Drawdowns are used in standard tests in many industries, including paints, inks, adhesives, paper, electronics, pharma, food, packaging, and a host of others. They are also specified in many standards, including ASTM D823, ASTM D3258; Federal Specifications TT-P-29, TT-E-50BA, PD-220A; FTMS 141a; JAN JAN-P-630, JAN-P-700; and Military Spec. MIL P-13341.
Color, gloss, opacity, pigment dispersion, wettability, dry weight, drying time, scrub resistance, pigment and dye uniformity, clarity, adhesion, and a host of other properties are measured on the completed drawdowns. One of the reasons that high quality and precision Drawdown Bars, (also known as Bird Type Applicators and Doctor Blades) are used in making drawdowns is to ensure that a consistent film thickness is achieved, since most coating properties are thickness dependent.
The first step in determining which drawdown bar to use is to determine the optimal film thickness for your application. This can be accomplished by experimenting with various film thicknesses until you determine which film thickness yields the optimal desired product performance. A good tool for this is the Gardco Microm Film Applicator an adjustable thickness applicator that can be used to create varying film thicknesses with very high precision.
It is important to remember that the wet film thickness and dry film thickness of your drawdown will be less than the gap clearance of your applicator. There are many variables in the drawdown process. Actual wet film thickness can be 40% to 90% less than the gap, and dry film thickness can vary even more. Variables such as gap clearance, coating viscosity, percent solids, solvents used, drawdown speed, surface energy and substrate absorptiveness all influence wet film and dry film thickness. ASTM D823 contains more information on this subject. As a rule of thumb, the Theoretical Wet Film Thickness of a bar is roughly 50% less than the gap clearance listed for the bar. So if you have a gap clearance of 6 mils, the theoretical wet film thickness will be approximately 3 mils.
Once you have determined the optimal film thickness that you would like to use, determine the bar width that you need, depending on the width of the substrate that you will use for the drawdown. The most common substrates are Drawdown Charts, which are available in multiple sizes, but almost any uniform, flat substrate can be used. Try to use a standard width drawdown bar if possible, as these are readily available from stock and are your least expensive option. Also be sure to use a flat, consistent surface under your substrate, such as a Glass Drawdown Plate or vacuum plate. Using an Automatic Film Applicator is the best choice to achieve consistent results, as film thickness can vary in a manual drawdown based on the speed and pressure applied by the operator.
Finally, be sure to select a one piece 440 grade stainless steel bar, as they are the easiest to clean and maintain, and will last the longest time.