Which Oven Technology is Right for You?

By Rich Saddler

Generally speaking, there are two types of powder coating curing processes, thermal (heat) energy and ultraviolet (UV) energy. An understanding of these technologies combined with several factors of consideration will help in determining which type of cure oven is best suited for your process.

Thermal technology is subdivided into two types: convection and infrared. Most ovens use a burner that receives natural gas or propane from a supply line, mixes the gas with air, and ignites the mixture to generate a flame that heats the air inside the oven. This heated air is circulated through the oven and provides heat to the powder coating and the substrate. As the powder coating increases in temperature, a chemical reaction takes place and the heated coating goes through a melt, flow, and crosslink process, and ultimately cures to a protective, hardened surface.

Convection Curing
Convection curing is the workhorse of the industry. Convection energy typically comes from natural gas or propane sources. In limited applications, electrical convection heating is used, but usually only where natural gas or propane is not available. When considering convection curing, the mass of the part being powder coated is key. As the convection energy heats up the surface of the part, that energy moves towards the core of the part to allow the part to reach equilibrium. This can extend the time required to heat the part and coating to allow the powder to go through the curing cycle.

Infrared Curing
Infrared curing utilizes energy in the infrared spectrum to heat the coating. This energy is located between visible light and microwave on the electromagnetic spectrum. The infrared energy is transmitted directly from the emitter to the part. Based on the surface of the part, this energy can be reflected off the surface, absorbed into the surface, or transmitted through the surface. The absorption, reflection, and transmission amounts are based on the spectral characteristics of the coating. The way to quantify this amount is based on a scale of 0 to 1. A perfect absorption is a black body which has a value of 1, while a very reflective surface like polished chrome or metal would have a low value of close to 0 and would not absorb much infrared energy.

To generate this infrared energy, the energy source can be natural gas or propane like the convection curing process, but electricity is often used because of the ability of electrical infrared bulbs to provide infrared energy. Although infrared energy is a continuous range, it is subdivided into wavelength spectrums for better classification. For purposes of this article, we are going to focus on short, medium, and long wavelengths. While the specific dividing lines between the three ranges are often a topic of debate, generally accepted divisions are short wave 0.76-2 μm, medium wave 2-4 μm, and long wave 4 μm-1mm. General temperature ranges for these classifications are 2,150-6,400 degrees Fahrenheit for short, 850-2,150 degrees Fahrenheit for medium, and 250-850 degrees Fahrenheit for long (Figure 1).Medium and long wavelength infrared energy can be produced by ceramic emitter panels with natural gas as their energy source. These ceramic panels not only produce infrared energy, but they produce convection energy as well. Short wave infrared energy is produced by infrared T-3 style bulbs. The short-wave energy is very intense heat energy and must be controlled so that it does not overcure and burn the powder coating. The absorption properties of the coating and length of time the coating is in front of the lamps will impact the curing process.

Ultraviolet Curing
Ultraviolet energy is on the other side of the visual light portion of the electromagnetic spectrum from infrared energy. It is located between visible light and x-rays. To provide a smooth coating, the parts are usually heated to a temperature which allows the powder coating to go through the melt and flow stage in either a convection or infrared oven. However, due to the formulation of UV-curable powder, the coating does not crosslink with thermal energy only. The UV energy/bulbs activate the photoinitiators in the powder to crosslink the coating. The UV bulbs use electricity as their energy source.

One of the benefits of the UV process is that the coating does not need to be subjected to a high temperature to cure. The melt flow stage of a UV coating needs to reach approximately 250 degrees Fahrenheit to gel and flow. The UV energy does not transfer thermal heat into the coating.

Which is Best for Your Application?
A significant benefit of convection curing is that it reaches all surfaces of the part. Meanwhile, both infrared and ultraviolet energy are line-of-sight cure processes. Any hidden surfaces to the infrared or ultraviolet emitters will not receive the energy necessary to cure the coating. Often emitters will be placed in different positions and angles to allow the energy to reach surfaces that are not directly facing the emitters. If the product you are coating is highly varied with different colors, shapes, sizes, and material thickness, a convection type cure which is designed for your parts would provide the most consistency and flexibility.

Often a section of medium to long wavelength infrared oven installed ahead of a convection oven will provide a ‘quiet zone’ of heat into the coating that begins the melt and gel phase of the cure. After the coating begins to gel, the part then moves into the convection portion of the oven where higher velocity air will not dislodge the powder paint from the substrate and impact the visual and performance characteristics of the final product. The infrared section of the oven can also reduce the amount of time required in the convection oven and reduce the overall size of the combination oven as compared to a convection oven only.

Heavy, thick products like street light poles and lift truck counterweights which would take hours to absorb enough heat to cure the coating in a convection oven are ideal candidates for infrared oven curing. The infrared energy is transferred into the coating and the surface of the product without heating up the entire substrate material for the coating to cure.

If your product is all one size, color, and material thickness, a short-wave infrared oven can provide quick, high intensity cure in a small amount of space. Detailed controls would be required to modulate the intensity of the heat and minimize the potential for overcure.

For product that has heat sensitive substrates such as plastic and wood, a UV-curable powder would provide a durable finish with a minimal amount of heat into the core of the substrate. UV-curable powder coating is also a candidate for large mass products that would require a significant amount of thermal energy to heat up the substrate and cure the coating.

Keep in mind, a powder that is formulated for convection cure can also be cured with infrared and vice versa due to the thermal energy properties of the coating. However, powder to be cured with the UV process must be specially formulated.

As always, working with your coating and equipment suppliers to test the different curing options and evaluate the performance of the coating based on your specific specifications is recommended to verify that you select the best material and process possible.

Rich Saddler is finishing consultant with Industrial Finishing Solutions, LLC.