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FLEXO Magazine : November 2013
TeChNOLOGIES & TeChNIQUES Thin Layers, big impact How Cascading Arc Plasma Coatings Help Deliver The Highest Efficiency by Steven Ort and Dave Hawley There’s something about a crisp, bold print that catches the eye and commands attention. However, before the ink ever reaches its destination, it travels through several components in the printing process. One of these— the anilox roll—determines the amount of ink transferred onto the printing plate. As a result, any defect on the roll could translate to visual blights on the finished print. Plasma spray technology, particularly cascading arc plasma, is a highly effective solution for producing the dense and homogeneous surfaces needed for successful print jobs. These roll coverings ensure the laser-engraved anilox roll has a smooth, blemish-free surface that produces attractive prints to maximize potential profits for companies. PLASmA SPrAY HiSTorY Plasma spray is a proven materials processing technique for producing protective and surface enhancing coatings using a plasma jet. When first introduced in the early 1960s, plasma spray was celebrated as an innovative method of applying coatings of ceramic and other high-temperature melting materials. Hailed for its ability to use a wide range of feedstock materials with higher melting temperatures, such as alloys and ceramics, the plasma spray gun gained rapid success for critical coating applications in the power genera- tion, commercial aviation, military and other industries. Today, plasma spray is considered one of the most versa- tile thermal coating processes available, producing high- VERSATILE, PROTECTIVE & BLEMISH FREE • Cascading arc plasma is an evolved form of plasma spray that is 60 to 400 percent more efficient • Provides a uniform and consistent coating for anilox rollers • Cascading arc plasma gun outlasts traditional plasma spray guns, often up to 200 hours • Fixed arc length has the advantage of stabilizing the plasma plume and eliminating the extreme amplitude power oscillations • Stable plume produced by cascading arc plasma guns significantly increases deposit efficiency and quality • Stable arc in cascading arc plasma guns causes less damage within the gun, allows the parts to last longer and creates a genuine, repeatable production process performance coatings with superior durability and reliability. The plasma spray process uses a combination of heat and velocity to create a coating from fine, powdered material feedstock. An arc forms between the anode and cathode, where the gas flows and becomes ionized, or stripped of its electrons. This creates a plasma plume into which powdered spray material is injected, melted and accelerated toward the substrate surface. Plasma spray has continued its evolution into what’s called cascading arc plasma. With its enhanced efficiency, cascad- ing arc plasma saves costs when applying coating materi- als to paper manufacturing rolls, as well as gas turbine and airframe components, engine and drive train components, hydroelectric and steam turbine components. Compared to traditional plasma spray, cascading arc plas- ma’s reliability allows for tighter control of coating process windows, important for critical components and long spray runs. In particular, this technology benefits the printing indus- try, making parts last longer and function more efficiently. WHY CASCAding ArC PLASmA? Because of arc instabilities, conventional plasma spray guns are prone to producing varying temperatures within the plasma jet. However, cascading arc plasma guns are able to provide a uniform heating platform with steady temperatures. The cascade fixes the electric arc length, which provides it with a starting path over a series of electrically neutral rings within the arc chamber. Once the gun is ignited, only the common front anode, or nozzle, is electrically connected to the power supply. The fixed arc length has the advantage of stabilizing the plasma plume and eliminating the extreme amplitude power oscillations (in the 3-to-5 kHz-range) that result from an unstable arc. As a result, cascading arc plasma generates homogenous ionization, creating a consistent coating for a part. Consider baking bread in a well-controlled oven, compared to a faulty oven with fluctuating temperatures, which would produce variable outcomes. Traditional plasma’s inconsistent coating also limits the amount of material that can be injected into the flame in a given period of time. As a result, cascading arc plasma has a higher efficiency than traditional plasma coat- ings with improvements generally measuring between 60 and 400 percent. ComPAring PLASmAS The uniform heating platform leads to higher coating throughput and productivity, or higher volumes of coating material applied per unit of time. Cascading arc plasma’s consistent stream of gas heats a greater percentage of pow- der, compared to traditional plasma. This allows the powder to be injected at a higher rate. With more material landing on the part, manufacturers achieve an overall reduction in mate- rial costs and processing time. The amplitude and frequency of the oscillation is directly linked to the heating and flight path of each powder particle. As a result, the stable plume produced by cascading arc plasma guns significantly increases deposit efficiency and the consistent quality of the coating. In addition, this also cre- ates a highly reproducible coating process, making it easier for the technician to achieve uniform results. The process window for cascading arc plasma guns remains comparatively stable over a longer period of time, enabling extended spray runs and reduced maintenance. Due to higher process drift, which is an indication that gun parts need to be replaced, conventional plasma guns last 15 to 20 hours before needing maintenance. However, the stable arc in cascading arc plasma guns causes less dam- age within the gun, allows the parts to last longer—often up to 200 hours—and creates a genuine, repeatable production process. Ultimately, the increased productivity depends upon the materials being sprayed. Cascading arc plasma guns have a greater material feedstock utilization, which reduces the time and cost associ- ated with spraying a part. Just 10 years ago, a customer may have needed several different guns to accommodate the wide range of applications. Now, a single cascading arc plasma gun can cover all applications except for internal spray processes. Diagram of a single-cathode, conventional plasma gun, which does not constrain the arc attachment, resulting in non-uniform heat distribution of the powder particles in the plasma jet. Diagram of a multiple-cathode cascading arc plasma gun, which divides the energy input across three arcs and constrains them, thereby creating uniform heating of the powder particles and long gun component life without process drift. 58 FLeXO NOVEMbEr 2013 www.flexography.org www.flexography.org NOVEMbEr 2013 FLeXO 59