In this column, I will discuss the control plan for a generic washer used for cleaning heat-treated parts.
Proper cleaning involves not only removing the various soils from the part, but also removing any residual cleaner residue by proper rinsing. This is a critical step and is often the cause of many problems. One-stage washers, with only a wash cycle, can leave a cleaner residue on the parts. The cleaner residue must be removed to prevent staining and attacking of parts by the residual cleaner residue. A good analogy would be a dishwasher full of dishes — the soap must be rinsed off the plates so as not to impart a chemical taste to food. Parts must be rinsed to remove any residual cleaner from their surface. Failure to remove cleaners during the cleaning cycle can result in problems such as caustic burn or rusting (Figure 1).
Improper control of the washer and rinse station can contribute to poor part appearance, as well as damaged parts. Residues that would be easy to remove before tempering become impossible to remove after tempering. Often, shot blasting is performed to clean surfaces, but this can cause part damage to delicate threads or fine details.
The general effect of cleaning process variables on cleaning are shown in Figure 2.
Control Plan for Washers
It is important that the washer system be properly monitored to achieve the desired cleaning. A disciplined approach to washer and cleaner maintenance is critical if quality cleaning is to be achieved.
There are two types of things to verify in a washer control plan. There are chemical items, which have to do with the soap, and there are mechanical items that are associated with the physical washer. We are going to concentrate on the cleaner side of the washer system.
The chemical items include concentration, pH, biologicals (fungus and bacteria), chlorine or chlorides, water hardness, and temperature.
The concentration necessary for good cleaning is geared to the method of agitation. Thus, disregarding any foam problems, it is possible to get similar cleaning from the same alkaline cleaner at 8 to 12 oz./gal. by soaking, 1/2 to 1 oz./gal. by spraying, 1/10 to 1/2 oz./gal. by steam cleaning, or 2 to 4 oz./gal. in a high-pressure water spray at room temperature.
The soap concentration should be properly controlled. More soap is not necessarily better, as it can leave residues, or excessive consumption. Follow manufacturer’s recommendation for proper concentration control.
The pH of a cleaner solution generally follows the concentration. For an alkaline cleaner, as the concentration is increased, the pH generally follows. Proper pH control is important to prevent rusting of parts or excessive residues. Again, follow manufacturer’s recommendations.
Biologicals (bacteria and fungus) are measured using dip-slides. While not directly related to the quality of cleaning, the presence of biologicals can produce some unpleasant odors such as a strong mildew smell or the smell of rotten eggs. Proper control of biologicals can reduce operator discomfort and a stinky shop. Use of biocide and fungicides is important to controlling these biologicals. There are also biostable cleaners on the market that do not support bacteria and fungus and reduce the reliance on hazardous (and expensive) biocides and fungicides.
Chlorides and Chlorine
Chlorides and chlorine can cause rusting on parts and the washer if the concentration gets too high. Chlorides and chlorine come from the water used to make up the bath. Evaporation of the water in the washer results in increased concentration of chlorides in the bath, and fresh additions of water further increase the concentration. If rusting is a problem, you can add corrosion inhibitors to prolong the life of the bath. However, should the chloride or chlorine levels become excessive, it may be necessary to dump the bath and recharge it with fresh cleaner. Depending on the source of the water, it may be worth considering the use of reverse osmosis (RO) water to extend the bath life.
Water hardness depends upon the quantity of chemical salts dissolved in the water. These salts, in the presence of heat or alkalinity, may react to produce insoluble material, which precipitates out of solution in the form of hard water scale. Unless this formation of hard water scale is prevented, several undesirable results are possible. The major undesirable result is the buildup of hard-water scale. This scale may be precipitated on the heating coils of wash tanks, reducing the efficiency and wasting energy and money.
Scale may build up in spray nozzles and on belt conveyors, impairing cleaning efficiency. The effectiveness of the spray is reduced, and scale may block the solution from reaching and cleaning the objects on the belt. Scale may also build up in the rinse tanks in extremely hard water situations due to alkaline carryover of cleaner into the rinse tank. Hard water salts regularly react with fatty acid soaps to yield insoluble residues on parts being cleaned. The harder the waters, the greater the susceptibility to rusting of parts and the washer. Hard water is not conducive to good cleaning.
Water hardness, in ppm of calcium carbonate (CaCO3), is determined by the following equation:
There are several other equations used to calculate water hardness, but this is the most common. A variety of different test kits can be obtained to measure the hardness, or ICP (Inductively Coupled Plasma) analysis can be used.
The effect of temperature depends on the type of soil and cleaner. For most oil type soils, an increase in temperature reduces the viscosity of the oil, makes it more mobile and therefore more easily removed. There is a well-established principle that the rate of chemical reaction is doubled for every 10°C or 18°F increase in temperature. Most industrial cleaning is carried out at temperatures of 140 – 180°F. However, excessive temperatures can result in “flash rusting” of the active surface. Therefore, most cleaners are designed to operate at temperatures around 140°F or lower.
A typical control plan for washers is shown in Table 1.
In this column, I described a generic control plan for the proper control of washers, including washers used before and after heat treatment. With proper control, excellent cleaning and long bath life will result.