In-line spray or immersion cleaning machines are the most common methods for cleaning stamped metal parts. Both systems use water-based cleaners that are either acid or alkaline. This type of cleaning is used to remove stamping and drawing fluid residue prior to a finishing operation. Most product residue washes easily in this type of system. Immersion alkaline or acid cleaners are mainly used prior to plating, anodizing, and chromating. Spray alkaline or acid cleaners are normally used prior to painting operations.
In addition to the type of cleaning system used, there are other physical and operational factors that affect cleaning. Proper application of the following factors optimize cleaning effectiveness.
In an aqueous cleaning process, the removal of residue is due to detergency rather than solvency. Residues are removed by chemical displacement, rather than being dissolved from the surface of the part by the cleaner. After displacement, the residue may emulsify with the cleaner, it may separate or settle as sludge, or it may rise to the surface. For these reasons, the cleaner is constantly being depleted of it’s detergency, and needs to be replenished with new cleaner solution at regular intervals. Many cleaning problems disappear when the cleaner is properly replenished and maintained.
The length of time that parts remain in the cleaner solution is significant. The longer the dwell time, the more effective the cleaning. Heavy residues, oils, and waxes require a lone enough soak, or spray time, to work through the layers of residue. Time cycles vary depending on the residue load and the type of residue on the parts to be cleaned.
The chemical activity of a cleaner is a direct function of temperature. For each degree Fahrenheit increase in temperature, there is a 2.6% increase in chemical activity. The higher the operating temperature, the better the cleaning. Typical washing systems and alkaline cleaners work best in a 140°F to 160°F range.
The rate of cleaning is a linear function of the concentration of alkaline or acid cleaner. Proper concentration levels must be maintained to guarantee effective cleaning.
Spray cleaning can be more effective than immersion cleaning because of pressures and volumes aiding in residue removal. Immersion with ultrasonics, or electrolytic action, is more effective than normal immersion due to increased surface activity.
The importance of thorough rinsing after cleaning cannot be overstated. Rinsing is a dilution process, and it’s function is to remove water-soluble material from the surface of the part after cleaning. Most water-rinse stations include some type of rust inhibitor to protect the metal part between finishing operations.
All contributing factors must be balanced to optimize the quality of the completed coating of the finished part. Again, these factors include a) mechanical characteristics of the cleaning system, b) type and composition of the cleaner, c) it’s operating parameters, and d) the composition of the lubricant used to stamp and draw the metal part. Alkaline cleaners are being formulated to be compatible with all types of water-soluble oils, semi synthetics and synthetic lubricants.