For the younger readers, the phrase “Cool it. goes back to the 60’s and 70’s, and often was used to settle down an unruly situation. When you were dealing with an agitated person, it was common to shout out “Just cool it, man!” This phrase was replaced later with “Chill out, man!. and eventually to just “Chill!” Of course, those of us who are even older remember that the phrase “Cool. was used to describe our approval of something that was really “Neat,. way before the term “Rad” (radical) was even in the lexicon of hip slang. But I digress.
It seems that our concerns about properly curing the powder coating on a part overshadows the cooling of the part, even though it has equal importance in a well-designed powder coating process. So I thought I would provide you with the information necessary to give you a perspective about powder coating system design that you may not have properly considered before.
Why do we want to cool parts off?
As most everyone knows, 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 exit most dry-off ovens at temperatures of at least 212°F or often even hotter.
This part temperature is not recommended for thin film (< 3 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 base coat and ensure proper intercoat adhesion with the top coat. Most gel ovens are designed to heat the part to > 250°F to gel the powder base coat. However, the top coat powder application should be per-formed at the aforementioned < 100°F for the same reasons discussed above.
Final cure of most powders occurs at part metal temperatures of > 300°F and often > 400°F in shorter oven designs. 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 because 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 that roller transport systems do not damage the resultant finish.
These are all very valid reasons why cooling a part is necessary to obtain the quality of finish required by most operations. In fact, one can say that cooling a part is equally as important as heating it, as far as the powder coating process is concerned.
How does part cooling work?
The physics of heating the parts for dry-of base coat gel, and top coat cure is the same as what is required for cooling the parts. The heating and 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 or 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 or cool the parts. So if it takes 5,000 BTUs to heat a part from ambient to 250°F, it will take the same amount of 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 in convection ovens. However, the source to cool parts is often a non-active source, such as cool ambient air from inside or out-side the plant. This can be effective if it is coupled with high air velocity to speed the cooling process. Asa 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 then when thinner, lighter parts are powder coated. This is when serious consideration to part cooling is required in proper powder coating system design.
How do you design a part cooling system?
There are several alternatives to accomplish part cooling in a powder coating system design. The first method, which is also the simplest, is 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 set the part outside the oven where it wont come into contact with anything until it has cooled. In a conveyor system powder coating process, this is done by having enough conveyor track or rail to allow the part to cool at the design line speed of the process. 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. Imagine a conveyor pro-cess that is operating at 15 fpm coating heavy mass parts that require 30 minutes to cool to the desired metal temperature, requiring 450 feet of conveyor…WOW!
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 to do with all the 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 noninsulated walls to contain the heat while fans take filtered air from outside the plant to cool the parts and exhaust the resultant hot air from the chamber back outside the plant. The fans used to exhaust the air are typically sized to remove at least 10 percent more air than the supply air fans to ensure heat containment within the chamber. Duct-work 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 that the air is chilled rather than being at ambient temperature. Chilling the air significantly reduces the time needed to cool the past by increasing the temperature difference between the part temperature and the cooling air temperature. This will reduce the floor space required to site 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.
Selecting a cool-down 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. Selecting the wrong cool-down method can lead to inefficient process designs that require inordinate amounts of floor space or, even worse, produce hotter than anticipated parts.
To paraphrase Goldilocks from Goldilocks and the Three Bears: “It’s all about the just-right temperature when it comes to porridge.” When you think about it, the same philosophy applies to a properly designed powder coating system, and that’s pretty “Cool!”
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For further reading, see the “Index to Articles and Authors 1990-2013,” Reference and Buyer’s Resource Issue, Powder Coating, vol. 24 no. 6 (December 2013), or click on the Article Index at www.pcoating.com. Articles can be purchased online. Have a question? Click on Problem Solving to submit one.
Nick Liberto is president of Powder Coating Consultants (FCC), a division of Ninan, Inc., 1529 Laurel Ave., Bridgeport, CT 06604. Established in 1988, PCC is an independent engineering firm specializing in the use of powder coating technology. Nick has more than 3 decades of experience in the powder coating industry and is a member of many industry associations, including the Application Equipment Technical Committee of the Powder Coating Institute. A registered professional engineer in Connecticut, he holds a bachelor’s of science degree in mechanical engineering with a minor in physics. He can be contacted at 203/366-7244; email firstname.lastname@example.org; website www.powderoc.com.