What to Look for in a Curing Oven

Q: The curing oven on our powder line is more than 30 years old, and we are in need of a longer cure time. We are beginning the search for a source for a new oven. Do you have any advice that will help us evaluate the features and benefits of cure ovens?

A: Buying a new oven is challenging because it is not something that you will do frequently. You may buy only one cure oven in your lifetime. Start by considering what you like and do not like about your current oven. You said it needs to be longer, so you have either added heavier parts, sped up the line or both. Or maybe it was never big enough to start with, not an uncommon situation. So first make sure the new oven is big enough to handle your heaviest parts and manage the line speed that you need.

Make sure the heat containment is well thought out. I personally favor longer vestibules at the oven entrance and exit to help better separate the cool air and hot air. Sometimes the distance between the outside of the oven and the heated zone is too short, and it is very hard to contain heat. Oven air curtains (high-velocity air fired across the opening to create an air barrier) can help, but they have some deficiencies. They must fire air at high velocity, and that can disturb powder at the entrance. They cannot maintain that high velocity for a great distance, so they are increasingly ineffective as the opening size gets larger. They need a large motor, and the electricity they use may exceed the gas savings. Still, they can be helpful on small oven openings if the vestibule is long enough.

Heat-relief hoods (a capture hood over the opening with a stack to the outside) can be useful for funneling the heat that rolls out of the oven out of the building. They can help reduce heat gain in the building, but they, too, have some weaknesses: They do not save any gas, and they are less effective if the building is under severe negative air pressure.

Check all the numbers on the oven quote, comparing Btu per hour, recirculation fan volume, exhaust volume, motor sizes and insulation thickness. This is not to see which quote has the biggest numbers but rather which one has the right numbers. We always want at least 3 inches of insulation, but 4 inches is much better. To understand the burner size and fans sizes, you likely will need some input from a consultant or other source. The Chemical Coaters Association International offers some books on that subject, or you can look for articles on pfonline.com that contain oven sizing information. The “Powder Coaters Handbook” from the Powder Coating Institute also will have some information that may be useful to you.

Another consideration for a new oven system is the controls. Check out if it is PC-controlled or uses a touchscreen, and then see how it actually works. The proposed oven could have some great features if you are working with the right source.

A final item to consider is the use of infrared (IR) heating at the entrance of the cure oven. Some IR could be a great addition for heavier mass products. It can also help with other factors like powder flow and gelling the powder before it hits the moving air inside the convection oven. Make sure any oven supplier who is recommending IR understands the technology and some of the science with regard to size, the way it is controlled and the type of emitter used. If the supplier offers low-cost IR with no idea of its function, it is not a good sign.

Go out into the field and see some ovens; talk to their owners and find out as much as you can, especially about ovens from suppliers you are considering. Check out references and do some research on features. Make sure the oven you eventually choose is big enough; do not go with low price as the guiding principle. Make sure you understand the value of the features and you will get a much better payback on the investment.

Q: We have been in the powder business for a few years now and have learned a lot about running the line and turning out a good powder finish. We have made progress over the years in reducing our rejects, but we still find that we get too many defects that we cannot identify, and that makes it hard to eliminate them. Common problems include dirt in the film, pinholes in the coating and craters. How can we better identify the defects so we can work on solutions to reduce or eliminate them?

A: I have spent many hours examining these types of defects and figuring out root causes to help resolve their sources. It is challenging, but there are systematic ways to categorize a defect and work towards identifying the root cause and solution. Knowledge and experience play a role in this, but if you start with some basic methods and work systematically, you will learn and improve your skills over time.

Let’s start by considering what you call “dirt.” I would separate that word into a number of different types of particulate, something standing up in the film. Look for practical identifiers like:

• Where is the particulate in the film, on the substrate or on the surface? This will help to connect the source to a specific step in the process, like pretreatment, application or in the oven.
• What are its size, shape and color? This also will help better define potential sources.
• Are there any patterns that can be identified, like colors, locations on the part, process variables, and days of the week or times of day? Patterns help lead to sources.

Categorize the particles if you can find familiar features. Use magnification to get a better look at them. A simple microscope can be one of your best tools for dirt identification. Try to find something in or around the system that matches the particle. For example, suppose you find a red flake on the finished surface. You inspect the oven and find that some exposed steel is rusting and flaking directly above where the parts were curing inside. You compare the magnified flakes from the oven and the part defect, and they seem to match. That may reveal an area of the system that needs to be fixed. Not all particles will show such neat connections, but you get the idea.

In addition to more formal identification of particulate, be sure that you are running a clean operation. Good maintenance practices can help limit dirt sources and reduce defects.

Pinholes and craters are somewhat more challenging. They may be caused by metal porosity or some contaminate on the surface. If the part is a cast metal, you should consider outgassing as a source for pinholes. If the metal is formed or welded, it may have porosity or contamination. You may want to turn to lab analysis to see what you can learn about these types of defects. Scanning electron microscopy/energy dispersive X-ray spectroscopy, for example, may reveal characteristics about the crater that tell you the possible source. Microscopy alone can show depth, size and other features that may tell you what the defect is and where it came from.

I would suggest that you assign someone in your facility to work specifically on defect analysis on a regular basis. A person that spends some of their time each day with a microscope and builds a library of defects can be very valuable in helping reduce defects. They can see patterns quickly and help to guide resolution. They can supervise lab testing or sampling on the line to work on defect discovery. They do not have to work full time at this, but an hour or two a day can be very effective.

Also look for presentations at conferences and other sources of information about formal defect identification, or hire a consultant to help train your staff in methods of inspection and identification. A little investment in training can save a lot in rework and scrap. A good defect analysis person can pay dividends for many years.