Powder on Heat-Sensitive Substrates

Posted on Wednesday, April 19, 2017

When it comes to powder coating heat-sensitive substrates, many factors must be considered to ensure the proper finish.

By: Marty Sawyer

In the world of oven design, there’s engineering, there’s experience, and sometimes there is even a little art. When you add the challenge of working with heat-sensitive substrates, it becomes exponentially more difficult. When finishing a heat-sensitive substrate, all types of heat technologies must be considered. Powder coating for these applications requires an investment in process design to ensure a good outcome, and the answer is seldom based on an engineering chart. For the best outcome, the business as well as operational needs need to be considered.

For the purposes of this article, a heat-sensitive substrate is a part that by its composition or design is very reactive to applied heat. When working with a piece of steel sheet metal, there is not much an oven can do to harm the part. If finishing with powder coating, the powder would eventually burn once it is past its overbake protection, but the part itself will be fine. When a part is made from plastic, fiberboard or even wood, however, it is going to be a challenge to incorporate heating to the process. To powder coat a plastic part, for instance, the part would melt long before it would reach the 450°F required temperature to gel and cure most powders. Heat-sensitive issues can also apply to any part that restricts your powder line design such as part geometry. As well, some metal assemblies can contain a material that is heat sensitive, such as the foam core of a door.

So, how can powder be used on these types of substrates and still not adversely affect the part? The easiest answer might be with the powder itself. Low-temperature cures have opened many possibilities for sensitive parts to be powder coated, and if the part can handle those temperatures, then the problem may be solved. However, if powder selection is not the answer, or if you are designing a coating line from scratch, then the next challenge is to review oven design. Most powder cure ovens, whether they are convection or infrared (IR), are typically designed to work with metal parts, which are quite receptive to applied heat. If the part is heat-sensitive, though, oven design becomes crucial as the heat must be controlled to get an optimum result.

In designing an oven for a sensitive environment, the first piece of data needed by any experienced oven designer is knowing how sensitive is sensitive. Like many works of art, this is a relative term. What percentage of temperature variation would be acceptable? In dealing with heat-sensitive materials and substrates, the first challenge is to identify the level of sensitivity. Some oven designers have test facilities in-house that can identify the particular heat characteristics of your part. With this knowledge, you can work on the best heat technology to heat your parts.

Factors that Make a Part Heat-Sensitive

First, precise control of the temperature is required by the part. When a part is very reactive to heat, precise control is required to ensure not overheating the substrate. Convection heat is the common form of heat technology used for powder coating applications. It is easy to use, but the response rate to control temperature is slow.

The highest level of control is with electric IR, which utilizes noncontact sensors or optical pyrometers to read the face temperature of the part by using a laser intercept light to “see” the part and gather readings at precisely when the target part is positioned in front of the reader. The optical pyrometers readings are fed into a proportional integral derivative (PID) temperature control loop as part of the programable logic controller (PLC). The function of these controls is to anticipate the heat energy needed and then the silicon controlled rectifiers (SCRs) increase or decrease the energy flow as needed to maintain the required heat output. The result is the heat output is modulated to maintain the selected setpoint temperature. Depending on the part needs, this adjustment can be done from seconds to instantaneous.

This near infinite level of regulation is what delivers the critical control of the application of heat for the sensitive substrate. MDF wood panels for instance, have a glue that binds the layers of wood fibers together which inherently contain moisture. The heat needed to cure the powder must be specifically controlled to prevent the evaporation of the moisture in the fibers which can cause bubbling or cracking. Powder coating on plastic or fiberboard can also have the similar constraints of too much or too little applied heat which negatively affects the product finish quality.

The second challenge with heat-sensitive substrates is when the maximum temperature cannot be exceeded because it has a thermal limitation. Some parts have a maximum temperature where if the applied heat exceeds this point, the part deforms. Plastic is the most common part with this limitation. The solution to this is varied depending on the particulars of the part. If the part temperature must never exceed a set temperature, then convection may be the easiest solution. A convection oven temperature can be set and processed parts will not be elevated beyond that setpoint. Convection also has very effective topline control on the heat and if that is the main challenge of the part then this can be an effective and a relatively easy answer.

Ultraviolet (UV) light ovens can also be a good choice for these type of parts. UV ovens cure through only light energy and can be very forgiving to heat-sensitive substrates. UV ovens can be a very good choice if this part is the only part processed on that line and the operating costs are within an acceptable range. The challenge is the powder must first go through a convection or infrared oven to flow the powder before it can be cured with the UV light. While the curing aspect happens at a very low temperature, the first powder flow oven still introduces considerable heat that must still be managed.

A third factor that makes a part heat sensitive is narrow thermal range tolerance, which again makes control a critical factor. Most steel parts are not affected by changing temperatures. However, some parts may only be able to tolerate a 15- 20°F temperature range to complete the required work and sometimes it’s only just a few degrees. Infrared is the typical choice for these parts, as a more definitive level of temperature control is needed. To provide additional control, IR ovens are typically designed to have “zones” of heat control. This allows powder to get very quickly gelled and then the following zones either top to bottom or front to back to finish the cure while still not adversely affecting the interior componentry of a part with excess heat.

Examples of parts with a narrow thermal range are refrigerant compressors or propane cylinders. These are assemblies made of mild steel and if manufactured and coated prior to final assembly, they present a fairly common powder coating operation. The challenge had been the heat sensitive interior seal that would degrade if subjected to temperatures above 180°F. This could not be accomplished using a typical convection oven design. The industry, though, has found a production cost savings by fully assembling the part and then powder coating. Using catalytic or electric IR, the parts can be fully assembled and cured with no damage to the heat-sensitive componentry. Due to the energy transfer speed of IR and controllability, the powder coating is fully cured before the heat sink of the part affects the heat-sensitive interior seal.

Other Considerations

Other issues that might create heat-sensitive issues in your process are complex metal parts, which can also exhibit some similar challenges. Due to the thicks and thins of a complex weldment, the part can be sensitive to some oven curing technologies. Shortwave electric IR, for example, typically has tremendous difficulty curing these parts because it will burn the thin or better seen sections before the thick areas can receive enough heat energy transfer to fully cure. Although convection heat may appear to be the straightforward answer for these types of parts, long- to medium-wavelength catalytic IR and mediumwavelength electric IR can often successfully do the job if properly designed.

It is always worth noting that on the opposite end of the spectrum are heat “insensitive” parts. These are parts that by the sheer size or thickness of the substrate they are very difficult to heat. Imagine the BTUs needed to powder coat an anvil or a forklift counterweight. These parts have such a heat sink from their interior mass, that the BTUs of heat required to heat them is enormous. Convection, which requires the whole part be brought up to temperature, is a very slow and expensive process for these types of parts. These parts are typically painted vs. powder coated because of the cost to counteract the heat insensitivity. Typically, some type of IR must be used to heat the powder to sufficient temperature before the heat sink can pull away the BTUs of the applied heat. For these applications, electric medium wave is typically the best choice as it has enough power to counteract the heat sink (i.e., can transfer energy at a rate that exceeds the substrate’s ability to conduct the energy internally), but still can be cost effective to operate as these ovens operate at basically full capacity for the process so the operating costs can be substantive. Curing these parts, though, may be better using electric preheat or boost and gas catalytic to finish the cure to bring the operating costs into an acceptable range.

Heat-sensitive substrates are a difficult challenge for coating applications. The solution is seldom simple or straightforward. To say one technology or one design will work in all applications is seldom the case. The answer for the best oven technology is “it depends.” The answers usually lie in the thorough analysis of the entire process. This means the operations, the quality needs, the operating costs, business priorities and most importantly the parts themselves all must be cnsidered. If you are working with an experienced oven manufacturer, they should be able to help you define the process and create a testing plan and provide the process testing to ensure acceptable results. Often, these solutions are custom designed for your application based on years of experience with similar parts as there is a fair amount of art to these types of process designs. The best recommendation is to do your research on your needs, and test and select an experienced partner. That is the best recipe to maximize your production and have a successful curing operation.

Marty Sawyer is CEO of Trimac Industrial Systems LLC, Kansas City, Mo. She can be reached via email at msawyer@trimacsystems.com.