Correlations between powder wettability and part colour in the High Speed Sintering process

Abstract High Speed Sintering (HSS) is a powder bed fusion additive manufacturing technology that relies on inkjet printing of an infrared (IR) radiation absorbing material (RAM) onto a polymer powder bed in order to produce cross-sections which can be selectively sintered using an IR lamp. The RAM used in HSS is carbon black, which is suspended in a petroleum-based carrier fluid to form an ink. The use of carbon black in this way means that any parts currently produced by HSS are inevitably some shade of black or grey depending on the amount and distribution of RAM on the part surface. As well as affecting aesthetics, the effect of printing different densities of RAM has previously been shown to influence part sintering and subsequently mechanical properties, which demonstrates the importance of RAM distribution in determining the final properties of parts made using HSS. Anecdotally, it has also been observed that parts produced with the same type, amount and printed density of ink, but using different polymer powders, can be significantly different colours. This suggests that the wetting interaction between the ink and the polymer powder is also significant factor in determining the distribution of RAM on a part surface. However, to date there has been no investigation into the nature of this relationship. By measuring part colour, in this work we quantify differences in RAM distribution when different polymers are processed via HSS using the same printed density of ink, with parts made from commercially available powders of poly(propylene) (PP), pol(ether-b-amide) (PEBA), poly(styrene) (PS), poly(amide)-11 (PA11), and poly(amide)-12 (PA12) having Relative Luminance (i.e. brightness) of (5.3  ±  0.2)%, (5.5  ±  0.5)%, (12  ±  1)%, (15  ±  2)%, and (18  ±  1)%, respectively. The contact angles of the ink on the powder beds were measured in a simple model system, and these were found to be (46.1  ±  0.8)°, (80  ±  3)°, 82°, (90  ±  2)°, and (93  ±  2)° for PP, PEBA, PS, PA11, and PA12, respectively. With the exception of one anomalous data point for PEBA, it is demonstrated that part Relative Luminance can be correlated to contact angle, and that we can describe that relationship to good precision using a semi-empirical model. Knowledge about the effect of ink wettability on RAM distribution should prove useful for continuing the optimisation of the parts obtained by HSS.