“We need the part to be clean.”
This is a common sentence often used by metal finishing managers when referring to their pretreatment cycle and while choosing a cleaner for their process.
But what is clean? And how do we define it? Can a metal be too clean?
From parts washing between assembly steps at large automotive manufacturers to clean assembly rooms to parts involved in paint and plating pretreatment, the word clean takes on several definitions and concepts.
This article will give you a clear answer on the following:
Let’s get down to it, what is clean?
A clean surface is a surface free of contaminates that directly affects the finish or the finishing operation.
In both rinsing and cleaning, it is important to use the cleanest and most economical possible water available to rinse with. This helps ensure that the surface is free from as many contaminates as possible before processing further.
It is rare for a surface to be considered “too clean”, but there are several “what if” questions if the surface is not clean enough.
Many quality issues can be avoided when a part is truly clean as it will be free of all contaminants. Too many shops accept unclean parts, not understanding that there could be long-term impacts on their finishes and/or process tanks.
The metal finishing industry is seeing changes in soils, product loads, and within the process before and after the cleaning steps. In many cases, finishers will utilize the same cleaner they’ve historically used without verifying that it is still sufficient to handle these changes. This passive decision may impact without considering the financial impact, increased usage, bath contamination, and increased rework.
In the metal finishing industry, cleanliness is a top priority and one that requires careful consideration.
Part washing is the first step in almost all metal finishing processes and if this step is not done properly and up to standards, the odds increase that there will be failures farther down the process.
Unclean parts can cause a variety of issues including:
When the final assembly fails, problems can include metal chips blocking orifices, coatings contaminating processes the parts may be used to transfer, contamination of other process tanks, and build up in coating tanks.
This can cause more problems in the future and could destroy plating baths, weaken plating efficiencies, and even shut down entire process lines, leading to dumping all tanks and power washing the line.
For decades, industries have just used cheap and simple Sodium Hydroxide (NaOH) solutions for cleaning, but we know that a more successful cleaning, though more expensive upfront, shows significantly improved finishing and could save you thousands of dollars.
Traditionally, the first and simplest sign to look for in a clean surface is a “water break-free” part emerging from the rinse steps after the soak and electro-cleaner.
A water break-free surface should have a coating of water adhering across the entire surface. Soil left on the surface, oil or paraffins in most cases, will often cause water to “break” or aggressively surround an area or spot on the surface part, exposing a bare surface without any water on the spot or break area.
The surface isn’t bare at all; a majority of the time this means there is a contaminate covering the surface at the break area. This can be observed by submerging a part in water before processing.
Observe the water on the surface of the part as you remove the part from the bucket. You may observe water breaking, beading, and, in extreme cases, even oil glistening on the surface of the part or an oil slick on the water.
Even if your parts are passable with water break present and don’t experience adhesion or appearance issues, your concerns are far from over. What long-term harm does soil accumulation have on the finish tank?
Soil accumulation in process tanks can lead to several problems in the long run: conductivity inefficiencies, increased proprietary additive usage, increased anode bag consumption, and other costly effects. This will cost you thousands of dollars per year if left unnoticed and unaddressed.
It is important to note that some surfaces require a sufficient and sometimes robust electro-cleaning step to become water break-free and effectively remove surface contamination.
Luckily, you can see this without doing a CSI style examination on what contaminates remain on the surface of parts post-soak cleaning. A simple lab procedure can typically determine if you need to remove the contaminate during the electro-cleaning step.
Do not confuse surface drying with water break, however. Surfaces drying can also cause finish issues. Remember our definition of clean states “contaminate”, not just specifically an oil or oxide, but the generic term “contaminate”.
The contaminate can come from the cleaning tank itself. Dried-on caustic can be a contaminate, leaving patterns in the finish and causing quality issues.
KEY INSIGHT: It is important to consider cleaning temperatures, exit rinses, seasonal changes, and other parameters in the pretreatment processes.
In addition, accumulation of the soil removed in the soak cleaner has been seen to contaminate the electro-cleaner if not properly removed by rinsing, skimming, or solution recirculation.
It is also important to note after acid pickling, the surface may give the appearance of water break after rinsing. This is not the water break we referred to in earlier process steps; this water break is more of a shedding of water from a clean surface, not the repelling of water from a contaminated surface. When you compare the two, they look very different from one another.
A more detailed study of this phenomenon should be conducted to determine if it is affecting your process. Analytical testing can help determine if the use of acid wetter, better rinsing, or other in-line adjustments prior to finishing will produce a cleaner surface.
There are several advanced methods for determining cleanliness. However, we find the most practical and telling sign in finishing operations will be observing, interpreting, and knowing the signs of a water break-free surface for your specific finishing operation.
By taking these steps and taking this advice, you can avoid spending thousands of dollars on painful testing, reworks, and tank dumps that could have been avoided, had you just read the sign.
To learn more about how to make sure your parts are clean, reach out to DuBois’ technical experts.
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