This fact sheet series is produced by the Minnesota Association of Metal Finishers & Minnesota Technical Assistance Program for metal fabricators and their platers.
Dried-on Process Fluids and Fluid Combinations
Fact Sheet #4
Films of dried-on process fluids interfere with the bonding of plating to part surfaces. Tough films are formed when some types of coolants, lubricants and rust preventatives dry on surfaces, or when a waterbased fluid is applied over an organic fluid already on the surface. These tough, transparent coatings are similar to varnish and are best removed by vigorous cleaning. Standard degreasing will not remove these tough coatings.
Lubricant Causes Plating Rejects
For many years a manufacturer was sending sheet-steel parts out for plating. Part designs had not changed significantly and processing methods, although more efficient and more precise, were very similar to past practices. Recently, the rate of rejects had increased. On occasion, tighter delivery deadlines were missed and their primary plater complained more about processing problems.
The manufacturer discussed the increasing problems with its plater and identified a number of possible causes and solutions. One common thread to the problems was the increasing numbers of parts that were not successfully cleaned in the plater's degreasing steps. The entire surface of many parts appeared to be coated with a thin oily slime which the plater's vapor degreaser seemed to dry on rather than clean off. The plater's subsequent alkaline and electrocleaning steps were not able to remove the coating either.
A surprising similarity was that all these parts had been processed with waterbased coolants. By themselves, coolants clean well in conventional alkaline cleaners. But, the problem was traced to using coolants over mineral oils applied at the steel mill to prevent corrosion. The mixture of the oil and the water-soluble coolant created a film that was much more difficult to remove than either material individually.
With the cause identified, the choices were to:
- avoid mixing oils and coolants on surfaces;
- improve cleaning at the manufacturer, or
- hand wipe the parts at the plater. The manufacturer determined the best solution was to remove mineral oils at its facility before using the waterbased fluids.
In-process cleaning involved extra processing but was accomplished with no additional labor. The operators were able to clean the part blanks in mineral spirits at the presses before loading them. The stamping coolants provided enough rust protection until plating and could be removed with little problem, as long as parts were not stored for extended periods.
Problem Sources and Solutions
The problem in the example above was applying a water-soluble coolant over the lubricating/rust preventative oil applied at the steel mill. The manufacturer knew the mill oil was there but disregarded it because it had always been there. Previously, they applied a cutting oil over the mill oil--two very similar chemicals. They switched to applying a waterbased fluid over an oil--two dissimilar chemicals. This problem may have been compounded by the fact that mill oils generally are on surfaces for long periods of time. And, the oil may have partially oxidized, picked up dirt or been affected by extreme temperature changes.
Dried-on process fluids can have similar problems when fluids are applied during product fabricating. When machining fluids remain on surfaces, the fatty chlorinated and sulfurized oils tend to oxidize slowly in the presence of air to form a varnish. Waterbased fluids leave their surfactants, chemical additives and sometimes oil on the parts.
These films cause problems when parts are going to be painted, plated or otherwise coated by materials requiring a good bond to the base metal. When parts will be coated, the best solution is to avoid the problem entirely--generally by cleaning before the problem becomes evident. The following suggestions may also help to avoid this type of problem in your shop.
Fluid Combinations Solutions
Remove mill oils before processing metal stock using other fluids. Mill oils tend to have ingredients that are waxy or easily oxidized into varnish-like coatings, making them difficult to remove. Machine parts could be dip rinsed in mineral spirits or a waterbased cleaner by operators as they load blanks into the machine tool. Sheet-metal parts may require a separate wash step.
Use mill stock that has minimal residual oil to minimize problems. At least one Minnesota company has success with aluminum sheet stock. This may also work with steel sheet stock.
Store metal stock carefully and purchase quantities based on use. Mill oils remain relatively easy to remove after being applied at the mill for up to six months under ideal storage conditions. Exposure to sunlight, extreme temperatures, moisture or dirt can greatly shorten safe storage time.
Remove the surface of metal stock during machining.
Fluid Selection Solutions
Use machine fluids which are similar to the mill fluids--oil over oil, waterbased over waterbased mill coatings--for all machining and forming operations whenever the mill finish remains and plating follows. This will increase the chance that the machine fluid will soften the mill coating.
Try evaporative lubricants that leave no new residue.
Use the same lubricant or coolant for all fabricating steps as much as possible.
Form metal parts without fluids.
- Studies at Michigan Technological University found that for aluminum machining (except boring holes) fluids did not significantly cool or lubricate. Fluids simply moved chips, which can be done instead by blowing air or vacuuming.
- A machine tool company has machined gray cast iron without cutting fluids. The keys were using: 1) high tool speeds, where chips remove the heat generated, and 2) tools that have a soft, glide coating.
Apply cutting fluids by drip, mist or through-the-tool to minimize the machine fluids present. Avoid flood application.
Use deionized water or at least softened water to mix coolant batches and makeup solutions. This avoids the build up of hard water salts as water evaporates over time and leaves behind minerals that were initially present. These salts can precipitate a hard scale onto surfaces. When combined with coolant ingredients, they can dry to a difficult inorganic/organic soil. The longer you try to make your coolant last, the more important it is to prepare solutions using deionized water.
In-Process Cleaning Solutions
Remove coolants and other process fluids as quickly as possible to avoid forming a tough, varnish-like coating which requires removal before metal finishing. Where possible, machine operators should dip rinse parts in water, a mild alkaline cleaner or mineral spirits during processing.
Maintain cleaning baths to control oil buildup. Oil can be removed from working waterbased cleaners by: a) skimmers or coalescer/skimmer combinations, if the cleaner is selected carefully; or b) ultrafiltration, in most cases. Look for cloudy solutions or oil films floating on the surface as indicators of contaminants. Try a two-stage dip wash where the second stage stays relatively clean and reduces the volume of solutions consumed.
If rust proofing is needed, check your rust preventive preference with your metal finisher.
Clean or rinse parts to remove or reduce the amount of surface contaminants after each fabricating step whenever possible.
Fatty oils react with air over time to leave a varnish-like coating. Try to minimize the fatty content of process oils. Look for mineral oils, "fully saturated" oils, paraffins or alkanes. Specifically try to avoid oils containing free fatty acids, like oleic acid or stearic acid, unless a strong alkaline cleaner can be used to remove them before they dry.
Mill oil is often a waxy or fatty oil that may be overlooked by a parts manufacturer. Shops rarely know or have control over the type of mill oil present. Be aware and make proper choices related to mill oil, like selecting the machine fluids to apply over it or the cleaning steps to remove it.
Heavily sulfurized or chlorinated oils oxidize easily to form a varnish-like coating and can corrode surfaces. Alternative extreme pressure ingredients exist. While they are often initially more expensive, they can generally be justified because they are easier to remove from surfaces and easier to dispose.
Reactive metals will cause fatty acids, as well as sulfurized and chlorinated oils, to dry on-- adhering firmly--over shorter periods than less reactive metals. Judge reactivity by the alloy's tendency to rust or corrode. Cast irons and low-carbon steels tend to be reactive.
Buffing compounds are generally a chemical mixture with both oily organic components and inorganic or solid components. Compounds with a stearate component are particularly difficult to remove.
- Clean off buffing compounds immediately after processing.
- Try liquid or greaseless compounds.
- Avoid overheating. Use care in selecting wheel lubricants.
Marking inks designed for layout are easily removed. Avoid permanent markers like Magic Markers or Sharpie pens.
Tape leaves adhesive residues as it ages. Cheaper masking tapes and strapping tapes with fibers tend to leave more residue. Alternatives to tape include plastic, shrink- or stretch-wrap films; twist ties; or electrical tie-wrap. If tape must be used to package metal parts, wrap parts in paper first or use a high quality masking tape specifically designed to leave no residues when it remains on surfaces for extended periods.
Walter Olson, Michigan Technological University.
 Tad Stein, Makino Inc., Mason, Ohio, 513.573.7458.
 Peter Bartos, Guehring Automation Inc., Sussex, Wisconsin, 414.246.4994.
Get it Plated Right Series
- Introduction: Cleaning and Design for Plating
- Fact Sheet #1: Cleaning Processes - Capabilities and Limitations
- Fact Sheet #2: Particulate Contamination on Part Surfaces - Eliminate to Reduce Plating Rejects
- Fact Sheet #3: Selecting Materials for Plated Parts
- Fact Sheet #4: Dried on Process Fluids and Fluid Combinations
- Fact Sheet #5: High Temperature Processing Burns on Soils
- Fact Sheet #6: Holes, Seams, Threads, Recesses and Tubing Assemblies
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