Issues affecting rod performance
Q. How do I know when a rod is worn?
You will discover that a rod has worn when you notice less coating being applied. As the wire or thread surface wears down, the open areas become smaller and thus no longer apply a sufficient quantity of coating. Coating build-up in the rod grooves can also diminish rod performance. When some thread designs are used with more viscous or high solids coatings, the coating can build up in the grooves. This will reduce the area of the openings and thus the metered amount. If this happens, cleaning the rod will usually restore performance.
Q. What does hard chrome plating do for a metering rod?
Hard chrome is much harder than stainless steel, and therefore, wears much more slowly.
Q. How can I maximize rod life?
You can maximize the working life of a rod by choosing the appropriate rod for your application, keeping the rod clean, and by adjusting the pressure against the rod to maintain optimal metering without causing excessive rod wear. Thread design plays a major role in the working life of a rod. Buschman engineers can work with you to create long lasting rods that suit your application needs.
Q. What is the best way to clean a metering rod?
Routine rod cleaning will minimize streaking and other defects while maximizing coating uniformity and rod life. If using water-soluble coatings, after use, immediately immerse the rods in warm water mixed with detergent (Liquid Dish Soap). If using caustic solvents in your coating, such as MEK (methyl ethyl ketone), Toluene, alcohol, turpentine, acetone and lacquer thinner, immerse the rods immediately after use in a container of the same solvent used in the coating. Once the rods have had enough time to soak, use a very soft and fine brass bristle brush and gently scrub the rods in the direction of the wire or threads.
Q. What is the best way to store a metering rod?
Metering rods should be stored in the original packaging until they are used. Care must be exercised when handling rods to avoid damage such as nicks and scratches, which can affect the quality of coating. Used rods should also be stored in a manner that will protect them from damage
Q. How do I maximize uniformity, wetting and/or flow of the coating?
Maximum uniformity of coating is achieved when a rod delivers the appropriate wetting speed. Because of surface tension, a rod with more narrow and rounded threads, whose open areas are closer together will deliver a faster wetting speed than one whose openings are farther apart. When dealing with low viscosity coatings with low solids, a variety of rod profile shapes will deliver a satisfactory wetting speed. When more viscous coatings are used, or coatings with higher solids content it becomes critical to select a rod profile that will deliver a fast enough wetting speed to achieve a uniform coat. Buschman engineers can work with you to create a rod that delivers the best wetting speed for your application needs.
Q. How do I minimize streaks and other cosmetic defects in the coating?
Make sure that your rod is turning in a direction opposite to that of the web. This reduces film splitting, which can cause streaking. Handle rods carefully to avoid bending the rod and to prevent scratching or nicking the rod surface. Using a thread profile shape that delivers the proper wetting speed for the coating solution will produce a uniform coating that is free of streaks and other defects.
Q. What causes streaks?
Streaks are usually found in coatings with a high percent of solids content. Make sure that you choose a thread profile designed to maximize wetting for your application. Such coating solutions may also separate or dry out before reaching the rod. If that is your problem, add a water retention agent to the solution to reduce streaking.
Q. What causes light and dark shadow marks in the coating?
Shadows and/or coat weight variations are often associated with bent rods or uneven
wear of the rod, holder or roll cover.
Q. Why turn metering rods?
Turning the metering rod evens out surface wear and helps in the distribution of the coating.
Q. At what direction does the metering rod turn?
Rods generally turn in the opposite direction of the web or roll. This reduces film splitting,
which can cause streaks.
Q. What RPM should a metering rod turn?
A metering rod generally turns somewhere between 10 to 90 RPM depending on the application.
Rod speed is customarily determined through trial and error methods.
Q. How do I minimize air bubbles in the coating?
Air bubbles are often caused by splash and foaming caused by the excess coating returning to the pan. The use of proper screens to filter out air bubbles and clumps of particulates can minimize this effect.
Q. Why is it more difficult to use metering rods when coating onto films such as polyester or polyethylene?
Films are generally less porous than paper, leaving most or all of the coating on the surface making films more susceptible to streaks and rod lines. To achieve a smooth and uniform coating on films, we recommend using a wire-wound rod or a grooved rod with a custom thread profile. Our engineers are ready to work with you to develop the appropriate profile for your application.
Q. What application variables affect coat weight other than the wire or thread size of a metering rod?
The open area of a metering rod is the main variable, which controls coat weight. Other variables controlling coat weight are the profile or shape of the thread, bladder pressure or tension on rod, hardness of roll cover, viscosity of coating, percent of solids, temperature, speed of web as well as a variety of lesser influences.
Q. How do rod holders affect the coating?
Rod holders can be an intricate part of the coating process. The best rod holders are hot cast in full-length molds instead of being extruded. This gives the rods a more uniform and consistent fit along the entire length of the holder, which reduces rod vibration, increases rod and holder life and promotes a more uniform coating.
Q. How do I increase or decrease coat weights without
changing rod sizes?
Minor changes in coat weight can be made by adjusting viscosity, percent solids, rod pressure, roll cover hardness, coating temperature, web speed, or thread profile on the metering rod. However, for significant changes you will need to change to a different thread size. Top of page
