Common-Sense Critical Cleaning

Most of you are already doing critical cleaning, and a common-sense approach to cleaning can yield big dividends. Invest a few minutes to explore some ideas to optimize your industrial cleaning process.

In the broadest sense of the word, cleaning is the removal of soil. Soil is matter that is out of place. Soil may be introduced inadvertently as a product is assembled, or it may be a process material, such as a lubricant or a polishing compound, that has a useful purpose at a particular point in the process but must eventually be removed so that the product can function properly.

Critical cleaning or precision cleaning is sometimes described as cleaning parts that look pretty clean to start with. By that definition, cleaning as part of a silicon wafer fabrication process would be critical cleaning.

You probably do not run a wafer fab facility, but most of you probably do critical cleaning. Often the most critical cleaning steps occur in the shop, not in the cleanroom. At BFK Solutions LLC Independent Consultants, we consider critical cleaning to be any cleaning step that is crucial to achieve appropriate surface quality and surface function in a high-value product.

A high-value product does not necessarily mean an expensive product; it’s any product where, if the surface is not “just so,” the overall costs become high. Many manufacturers do critical cleaning rather than general cleaning. If your product has value to you or your customer, and if you need to achieve an appropriate surface quality, your business benefits by treating the product as one that requires critical cleaning.

Why Are You Cleaning?

Optimizing cleaning processes can be one of the easier and more cost-effective investments you can make to achieve a quality product. Cleaning, like any other step in manufacturing, should be value-added, and a good first step in making it so is to figure out why you’re cleaning.

Hopefully, it’s not because Mom said so—unless Mom owns your company. Too often, cleaning is treated as a marginal, unproductive process step. Cleaning equipment may be budgeted last, and new hires with little or no understanding of the process may be asked to manage and operate fairly sophisticated cleaning equipment.

If you think about it, a product has properties of materials of construction, configuration, and surface. Part of achieving the right surface for your product involves choosing the right cleaning process(es) and cleaning at the appropriate point(s) in the process.

One of the first things we do in evaluating a manufacturing process is to assess the value of a particular cleaning step. Try walking through your plant and looking at the entire manufacturing process flow, just as if you were an outside visitor. It may be apparent that a cleaning process step could be eliminated or that it should be moved to another place in the process. Of course, it is wise to test before making any drastic changes.

Keep in mind that more cleaning is not always better cleaning. A higher temperature, for example, is not always more effective. For example, with ultrasonics, there is an optimal temperature that depends on the cleaning chemistry. And as temperatures near the boiling point, the ultrasonic bubbles become filled with vapor. They therefore become “cushy,” so they do not implode effectively. This translates into less-than-optimal cleaning. Too much cleaning power can translate into product damage.

Clean Sooner, Not Later

While we advocate cleaning only where necessary, we are proponents of immediate cleaning. Procrastination can be costly.

In general, for both particles and thin films, leaving a soil on the part results in that soil becoming more adherent. Dried soil is difficult to remove. Think of that lasagna pan you left on the stove overnight! With heat and time, soils can caramelize; this makes them very difficult to remove. If several different manufacturing chemicals or products (potential soils) are used with no cleaning steps in between, the soil residues can actually interact, making them more difficult to clean.

Using a water-soluble lubricant or coolant may not be the solution. Semi-synthetic and synthetic lubricants contain additives that have to be cleaned for the proper functioning of the product. Even water-soluble metalworking fluids may become less soluble after exposure to the heat and force of typical machining operations, and they are apt to become even less soluble on standing.

Some manufacturers have adopted costly and effective cleaning systems, but they undermine the effectiveness of those systems by eliminating the immediate removal of soil using simple bench-top or hand-cleaning processes. Having small cleaning tanks in the metalworking area to immediately remove excess soil can expedite the entire cleaning and manufacturing process.

Of course, such simple cleaning processes must be thought through as well. The cleaning chemistry has to be effective. It must not damage the product. The cleaning chemistry can itself leave an appreciable, adherent residue. This means that it makes good technical and economic sense to select the right cleaning products for what is often termed initial, or handwipe, or benchtop cleaning. Select products geared for industrial use, and choose suppliers that provide a consistent product and that offer good product support.

Identify the Cleaning Process

The step in the manufacturing process that actually does the cleaning may not be identified as the cleaning step. Basically, cleaning consists of washing (sometimes also called cleaning), rinsing and drying steps. Cleaning, particularly washing and rinsing, involves chemistry, force, temperature, and time.

In more than one instance, our company has helped clients correct problems with surface finish defects by identifying a “hidden” cleaning process. For example, one company was searching for the source of finish defects that sporadically appeared on a range of painted and deposited finishes. When we looked at the entire molding, cleaning, and finishing process we saw a very large, well-functioning aqueous cleaning system. In the corner, we also found a series of tanks with an array of buffing wheels. Most of the product was subjected to this “rough buff” process after initial fabrication. After buffing, the product was placed in a tank of hot water containing an aqueous cleaning agent, where it was subjected to additional agitation. It was then immediately moved to a hot water rinse tank.

The rough buff looked suspiciously like a cleaning process, even though it was thought of as a polishing step. By tracing back through the records, it became apparent that product with acceptable finish all went through this “rough buff” step.

The polishing process was serving as a cleaning process. Moreover, it was done early on in manufacturing, so soils did not have the opportunity to become adherent. Since most of the product already went through the buffing process, adding the step for all product was the reasonable solution.

In other instances, buffing and polishing is performed without adequate rinsing. Particles of the media are retained on the product, and these particles become very adherent. In such cases, an extra, extensive cleaning step is needed to re-clean the polished product. If the cleaning step were performed immediately, this step could be avoided.

Plan and Budget

As previously mentioned, the cleaning process consists of wash, rinse, and dry steps. Washing removes the soil from the surface of the part. Rinsing removes residual cleaning agent; and it may also serve to continue the wash step. Drying removes the rinsing agent—typically water or water with a corrosion inhibitor.

Each step of the cleaning process requires specifically designed equipment, and you should plan and budget for each step. Consider the following questions:

Do you need a rinse step? If you’ve been washing without rinsing, perhaps because of corrosion concerns, it may be appropriate to re-evaluate the decision. If residue, including cleaning agent residue, can interfere with your product performance, adding appropriate rinsing with corrosion control may be the way to go. Sometimes more than one rinse step is needed.

Are you filtering and/or recycling? Sometimes, a small investment in filtration technology can greatly extend bath life and bath quality.

Do you need a drying step? Too often, particularly with aqueous processes, we ignore the drying part of the cleaning cycle. Without drying, parts can become recontaminated with particles and/or thin film. If you have complex parts with lots of blind holes or if your assemblies are made of mixed materials, including non-metals, drying can be essential to product to performance and quality; and drying can help avoid corrosion problems. It is possible to retrofit drying systems, but it is usually more efficient to design the dryer as part of the cleaning system.

Even with a drying system, large parts may need cool-down time to allow ready handling. Some of our clients find that a lower drying temperature is sufficient. If not, it may be possible to arrange the material handling system such that the transport time allows the parts to cool down.

More Considerations

Fixturing is another consideration, and fixtures are ideally designed to minimize re-racking. More than occasionally, we see assemblers carefully removing parts from one set of fixtures, setting them up in a second set of fixtures for cleaning, and then removing and refixturing for the third step.

By considering cleaning as part of the overall process, it is often possible to cut process time significantly. One company made a significant capital investment in automated, well-contained equipment. The cleaning cycle time increased by 25%, from 45 minutes to one hour. However, the new cleaning process was integrated into the overall build process in such a manner that overall process time decreased by 50%.

Look at the impact of changes in soils on the overall process. Metalworking fluids are constantly evolving. In some cases, with some synthetic lubricants, it may be desirable to eliminate a solvent cleaning step. Perhaps, after using the new fluid, you see more particle fines or adherent chips. Overall, the fluid may be desirable; but adding a benchtop step may expedite overall cleaning.

Be sure to involve shop-floor personnel in the cleaning evaluation and change process. One of the first things we do in visiting a new facility is to meet the people doing the actual work. An experienced operator will see process problems before anyone else; sometimes their eyes can even pick up contaminants a sophisticated analytical test misses. Experienced workers can contribute excellent ideas that make the new cleaning process a success.

What’s in Your Tank?

Here are some typical responses to the question of why a company selected a particular cleaning process or chemical.

• I have no idea; we’ve always done it that way.
• One of our customers told us to use that cleaning agent.
• The safety/environmental people said we had to.
• Corporate said we had to.
• The sales rep put together the process.
• Our consultant put together the process.

Too often, people have no idea what chemical is in their cleaning tank. Aqueous cleaning agents are complex formulations containing not only water, but also organic and inorganic chemicals. Given that most metalworking fluids are also mixtures of organics and inorganics, and remembering the adage “like dissolves like,” you get the most effective cleaning by matching the cleaning agent to the soil.

To sum up, you do not have to become an expert in solvency or in equipment design; but you do need to understand the cleanliness and surface quality requirements of your own product line in the context of your process. It is important to have vendors that will provide good product support; but it is unwise to simply adopt a suggested product without question. Make sure the cleaning chemistry is truly adapted to the materials you use. Confirm that the cleaning equipment is adapted to your product line. This may involve a small amount of testing and varying of time, temperature, force, and cleaning chemistry (including concentration).

A small investment in time can result in big process improvements.