Reflectance and transmittance of TrueForm TM powder and its composites to CO 2 laser K.M. Fan, K.W. Wong and W.L. Cheung Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China, and I. Gibson Department of Mechanical Engineering, National University of Singapore, Singapore Abstract Purpose – The purpose of this paper is to report on a study of the effect of glass microsphere (GMS) and potassium bromide (KBr) powder as an additive on the reflectance and transmittance of TrueForm TM acrylic-styrene co-polymer (TF) powder to CO 2 laser during selective laser sintering (SLS). Design/methodology/approach – GMSs and KBr powder were chosen because glass is opaque to CO 2 laser while KBr is transparent. The GMSs were treated with silane coupling agent and hydrofluoric acid to study the surface effect on the optical properties of TF/GMS blends. KBr powder was blend with TF powder in an attempt to modify the penetration depth of the laser in the powder bed. An integrating sphere was used to measure the reflectance of the powder bed. In the measurement of transmittance, a power meter was placed below the powder layer, which was supported by a KCl disc, to register the transmitted laser energy through the powder layer. Findings – For the TF/GMS blends, smaller GMSs gave a higher reflectance while the surface treatments had little effect. The transmittance of both the polymer and the blends were very low. Although bulk KBr is highly transparent to CO 2 laser, adding 30 vol% of KBr powder to TF hardly increased the transmittance of the powder bed. Research limitations/implications – Experiments were carried out on a modified laser engraving machine rather than a commercial SLS machine. The laser energy density used was lower than that for normal SLS processes and no significant changes of physical condition of the powder bed were inflicted. The results only indicate the optical properties in the initial state. Practical implications – The effects of transparent and non-transparent fillers on the optical properties of the powder bed are presented. Originality/value – This work furthers the understanding of heat absorption behavior of the powder bed during SLS. Keywords Powders, Composite materials, Lasers Paper type Research paper Introduction During selective laser sintering (SLS), the optical properties of the powder material will influence the heat transfer within the powder bed and hence its fusion behavior. When the laser beam strikes the powder bed, part of the laser energy is reflected and the rest absorbed by it. The transmittance of the powder affects the energy distribution within the powder bed. For powders with a low transmittance, most energy will be absorbed near the surface and the thermal gradient along the z-axis, into the powder, is large. It was reported that the maximum absorption takes place near, but not exactly on, the powder bed surface (Laoui et al., 2000; Wang et al., 2002). The depth of this shift depends on powder characteristics and laser source. The absorption depths in dense materials, except for transparent materials, are rather small, typically of the order of 10 nm-1 mm over the entire range of laser wavelengths of interest (Tolochko et al., 2000). In powders, however, the inter-particle spaces may facilitate penetration of the laser beam. The powder absorptance is dependent on the wavelength used and it changes with time as a result of changes in the thermo-physical properties of the powder bed. The absorption lengths in polymers change significantly with temperature (Keller et al., 1998). The penetration depth of a laser beam into a powder system is also related to the reflectivity of the powder material. Wang and Kruth (2000) developed an analytical ray-tracing model to study the energy absorption in SLS of metal powders. In their simulation, each ray has a certain amount of energy and at each impingement on a particle, part of the energy is absorbed and the rest is reflected. The reflected ray continues to impinge on another particle further down the powder bed and attenuates through the depth. The model yields better results when including specular reflectivity of the powder particles than when using purely diffuse reflectivity. A low penetration depth will result in most of the laser energy being absorbed near the surface and a large thermal gradient exists through the layer thickness. This may induce curling and distortion of the sintered part. In contrast, if the penetration depth is much greater than the powder layer thickness, some The current issue and full text archive of this journal is available at www.emeraldinsight.com/1355-2546.htm Rapid Prototyping Journal 13/3 (2007) 175–181 q Emerald Group Publishing Limited [ISSN 1355-2546] [DOI 10.1108/13552540710750924] This project was supported by a CERG grant of Hong Kong Research Grants Council (Project code HKU 7187/03E). Received: 1 July 2006 Revised: 7 March 2007 Accepted: 7 March 2007 175