C4: Critical cleaning for contamination control
Clean manufacturing involves cleaning surfaces of particles, NVR, micro-organisms, fibers, and other undesirables. Last month’s column investigated the phenomena of boundary layers and explained the fact that the velocity of fluid flow is zero at each surface containing the flow. This means that there is a velocity gradient from the wall to the center of the flow channel. Thus, the fluid velocity is at its lowest nearest the surface being cleaned. Naturally, fluid momentum (mass times velocity), is also minimized.
A consequence of zero flow rate at the surfaces to be cleaned is that it is difficult to make rinse fluid effect an aggressive scrubbing action on the surface from which particles are to be removed. The nearby fluid is moving too slowly to transfer significant momentum to particles and remove them from the wall surface. In this continuation of the discussion, options will be considered when faced with this problem.
What Are The Choices?
More flow. What if a higher flow rate is used? That does shrink the boundary layer over a broad range of flow rates, as Figure 2 shows. But the water demand would drown a fish! Re member, all this water must be as clean as the surface to be produced.
Move the supply. Notice that the boundary layer is least thick at the point where the flow starts. What if the flow supply along the part were periodically stopped and started? That would have the effect of creating a new boundary layer that is significantly less thick. This scheme would be difficult to implement. But note in Figure 3 how the thinner boundary layer would allow more access to particles.
Direct impingement. This approach could be practical, depending upon the rigidity of the part surface. In this instance, the horizontal flow of the boundary layer is penetrated with fluid jets. The questions that arise using this method are whether the part, which is often frail, can stand impingement of the fluid jet. And can the part surface, which is often polished, stand the bombardment of fluid containing particles previously removed?
Megasonics (high-frequency pressure waves). While the “no-velocity at the wall” rule can't be changed, a horizontal flow field with a fluctuating velocity could be used. This would be pressure surges and declines produced by megasonic transducers. Figure 4 is a conceptual drawing of this. It shows the influence on the boundary layer of pressure waves produced by megasonic transducers. Scientific opinion is, however, mixed about the exact mechanisms involved. The flow eddies created by the roughly 250 kHz megasonic waves do penetrate the boundary layer. Remember, pressure waves produced by megasonic transducers do not cause cavitation as do pressure waves produced by ultrasonic transducers.
Clean manufacture involves coping with boundary layers. They can hide submicron particles and they limit the available force to dislodge particles. Mega sonics is one of a few effective options.