This fact sheet series is produced by the Minnesota Association of Metal Finishers & Minnesota Technical Assistance Program for metal fabricators and their platers.
Particulate Contamination on Part Surfaces: Eliminate to Reduce Plating Rejects
Fact Sheet #2
Particulate contamination generally makes the appearance of plated surfaces rough and uneven. It can also cause more serious localized defects by interfering with the bonding between the plate and base metal. Contamination can be caused if maintenance schedules are not followed, in-process cleaning is inadequate, or the electrical grounding of process equipment deteriorates. Particles can originate from sanding, grinding, machining, deburring, and buffing operations.
Straightliner Maintenance Lapse Increases Plating Costs by 40%
Late in the day-shift, a sheet metal stamper called its plater to schedule straightlined parts for zinc plating and a clear chromate finish. The 400, 9-inch-by-12-foot parts would arrive at 11 p.m. and be needed by 7 a.m. the next day. The job had been run successfully before so the price was quoted and the job scheduled for the third shift.
The third shift supervisor noticed that the parts were coated with a grimy film so he plated samples. The samples looked dull and cloudy and were fuzzy--much different than previous jobs. Metal finishers call this general fuzzy-like coating graphite.
Vapor degreasing removed some of the oily components of the graphite but caused the remaining coating to adhere more tightly. This coating proved impervious to the electrocleaning and acid cleaning steps.
The choices were to: 1) hand wipe each part at the cost of $0.50 each, and try to resolve the cost problems in the morning, or 2) ship the parts back to the fabricator in the morning for further cleaning (at perhaps a $0.50 per part), incurring extra transportation costs and missed delivery deadlines.
Because the plater had a good relationship with the sheet metal fabricator, they hand wiped and plated the parts. The job cost 40% more than quoted. But, if the dirty parts had been processed without manual cleaning, the job would have been defective. Rework at the plater to strip, clean and replate would cost 250% over the original quote and delivery deadlines would have been missed.
The source of the dirty parts was traced back to one of the fabricator's straightline sanding machines. Its wash solution had not been changed on schedule, causing it to contaminate parts with metal fines, abrasives and oil. Better maintenance procedures could have prevented a 40% surcharge.
Problem Sources and Solutions
While the problem encountered above was traced back to a lapse in maintenance procedures, a number of situations can cause particles to stick firmly to parts as they undergo further processing. In the following section, typical causes of particle contamination are listed and a number of possible solutions are proposed.
Vapor degreasers have been a primary precleaning step at metal finishing shops. They work very well at removing oils and other organic liquid films that readily dissolve in the solvent. However, it is a physically mild cleaning method which does not remove particles from surfaces well, unless the degreaser has been augmented with ultrasonics, boiling sump immersion or sprays. Even with augmentation, cleaning and plating become difficult when small particles adhere to surfaces.
Abrasive Equipment Maintenance Solutions
Wide-belt deburring machines tend to be troublesome sources of particle contamination when they are not maintained or correctly selected for the job. Polishing, grinding, vibratory deburring, and straightlining are other common sources of particulate contamination. The following steps can help prevent contaminants:
- Follow the equipment manufacturer's recommendationsfor equipment maintenance and select the appropriate abrasive media.
- Clean equipmentat least once a shift
using a coolant spray for wet sanders or a vacuum for
dry sanders. Clean:
- Conveyor belts. Look for belt discoloring or images transferring from the belt's texture onto parts.
- Hold-down rolls.
- Squeegee rolls.
- Clean heavily oiled parts before abrasive operations. Dry abrasive use is incompatible with wet or oily parts. Filtration systems on wet machines are not designed to remove oil. Oils will circulate and build up in the system and recoat parts. This may create a mixed soil that is difficult to remove (see fact sheet #4).
- Replace worn or foul abrasive belts and media.
- Change coolants and process fluids on abrasive equipment and clean out sumps when fluids appear discolored or no longer leave clean surfaces.
- Maintain equipment cleaning and
dust control features:
- Coolant flow on wide-belt sanders should be at least eight gallons per minute per foot of belt width, and sprayed on both the abrasive belt and the product. Plugged or damaged spray nozzles should be looked for and repaired.
- Dust exhaust on dry sanders should be at least 300 cfm per foot of belt width to remove particles from the work area. Remember that particles left in the work area shorten abrasive life as well as soil parts.
- Educate operators that their workmanship not only affects the reject rates from their own operation but, in many cases, also affects the reject rates of subsequent processing steps. Try to give operators feedback on how well their parts fare through the entire production process.
- Prevent metal fines from embedding into part surfaces of different metals or alloys. Change grinding and polishing wheels, and abrasive belts, when jobs change and different metals are used. Or, schedule jobs-to-be-plated after belt changes and less critical jobs for the end of a belts life. Longer-lived abrasive media need to be dedicated to specific metals.
Equipment Grounding/Magnetic Adhesion Solutions
Inadequate grounding can induce magnetic fields in both equipment and the parts being processed. Magnetized ferrous parts hold particles onto surfaces, preventing their removal by standard cleaning methods.
- Check the grounding of wire forming machines, wire spot welders, sanders and other equipment periodically with an ohmmeter. Measure the resistance from the work fixture, the equipment's metal frame or housing, and the equipment's neutral and ground connections to a known building ground connection.
- Correct grounding if readings exceed zero ohms. Most often, grounding problems are caused by loose electrical connections or broken wires, caused by equipment vibration. If vibration is excessive, work to identify and correct those causes.
- Demagnetize susceptible parts after using a magnetic fixture and after spot welding wire. Tool steels and cast iron alloys are the most susceptible to retaining induced magnetic fields. Determine if parts are magnetic by using a gauss meter. Parts with readings higher than one gauss should be demagnetized. Parts with readings above two gauss will be very difficult to chemically or mechanically clean. Some platers have demagnetizing equipment. Shops with magnetic particle inspection equipment can generally use it to demagnetize parts.
In-Process Cleaning Solutions
Some fatty oils, coolants and other waterbased process fluids dry to form a very tough coating which can glue particles onto surfaces. These coatings can be similar to varnishes and are best removed by vigorous chemical or abrasive stripping operations. Like cement on tools, organic soils and oils are easiest to remove when they are fresh; cleaning gets increasingly more difficult with time. Although the soils and oils generally take longer than cement to set up, they need to be removed as quickly as possible to avoid cleaning problems.
- Wash parts after grinding, straightlining, wet sanding and deburring. These operations commonly place small particles on surfaces. Where possible, machine operators should dip-rinse parts in plain water, a mild alkaline cleaner or mineral spirits while processing them.
- If rust proofing is needed, check your rust preventative preference with your metal finisher
- Maintain wash baths to control solids buildup. Look for particle contaminants on parts surfaces using a wipe test. To reduce the volume of solutions consumed, try: 1) using a two-stage dip where the second stage stays relatively clean; or 2) filtering the bath.
Where in-process cleaning is not possible, expedite shipping and finishing of jobs so the finishing shop an lean the parts before soils set up.
If finishing problems arise, be prepared to discuss manufacturing processes with the metal finishing shop. Particularly important will be changes in:
- Process fluid brands. Also, be careful of "new and improved" formulations of your current brand.
- Fluid suppliers.
- Fluid preparation and maintenance procedures. Changing fluid concentrations, biocides or the method of coolant recycle can occasionally cause problems even if the change is within the manufacturer's recommendations.
- Operators. Especially if they are allowed to choose or modify the fluids they use.
- Feed rates or cutting and processing speeds.
- Metal suppliers or alloys.
When a problem occurs, consider all possible sources. Many of the changes described earlier will seem insignificant and most of the time they will be. Good metal finishers will review their shops for similar changes and problem sources. The objective is to find an economical solution to the problem as quickly as possible and implement it regardless of where the implementation occurs.
We would like to thank Timesavers Inc, Minneapolis, Minnesota for their recommendations on maintaining abrasive equipment particles from the work area.
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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