Over the last several years, there has been a trend among oems toward making sub-tier suppliers more responsible for the cleanliness requirements of parts and assemblies. In addition to this, the very importance of part cleanliness is growing. Many producers that might once have dismissed cleaning, discover all too late that their highly engineered, closely toleranced devices can be rendered inoperable by a particle too tiny to be seen with the naked eye. For these various reasons, engineers and quality managers, who may not have considered the topic before, are now giving serious attention to cleaning. A look at some basic information on precision cleaning is given below.
Parts cleaning is a broad field. It ranges from gross cleaning of heavy industrial components to critical cleaning for the space program, with a wide spectrum of options in between. Many factors determine which cleaning processes best fit a particular need.
One basic question is:
Just how clean do your parts need to be, and who is going to determine this? In the event that the customer has imposed a cleanliness specification, then at least this offers a place to start - cleanliness is now a matter of conformity. But if not, then the shop will need someone experienced in selecting an appropriate cleaning process. A job shop specializing in this field may be appropriate here. Parts sent for evaluation can be tested to determine the existing level of cleanliness (or lack thereof). After an experimental cleaning run, the part can be tested to quantify the cleanliness level that results. The functionality of this clean part can then be tested in the application. If the performance is satisfactory, then this level of cleanliness, along with the methods used to obtain it, can provide a benchmark.
The next factors to be considered are the equipment and the process, factors that will ultimately be determined by the volume and the type of contamination, along with the material and complexity of the part. There are several options. For those hoping to perform cleaning in-house, most of these entail significant capital expense. Therefore, before going too far, it is always best to determine the nature of the pollutant to see if it can be eliminated from manufacturing. Even if it can, however, precision cleaning may still be necessary to ensure the smooth operation of a closely toleranced mechanical assembly.
Cleaning options include vapour degreasing, pressure washing, CO2 snow, and even mechanical blasting followed by a filtered rinse. These are common ways to achieve a high level of cleanliness. For removing the smallest and most tenacious particles, particularly on parts with complex geometries, another potentially effective option is ultrasonic cleaning. The frequency r this process is tailored to particular particles. (The rule is: the fine the particles, the higher the frequency.) However, lower frequencies can shatter delicate material, or (even worse) can introduce microcracks that escape detection, so this option isnt universally suitable.
Care after cleaning
After cleaning, dont underestimate the importance of the remaining processes. Rinsing, drying, preserving, and packaging all present their own grace. If you need spot-free parts, for example, you will need a deionised water system. Drying of the parts can be the most time consuming step of all. Options range from blowing down with shop air to vacuum bake-out ovens. One particularly convenient and cost-effective device is a portable dryer consisting of a small electric turbine producing heat via mechanical friction, with the intake filtered to three microns to ensure purity at the output end. The user simply blows the parts dry without shop air, and therefore without having to worry about recontamination resulting from a faulty oil trap on the shop air compressor.
What comes next is quickly becoming the prominent factor in much of the cleaning industry, and thats the quantitative verification of the cleaning process. There are two commonly accepted methods of obtaining cleanliness samples for this purpose. One involves spraying the test part with a sub-micron-filtered solvent from a pressurised spray can, collecting the seepage and running it through a vacuum filter funnel that deposits the debris onto a special test filter. The second method is to sonicate the parts in solvent, remove the parts from the bath and rinse them with additional solvent, collecting and filtering the effluent as in method one. The most commonly called-out particle analysis criteria are count, maximum particle size and weight-and its not uncommon to see a combination of two or even all three.
It would be a disaster to develop a process and have it installed, only to find out that the municipality wont issue a permit to run it. Questions to consider here include: What will you be removing from the parts, and what are you using to remove it? Do you need local, state or federal permits to store, use, and dispose of your chemistry? What will you do with your spent cleaning solutions and rinse water, both of which will contain the contaminants removed from the parts? Will one or both of them be reportable as hazardous in your waste stream? (It can be cost-effective to purchase an evaporator, by the way-which will greatly reduce the volume of wash and rinse waters). What about the effect of this chemistry on your employees-is it safe? What are the exposure limits? Some alkaline cleaners are highly caustic and can burn the skin and irritate the respiratory system with very limited exposure.
Dont fail to unclog tank
Sometimes the obvious is overlooked. That can be the case for metalworking fluids. Clean metalworking fluids perform better and last longer that dirties fluids. Unfortunately, thorough tank cleanout-a vital step in the process of recharging the tank on an individual machine or central system-is often neglected. A tank filled with contaminants such as metal swarf, oil, mould growth and pockets of bacteria will cause deterioration of the fresh fluid. Removing these impurities with a thorough cleanout procedure ensures that the fresh fluid can perform at peak performance levels.
Here is a ten-step procedure, in cases where complete tank drainage is possible.
· Drain the entire tank or central system
· Remove debris from sumps, return trenches, oil pans, and filtration units
· Fill the system with water (warm water is best) sufficient to circulate through all lines and machines
· Add a cleaner such as Cirnclean 30 (one part cleaner to 50 parts water concentration) and circulate for 2 to 8 hours. While the cleaner is circulating, brush all trenches and filter elements, and scrub machines and oil pans
· Steam clean areas where swarf and/or oil may not have been removed by the cleaner
· Drain the cleaner after cleaner circulation is completed
· Refill the system with enough water to circulate through to remove any remaining cleaner or swarf. If the rinse water is extremely dirty, rinse a second time. All lines and sumps should be drained at this time
· Charge the system with the required amount of water
· Add fresh metalworking fluid to the recommended concentration
· Circulate the metalworking fluid to ensure it is properly mixed before starting production. For precision grinding systems, continue circulation until the fresh fluid reaches room temperature
When a complete system drain is not possible, incorporating as many of these cleanout procedures as possible is recommended. To ensure long coolant life and effective performance, tank cleanout should not be sidestepped when recharging metalworking fluids.
|Posted : 9/3/2005|