Residue is the enemy of a properly-functioning product. Manufacturers demand a “perfectly clean” surface. We are experts in critical cleaning – but critical cleaning is not the same as achieving zero residue. Rather than aiming at a perfectly clean surface, aim for the right cleanliness relative to subsequent storage, assembly and/or coating processes and to the performance requirements of the final product. A perfectly clean surface is unrealistic; and it may be undesirable. Rust preventative, for example, is a purposeful, desirable residue.

Inadvertent residue
Soil may be deposited on product surfaces inadvertently. For example, particles or oil mists from a neighboring facility can waft through air ducts and deposit on surfaces. Handling product without wearing gloves or gowns causes fingerprints and skin flakes to deposit. With careful engineering controls and employee practices, it is possible to minimize these inadvertent contaminations.

The entire cleanroom industry is built around avoiding inadvertent contamination by the environment, by processes, and by people. Cleanrooms are an essential part of high-value manufacturing. However, a cleanroom will not remove contamination already on the part. As our colleague Scott Mackler advises, all a cleanroom can do is slow down the rate of product contamination.

Semi-avoidable residue
Other residue arises from the manufacturing process itself. Surface residue can come from process fluids such as metalworking fluids. Dicing processes generate particles. Polishing compounds contain particles. These particles contribute to surface residue. Some residue can be avoided by strategic process design. Co-processed product, where many different metals are cleaned in the same cleaning machines, can result in cross-contamination. It might be more effective to segregate the processes.

Sequentially-processed products can be a source of contamination. In pharmaceuticals, where the same vessels and auxiliary equipment may be used for a number of different products, there is concern that the Active Pharmaceutical Ingredient (API) from one product could contaminate subsequent products. API is a primary residue of concern. With complex products, other residues that could contaminate subsequent batches may also need to be dealt with.

The cleaning agent itself can leave a residue. This is why the cleaning process typically consists of washing, rinsing, and drying steps. We have discussed the importance of multiple rinses to completely remove aqueous cleaners. Some solvents are self-rinsed and dried in the vapor zone of the degreaser. Water and solvents that are not removed by drying become a residue that is adsorbed or absorbed. Subsequent outgassing may compromise the product performance.

Additive manufacturing
Additive manufacturing may change the way we look at the type and location of residue. Metal powders can be trapped within a porous structure. While in some cases, residue of metal particles may be dealt with by heat, in other instances the integrity of the product could be compromised. In such cases, residue of powder must be removed by cleaning. When the goal is to use additive manufacturing to produce an entire, finished product like an automobile or an airplane, the reality is that post-printing processing is often required. Post-processing results in residue, including particles and process fluids, like metalworking fluids, that can become trapped in the product.

Residue control – or maybe out of control!
Manufacturers doing specialized surface coating and deposition may have no control over the residue on components and parts that the customer provides. Final assemblers may specify materials, dimensions, and structural properties of components, but they may have very little control over residue. This is an even greater problem with complex supply chains. Even if the cleaning process is called out, the cleaning specs may be out of date. They may not reflect currently-available cleaning agents and cleaning processes. Specs may not accurately reflect currently-available metalworking fluids in part because of air quality regulations to reduce volatile ingredients. In response to the regulations, some metalworking fluids have been reformulated. This translates to more residue.

The product may itself interact with the environment to produce residue. Some cleaning or surface prep processes involve removing surface modifications that result from interaction of the product surface with the environment. Examples include aluminum oxide or beryllium oxide. Reaction of steel with water can result in corrosion; corrosion can be thought of as a residue.

Creating Residue
Replicating actual undesirable residue as it exists on the surface of the product is not as simple as it might seem. On the surface (pun intended!) it might seem counterintuitive to purposely create surface residue. However, it can be useful to do so to simulate actual process conditions. Designing a cleaning process based on applying water soluble lubricant to a flat surface won’t reproduce the residue that’s left on the surface after machining, handling and after storage. Storage conditions, including exposure to air, airborne molecular contamination, and particles, moisture, and heat, changes the residue.

Residue is indeed the enemy. Understand the nature of the actual surface residue and you will be on your way to successful critical cleaning.

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