Chemical Engineering Science 60 (2005) 413 – 421 www.elsevier.com/locate/ces Acoustic initiation of powder flow in capillaries ShoufengYang, Julian R.G. Evans ∗ Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, UK Received 13 January 2004; received in revised form 1 May 2004; accepted 29 July 2004 Available online 30 September 2004 Abstract Second generation solid freeforming devices will have the capability to render both shape and the spatial arrangement of composition directly from a computer file. In order to build three-dimensional functional gradients in selective laser sintering it is necessary to have a computer-controlled mixing and dispensing system. We report such a system based on the horizontal acoustic vibration of vertical capillary tubes that provides both switching and flow rate control. An orchestra of such tubes can be constructed to deposit a multi- component system onto a building platform. Our concern is the initiation of flow. It requires an “attack” waveform to break the domes that provide flow arrest and release the potential energy of powder above the dome. The intensity of attack also influences the mass that flows out before a new dome forms, i.e., the response time of the valve. Neither the extension to vibration caused by ringing nor the wave amplitude account for the over-run of the valve. Much better correlation is obtained with acceleration and with calculated kinetic energy of horizontally vibrating particles.  2004 Elsevier Ltd. All rights reserved. Keywords: Powder flow control; Solid freeforming; Acoustic vibration 1. Introduction In solid freeforming procedures, a three dimensional shape is built by adding rather than abstracting material. Neither confining surfaces nor plastic deformation are used. In selective laser sintering (SLS), shown schematically in Fig. 1, the shape is built from layers of powder by com- puter control of the position and intensity of laser light which sinters, or more often melts, selected areas in each layer (Bourell et al., 1992; Conley and Marcus, 1997). The non-forming areas remain to support subsequent powder layers so that overhangs and re-entrant profiles can be constructed. In second generation solid freeforming, the spatial ar- rangement of composition will be held in the computer design file and delivered to manufacturing peripherals that can meter and dose a range of different materials thus ∗ Corresponding author. Tel.: +44-20-7882-5501; fax: +44-20-8981- 9804. E-mail address: j.r.g.evans@qmul.ac.uk (J.R.G. Evans). 0009-2509/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.07.124 providing three dimensional functional gradients. This is al- ready possible in ceramic ink-jet printing (Mott and Evans, 1999; Mohebi and Evans, 2002). In SLS, where the building platform is often preheated to reduce residual stress devel- opment, methods for metering and dosing of small quanti- ties of dry powder are needed so that each powder layer can be compositionally patterned. There are many mechanical devices to achieve this aim originating, inter alia, in the polymer processing, pharma- ceutical and food industries ( Yang and Evans, 2004c). They include suction tubes which aspirate a fixed volume of pow- der and then dispense on to the target, gravimetric dosers, various forms of auger, pneumatic dosers and even electro- static methods of delivering a 2D image. In the present work, a technique was selected that offered simplicity, computer control and freedom from tribological problems associated with seizure of moving parts. The powder is held in an open capillary and flows only when the capillary is subject to continuous wave vibration. This simple method originated with the sand paintings of the Navajo Indians (Pegna et al., 1999) but has been identified as an ideal approach to pow- der metering and dispensing partly because of the ease with