Is ink heating a relevant concern in the High Speed Sintering process?

Abstract

High Speed Sintering (HSS) is a novel polymer additive manufacturing process which utilises inkjet printing of an infrared-absorbing pigment onto a heated polymer powder bed to create 2D cross-sections which can be selectively sintered using an infrared lamp. Understanding and improving the accuracy and repeatability of part manufacture by HSS are important, ongoing areas of research. In particular, the role of the ink is poorly understood; the inks typically used in HSS have not been optimised for it, and it is unknown whether they perform in a consistent manner in the process. Notably, the ambient temperature inside a HSS machine increases as a side effect of the sintering process, and the unintentional heating to which the ink is exposed is expected to cause changes in its fluid properties. However, neither the extent of ink heating during the HSS process nor the subsequent changes in its fluid properties have ever been investigated. Such investigation is important, since significant changes in ink properties at different temperatures would be expected to lead to inconsistent printing and subsequently variations in part accuracy and even the degree of sintering during a single build. For the first time, we have quantified the ink temperature rise caused by unintentional, ambient heating during the HSS process, and subsequently measured several of the ink’s fluid properties across the ink temperature range which is expected to be encountered in normal machine operation (25 to 45 ∘C). We observed only small changes in the ink’s density and surface tension due to this heating, but a significant drop (36%) in its viscosity was seen. By inspection of the ink’s Z number throughout printing, it is concluded that these changes would not be expected to change the manner in which droplets are delivered to the powder bed surface. In contrast, the viscosity decrease during printing is such that it is expected that the printed droplet sizes do change in a single build, which may indeed be a cause for concern with regard to the accuracy and repeatability of the inkjet printing used in HSS, and subsequently to the properties of the polymer parts obtained from the process.

Evaluation of Polyethylene Terephthalate Powder in High Speed Sintering

“…It can be assumed that this arose from the carrier component of the ink rather than from the carbon black. It is possible that the carrier liquid (petroleum distillates [ 15 ]) from the ink acted as a plasticiser lowering the T g and this caused more of the PET in the part to crystallise during the heating scan (this would be an exothermic event). Examining the DSC curve of a PA 12 part manufactured by HSS, an endothermic event around its T g could not be observed and it has not been reported in the literature.…”

“…The heat applied during the travelling sintering lamp’s pass causes a rise in the ambient temperature of the whole machine, including in the ink-jet print heads and associated ducting. Hence, during manufacturing, temperature gradients are set up and they vary with building time which can then lead to differences in the parts that are being built [ 14 , 15 ]. This would be a matter for consistency of product quality and would pose a problem for acceptance as a manufacturing method.…”

“…The second issue is a rise in the temperature of the ink. The ink used in HSS is a suspension of carbon black (RAM) in a mixture of petroleum distillates [ 15 ]. The ink is continually circulated through the print heads and droplets are delivered to the powder’s surface using a drop-on demand piezoelectric cartridge.…”

“…The ink is continually circulated through the print heads and droplets are delivered to the powder’s surface using a drop-on demand piezoelectric cartridge. Williams et al [ 15 ] investigated the effect of the ink’s temperature during printing on PA 12. This challenge has been surmounted in the voxeljet VX200 HSS machine as an ink temperature regulation system has been implemented.…”

“…They probed various aspects of HSS on part properties such as the amount of ink deposited [ 12 ], the effect of build orientation [ 13 ] and the effect of bed temperature and infrared lamp power on the mechanical properties of parts [ 14 ]. Moreover, the effect of heat on the ink’s viscosity and the possibility of part variation with manufacturing time [ 15 ], the spectral emission properties of different infrared lamps [ 16 ], thermal gradients in HSS and variation in the flatness of parts made by HSS compared with LS [ 17 , 18 ], and the effect of infrared lamp power and depth of sintering [ 19 ] were investigated. Most of the work on HSS was conducted with PA 12 (the same as with LS of thermoplastic powders).…”