Fluidized Bed

Question:

I work for a custom PVC coating and molding company. We do dip molding and coating. We are now trying to get into some powder coating using PVC powders in a fluidized bed. We have been using our current ovens to attempt this process. We have built our own tanks to roll our material. We have struggled to stay consistent in our processes. We need help to understand what the rules are. How much heat? How much roll in the material? Pin holes. Do we need to prep the parts prior to dipping? One day we think we have it, then the next day the parts run differently. We need a resource to tell us if we have the right stuff or not, and we need to know the ground rules. Are there any resources around Minnesota that we could use as a consultant? What literature do you recommend? At this time, any of us using powder hate it because we don’t know what we are doing. Any suggestions? E. K.

Answer:

Well, it just so happens that my chosen profession is being a consultant. I travel to Minnesota quite often. Maybe you had a bad experience using consultants in the past. That’s probably why there are so many jokes about consultants. I have heard most all the jokes about consultants. Like: “What’s the definition of a consultant? Someone who knows 1001 ways to make love but doesn’t know any women.” My favorite one is “How can you spot a consultant? He’s the guy with a briefcase that’s from out-of-town.” Anyway, let’s get to your question.

Performing fluidized bed powder coating application is an art more than a science. It all starts with a well-designed and constructed fluidized bed. Most people who design and build their own fluidized beds are unsuccessful in their first try. I will assume that your fluidized bed is poorly conceived. So, first I recommend that you contact someone who makes these devices to get one that works properly. Go to www.pfonline.com/suppliers.html to get some supplier names.

A well-designed fluidized bed will fluidize the powder to a low simmer evenly throughout the entire container. A “rolling” situation is not what you want to see when you talk about the condition of the powder within the bed. If the fluidized bed is too violent, it will cause inconsistent coating on the part where the air bubbles brush past the part’s surface. A violently fluidized powder bed will cause the powder to become airborne and migrate throughout your plant, as well.

Check with your powder supplier for the proper part preheat temperature. It is normally below the melt point of the powder when you want thinner film builds and closer to the melt temperature for higher film builds. Thicker film build up will cause the pin holes you are seeing on the part surface.

How long the part is in the fluidized bed depends upon the part temperature and the desired film build. Higher film builds require more time than lower film builds. In all cases, the time is measured in seconds or minutes, not hours.

As far as prepping the parts before coating, you should always make sure the parts are oil and dirt free to ensure good adhesion and eliminate coating contamination. Some thermoplastics (like PVC) require that the parts have a liquid primer applied to the surface before coating to provide better adhesion between the part substrate and the powder coating. Verify this with your powder supplier.

As far as literature, there are two books that will help you understand the principles of fluidized bed powder coating (or powder coating in general for that matter). The first one is Powder Coating; The Complete Finisher’s Handbook, Nicholas Liberto P. E. Editor. The Powder Coating Institute publishes this book. The second book is The User’s Guide To Powder Coating, Nicholas Liberto P.E. Editor. The Society of Manufacturing Engineers publishes this book. Both books are available through www.powdercoat.com.