Vol.11 No.2 2018

報告:Additive manufacturing of ceramic components (大司)−87−Synthesiology Vol.11 No.2(2018)respectively. The powder flowability is also important for making a suciently lled powder layer. Because the layer is formed only by smoothing the powder by a squeegee, etc. without pressure, the powder density of the layer is almost equivalent to the static bulk density of the powder. It should be noted that since the mixed powder contains polymer binder as well, the density of ceramic powder itself is further lowered. It is, therefore, essentially required to have a well filled powder layer to get high densities of the resulting green and sintered bodies, which also leads to reduction of undesired deformation/distortion. It has been known that proper combination of coarse and ne powders leads to closer packing; however, powder owability is generally degraded when containing ne powder.In the lamination, the first step is formation of the powder layer, whose thickness is to be determined from the mixed powder size; it is preferably 1.5 to 5 times larger than the powder’s maximum size. The thicker the layer is, the higher the production rate; however, it results in larger steps of side surfaces. The thickness also should be determined so that laser-heat is suciently transferred to the bottom of the layer. Insufficient heat transfer causes a large temperature distribution thickness-wise, frequently resulting in warping and inter-layer delamination of a green body. Next, pre-heating mixed-powder is made before lamination, depending on the binder’s melting point. For example, when employing wax-based binder whose melting point is 80–120 ℃, temperature difference is usually small thickness-wise and successful lamination is easily attainable without pre-heating. On the other hand, for nylon-based binder with melting point of 150–200 ℃, higher laser power or slower laser scan is usually required to melt it, which leads to a large temperature difference thickness-wise and frequent appearance of warping as described above. An example of warping which was observed in a green body with nylon-based binder is shown in Fig. 7 (a). Pre-heating the mixed powder closely to the melting point is eective for avoiding such warping. Laser irradiation conditions including laser type, power, scan spacing, and scan speed should be carefully chosen depending on the types of the polymer binder and ceramic powder, etc. An issue often occurring during the lamination is sliding of a green body embedded in powder and formation of a gap, as shown in Fig. 7 (b). This sliding is Fig. 7 (a) Warping observed in a green body with nylon-based binder, (b) Sliding of a green body and formation of a gap, (c) Successful formation of a green body embedded in powder. The body is rectangular-shaped with 7 mm width and 50 mm length.(a)(b)(a)(c)(b)Fig. 6 Preliminary powder test for lamination, (a) Smooth surface with flowable powder, (b) Dimples and cracks with less owable powder

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