By Mike Maxwell
Fluidized bed powder coating has been in use since its inception in the late 1950s. It is not difficult to find many examples of its continued use for important products that surround us every day. However, even with the longevity of this technology, there are still very few clear guidelines on how to develop a professional coating system of this kind.
The fundamental outline of the fluidized bed process involves cleaning the part, heating the part to above the melting point of the powder to be applied, and dipping the part in a tank of fluidized powder. There are several other important details to consider, but let’s take a look at the powder application area first.
The Fluidized Bed Powder Coating Applicator
The fluidized bed powder applicator is a specially constructed tank that is filled with dry powder in its upper region with an open plenum in the lower region, and a permeable membrane that separates the upper tank from the lower (Figure 1). Air is supplied to the lower plenum and allowed to diffuse through the membrane in such a way that it also permeates the powder to form a multi-phase fluid. In simpler terms, the air flows through to make the powder fluffy.
Tank Sizing & Material of Construction
Tanks are typically rectangular but may also be cylindrical or take on more exotic shapes. The size and shape of the parts being coated should drive this decision.
It’s fairly easy to size a tank. There should generally be approximately six inches of clearance all the way around the part(s). Of course, this greatly depends on their size and how many will be dipped at once. It is wise to consider an additional allowance for space underneath the parts in case any fall off into the tank. While you want to ensure adequate space to coat your parts, it is also important to minimize the size of the tank for several reasons. Powder inventory, contamination risk, air capacity costs, and tank duplication costs are all factors that benefit from smaller tanks.
Fluidizing tanks have been constructed from a wide array of materials from plywood to plastic. However, plain carbon steel or stainless steel tanks have the greatest durability and have qualities beneficial to sustaining reproducible coating quality. It’s important to note that welded metal construction tanks should be painted to prevent corrosion, but the surfaces that come in contact with fluidized powder, the inner upper tank, should be bare and oil free. Metal tanks will also experience less distortion from fluidizing air pressure in the lower plenum.
Fluidizing Membranes
Selection of an appropriate membrane is one of the most critical, and most overlooked, steps in the system design. Air permeability is the important feature to consider when selecting the membrane material.
Fluidizing media options include sintered porous plastic plates, permeable fluidizing fabric, and sintered porous metal plates.
A word of caution: Many powder coating operations are familiar with the fluidizing hoppers used to supply powder to their spray guns. This often leads to ambitious DIY fabricators trying to scale up hoppers into larger fluid bed dip tanks. This rarely works well. Fluidizing hoppers frequently use the sintered porous plastic plate as a membrane. These materials have very low air permeability compared to other options. That means they also require higher operating pressures that can be out of balance with smooth, uniform fluidization. Fluidizing membranes are not good structural materials and will experience considerable loads from air pressure during operation or supporting the weight of the powder otherwise. It is important to consider the addition of structural supports to reduce deflection. The support grid can also serve to protect the material if any hot parts fall off into the tank during coating operations.
Fluidizing Air Supply
The air supply can be from compressed air or a blower. Evaluating which to use and when is best determined by a combination of some reasonable rules of thumb.
Air routers are high consumption tools commonly used in fab shops, intermittently at about 30 cubic feet per minute (cfm). A rotary screw compressor can deliver about four to five cfm per one horsepower (hp). So, an air router may consume in the neighborhood of five hp on occasion but does not truly run continuously. We might then set a guesstimated upper limit for using compressed air as the fluidizing air supply at about 30 cfm. That would be appropriate for a tank approximately 15 inches wide by 15 inches long without regard for depth. This is not a hard rule but does serve as a fair point of reference.
For larger tanks, blowers are far more cost effective. The blower selection is made based on the air volume and pressure required by the tank size, the permeability of the membrane, and the critical velocity of the powder being fluidized.
A Brief Example
Assuming a tank that is 24 inches wide by 36 inches long, the horizontal cross section would be six square feet. Aiming for a critical velocity of 15 feet per minute (fpm) would then give an estimated 90 cfm of fluidizing air volume. In order to complete the picture, we also need to estimate the pressure requirement. The membrane selection permeability and an estimated weight of the powder will help to answer this. For permeability in units of cubic feet per minute per square foot at a given pressure, we can find a material with 0.5 cfm at 0.5 inch water pressure per square foot. A membrane of this size would emit about 90 cfm at 17 inches water pressure. The estimated weight of the powder (≅220 lbs.) over this surface would result in additional pressure of about seven inches water pressure for a total estimated back pressure of 24 inches water pressure. With those performance criteria, something near a 2 hp blower would be in the right ballpark.
Additional Application Equipment Features
A fluidized bed powder coating applicator can have several other features. The tanks can incorporate a wider flare at the top to keep more powder in the tank. In this case, casters or leveling legs for tank positioning should be considered. Large tanks can include forklift pockets or lifting lugs for maintenance functions.
Mechanisms for part agitation and removal of excess powder can be incorporated into larger capacity automated systems. In addition, a level sensor and powder pump can automate the replenishment of powder during long production runs. Maintaining powder level also ensures consistency of the fluid density and coating applied. There are many options and recommendations for the addition of vibratory agitation of the tank. However, if your tank system is designed properly, this measure is unnecessary.
Sensors to monitor the air supply velocity and pressure can provide helpful feedback for process control. There are temperature sensors that can monitor the surface temperature of the fluidized powder to give feedback for process control. The fluidizing air may require either heating or cooling of the air stream. Some powders readily absorb water in humid conditions and these measures can mitigate such negative effects.
For high-capacity automated production lines it is possible to incorporate shuttles to swap colors on the fly by switching tanks.
Fluidized Bed Coated Parts
Fluidized powder is a multi-phase fluid, but that doesn’t mean it behaves entirely like a liquid. If there are any significant obstructions to the airflow, then the powder will stagnate and accumulate. Parts with large flat horizontal surfaces, like a deep drawn pan, are poor choices for fluid bed dip coating as it would act like a scoop. Wireform weldments compose a large category of viable parts for coating in his manner. Parts in this class include sports facemasks, dishracks, refrigeration racks, point of purchase display racks, shopping carts, some outdoor furniture, etc. Insulation of electrical conductors like busbars is another common use for fluid bed coating. These types of parts typically require heavier film builds than electrostatic spray can apply. In addition, back ionization and Faraday cage effects from spray applicators usually provide inconsistent results on these types of products.
The process requirement of preheating parts before fluidized powder application implies that this method is primarily for metal parts. Though steel is the most common material coated, copper and aluminum are also viable choices.
How parts are handled for conveying is an important fundamental of fluidized bed powder coating. In most cases the part hanger becomes encapsulated with coating and is entrapped with the part. There is so much attention to detail required in designing the part hanger concepts for this application process that an entire article could be dedicated to outlining only those strategies. In most cases, it is imperative that part hanging considerations be integrated with part design.
The Complete Fluidized Bed Coating System Layout
The overall layout for a fluid bed coating style paint shop is very different from electrostatic spray. Generally speaking, the process workflow for unmasked parts is loading, cleaning, drying/preheat, dipping, post heat, cooling, and unloading. The process requirements for these steps affect the specifications for conveyor, part washer, and ovens.
The way in which the parts are dipped can be segregated into three general layout types, each of which has its own speed limitations and capital equipment costs.
Manually dipped—The parts are manually manipulated through the powder. Very little equipment is required for this, for example, a conveyor is not always necessary. This approach is best for low production quantities and does not work well when the timing of the dip requires automated precision.
Lift dipped—The parts are dipped in the powder by lifting the fluid bed up to the parts while they remain on a stationary conveyor. Process speed is limited by the ability to raise and lower the entire tank. This generally works well for medium volume production requirements and in situations where the timing of the dip process requires automated precision. It is not feasible for very large dip tanks.
Lowerator dipped—The parts are lowered with an automated conveyor into the powder. This method works well to achieve very high production throughput with automated precision but is significantly more costly than manual dipping.
Airflow around the applicator tank should be managed just as importantly as it is with electrostatic spray booths. A cross draft airflow exhaust system is often the only approachable strategy to collect any powder that might escape the top of the tank.
Process heating of the parts also has a significant impact on the equipment layout and specifications. The preliminary needs always come from the powder supplier, but the thermographic profile for the way the parts heat and cool should be measured before designing a system layout. The timing for when the part is at the right temperature to be dipped and melt powder is deserving of diligent study so that measurement-based decisions can be made in the design phase.
The Professional Difference
It is easy to search the general concepts for fluidized bed powder coating on the internet. There is enough information out there to figure out ways to get started. For the most part, this type of powder coating can be very forgiving and easy to accomplish.
However, there is a significant difference between an ad hoc DIY fluid bed coater and a professional fluid bed coating organization that can reliably produce thousands, or millions, of high-quality parts per year. The concepts and considerations presented can help accelerate the move towards a more professional fluid bed coating operation.
Knowledgeable finishing systems providers and consultants can help get you across that finishing line.
Mike Maxwell is president of Larea Corporation.