- May 01, 2012, Mark Miller
I had a great physics professor in college. When I was considering what area of engineering to go into, he explained the world of technology in a simplified model—everything deals with flow. Electrical engineers deal with the flow of electricity through wires; civil engineers deal with the flow of traffic through streets; industrial engineers deal with the flow of manufactured materials through a plant; and chemical engineers deal with the flow of fluids through a pipe. So I decided to become a plumber!
As I have experienced fluid coating in industry over the years, I would like to think that the fluid dynamic problems I have encountered are more highbrow than a monkey wrench and cleaning up after plugged flow, but alas, the world of chemical engineering is sometimes no more than cleaning out the fouling in a stainless steel pipe and making sure the material flows downhill. But how does being a good plumber help you be a great coating technology expert? Air.
One of the biggest defects seen in coated products is air. Air can be introduced through entrainment or entrapment. Air entrapment is when the air is trapped between the fluid and the substrate. This is an important consideration in coating head process control but not as difficult to overcome as the plumbing problem—air entrainment. Entrainment occurs at some point prior to the entrance to the coating station. And what does the fluid pass through before the coating station? Pipes (see where this is going?). If you look at the space between the pump and the coating station, the analogous science of plumbing provides an important insight into what is going on with the fluid and air that may exist in the system.
So let’s look at the layout of the piping between the pump and the coating station as a plumber. How many turns, elbows, and angles should we put in the pipe system? As few as possible. Does it make sense to have the pipe go up, down, and all around the manufacturing facility before making its way to the coating station? No. The simple rule of thumb is to make the fluid travel as short a distance as possible and always move up hill. Traveling a short distance makes sense because you don’t want to have to waste material and clogs can be cleaned and issues can be found quicker. Moving up hill makes sense for a couple of reasons:
- the fluid/air mixture will have the air rise to the surface and the fluid drop to low settling areas
- the coating station standing at the high point of the fluid flow path allows for a relief valve for the air to rise and release so that the system is purged
Imagine a roller coaster of pipes carrying a viscous fluid that can hold air for a long period of time. At the peaks of the roller coaster run, air will sit and slowly release over time. The result will be a coating defect that lasts the length of the manufacturing run with an intermittent coating defect that frustrates the coating operator and quality inspector for the entire shift. So when you are laying out the fluid delivery system for a coating operation, put on your plumbing hat and envision the flow of fluid. The more attention to detail that the operations engineer pays to plumbing, the fewer headaches the operators will have as products are coated day-to-day.
If air entrainment becomes unavoidable, be sure to read “Coating Matters” from August 2011 to find some solutions.
Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. Contact him at 715-456-9545; firstname.lastname@example.org; www.coatingtechservice.com.