Acoustic control of powder dispensing in open tubes Shoufeng Yang, Julian R.G. Evans * Department of Materials, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK Received 27 November 2002; received in revised form 30 September 2003; accepted 20 October 2003 Abstract A multiple powder metering and dispensing device has been constructed to create functionally graded materials by the solid free-forming process known as selective laser sintering. The device makes use of vertical open tubes through which the flow rate of dry powder is controlled by the characteristics of acoustic vibration in the 20 – 200 Hz region. Flow is initiated by high amplitude ‘attack’ waveforms and stops when vibration ceases. This simple but effective device raises many questions about the factors that cause flow arrest. The effects of tube diameter, amplitude, mechanical damping and particle size distribution are investigated. By selecting optimum parameters for a metallic powder, the overflow (discharge from the open tube after silence) was brought below 1 mg. D 2003 Elsevier B.V. All rights reserved. Keywords: Powder flow; Solid freeforming; Powder metering 1. Introduction Solid free-forming (SFF) is defined as ‘the creation of a three dimensional shape by point, line or planar addition of material without confining surfaces other than a base’ [1]. Several computer controlled methods of solid free- forming have become established for metals and ceramics: selective laser sintering [2], stereolithography [3], laminat- ed object manufacture [4] and multi-layer printing [5,6]. In selective laser sintering, a layer of powder is placed on the building platform. The forming areas are scanned by a laser which consolidates the powder. The non-forming areas remain to support subsequent layers providing for overhangs and cavities. Second generation SFF techniques allow both composition and shape to be downloaded directly from a computer file so that functionally graded materials (FGM) can be assembled. Methods for multi- material deposition on the building platform are therefore needed. One dimensional FGM can be made by laminat- ing different tapes [7] or by depositing different layers of powder in 3D printing [8]. Stepless functional gradient can be assembled using direct ceramic ink-jet printing [9] and the equipment can easily be modified to deliver three dimensional gradients [10]. Such structures can be made by selective laser sintering (SLS) if it is possible to meter and blend metal powders before deposition on the building platform. There are several distinct methods of powder metering [11] and they include pneumatic aspiration–ejection meth- ods, plunger operated dispensers, automatic weighing devi- ces, augers and electrostatic dispensers. The method selected for this work has its origins in the sand paintings of the Navajo Indians [12] but with somewhat more instrumental control. It has the advantage of no relative moving parts that can both wear and seize up with fine powders. It offers both flow rate control [13] and switching solely by computer- controlled actuation in the form of acoustic vibration from an electromagnetic transducer. Although there is significant literature on vibratory dispensing in open belt conveyors, it is surprising that little attention has been given to the phenomena of flow control and switching in open capillaries in view of the technical potential in areas as diverse as pharmaceuticals, pigments dosing, powder metallurgy and ceramics. It also has the potential for both powder characterisation and quality con- trol procedures since flow and arrest behaviours are depen- dent on particle characteristics. In the studies that have been published, Staffa et al. [14] show that flow rate in a vibrating Hall flowmeter 0032-5910/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2003.10.008 * 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). www.elsevier.com/locate/powtec Powder Technology 139 (2004) 55– 60