FT Archives

Powder Coating

Weld Area Coverage Problems

We manufacture outdoor furniture. Many of our products are made with wire that is welded to form a mesh seat or other surface. The points on the parts where the wires cross have always given us trouble with coverage. We started to E-coat everything to reduce failures from rust at the welds. This has helped enormously, but we still have trouble with coverage in the weld area. We use tribo guns in an all-manual operation. Is this the correct gun for this type of product? Will automatic spray guns do a better job in the weld area?

This is difficult to answer because the application issues are numerous. Wire coating faces problems with Faraday areas as well as aerodynamic issues that can occur as the powder and air travel around the wire and intersections. Unfortunately, E-coat, although providing additional protection in difficult areas, adds to the application problem. Having the parts E-coated at a different location and then rehung on your line increases the resistance, making electrostatic dissipation more difficult. The E-coat can be charged by the ions generated with corona charging equipment and, as a result, make penetration and coverage even more difficult at the welds and wire intersections. The advantage of the tribo guns is the minimal amount of external charging field that the guns generate. The disadvantage is the relatively poor transfer efficiency and high labor required. The effects will be the same with a corona gun that has to be "tuned" back so far to reduce the charging issues enough to allow coating those tough areas.

Is tribo the only way or best application? It may be. However, the only way to determine which is the best is through testing with more than one formulation with a number of conditions, such as potentially preheating the parts to enhance performance in those weld areas. Automatics will allow you to apply more consistently, although it is probably not realistic that automation by itself will eliminate the need for manual reinforcement. Another alternative is to explore other methods of rust prevention, such as conductive primers. Take a close look at the fabrication and design of the products as well. Maintaining good spacing practices for the wire (minimum 1.5 times the wire diameter), high quality welds to eliminate trap areas and clean cut off of the wire, bar or other materials that will reduce sharp areas, the first to fail, will enhance the application.

Possible Causes of Sieving Problems Numerous

We are having problems controlling our rotary sieve. We empty the waste bag two or three times a day, and there's a lot of scrap powder. We use a 100 mesh screen because we want to be sure there's no dirt in the reclaimed powder. Is this the right screen size? Is there some other feature of the screen that can be adjusted to avoid so much lost powder?

Whenever I hear this, what pops into my mind is that nobody likes to throw away powder, and what changed to cause the situation? If you don't know why you are using 100 mesh, then test a 60 or 80 mesh to see if it meets quality requirements and allows increased flow. Also, be sure to understand the type of contamination that you are trying to remove, where it comes from and its particle size relative to your target powder particle size.

If you must use this level of sieving, analyze the powder going into your scrap bag to see if it's really "bad." Determine the size and type of particles. Some situations in your system can cause the same result, such as moisture or a change in product density or quality.

Other areas that can affect sieve performance:

• Loose, stretched or improperly tensioned screens. Check the seating of the screen on each end. Verify that the screen is "drum" tight.

• Verify the correct screen model is being used and that the gap between the blades and the screen surface meets the manufacturer's specifications.

• Good sieving performance requires a balanced atmosphere. Venting or isolation of the screening process from undue pressure is critical.

• If the sieve is connected directly under a collector, it may be under high vacuum and require isolation by utilizing a rotary airlock.

Sieve sizes are based on the volume needed to pass a specific opening area. If the volume has increased but the screen surface area has not, you will lose more powder. Be sure to check with the manufacturer regarding the capacity of your unit for the mesh size you need.