| contents page | news | events | advertisers’ index | contact | Green compact Resulting MEX-part MIM and sinter-based AM Green compact ~20% Resulting VPP-part ~17% <<<<<<<<<<< <<<<< Fig. 8 Sintering factor of (left) MEX and (right) VPP parts powder. Technically, BJT produces a brown rather than a green part and, therefore, no ‘main’ binder has to be removed by solvent or catalytic debinding. All remaining binder is removed in the sintering furnace. Debinding of VPP green parts The same is true for VPP, but for different reasons. Due to its chemical structure, the employed photopolymer cannot be removed by solvents or in a catalytic reaction and has to be removed during a thermal procedure that is integrated in the sintering run. Thus, when adhering to MIM terminology, all binder in VPP parts is ‘main’ binder and no ‘backbone’ binder is employed. Thermal debinding and sintering of sinter-based AM parts For all three described sinter-based AM technologies, the remaining binder is removed in a thermal procedure as part of a two-step operation in a MIM sintering furnace under protective atmosphere. The parts are held at temperatures suitable to fully decompose the Vol. 14 No. 1 © 2020 Inovar Communications Ltd used polymers thermally, typically in a range of approx. 200–350°C. Depending on geometry, but also on the employed feedstock system, the holding times will vary significantly, with VPP (~50 vol.% binder content) having longer debinding times for a geometrically similar part than BJT (2–8 vol.%). A constant flow of protective gas ensures that the decomposed binder residues are removed from the samples and transported outside the sintering retort through a piping and exhaust system. As illustrated in Fig. 7 with the help of a temperature vs time diagram, in the sintering step the parts are quickly heated to a temperature near the melting point of the material to densify the particles using capillary forces. Reduction of the surface free energy alters the geometry of the dense object by non-densifying and densifying mechanisms [32]. This results in nearly pore-free samples with complex shapes and finegrained microstructures for all three compared technologies. The sintered parts are about 14–22% smaller than the green compacts [33], with the exact sintering shrinkage factor depending on the green part density of the material and manufacturing technology used (Fig. 8). For MEX and VPP, the shrinkage factor itself is highly reproducible, with shrinkage being similar in all three dimensions [12, 34]. This allows a simple scaling of the CADdrawing or scanned data as part of the construction step, as is well known from MIM products. For BJT, there are strong indications that the shrinkage is larger in the z-axis, leading to anisotropic sintering behaviour [35, 36, 37], which is more difficult to address in the design stage. The reason for this may be complex fluid/powder interactions during the Binder Jetting procedure. As has long been recognised in MIM production, it is preferable for parts made by all three technologies to have at least one flat surface for support during the sintering process to prevent distortion. If a chosen geometry does not allow such support, a matching sintering support may be needed. The support must be made from the same material as the desired part. It must also be in the green state before sintering, as sintering shrinkage has to be taken into account. March 2020 Powder Injection Moulding International 75

PIM International March 2020