder printing with laser sintering and binder jetting systems Material jetting systems are currently used mostly for polymer jetting so that there is a limitation on the number of materials since only jettable polymers that have good flowability at restricted temperatures can be used. In particular, processes such as Objet’s polyjet technology has inherent limitations on the selection of base materials since the jetted liquid needs to have certain properties of viscosity and curing temperatures in order to be jetted. However, the resolution of the process should be improved to expand its range of applications. More emphasis should be placed on the development of inkjet printing systems which are able to produce high-quality true 3D multiple material components from functional materials, such as metals, ceramics, and smart materials so as to be able to achieve the required thermal, mechanical, and electrical properties for the MEMS/MOEMS industry. The applications of inkjet printing in multiple materials fabrication will expand with the emergence of new jetting techniques such as electrostatic and with better understanding of the process and the utilization of efficient modelling techniques. Future work may focus on the