More than a hundred years after the wire wound coating rod or "Mayer" rod was invented, they are still used in some paper coating operations.
But as coating applications evolve and demand higher performance, today's users are finding that wire wound Mayer rods are less and less able to meet their needs.
Modern paper coatings are more abrasive, contain a higher percentage of solids, and can coat a wider variety of substrates. In order to effectively apply these types of coatings in more complex applications and meet customers' increasingly stringent quality standards, more and more paper mills, linerboard manufacturers and converters are transitioning to roll-formed or grooved bars.
Are wire wound rods still the right choice for your spray operation, or should you switch to roll formed rods? How difficult was the transition?
Here are some factors to consider.
How Rolled Metering Rods Are Modernizing Paper Coating
Wire wound rods have been the metered rod standard for paper coating for decades. Then, in the 1980s, the roll-formed metering rod was invented, with engineered threads built into itself.
Unlike wirewound rods, which are limited to profiles consisting of circular cross-sections placed side-by-side, the shape of the roll-formed profile or the thread shape can be adjusted to suit the application, as shown in Figure 1.
Rollform metering bars have transformed the paper coating process and set a new standard for the industry by allowing users more accurate control over their coating application and extending the life of the wire rod.
Gain control of key rod performance variables
Both wirewound and grooved rods control application volume by the valley area between wire or thread peaks. However, unlike wire, the shape of the thread profile on roll-formed or fluted rods can be custom designed and manipulated to precisely control key rod performance variables to tailor the rod to a specific coating application .
Optimizing the threads on the metering rod will improve coating quality and efficiency in most cases. translates to:
Longer run times between lever changes;
more uniform coating;
Higher quality coated products;
More efficient coating operation.
Plus, extruded or slotted rods are more durable, easier to clean and have a more uniform finish than wire wound rods . No winding breaks, no tight spaces between windings to collect dried coating, large coating particles and air bubbles.
These benefits are usually realized immediately when transitioning from wire wound metering rods to roll formed metering rods.
Common Coating Challenges: A Comparison of Wirewound and Extruded Rod
Paper coating involves many common problems and quality issues. The table below shows how wirewound rods and formed rods compare in addressing these issues:
| Wirewound | vs. | Extrusion |
| Unable to make adjustments to outline shape | Coating smoothness and uniformity | Thread shapes can be engineered to optimize application, providing a higher quality finished coating. |
| Sharp valleys between wires are difficult to clean | clean | Open root shape of threads for easier cleaning |
| Sharp valleys between wires easily clogged with coating particles and debris | rod contamination | Thread shape can be optimized to keep rods clean during operation |
| Smaller diameter coils can break easily during handling and use | durability | Roll-formed rods have no broken wires and are more durable |
| Steel wire is softer than roll-formed threads. Therefore, the winding wears out much faster | service life | Harder surface finish and engineered thread crests for longer rod life |
| The overall diameter of the rod is increased by twice the wire diameter. This can cause drive alignment and other issues | outer diameter | The outer diameter of rolled rod is much less variable than that of wire wound. Eliminated alignment issues when changing dimensions |
Extruded rods provide valuable quality and efficiency advantages
If you are still using wire wound rods for your wire rod coating application, you are missing out on the significant quality and efficiency benefits of rolled or grooved coated rods.
The image below shows a wirewound profile compared to two distinct thread profiles.
Profile A has elongated peaks to better enhance surface contact with the substrate being coated. Extending the crest in this way creates a rod that lasts longer as the surface area in contact with the substrate increases.
Profile B has an extended root region. This elongated root is ideal for high solids and high viscosity paints that might otherwise not flow well through a wire wound rod or a rolled rod with a more closed root.
The correct thread profile allows you to increase the percentage of solids in the coating, reducing drying time and energy costs. If your paint is very low viscosity, the threads can be designed to extend the life of the metering rod itself.
How to Transition from Mayer Rods to Grooved Metering Rods
For a growing number of paper coaters, the benefits of rolling or slotting bars are obvious.
Fortunately, the transition from wire to slotted rod was painless. In most cases, a suitable grooved rod can simply replace the wound rod without any modifications to the equipment.
The first step is to determine the correct thread size for your application. The dimensions of the Buschman grooved rod thread are the same as that of its coil. Thus, a #10 Buschman grooved electrode will apply the same amount of coating as an electrode wrapped with 0.010 inch wire. In fact, grooved metal rods can apply more coating volume than wires due to the more efficient (higher throughput) fluid flow through the threads than through the narrow roots between the wires.
After determining thread size for coat weight, the next important factor to consider when selecting a profile stick is the thread profile shape (aka profile). The shape of the thread will have a major impact on the quality of your coating, as well as on the efficiency and cost of other parts of the operation.
