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.