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?????? Cleaning Vacuum Cycling Nucleation productionmachining.com :: 45 100 80 60 40 20 0 0 0.002 0.004 0.006 0.008 0.01 0.012 Inches from Solid Surface Percent Concentration of Bulk Fluid Boundary Layer Concentration Profile a solvent or an aqueous solution) the components to be cleaned. e reduced pressure results in the formation of vapor bubbles at the surface of parts, much like bubbles forming on the inside surface of a boiling pot of water. Typically, nucleation sites for bubble formation are found where imperfections, crevices or foreign particle material exist. Continuous evacuation of the chamber with a vacuum pump produces steady bubble formations on the solid surface; pulsing can also be applied to facilitate surface mass transfer. As opposed to conventional methods used to reduce the fluid boundary layer near a surface, VCN bubbles nucleate at the solid surface and grow inside this boundary layer, subsequently disrupting the boundary layer. e mecha- nisms involved with these growing bubbles result in the movement of the boundary layer fluid (containing product chemicals or material removed from the surface) out into the bulk fluid; when the bubble grows large enough, it detaches from the surface and creates a flow of bulk fluid back into the surface region. is type of forced convective mass transfer has been demonstrated to be up to an order of magnitude stronger than jetting or ultrasonic agitation. e low concentration of chemical in the bulk solution is brought to the nucleation site as a transiently "high" concentrated chemical that assists with surface mass transfer activity; as a result, any chemicals used in VCN are always maintained at a much lower concentra- tion than used in typical surface preparation methods. An example of this phenomenon is shown in Figure 2, where a low concentration of chemical in the bulk solution exists. As the bubble is formed and released, the chemical is convec- tively brought to the surface, resulting in a higher concentra- tion of chemical at the surface. Intricate parts typically found in the medical device industry have cavities or recesses that involve more compli- cated boundary layer issues and require even more bulk fluid agitation and higher chemical concentrations to achieve the desired results. Ultrasonics are used to penetrate small recess areas. e problem is that ultrasonics can only act on recesses near the surface, and internal surfaces never see sonic bubbles. e diffusion process into deep recesses is extremely slow, and complete cleaning or surface treatment is rarely accomplished. Unsatisfactory surface treatment, high product rejection rates, long processing times and increased liability risks can lead to expensive products. Tight areas are ideal nucleation points to grow VCN vapor bubbles. e high surface area to fluid volume ratio and absence of natural convection to cool the nucleation area help grow effective bubbles. e vapor bubbles force fluid from the part's internal structure, and when pressure is applied, new bulk fluid enters the part. Cycling of vacuum and pressure removes contaminant or delivers fresh chemical for treating an internal surface. :: Fig. 1: This example shows a typical concentration profile developed for a chemical reacting at the surface (zero concentraion at the surface). :: Fig. 2. The VCN process results in the formation of vapor bubbles at the solid part's surface where typically nucleation sites for bubble formation can be found in the form of imperfections, crevices or foreign particle material.