This fact sheet series is produced by the Minnesota Association of Metal Finishers & Minnesota Technical Assistance Program for metal fabricators and their platers.
Cleaning Processes: Capabilities and Limits
Fact Sheet #1
Metal finishers clean all parts before plating. Clean part surfaces are crucial to producing a uniform, defect-free coating that meets specifications. Surfaces must be microscopically clean to insure a molecular bond between the base metal and the plated deposit. When cleaning is inadequate the plated surface is likely to be defective.
Precleaning Changes Save the Job
A large manufacturer of kitchen cooking utensils, like spatulas and spoons, had their parts nickel-chrome plated by a job shop. Parts were formed from sheet-metal and deburred in very large manufacturing runs. The plater started the cleaning process with trichloroethylene vapor degreasing to remove most of the mixture of cutting, stamping and forming oils found on the surface. Then parts were plated on an automatic line that included an alkaline soak, electrocleaning step, acid pickling bath, followed by nickel and chromium plating baths.
At first the process produced a smooth, reflective bright nickel-chrome finish with few defects. However, after a time, an irregular line began to show through the plate. This cosmetic defect caused a 50% rejection rate. Because of the quantity of parts on hand, an improved cleaning method for the remaining unprocessed parts had to be identified.
A dried on mixture of machine fluids and deburring solids was the suspected cause. Because vapor degreasing has been known to cook soils on to surfaces, the degreasing step was omitted for a short trial run. This yielded a minor, but promising, improvement. After several attempts, a revised procedure using a hot, concentrated caustic soak for an extended period was implemented in place of vapor degreasing. The lines did not recur.
For this large job, setting up a customized cleaning step was effective and economical. But this incident convinced the manufacturer to 1) minimize the time that parts sit in inventory--where soils have the chance to dry on, and 2) try to send a cleaner part for plating.
Cleaning Issues and Choices
Soils on parts can generally be placed in one of three categories:
- Organic soils (lubricants, oils, grease, resins, fluxes and polymers) tend to be spread in thin, barely visible films over parts surfaces.
- Inorganic soils (heat and weld scales, carbon, dusts, rust and other oxides) form or settle on surfaces exposed to air, sometimes at high temperatures.
- Mix of organic and inorganic soils (buffing compounds, fingerprints, shop soils and some drawing compounds) can be among the most difficult contaminants to remove.
There is no universal cleaner for all these soils--rather there is generally a best formula for each type. Typical types of cleaners include: solvents, emulsions, waterbased cleaners (acid, neutral or alkaline).
Selecting the proper cleaner is necessary to remove specific soils efficiently and effectively. Carefully consider the following factors:
- Soil to be removed.
- Metals processed.
- Surface condition or properties needed for subsequent operations or the end product (cleanliness, texture, hardness, etc.).
- Effect of previous processes on the metal and soils.
- Part size and surface area.
- Susceptibility to corroding.
- Operating and investment costs.
- Environmental impacts and regulations.
- Materials handling methods and limits.
- Capabilities of existing facilities.
Plating lines have cleaning steps that are optimized for the types of parts and soils generally processed, and that are compatible with subsequent plating chemistries and the wastewater treatment system.
Chlorinated solvents have been widely used as a first cleaning step because of their ability to dissolve most organic soils and remove heavy films of oil and grease. However, increasingly stringent regulations are pushing industry to look at other cleaning systems where possible.
Alkaline cleaning has been a mainstay of platers' final cleaning steps to expose and activate the metal surface. Nearly every plating line has an alkaline cleaning step. Most metal finishers also have additional off-line cleaning capabilities not tied to a dedicated plating line. Off-line processes typically require more labor and can include degreasing, hand wiping, pickling, stripping baths and occasionally polishing or abrasive blasting.
In some cases, metal finishers can modify cleaning processes and procedures to solve difficult problems. They may change cleaner formulas, concentrations, bath temperatures, soak times or how vigorously the cleaner is applied. Generally these will increase plating charges and turnaround time. In extreme cases, part dimensions, surface texture and surface properties will be affected by nonstandard cleaning methods. For other difficult cleaning problems, modifying fabricating processes makes more sense so that the amount of contaminants is reduced or the difficult soil is eliminated.
Metal finishers generally have a wide range of cleaning capabilities. But, few of these cleaning methods are economical when used on a regular basis. Using common metal working fluids and fabricating techniques, and sending out the cleanest parts possible, allows platers to use standard cleaning methods at their end. Identify and solve difficult cleaning problems through cooperative efforts. It's in everyone's interest--saving headaches, time and money.
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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