In this article, we will go over the six key factors to a successful cleaning process and give you some Pros & Cons to consider for each
By David Geis, Product Manager
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.
Keep in mind, there is no “one size fits all” option for using aqueous cleaners to clean parts during or after manufacturing or nondestructive testing.
You get to decide what is right for your application considering time constraints, equipment and personnel available, quality requirements, budget and maintenance to develop the right fit for you.
The six primary factors that affect the success of a cleaning process, and their interrelations, are commonly called The Cleaning Equation.
Agitation × Chemistry × Temperature × Time + Rinse + Dry = Clean
The key is balancing these factors to get the results you need.
1. Agitation
Spray and ultrasonic agitation will clean faster than simple immersion.
The physical action of moving the part in the cleaning solution (or sometimes moving the cleaning solution over the part) 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 labor, 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 labor 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 and specialty coatings or plating may be damaged
Higher equipment expense and maintenance requirements
2. Chemistry
The type of cleaner will affect how aggressively soil is removed from a part surface.
A cleaning solution can be very simple (soap and water) or very complex (aerospace qualified cleaners).
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 broad 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 (in the case of aqueous cleaners, the solvent is water).
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 a surface.
Builders are different chemicals that assist the surfactants in different ways, such as breaking down greases and oils, maintaining the pH level in the solution, 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, restricting or enhancing the formation of foam, increasing or decreasing viscosity, or even giving the solution a pleasant fragrance or color.
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 oxidize the metal surface of a part (i.e., rust) or even etch into the metal itself.
Caustic cleaners are effective against heavy grease and carbonized 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 a 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 very important 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 or damage to the part later on.
Different Types of Cleaner Chemistry
Use the least aggressive chemistry which will still get the part to the desired level of clean.
Neutral Aqueous Cleaners (pH 7-9)
Examples: Daraclean 212, Daraclean 235, Daraclean 236
Advantages
- Very free-rinsing, no residues
- Safer for operators
- Good for delicate alloys
Disadvantages
- 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)
Examples: Daraclean 282 and Daraclean 282GF
Advantages
- General, all-purpose cleaning
- Safe for use with multiple metals and alloys
- Low foam levels for spray equipment
Disadvantages
- Not as good for heavy carbonized soils
- Need to rinse well to prevent residues
High-Alkaline Aqueous Cleaners (pH 12-13)
Examples: Daraclean 200 and Daraclean 283
Advantages
- High strength for heavy grease and carbonized soils
- Effective at low concentrations
Disadvantages
- Can damage conversion coatings and soft metals
- Must rinse well to prevent corrosion and residues
Solvent Cleaners
Example: SKC-S
Advantages
- Excellent cleaner for oil and grease
- Leaves little to no residue without rinsing
- Fast and easy to use
Disadvantages
- Can be hazardous and bad for the environment
- Not effective on mineral scales
- Not recyclable or reusable
3. Temperature
Higher temperature will increase the efficiency of the cleaning solution.
In general, you get 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.
It is important to keep these things in mind when designing a cleaning process using high temperatures.
Advantages of Higher Cleaning Temperatures
- Better cleaning results, cleaning is faster and easier
- 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
4. Time
The time required to get parts clean can be minimized 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.
In an ideal world, the cleaning process would take as long as necessary to ensure the part is clean. However, in the real world, 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 labor costs, most companies will strive to decrease the overall production time for a part or finished good. Thus, there will always be pressure to minimize 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 (spray cabinet, soak tank, etc.), and the chemistry of the cleaning solution. The entire cleaning equation needs to be considered when addressing issues of this type.
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
5. Rinsing
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
6. Drying
Drying should not be neglected since water left on a part can cause corrosion or interfere with post-cleaning processes.
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 or cause long-term equipment corrosion.
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