Elsevier

Powder Technology

Volume 301, November 2016, Pages 1270-1274
Powder Technology

Dust reduction in abrasive jet micro-machining using liquid films

https://doi.org/10.1016/j.powtec.2016.08.002Get rights and content

Highlights

  • A novel concept of covering the target with a liquid film in AJM is proposed.

  • Water, glycerin and a polymer solution were used to improve process cleanliness.

  • The percentage of trapped erodent increased with increasing initial film thickness.

  • The characteristics of machined channels did not change when water was used.

  • With glycerin the erosion rate and roughness decreased and waviness increased.

Abstract

Abrasive jet micro-machining (AJM) uses a high-velocity particle jet to erode features in target substrates for a variety of applications, including micro-electro-mechanical and micro-fluidic device fabrication. AJM can result in a dusty environment due primarily to airborne, rebounding abrasive particles that eventually settle. This paper proposes a novel concept of covering the target with a layer of liquid in order to improve the process cleanliness. Films of water, glycerin, and a polymer solution were used to investigate the effect of liquid viscosity and film thickness on the percentage of captured particles, and also on the depth, width, erosion rate, roughness, and waviness of abrasive jet micro-machined channels.

The glycerin film captured up to 61% of the rebounding particles during the machining of micro-channels. The channel depth, width, erosion rate, and roughness decreased, and the channel centreline waviness increased. Films of the long-chain polymer solution and of pure water absorbed up to 42% and 36%, respectively, of the rebounding particles, while not significantly changing the channel depth, width, roughness, and waviness. For all liquids, the percentage of trapped particles increased with increasing film thickness. The results showed that AJM with the target covered by a thin liquid film is a viable way of increasing process cleanliness by decreasing the amount of airborne particulates.

Introduction

Abrasive jet micro-machining (AJM) uses an air jet to accelerate solid erodent particles toward the target surface at velocities reaching 300 m/s [1], [2]. It has been used to make micro-electro-mechanical devices such as inertial sensors [3], electronic devices [4], and micro-fluidic components for capillary electrophoresis chips [5], [6] and biochemical separations [3]. Normally AJM is performed inside a blasting chamber which is connected to a dust collector in order to avoid contamination of the air and surrounding equipment due to airborne particles. The use of ever smaller abrasive particles in order to reduce the size of machined features, leads to longer particle settling times and potentially larger areas of contamination according to Stokes' law.

Operators normally wear particulate respirators in order to avoid lung irritation [7]. Mitchell et al. [8] reported that exposure duration, particle size, and dust concentration were all relevant to the development of pulmonary fibrosis caused by exposure to very fine aluminum powder. Occupational limits exist in several countries for exposures to aluminum dust and aluminum oxide. For example, the legal airborne permissible exposure limit to aluminum powder required by the U.S. Occupational Safety and Health Administration (OSHA) is 5 mg/m3 as respirable dust, and 15 mg/m3 as total dust averaged over an 8-hour work shift [9]. It is therefore of great interest to limit the concentration of airborne particles generated during the AJM processes.

The introduction of a liquid film on a target surface can trap debris, but can also alter material removal mechanisms and the characteristics of a machined surface. For example, Ren et al. [10] increased the rate of laser ablation of silicon by using a continuously-sprayed water layer to remove debris. Similarly, Bärsch et al. [11] found that a liquid layer on tetragonal zirconia during laser processing trapped nanoparticulate matter while enhancing ablation rates and improving the quality of the laser micromachining.

High-pressure (100–300 MPa) abrasive water-jet machining (AWJM) can be performed with both the nozzle tip and the workpiece submerged in water in order to reduce noise and contain debris and abrasive dust, as well as to modify the geometry of the machined features. For example, Haghbin et al. [12] found that the channels machined in stainless steel and aluminum using the unsubmerged jet were significantly wider than those machined with the submerged jet, because of increased drag at the jet periphery.

The role of fluid viscosity in erosive jet micro-machining was examined by Kowsari et al. [13] who compared the cross-sectional profiles of channels machined with low-pressure (2–14 MPa) abrasive slurry jets of alumina in mixtures of water and glycerin. It was observed that a 10% increase in the slurry viscosity (by adding glycerin to the water) resulted in a 19% reduction in channel depth. This was attributed to two factors: a) the abrasive particles had a greater tendency to follow the fluid streamlines rather than collide directly with the target, consistent with the increase in the momentum equilibration number caused by the increased viscosity [14]; and b) the impact velocity was decreased by the greater viscous drag on the particles in the stagnation zone [14].

The erosion rate of brittle materials is determined by the particle kinetic energy, particle size and the relative hardness and toughness of the erodents and target [1], [15]. The local impact angle of the particles also affects the erosion with more perpendicular impacts increasing the erosion rate of brittle materials [16], [17]. This is because increases in kinetic energy normal to the target surface lead to longer cracks beneath the impact site which serve to create larger removed chips [18]. One of the main differences between solid particle erosion in gas and liquid jets is the local impact angle of the particle strikes. When particles are carried in air, the local impact angle approximately equals the jet impingement angle [19]. However, as various CFD studies have shown, in slurry flows with low Stokes numbers, large stagnation zones can form at the surface, and the particles tend to follow the fluid streamlines leading to a much wider range of local impact angles [19], [20].

The present work investigated the effectiveness of films of water, glycerin and a polymer solution in trapping erodent particles during the AJM of channels in borosilicate glass, and measured the corresponding changes in the channel depth, width, erosion rate, roughness, and waviness.

Section snippets

Machining of channels in glass

The experiments were conducted using an AccuFlo air abrasive blaster from Comco Inc. (Burbank, CA, USA) with a blasting nozzle having an inner diameter of 0.76 mm which was held stationary at a nozzle-to-surface centerline stand-off distance of 20 mm. The target material was 3 mm thick Borofloat® 33 borosilicate glass (Schott Inc., NY, US) cut into 10 cm × 5 cm plates. The glass samples were attached to the bottom of an aluminum container (10.2 × 5.2 × 4 cm deep). Water, glycerin, or a 50 wppm (weight part

Film thickness during blasting

Fig. 3a illustrates the cross-sectional profiles of glycerin with initial thicknesses of 2, 4, and 6 mm in the vicinity of the impacting air jet (with no abrasives) as measured using a scan of the laser displacement sensor through the center of the circular jet footprint. The diameter of the jet footprint on the target surface decreased as the thickness of the liquid layer increased (Fig. 3a). Fig. 3b compares the profiles of water, glycerin, and the polymer solution with an initial thickness of

Conclusions

The novel concept of the abrasive jet machining of a submerged target was found to reduce airborne dust by about 61%, 42% and 36%, respectively, when blasting through glycerin, an aqueous solution of a long-chain polymer, and pure water. The liquids were selected to illustrate the effect of viscosity, elasticity and liquid film thickness on the percentage of captured particles, channel depth, width, erosion rate, roughness, and waviness.

It was observed that glycerin absorbed more particles than

Acknowledgments

The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada (grant number: STPGP 364911-08), the Canada Research Chairs Program, Micralyne Inc. and BIC Fuel Cells for their financial and technical support. We are grateful to Kavin Kowsari for preparing the polymer solution and his contributions to our understanding of the behavior of polymer solutions. The authors also thank Bjorn Michaelson for his assistance on conducting the experiments.

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    Both authors contributed equally to this work.

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