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.
Highlights
High Speed Sintering (HSS) is a polymer additive manufacturing (AM) process which aims to increase production rates and reduce machine costs relative to better-established polymer AM technologies, such as laser sintering [13]
Data was taken from the temperature sensor in the ink return feed, i.e. after the ink has passed through the printheads in the warm build chamber
Using fitted equations describing the ink’s fluid properties as functions of temperature, the reciprocal Ohnesorge number, or Z number, of the ink was calculated as a function of temperature throughout the build, and it was shown to remain in the range over which stable droplet formation would be predicted
Summary
High Speed Sintering (HSS) is a polymer additive manufacturing (AM) process which aims to increase production rates and reduce machine costs relative to better-established polymer AM technologies, such as laser sintering [13]. In HSS, parts are built up by sequential deposition of a polymer powder layer, inkjet printing of a liquid ink containing an infrared-radiation-absorbing material (RAM) onto the powder bed in a desired crosssection, followed by passage of an infrared (IR) heating lamp over the powder bed to selectively sinter the printed shape (i.e. where the RAM has been deposited). The ink used in HSS is generally a suspension of carbon black particles (the RAM) in a mixture of petroleum distillates [5]. While some effort has been made to expand the suite of polymer materials which can be used in HSS [2, 3] little attention has been paid to the inks and their properties.
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