Knowing your required cleaning quality requirements is fundamental to choosing the right cleaning agent.

To achieve optimal cleaning results, you need to consider two different kinds of soils:

1) particle contaminations, including their size, numbers and types and
2) the sort of filmy contaminations.

“Heavier” cleaning agents are effective in removing particulates that cannot be readily dissolved. They can literally “push” or ”float away” particulates and lift them off the surface. Aqueous cleaning agents have a density of approx. 1g/ml. Solvents may have a higher or lower density.

Filmy contaminations can affect the required surface energy of the metal surface, which varies depending on the industrial applications. For example, nitriding or crack detection requires a higher surface energy than standard coating or assembling. The required surface energy should therefore match the ability of the cleaning agent.

For precision cleaning where required surface energy can range anywhere from 38 mN/m to approx. 60 mN/m, both aqueous-based cleaning and solvents are capable of fulfilling the requirement.

Effective cleaning is based on the principle “Equal dissolves equal”. To achieve optimal metal cleaning results, the cleaning agent should be chemically similar to the contaminant.

There are polar (inorganic) and non-polar (organic) contaminations. For water-based (polar) types of contaminations such as coolant and lubricant emulsions as well as solids like chips, salt, residues of polishing pastes and particles, aqueous cleaners are typically the first choice.

These are available in neutral pH, alkaline and acidic formulations, and a huge variety of surfactants and other ingredients.

When removing mineral oil based, non-polar contaminations, such as machining oils, greases and waxes, solvent will commonly be the preferred cleaning agent.

Nowadays modified alcohols are also available; due to their both non-polar and polar properties, they are able to clean non-polar contaminations as well as certain polar contaminations. This type of solvents has a much broader application range compared to traditional non-polar solvents.

In water-based cleaning, cleaning agents which can be acidic, neutral or alkaline, is usually matched to specific metal types. Simultaneous cleaning of different metals can therefore be problematic and this can result in compatibility issues and in worst case: corrosion.

Sometimes reactive additives are added to water to modify metal surface in processes such as edging and pickling. However, at times such reactive additives can also act aggressively on certain metal surfaces where it is not intended.

Solvents generally have broad material compatibility, which means they are compatible with all kinds of metal, when used properly, making it a suitable option for universal cleaning.

If the component parts are tiny or have complex geometry or small crevices, solvent is often recommended due to its lower surface tension and viscosity which makes it easy to wet into and evaporate out of tight spaces.

With water-based cleaning, even tiny traces of residue moisture could give rise to issues such as challenges in subsequent production processes, corrosion, or growth of bacteria and related bioburden issues.

One commonly used indicator for rating the cleaning strength of a non-aqueous solution is the Kauri-Butanol (KB) value. It can range anywhere from very mild (10) to very strong (+1000). The higher the value, the better the relative cleaning power.

Nevertheless, a high KB value can also cause materials compatibility issues if the strength of the cleaning is proving too aggressive for certain materials. Please note that KB value is not applicable for water.

When cleaning geometrically complex components, both surface tension and viscosity of the cleaning agent should be considered.

Surface tension is defined as the attraction of molecules on a liquid’s surface. The higher the surface tension, the higher the tendency of liquid surfaces to shrink into the minimum surface areas possible.

Given that solvents have lower surface tension than water, they have greater ability to wet a surface (as well as evaporate out of tiny places) and can thus more effectively permeate tight clearance areas to remove soils.

Viscosity measures a liquid’s resistance to flow. The lower the viscosity, the better a liquid can flow around objects and get into (and out of) tight spaces more easily.

In water-based cleaning, significant energy is required for pre-treating and heating up cleaning water, operating high-pressure pumps, drying parts as well as treating wastewater.

Compared to the higher volatility of solvents, water is a slow-drying cleaner and requires more elaborated drying procedures aided by heaters or blowers. Water requires 10 times more of the latent heat of vaporization (2259 J/g) than that of solvents, which lie typically between 200-300 J/g.

Solvent vapor degreasing also requires energy to keep the entire operation under vacuum condition. As operating temperatures are reduced accordingly in a vacuum, the boiling point of solvent is also reduced, making evaporation quick and effective.

Vapor degreasing systems usually work vertically, compared to aqueous cleaning which often operates horizontally (a typical aqueous batch system may have one wash tank and 2-5 rinse tanks, with the number of rinse tanks depending upon the required cleaning quality). Water-based cleaning therefore tends to require a larger footprint (and more electricity to run). The humidity and heat generated by the aqueous cleaning system also needs to be counteracted by the air conditioning system, another energy cost factor that should be considered.

The capacity and cycle times of your parts cleaning method should match your throughput requirements.

Given that the purchase of any cleaning equipment is a long-term investment, you need to bear in mind your future growth. Because of the need for washing, rinsing and drying in aqueous cleaning, degreasing with solvent typically has shorter cleaning cycles in comparison.

Depending on the required cleanliness standards, additional mechanical actions or temperature treatment might be required in aqueous cleaning which can impact cycle time and throughput rate.

That said, with solvent cleaning, metal parts are cleaned in batches whereas inline aqueous cleaning allows the cleaning process to be integrated into the production line.

Since dirts and soils are emulsified and rinsed off in aqueous cleaning, aqueous baths that are not treated have to be replaced frequently.

In contrast, solvent can be recycled indefinitely via the distillation unit, which is built within the vapor degreasing system or as a separate additional system.

The continuous distillation and re-purification of the solvent significantly increases solvent lifespan and cuts down on waste volume. This again leads to reduced solvent consumption and solvent costs (therefore a lower cost per-cleaned-part).

If there is a large amount of oils to be cleaned off, the opportunity to separate solvent from oils (via distillation) can be a particular advantage, since solvent can be effectively recovered, leaving only very little solvent residues in the waste.

Generally, the higher the difference in boiling point/range between solvent and the oil, the better they can be separated in the distillation.

As an example, given that the boiling point of Perchloroethylene is significantly lower than that of oils (>250 °C), the solvent can be most effectively separated from oils in the distillation sump. Solvent content in the waste can be as low as just 1% (or even lower).

Basing your investment decision on the sole cost of the equipment or per-liter cost of the cleaning agent is a guaranteed recipe for distorted conclusion. To access the true economic costs of cleaning, you must look at both the acquisition costs and the ongoing costs.

Acquisition costs entail the price of the machine, installation, and retrofits. Operating costs include consumable solvents or detergents/additives, utility bills for electricity/water, process control and bath maintenance, floor space, waste disposal charges, not to mention labor costs in terms of hours and efforts in operation and process monitoring.

By taking into account all the cost parameters, a total cost-per-part cleaned can be devised, which is a much more meaningful and robust indicator of the cost efficiency of your parts cleaning system.

Comparison of Cost Factors

Whether it is aqueous or solvent cleaning, both processes require prudence and diligence from users in ensuring worker safety, environmental protection and regulatory compliance.

In aqueous cleaning, effluent treatment is a key issue. Although water tends to be associated with sustainability or “green”, it is a finite and precious resource. Certain water-based cleaners are also classified as more dangerous than specific solvent cleaners in their toxicological profile. Proper treatment of wastewater is therefore of crucial importance.

Even if the water-based cleaners are biodegradable, given that soils generally are not, the wastewater still needs to be treated and disposed properly, especially since it can contain anything from cleaning additives to oil, lubricants, fluxes, particulates etc.

Companies who fail to do so not only risk high penalties, worse still, their negligence can severely impact the ecological profile and the local communities.

Similarly, while solvent cleaning does not consume water, it still demands a well-defined solvent risk management approach across storage, handling and external recycling of used solvent/disposal of waste oils.

As solvents can give off volatile organic compounds (VOCs), the use of closed cleaning machines with inherent distillation unit, along with a closed-loop solvent delivery system, can significantly reduce air emissions and waste to an absolute minimum (solvent consumption is optimized simultaneously since less solvent is lost to the air and less waste is generated).

Modern closed cleaning machines are now engineered to avoid any interface between operator and solvent. In most cases the working chamber can only be opened when solvent volume falls below a defined limit. In addition, the vacuum operation model in closed machines also ensures that flammable solvents are safely handled.

Regardless of which cleaning method is chosen, there are risks involved. It is therefore vital to put in place adequate risk management policies where key aspects such as toxicological information, corrosivity, exposure risks, and flammability etc are carefully examined and considered.

Metal cleaning is more than simply achieving the desired cleanliness in a consistent and reliable manner. An effective cleaning process should fulfill the cleaning needs in the most economical way, where risks are properly managed and kept to a minimum, employee welfare and safety is looked after, and sustainability and regulatory compliance are well demonstrated.

Every industrial process is unique within its own specific context, so are the goals and the requirements. A cleaner that works excellently in one context may bring insufficient results in another, depending on the complexity of the parts, the contaminations, and the throughput requirements. Because of the many intertwined factors in play, it would be oversimplified to make an overall credible judgment of the superiority of one cleaning media over another.

No cleaning system is without environmental impact. They only manifest in different ways and we need to look at the big picture as a whole when considering various factors, such as the use of chemicals, water consumption, energy usage, recycling potential, waste handling and risk management approach etc. Ultimately, the goal should be to minimize emissions, pollution and waste as much as possible while still meeting the cleaning requirements.  

The performance of the solvent itself is just as crucial as the process and system within which it is used. Great technological advances in closed-loop solvent vapor degreaser are leading to environmentally sound and responsible cleaning options. Taking into account energy usage, solvent recyclability, waste treatment and disposal, as well as chemical impact on the environment, solvents when used in combination with modern closed cleaning equipment, can have an even better environmental profile than many other cleaning methods.

The investment in a proper cleaning set-up is a considerable financial commitment. It is also a decision with long-term impact. More than just a necessity, parts cleaning – when done properly – can deliver much more value than the mere technical function it fulfills, in terms of driving operational efficiency, savings and sustainability performance.