Production Coating Rods: Design, Materials, and Tolerances
Rod Types
Several different varieties of metering rods are available. The options are described as follows:
Smooth: These rods are not wound with wire; they are most often used to level previously metered coatings — notably those that do not flow out readily — on paper, paperboard, or plastic films. They are also used to apply clay coatings, lightweight coatings to fine papers, and for coating paperboard and linerboard. Coating thickness is controlled by pressure against a backing roll or by varying web tension. Smoothing rods can be precision ground to any required surface finish, and hard chrome coating can extend rod life significantly.
Single wound wire: This is the original, classic Mayer rod design, featuring a single strand of wire, tightly wound around the base rod. The wire (or rod) number corresponds to the wire diameter, measured in mils (thousandths of an inch). The coating thickness applied will be approximately one tenth the diameter of the wire. Please keep in mind that this ratio may vary considerably, based on a number of process parameters, including especially line speed, percent solids, and coating viscosity. This design is used for a wide variety of applications; it works best for relatively free flowing coatings that level readily and do not dry too quickly. These rods, available from #3 through #75 wire, can be stripped and re-wound.
Double wound wire: These rods feature a double layer winding, with a larger diameter wire wrapped directly on the rod, and a smaller wire wound over the top of this base winding. This results in wide, shallow cavities between the wire windings, helping to reduce clogging and machine-direction variations in coating thickness when applying higher viscosity and/or higher surface tension coatings. This design is more expensive, but is more durable than a single wound or gapped rod. These rods, available for wet film coating thicknesses equivalent to #10 through #110 wire, can also be stripped and re-wound, though the economics is questionable for all but large diameter rods.
Gapped: These rods use relatively fine wire, spaced apart rather than wound tightly together. Like double wound rods, this produces wide, shallow cavities between the wire windings, similarly reducing clogging and machine-direction coating striations on high viscosity and high solids content coatings. This design is less expensive but also less durable than double wound rods. Gapped rods are available for wet film thicknesses equivalent to #30 through #250 wire. Like double wound rods, they can be stripped and re-wound but the cost benefits are generally minimal at best.
Formed: Also known as a grooved rod, this design does not use a wire winding to meter the flow of the coating. Instead, it is roll formed from a solid steel bar; the size of the groove meters the coating thickness in the same fashion that the cavity between wire windings does on a traditional wound rod. The leading advantage is that the cavities are less likely to clog, and the cleanup is quicker than with a wound rod. They are widely used in paper mills and converting plants to apply pigments, adhesives, starches, and other coatings; they are especially effective with high solids content coating materials and/or fibrous substrates.
Formed rods are sized by wet film thickness, and are numbered in accord with the yield offered by wound rods. Please keep in mind that due to the shape of the cavity — formed rods have “U” shaped cavities, whereas wire windings produce a “V” shape — the actual coating thickness may differ somewhat between the two designs. They are available for wet film thicknesses equivalent to #3 through #95 wire. While there is a setup charge for small orders of formed rods, for large quantities, they generally offer an economic advantage.
Materials
Our Mayer rods are fabricated from 303 stainless steel rod, and are wound with 304 stainless steel wire. For special circumstances, we can also supply rods fabricated from 416 stainless steel and/or wound with 302 stainless steel wire.
For certain applications — especially for smoothing rods — hard chrome plating can significantly extend the usable life of metering rods. The thickness of the chrome plating can be applied from 0.25 to 2.0 mils (6.4 to 51 microns); thinner coatings are generally more appropriate for wound or formed rods, whereas a thicker coating on a smoothing rod will likely extend its life.
Specifications
Rod straightness: +/- .005″ per linear foot
Minimum rod diameter: 3/16″
Maximum rod diameter: 1 1/2″
Tolerance, rod diameter: +/- .0005″
Minimum length: 3″
Maximum length: 12′
Tolerance, rod length: +/- .125″
Surface finish (Ra): 32 micro inch guaranteed; typically 16 to 20 micro inch
Wire Diameter Specifications:
#3 – #6 rods: +/- .0001″
#7 – #9 rods: +/- .00025″
#10 – #20 rods: +/- .0004″
#21 – #35 rods: +/- .0005″
#36 – #48 rods: +/- .00075″
#50 – #75 rods: +/- .0010″