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Water is arguably the most important component in the pretreatment process comprising greater than 95% of the pretreatment process chemistry. Surprisingly, few finishers give much thought to water’s contribution and ultimate cost in their pretreatment operation.
Water’s primary role is to remove chemical/soil residue and contaminants from the work place. This permits the presentation of a contaminant free surface in order to promote optimum coating adhesion and performance. Clearly, water quality is of critical importance in order to achieve your customers implied or specified coating performance standards.
Water quality is directly related to the impurities or contaminants that it contains. A major problem area in the pretreatment process is by high total dissolved solids (TDS) accumulating in the rinse water. Excessive TDS in he water are deposited on metal parts during rinsing. As the water evaporates these impurities remain on the metal substrate. Another method of measuring water quality is pH. Generally, the lower the TDS and the closer the pH is to neutral (7) the best the water quality.
Of course, water quality comes at a cost. Poor water quality costs come in the form of adhesion failure, rust, warranty claims, excessive water treatment costs, and environment compliance issues. Maintaining superior water quality yields costs in the form of water purification equipment. Which costs would you prefer – the unexpected, unpredictable, possibly devastating cost of poor water quality or the predictable, budgeted, proactive cost of good water.
It is likely that all of us have given the role water plays in the pretreatment process a passing interest, but only a few of us understand that good water quality is the key to preventing paint problems. And fewer still monitor and control the quality of water in the pretreatment process.
Your choice should ultimately depend on your specified performance standards and whether or not you have the tolerance to handle field failures and dissatisfied customers.
Before tackling the heat vs. viscosity issue, let’s review the importance of viscosity.
When setting up a spray gun, the choice of air caps and fluid tips is highly dependent on the flow rate and viscosity of the coating being sprayed. The choice of fluid and air pressures is also based on the flow rate and viscosity. The lower the viscosity, the lower the air and fluid pressures. The lower the air and fluid pressures, the more efficient the spray gun. More efficiency translates to lower costs.
Many modern coatings are high in solids (low in solvent). The temperature effects on a high solids coating are the same as that of syrup. The thicker the syrup, the more effect heat has on its viscosity. As indicated above, if we have viscosity that varies due to temperature fluctuations, the atomization of the coating will also vary. Results of the variation may result in off color, dry spray, runs, etc. etc.
While a common method of lowering viscosity is thinning or reducing, warming a coating up is environmentally friendly and lower in cost. Many environmental laws limit the amount of solvent that may be added to the coating.
The maximum temperature that a coating should reach should be available on the “Technical Data Sheet” available from your coating supplier. Obviously caution should be used with catalyzed coating.
A sample temperature curve for a high solids coating is shown below.
At 50 degrees, the viscosity would be approximately 140 CPS (58 Seconds #2 Zahn).
Warming the coating to 100 degrees, the viscosity would be approximately 30 CPS (20 Seconds #2 Zahn).
The air and fluid pressures used at 100 degrees would be considerably lower than those used at 50 degrees. The net result is lower coating costs.
Circulation of paints and similar materials containing pigments or fillers requires fluid flows velocities high enough to maintain the contained particles in suspension. A Velocity of 60 feet per minute has been accepted as the minimum velocity to maintain suspension.
A wet film thickness gauge is designed to give the spray operator immediate feedback as to the film build just sprayed. In most cases, measuring the dry film thickness (DFT) provides little information as it is usually measured a considerable amount of time after the actual spraying. Many things could have influenced the DFT: operator fatigue, ambient air temperature, coating temperature, etc.
There are several types of WFT gauges available; the most common being the notch gauge (see figure 1). Others types including the eccentric disk, the rolling notch gauge and the 6 sided gauges are available from specialty vendors.
There are several issues that must be addressed when using a WFT gauge:
When placing the gauge on a freshly painted part, the gauge must be placed 90 degrees to the part. The operator also needs to be aware of variation of the surface that may influence the reading. For example, if the surface is not perfectly flat, one direction may give a more accurate reading than another.
To use the WFT gauge, place the gauge directly on the wet finished part (see Figure 2) and as described above. The notches will indicate the measured film thickness. For example, if the 1 and 2 mil notches are wet and the 3 and 4 notches are dry, then the measured thickness is between 2 and 3 mils (.002 to .003 inches).
The solvent in a coating will immediately start to evaporate after spraying. In order to achieve a common method of reading the coating thickness, a time frame will need to be established. Typically, one might measure the thickness 5 to 10 seconds after spraying. If another operator measures the thickness after 20 seconds, the results would be different even if the initial thickness was identical.
Reading with Clear Coats
A clear coating on a WFT gauge would be very difficult to read. The most common method of reading clear coats is to use the gauge as a stamp on a piece of absorbent (non-gloss) paper. Many companies use the stamp method as a way of documenting the WFT.
Creating Surface Defects
After using a WFT gauge to check the film thickness, the material may not flow to hide the area where the gauge was used. If this creates an undesirable defect, place a small sample of the material in line with the operators normal spray path. This sample should be sprayed along with the part. The sample then may be checked for WFT and DFT (after curing).
The Answer depends on the following:
– The percent solids of the coating
– The transfer efficiency of the equipment
– The thickness of the paint applied.
A gallon of paint will cover 1604 square feet at 1 mil (.001”) at 100 percent solids.
Let’s take the parameters above to calculate a sample production line.
The percent solids will reduce the 1604 to 641.6 square feet (1604 * .4)
The transfer efficiency will reduce the 641.6 to 385 square feet (641.6 * .6)
The film build will reduce the 385 to 192.5 square feet (385 * .5)
As you can see, our original 1604 square feet has been reduced to 192.5 square feet.
The major variable in the above is transfer efficiency. It can vary considerably depending on the following:
– What was the required quality of the finish? (sheen, orange peel, gloss etc.)
– What was the required mil thickness?
– What was the size and shape of the part?
– What was the spacing of the part?
– What was the line speed?
– Was proper spray technique used?
– What type of coating was used?
– What was the viscosity of the coating?
– What was the ambient air temperature?
– What was the spray booth air flow?
– What air & fluid pressures were used?
Using thee calculations above, an approximation of coverage per gallon can be determined
The life cycle of any paint shop is best expressed in four phases . . . .
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In this example we will compare an air valve and regulator although the same data could be applied to fluid valves and regulators.
In figure 1 below, both the valve and regulator can reduce the incoming air from 80 psi to 50 psi. In order to measure the output, a gauge is typically installed on both.
The problem with a valve is that when the incoming pressure varies, the output pressure will vary. The reason for this is the valve is simply a “choking device” and has no mechanism for regulation. A regulator, on the other hand, has a diaphragm and spring mechanism to maintain the pressures set.
Additional note on air regulators:
Air regulators are available in two distinct styles; standard and bypass. Typically air regulators have one inlet port and 3 or 4 outlet ports.
In figure 2 below, all of the outlet ports will change with the adjustment of the regulator knob on a standard regulator. In a bypass regulator, all ports with the exception of the port opposite the inlet port will change. As noted above, the installation of a gauge will show the regulated pressure. A typical use for bypass regulators is on a paint pressure tank. If it is a tank with two regulators, one is typically a standard and one is a bypass. The regulators need to be in the proper order for the tank to work correctly.
Air purifying respirators can be used safely and effectively to reduce exposures to common diisocyanates.
Misconception #1: Air-purifying respirators should not be used because diisocyanates have poor warning properties.
Misconception #2: Air-purifying respirators cannot remove diisocyanates.
Misconception #3: Air-purifying respirators are not approved for gases and vapors with poor warning properties
Misconception #4: Air-purifying respirators should not be used because diisocyanates are sensitizers or are “too hazardous”