Picturing Fixturing In Parts Cleaning
The fixture and the process have to collaborate, not fight one another.
You are setting up a new cleaning process with the goals of better yield, higher throughput and lower costs. Tests are promising. The cleaning chemistry provides good solvency; the cleaning equipment provides the right level of force. Drying time is within range.
However, testing a few parts to do a process evaluation does not ensure successful critical cleaning during production.
Here are three portraits—based on actual case studies—that illustrate the importance of choosing the right fixtures.
Portrait 1 – Overloading: Visualize a state-of the-art, automated batch cleaner—one ultrasonic wash tank, three rinses (the first two ultrasonics) and a drying chamber. However, the parts fail cleanliness inspection. What do they do to solve this problem? They repeat the cleaning process over and over and over, until the parts finally pass inspection. We notice that the baskets are crammed full of tiny parts. The techs explain that there is no problem, because the fixture is designed to handle the weight. We explain that the fixture, not to mention the process, is overloaded in terms of cleaning capability. It is unrealistic to expect that ultrasonic cavitation could reach the parts in the center of the basket.
Overloading fixtures is a false economy. In addition to considering parts configuration, keep process flow in mind. If you are thinking of purchasing the smallest system with the minimum number of fixtures to keep up with the overall rate of production, keep in mind that spikes in day-to-day process flow and increases in product demand may cause problems.
Portrait 2 – Busy Work: For the pre-cleaning step, production technicians carefully position each part into a fixture. They run the automated process. Next, they take each part out of the fixture, then move each one into a different cleaning fixture. Next, perhaps they have to passivate. Guess what happens? More laborious repositioning and refixturing. Refixturing the parts increases process time and costs; and each time the parts are handled, the potential for product damage or recontamination increases. Granted, it is not always possible to use one fixture for the entire manufacturing process. However, fixturing for cleaning as part of the overall build process will help with streamlining and quality.
Portrait 3 – Creeping Contamination: A contamination problem is getting worse. Inspection reports an increase in medium to large metal particles. There is no obvious change in the product line, the supply chain, or the cleaning chemistry. The equipment is running properly. But we look at the fixtures and see discoloration and erosion. Choose rugged materials of construction, because the fixture has to withstand repeated exposure to cleaning conditions. Otherwise, it may degrade and actually contribute contaminants to the process.
These three scenarios, summarized from actual observations, show the importance of fixtures in manufacturing. To avoid problems, consider the fixture design while planning the process. Parts may be placed in fixtures individually, or simply piled into a basket. In either case, consider fixture design. A fixture that is primarily mesh with little solid metal is usually preferable to one that has fewer access holes.
With ultrasonic cleaning, there are more considerations. With the exception of a few very hard plastics, it is almost always better to use a metal fixture. Plastic fixtures or even plastic clips can dampen the ultrasonic action to the point that the ultrasonic cleaning action you carefully engineered into the process is non-existent. The wrong mesh size relative to the ultrasonic frequency can also result in dampening. A mesh fixture may not always be the best approach for your application. For smaller, specialized applications where several different cleaning agents are needed within the course of the day, the best approach may be to clean the parts in separate electropolished stainless steel or glass beakers that are placed in a larger ultrasonic bath.
Plan and invest in the right fixturing, whether you are doing aqueous, solvent-based or non-chemical processes. Choosing the right fixtures for the parts being cleaned can make the difference between great surface preparation and unacceptable yield or poor product performance.
The fixture and the process have to collaborate—not fight each other. Strategize to figure out your best solution. For example, a basket that rotates may expedite cleaning, but not if rotation damages the parts. The parts may need individual, secure placement, the equivalent of a restraint system for a roller coaster. If the cleaning equipment has only an overhead spray, and if the parts need to be cleaned on all sides, and if the soil is adherent, your only choice may be to run the parts through the system two or more times, each time repositioning the part. This is time consuming and operator-dependent, so the least expensive cleaning system in terms of capital investment may be costly in the long run.
Barbara Kanegsberg and Ed Kanegsberg Ph.D., consultants with BFK Solutions LLC, are leaders in critical/precision and industrial product cleaning. For questions or to receive their newsletter, contact: 310-349-3614, firstname.lastname@example.org
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