Specifications Depend on Parts Being Cleaned

Because material is removed during the chipforming machining processes, contamination can never be entirely avoided.

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No matter the industry, the purpose of industrial parts cleaning is to ensure an adequate level of cleanliness for downstream processes such as assembly, coating, heat treatment and welding, or for flawless functioning of the part. Most people in manufacturing report that these levels of cleanliness have increased over the last few years. Terms such as fine cleaning, ultra-fine cleaning, precision cleaning and critical cleaning are used interchangeably to describe the process of achieving particulate cleanliness specifications. 

Interestingly, from the semiconductor industry to machining, the same terms for cleaning are used to describe completely different cleanliness levels: When cleaning wafers, particle sizes less than 10 microns are associated with precision cleaning. For other applications, precision cleaning starts with particle sizes less than 1,000 microns. It seems as if every industry defines precision or ultra-fine cleaning as the maximum attainable cleanliness level. This not only can be confusing, but it also can lead to exaggerated cleanliness requirements.

The achievable particulate cleanliness level indicated by equipment manufacturers can also be misleading for users. Yes, for wafers, optical lenses and other similar parts, cleanliness requirements such as “no particle larger than 5 microns” can be fulfilled. This doesn’t mean that after a machining process step, a residual dirt requirement of 150 to 200 microns can be met. In the semiconductor and optics industry, the outer surfaces of the parts are most relevant and are easily accessible. Not so with machined parts. Here, particulates are often hidden in the inside of the part and are often difficult to remove. One main risk is if a mobile particle migrates and causes a failure.

Unfortunately, no one can predict which particle will lead to failure. Thus, the cleanliness specifications for machined parts are usually determined by the design engineer, based on the size of the orifice a fluid must pass through, and particulate size limits are often quite stringent.

Costs for cleaning equipment rise with cleanliness requirements. So the outlays necessary for cleaning technology that reliably fulfills a specified requirement of “no particles larger than 600 microns” can be twice that of systems in which cleaned parts are contaminated with larger particles.

Economic optimization in the parts cleaning process is often pursued despite, or perhaps because of, the large investment involved. One approach to optimization is during component design because the geometry of the workpiece and the individual steps of the manufacturing process (turning, milling and assembly, as well as the ability to clean) are determined during this stage. During the subsequent production process, often complex part features prevent further improvements to cleaning.

Because material is removed during the course of chipforming machining processes, contamination can never be entirely avoided. The quality of cooling lubricants and machining fluids influences the quantity of chips, burrs and particles on the workpieces. Suitable purification/filtration prevents previously washed-away contamination from being returned to the component. A special rinsing step with the tool in the machining center—perhaps with purified fluid from a separate tank—can also help to reduce the number of chips. At first glance, this represents an additional expense. But it pays for itself later, thanks to shorter cleaning times and/or a longer bath service life, as well as better component quality. And residues that are removed after machining by means of mechanical pre-cleaning based on vibration, shaking, spinning or vacuum blasting do not place any unnecessary load on the cleaning agent.

In the case of multi-stage machining processes in metalforming and machining applications, intermediate cleaning steps prevent the accumulation of contamination, as well as any mixing or drying out of media on the workpieces. Complete deburring is also an important factor. 

Whether it is called precision cleaning or ultra-fine cleaning, when faced with stringent particulate cleanliness specifications, it is worth inquiring if a specified residual contamination target is necessary so an appropriate cleanliness testing method can be applied.