Cleaning generally consists of three components: cleaning, rinsing, and drying. Manufacturers involved in the practice of precision cleaning know that it is critical to understand each of these components and how they interact. The successful accomplishment of these steps is important to the overall cleanliness level achieved.
When washing a part, the contaminant is often removed through the introduction of a cleaning chemistry and mechanical force. Rinsing involves the removal of any residual soil and chemistry that remains after washing. In performing this task, it is important to do so without introducing new contaminants, such as dust or impurities in the water. Drying is the process by which residual rinse liquid is removed – hopefully, again, without introducing any new contaminants.
When people think of cleaning applications, much focus is typically placed on the washing stage of the operation. In precision cleaning, however, rinsing becomes a much more important step. The allowable contamination levels are lower, and spot-free drying is almost always a requirement.
Rinsing is a technology, just as washing is. It is measurable, controllable, and directly contributes to the effectiveness of the cleaning process. Effective rinsing can improve yield, reliability, and appearance; it is also an important factor in containing operation costs.
Rinsing removes two basic types of soils: 1) solubles, which encompass washing chemistries and other soils that dissolve in the cleaning media; and 2) insolubles, consisting of particulate dispersed throughout the cleaning media.
Rinsing is based on the principle of dilution. To develop an effective rinsing process, three questions must be answered:
- What is there?
- How much is there?
- How much residue is acceptable?
What is there and how much there is can be determined with analytical testing. How much residue is acceptable is a more difficult question – one that must often be answered empirically by the end-user.
Often, acceptable residue levels are defined by testing a cleaned part for acceptable performance in its next operation or use. If the part performs acceptably after being put through the cleaning process, the cleanliness level is assumed to be acceptable.
Effective rinsing is based on the successful completion of two tasks: first, the soils must be separated from the part; then, the soils must be prevented from redepositing onto the part. This can be accomplished by several means – for example, sparging the surface to remove buoyant soils, filtering the solution for particulate, and maintaining continuous dilution of solubles and fine particulate.
Separating the soil often requires mechanical energy, especially with parts having complex shapes, or those that are “nested” in blind holes and/or crevices.
If ultrasonic + agitation is used in the wash, it might also be helpful in the rinse. Many times, a higher frequency is used in the rinse than has been used in the wash. This facilitates smaller particle removal and reduces the potential of part damage.
Continuous filtration of the rinse baths is very important in precision rinsing. The level of retention of the filter (5 microns, 0.2 microns, etc.) should reflect the level of cleanliness required. In systems with multiple rinse tanks, the filter retention level is often reduced with each succeeding bath.
Effective rinsing is essential to the success of precision cleaning processes. The factors that most influence the rinsing process include:
- Type of soil (soluble or insoluble)
- Cleanliness specifications
- Agitation, flow rate, and filtration
- Overflow rate
- Rinsewater quality
- Equipment design
All of these factors must be taken into consideration when designing the optimum process for your needs.