Part Cooling Techniques & Why
Posted on Monday, June 1, 2015
Nearly everyone in this industry knows that properly curing the powder coating on a part is important. However, cooling the part is equally important in a well-designed powder coating process. Selecting a cooldown method takes as much forethought as selecting part heating systems for powder coating. Ignoring this important process step can easily result in uncontrolled film thickness, damaged powder coatings, and intolerant plant conditions. Following are ways to accomplish part cooling along with an explanation of why it is important to do so.
Why Cool Parts?
Of course, parts need to be heated to dry them after chemical pretreatments. They also need to be heated to “activate” polymer-based pretreatments and sealer chemistries used in many modern washer chemistries. These situations require the substrate to be heated to at least 212°F (the evaporation temperature of water at sea level) for drying purposes and to at least 250°F for polymer sealer and pretreatment activation purposes. This means that parts exiting most dry-off ovens are 212°F, or often hotter.
This part temperature is not recommended for thin film (≤ 5 mils) powder application, as hotter parts will attract more powder and have thicker dry film thickness (DFT). If the part is too hot, for instance above the melt point of the powder being applied, then the powder will sinter (partially melt) on the part surface, causing appearance issues (excessive orange peel). Ideally, if you do not want part heat to affect your powder coating thickness or appearance the part temperature should be below 100°F.
In two-coat powder coating systems the first coat needs to be gelled to flow the basecoat and ensure proper inter-coat adhesion with the topcoat. Most gel ovens are designed to heat the part to ≥ 250°F to gel the powder basecoat. However, the topcoat powder application should be performed at the aforementioned ≤ 100°F for the same reasons discussed.
Final cure of most powders occur at part metal temperatures ≥ 400°F. This means that the parts exiting powder cure ovens are often “too hot to handle” at the unload area without operator protection. Additionally, handling powder coated parts that are warmer than the Glass Transition Temperature (usually 250°F for most powder formulas) can cause surface defects as the coating is still “tacky” or soft.
In functional coating applications part cooling is imperative to allow part handling without damaging the applied coating. These processes often use induction coils to preheat parts to ≥ 600°F before the powder is applied and must rapidly cool the part to ensure roller transport systems do not damage the resultant finish.
How Does Part Cooling Work?
The physics of heating parts is the same required for cooling the parts. The heating/cooling of parts is a function of the substrate material (iron, steel, aluminum, brass, etc.), its accompanying specific heat value, the part mass (or weight), and the delta temperature between the target heating/cooling temperature and ambient (desired temperature minus the starting temperature). The result of this calculation is the energy units in BTUs (British Thermal Units), or appropriate metric equivalent value (calories, joules, etc.), required to heat/cool the parts. So if it takes 5,000 BTUs to heat a part from ambient to 250°F it will take the same energy to cool the part back to ambient.
The source of the energy to heat parts is normally a gas burner, oil burner, or electric heating element commonly used convection ovens. However, the source to cool parts is often a “non-active” source, such as cool ambient air from inside the plant or from outside the plant. This can be effective however, if it is coupled with high air velocity to speed the cooling process. As a rule of thumb, the amount of time to heat a part is roughly the same to cool it back down to ambient, if high air velocity is employed to aid the cooling process. This means that if it takes more time to heat heavier mass parts than lighter mass parts, it will require more time to cool them as well. As a result, part cooling is much more important when thicker, more massive, parts are powder coated than thinner, lighter parts are powder coated. This is when serious consideration to part cooling is required in proper powder coating system design.
Part Cooling System Design
There are several alternatives to accomplish part cooling in a powder coating system design. The first is the simplest: ambient cooling. This is where the part is exposed to the plant atmosphere long enough to allow it to naturally cool to the desired temperature. In a batch powder coating process, this is done simply by having an area to “park” the part outside the oven. In a conveyor system powder coating process, this is done by having enough conveyor track/ rail to allow the part to cool at the process’ design line speed. This can be problematic when dealing with heavy mass parts at high temperatures coupled with fast line speeds, as the time required to cool the part may require an excessive amount of conveyor.
Adding floor fans to move air across the parts during ambient cooling can help accelerate the cooling of parts. The efficacy of this approach can be impacted by the plant air temperature, which means on warm days the ambient cooling may not be what you expect. Another problem with this method is: what do you do with all this hot air you are dumping into your plant? If this energy load is excessive, it can make the powder coating area very uncomfortable to work in and can adversely affect the powder coating process itself. This is when it is time to look at another part cooling method.
The second method of part cooling is a forced-air cool-down tunnel. This device uses a chamber made from non-insulated walls to contain the heat while fans that take filtered air from outside the plant to cool the parts and exhausts the resultant hot air from the chamber to outside the plant. The fans used to exhaust the air are typically sized to remove at least 10% more air than the supply air fans to ensure heat containment within the chamber. Ductwork to evenly distribute the supply air and strategic placement of the exhaust fan inlets are frequently used to better control the cooling process. Installing paddle fans to promote more air movement around the parts is also effective to better manage and improve the cooling process.
Cooling tunnels can significantly reduce the time required to cool parts when compared with the ambient cooling method. The added benefit of removing the heat from the plant environment is another significant consideration for using a cooling tunnel. We have some enterprising clients in cooler climates that vent this captured heat to personnel areas for heating in the wintertime and outdoors in the summertime reducing their heating bills.
The third method of part cooling is a refrigerated-air cool-down tunnel. This design works the same as the forced-air cooling tunnel, except the air is chilled and not at ambient temperature. Chilling the air will reduce the time to cool the part significantly by increasing the temperature difference between the part temperature and the cooling air temperature. This will reduce the floor space required for the cool-down tunnel, an important issue in fast line speed processes or when very heavy mass parts are processed. Of course, the cost to refrigerate the air can be very expensive and must be justified prior to purchasing this process equipment element.
The fourth and last method of part cooling is a water-quench process. This design sprays cold water onto parts to cool them rapidly. Typically used in support of functional coating systems, such as Fusion Bonded Epoxy (FBE), this approach can leave water spots on the part surface. However, if you are coating concrete rebar or pipe coating, this is the only method that can overcome the residual part heat in the time the process allows for part cooling.
Now that you know the importance of these “cool” ideas, give some serious thought to this process step. In the long run, choosing the proper cooling method will lessen uncontrolled film thickness, damaged powder coatings, and intolerant plant conditions.
Nick Liberto, P.E., is president of Powder Coating Consultants, division of Ninan Inc., an independent technical consulting firm in Bridgeport, Conn. He can be reached at firstname.lastname@example.org.