Differences Between Tribo and Corona
When evaluating the two type guns for a specific application, there are some fundamental items to take into consideration. The differences between tribo and corona guns are outlined in this Manner.
Faradav Cage Effect:
Probably the most common reason to consider tribo guns for an application is the ability of the tribo gun to coat products with a high degree of Faraday cage effect areas.(See diagram #4.) Examples of these areas are corners of boxes, fins of radiators, and support seams on shelving. In these cases, powder is attracted to the flat areas of the product and forced out of the comers and seams due to electrostatic repulsion of similarly charged particles in the area or intense air flows. Tribo guns are well suited for this application because an ion field is not generated between the gun and the product It is the ion field which increases the electrostatic repulsion. This effect can be reduced in corona guns by operating the gun at a lower voltage output. This removes one variable from the application and becomes an issue of air flow
The powder output of a gun determines the amount of powder that can potentially be applied to an product. Corona guns can operate at low and high powder outputs due to the consistent charging capability. Tribo guns typically must operate at lower powder outputs due to flow restrictions. The flow restriction is a result of forcing the powder through multiple tubes, using air to rotate the powder around the inner tube, or having dimples to disrupt the powder flow through the tube. When the tribo gun is operating at a low powder output, the powder particles have more opportunities to impact the walls of the gun and become charged. At high powder output, the powder particles are moving at a higher velocity through the gun but the flow restriction limits the powder output.
Conveyor speed also plays a differentiating role between the two gun types. Tribo guns often need more guns to apply the same amount of coating as corona guns, especially at high line speeds. Corona guns have the ability to coat products at low and high conveyor speeds. Because tribo guns operate at lower powder outputs, more guns are required to apply the same coating thickness.
The type of powder required for an application is important to the type of gun used. Most powders have been developed to function with corona guns. This is especially important to operations that require frequent color change to many different type powders. Tribo guns, however, are very dependent upon the type of powder used because it must be capable of This transferring electrons between dissimilar materials in order to charge effectively has limited the usage of tribo to specific applications that only use powders formulated for tribo charging
Powder Finish Quality :
The powder finish quality each type gun can apply to an product is also different. Corona guns are very successful in achieving a consistent film build especially with thin film thicknesses. While other parameters such as room environmental conditions, conveyor speeds and powder outputs change, corona guns are capable of making adjustments to meet coating needs very consistently. However, corona guns can develop a very high charging field which actually limits the amount of powder that can be applied and maintain a smooth finish. A phenomena called back ionization occurs when the powder accumulating on the product dissipates its charge through the accumulated powder. The result is what looks like a small crater on the cured finish.
Also, with heavy powder thickness, a wavy look considered “orange peel” occurs. These conditions usually only occur with finishes of 3 mils or more. Tribo guns are not as susceptible to back ionization and orange peel because the powder particles are charged and no electrostatic field is developed. As a result, tribo guns can develop heavier powder thicknesses with a very smooth finish.
Corona guns tend to be more forgiving than tribo guns in harsh environments. Even though a controlled environment is recommended for all coating operations,occasionally this is not the case. Variances in room temperature and humidity effect the coating performance of both types of guns. Tribo guns are especially effected because as these conditions change so does the charging effectiveness of the gun The ability of electrons to transfer from the powder particles to the teflon material varies with changing conditions. This can result in inconsistent coating of the product over time. Because corona charging does not rely on the properties of the materials as much, they are not as effected by variances in environmental conditions.
[ Thanks for Michael J.Thies, please contact us if any doubt ]