While the basic idea behind water-based cleaning is simple, the details can seem complicated and overwhelming at the start. But this doesn’t need to be the case. There is no “one size fits all” option for using aqueous cleaners to clean parts during or after manufacturing or nondestructive testing. One gets to decide what is right for one’s application considering time constraints, equipment and personnel available, quality requirements, budget and maintenance to develop the right fit. The six primary factors that affect the success of a cleaning process and their inter-relations are commonly called ‘The cleaning equation’ – Agitation × Chemistry × Temperature × Time + Rinse + Dry = Clean. The key is balancing these factors to get the results one needs.
Spray and ultrasonic agitation will clean faster than simple immersion. The physical action of moving the part in the cleaning solution is called agitation. There are many ways to accomplish this, depending on the setup available, the arrangement and geometry of the part, and the type of soil being removed.
The agitation method is, typically, the first parameter of the cleaning equation that is fixed, since it involves the equipment and capital investment necessary to accomplish cleaning. Choosing one method of cleaning will dictate the constraints on time, chemistry and temperature.
The primary methods of agitation are hand wiping, spraying, soaking and ultrasonic. Soaking is the simplest and most basic means of agitation. Parts that need to be cleaned are simply placed in a bath of cleaning solution and allowed to dwell for long enough to remove any soils. While this method takes the least amount of labour, it also typically requires the most time of any cleaning methods.
Hand wiping of parts using a solvent and a rag or brush is the next simplest method. The solvent loosens soil and makes it easy to remove, while the physical act of wiping the part removes the soil. This method is the easiest to implement, but it can become costly in terms of labour-cost since an operator is doing all the work in removing the soil from the part.
Sprays are typically used in higher volume production lines where time is limited. Parts are placed in a chamber with an array of nozzles that spray cleaning solution. The action of the cleaning solution combines with the physical impact of the spray to effectively remove soils from the part surface. This method can be the fastest cleaning method, but it requires more equipment and engineering to implement in a shop. Ultrasonic cleaning uses sound waves to create microscopic bubbles in the cleaning solution, then collapsing them back on themselves. This creates a microscopic scrubbing action all over the surface of a part, that will break up and remove dirt. Costs for setting up and running ultrasonic cleaning tanks are typically higher than agitated tanks, but often produce better results with precision parts.
Advantages of increasing agitation
Moves cleaner to dirty areas
Breaks up the soil molecules
Moves soil away from parts
Increases wash efficiency, decreases wash time
Disadvantages of increasing agitation
Limited by the type of equipment available
Delicate parts & specialty coatings or plating may be damaged
Higher equipment expense and maintenance requirements
The type of cleaner will affect how aggressively soil is removed from a part surface. A cleaning solution can be very simple or very complex. The components used in a cleaner formulation depend greatly on the surface condition, the material of the part, the soil, the method of cleaning, and the next step the part will go through in manufacturing. In broader terms, the chemicals used in a cleaning solution include a base solvent, surfactants, builders and additives:
Solvents make up the bulk of the cleaning solution
Surfactants are used to ensure the part being cleaned is wetted by the solution, to pull soils into suspension, and to emulsify the soil so that it can be washed away from surface
Builders are different chemicals that assist the surfactants in different ways, such as breaking down greases and oils, isolating hard water minerals, or coagulating suspended soils for filtration and removal
Additives are chemicals that perform auxiliary functions in a cleaning solution, such as, providing corrosion protection, increasing or decreasing viscosity, or even giving the solution a pleasant fragrance or colour
When dealing with solid or caked-on soils, very aggressive chemistry is needed for effective cleaning. Acid cleaners are effective against mineral scale, but they have the potential to oxidise the metal surface of a part or even etch into the metal itself. Caustic cleaners are effective against heavy grease and carbonised soil, but they have the potential to strip off coatings and plating, or corrode softer metals.
It is important to understand the effect of aggressive chemicals on the surface and metallurgy of the part being cleaned to prevent damaging the part. Some cleaning solutions also deposit a film on the part being cleaned, especially when working at elevated temperatures. This can be very important to understand when a part is cleaned between manufacturing steps, since a residue film could potentially interfere with a future step in the manufacturing process. It is necessary to include a clean rinse at the end of the cleaning process to wash any remaining chemicals from the surface of the part to prevent unwanted interference to the part later on.
Different types of cleaner chemistry
Neutral aqueous cleaners (pH 7-9)
Safer for operators
Good for delicate alloys
Less cleaning power than more aggressive cleaners
More likely to foam, need to use de-foamer with spray equipment
Alkaline aqueous cleaners (pH 9-12)
General, all-purpose cleaning
Safe for use with multiple metals and alloys
Low foam levels for spray equipment
Not as good for heavy carbonised soils
Need to rinse well to prevent residues
High-alkaline aqueous cleaners (pH 12-13)
High strength for heavy grease and carbonised soils
Effective at low concentrations
Can damage conversion coatings and soft metals
Must rinse well to prevent corrosion and residues
Excellent cleaner for oil and grease
Leaves little to no residue without rinsing
Fast and easy to use
Can be hazardous and bad for the environment
Not effective on mineral scales
Not recyclable or reusable
Higher temperature will increase the efficiency of the cleaning solution. In general, one gets better results cleaning at higher temperatures than at lower temperatures. This is because nearly every step of the cleaning process happens faster and easier as temperature increases.
When cleaning is looked at from a chemical standpoint, all the reactions between the solution and the soil will occur at faster rates. Chemical reaction rate equations will show that the rate of cleaning is exponential with temperature, meaning that cleaning times can be lowered significantly if the temperature is raised.
Temperature affects the properties of soils as well. As temperature increases, the viscosity of a soil will decrease, allowing it to be penetrated more easily by aggressive cleaners and removed from the surface more easily by agitation. Oils will emulsify more easily as temperature goes up, and the suspended droplets will be smaller. On the other hand, increased temperature can be detrimental to a part surface. Higher temperatures mean more aggressive chemicals in the cleaning solution, allowing corrosion or etching to take place faster.
Additionally, high temperatures also increase the volatility of the cleaning solution, leading to evaporative losses. As the temperature goes above 125°F / 50°C, evaporation from a cleaning bath can be significant.
Advantages of higher cleaning temperatures
Better cleaning results
Increasing the bath temperature reduces foaming
Disadvantages of higher cleaning temperatures
Higher equipment costs, operating and maintenance costs
Has potential to harm the part surface by increasing the aggressiveness of the cleaner chemistry
High temperatures can increase residue if the bath evaporates when the part is removed from a hot bath
The time required to get parts clean can be minimised by adjusting the other factors in the cleaning equation. Results of the cleaning process depend greatly on the amount of time spent. Longer cleaning times typically mean a greater extent of cleaning. This can be understood in simple terms. The longer that a part is scrubbed or sprayed or soaked, the longer that the chemicals in the cleaner have to act on the soil, and the more soil will be removed.
However, there are often limitations to the time available for cleaning. Time limitations can be imposed in several ways. The most obvious limitation is imposed by the manufacturing process. A part in production can only spend so much time in each step without holding up the entire line, and cleaning is no exception. Since time spent in production translates directly to overhead and labour costs, most companies will strive to decrease the overall production time for a part or finished good. Thus, there will always be pressure to minimise the time spent cleaning parts.
Another important time limitation can be imposed by the details of the cleaning process itself. To remove stubborn or caked-on soils, cleaning solutions often must have aggressive chemistries or operate at elevated temperatures. These conditions can lead to erosion of delicate parts or corrosion on the part surfaces. When using aggressive cleaning solutions, a balance must be struck between effective soil removal and potential damage to the surface being cleaned. With certain cleaning processes and chemistries, there will be a maximum cleanliness that can be accomplished before soil is re-deposited on the surface. Several factors influence this limitation, including the amount of soil already suspended in the cleaning solution, the method of cleaning and the chemistry of the cleaning solution.
Advantages of increasing cleaning time
Chemistry has more time to break up soils for more effective cleaning
Advantages of decreasing cleaning time
Higher part volume through the cleaning process
Sufficient rinsing is needed to remove not just the soil, but also the cleaner to prevent residues. When using an aqueous cleaner and water-soluble chemistry, the proper rinse is critical to the final cleanliness of a part, for several reasons.
First, rinsing is the best way to physically remove soils from the surface of a part, particularly in soaking applications where there is little other agitation. In spray or wipe applications, rinsing is also important, especially when using chemicals that can corrode the part surface if left to dry.
Secondly, when cleaning a part in mid-production, the final rinse is critical to the condition of the surface of the part going into the next step. If the cleaner chemicals are not sufficiently rinsed away, they can dry on the part and impair any coatings, adhesives, plating, or other conditioning that the part must go through next. Lastly, sufficient rinsing in clean water will remove the chemicals, allowing the part to continue in the manufacturing process.
Advantages of rinsing
Removes cleaning residue, surface oil
Surface is prepared for next manufacturing step
Corrosion inhibitor can be added to rinse water to protect parts
Disadvantages of rinsing
Rinse water must be clean water
More than one rinse may be necessary
Tap water can leave water spots
Drying should not be neglected since water left on a part can cause corrosion. Drying the part surface is just as critical to a good cleaning process as the other steps, especially when the next step in the manufacturing process is NDT inspection.
With liquid penetrant inspection, any water remaining on the surface will interfere with the penetrant’s ability to get into surface discontinuities. A wet surface will negatively affect the sensitivity of penetrant inspections, so a heated dryer should be used.
Also, water remaining on the part surface can contaminate the next processing step, particularly with penetrants or magnetic particle oil baths like Carrier II, where water contamination can foul the equipment.
Advantages of drying
Prevents part corrosion since wet parts may rust
Eliminates water residue which would interfere with penetrant testing
Multiple drying options – air knife, air hose, oven (required before liquid penetrant inspections)
Disadvantages of drying
Higher equipment costs
Higher operating and maintenance costs
Long heat exposure in a dryer oven has the potential to harm coatings or delicate part surfaces