Infrared preheating of powder coatings on complex components
Compact, energy-saving oven designs
Powder coatings are melted using heat and are generally cured at temperatures of
around 180 °C either in convection ovens or using i nfrared heaters. Hot air and
infrared radiation have different heating properties. Infrared is a popular heat source
because of its ability to heat the parts quickly and because of the small amount of
space needed for infrared ovens. However, these systems must be designed carefully
to accommodate parts with complex shapes. Increasingly, combinations of the two
heat sources are used for powder coatings. The following examples show how the
powder curing process can be improved with infrared heaters.
1. Different heat sources
Convection ovens transfer heat using the medium of air. The higher the temperature and the
speed of the air, the greater the heat transfer. Because of the risk of the powder being
displaced at high fan speeds, convection ovens in the preheating zone generally operate at
very low air speeds. In addition, it is not possible to increase the oven temperature to a very
high setting, as there is a risk of the parts being overbaked (yellowing) if the conveyor stops.
As a result, the heat transfer values of convection curing ovens are relatively low. This
means that large ovens are needed, which take up a great deal of space
Infrared systems transfer heat without the need for a contact medium. They emit
electromagnetic waves with similar properties to light, which are only transformed into heat
when they meet the material. Infrared radiation has a significantly higher heat transfer
capacity than convection. Another benefit of infrared systems is their short response time.
Short-wave quartz heaters and medium-wave carbon heaters, in particular, respond within a
few seconds. Therefore, infrared ovens can be switched on and off and adjusted to suit new
products very quickly.
Like all plastics, powder coatings absorb infrared rays very effectively. The intense radiation
melts the powder and brings it to curing temperature very quickly. As there is no air
movement, there is no risk of dust contamination and the powder is not subjected to
turbulence or displaced from the product.
Because infrared heat is transferred rapidly and with a high output, in most cases a much
shorter oven can be used or the production rate can be increased. Infrared heaters have very
short response times. For example, Heraeus Noblelight’s short wave and carbon heaters
respond within one to three seconds. As a result, the heat is easily adjustable and this,
together with temperature measurements of the object being coated, helps to prevent
materials from overheating. In addition, it is possible to switch quickly between different types
of coating with different baking temperatures in the same oven. Energy savings can also be
made because the heat source is only switched on when it is actually needed.
2. Factors that influence infrared heating
The extent to which the energy emitted by infrared heaters heats the object and the
temperature reached by the lacquer depend on:
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angle of impact of the radiation
distance between the heaters
component, the pigment of the powder
heat conductivity of the component material
thickness of the component walls.
Two-dimensional parts with a uniform wall thickness, such as powder-coated aluminium or
steel panels, are the simplest to heat using infrared radiation. Highly three dimensional
products are difficult to heat with infrared heaters because specific areas may be in shadow
where the infrared rays do not reach them. Metals, and in particular aluminium and steel,
have high levels of heat conductivity.
This allows even complex three-dimensional parts to be heated with infrared heaters, as the
temperature differences balance out after a short period. In addition, infrared heaters can be
arranged in different positions in the oven with regard to their angle of impact and distance
from the parts and can also be used across different zones. For example, in the case of very
tall components, the heaters higher up in the oven can be operated on relatively low power
settings because the natural convection process will help to ensure that the upper parts of
the components are heated.
Temperature [°C]
Less power is needed to maintain the temperature of the oven than to heat a component.
Therefore, in the waiting area of an infrared oven fewer heaters or heaters with a lower
specific output are used.
In practice, infrared systems often allow the throughput of a powder coating line to be
increased. Where there is not enough space available to enable the existing curing oven to
be extended, an infrared booster can be the ideal solution. This is an infrared heating unit
installed before or in the entrance to the oven which ensures that the powder melts quickly.
Any temperature differences in complex components are balanced out during the
subsequent convection heating process. If infrared heaters that can be switched on and off
quickly are installed, they can be shut off when the conveyor stops or during breaks in the
production process in order to save energy.
3. Applications for infrared heaters are described below
3.1 Aluminum frames
A highly water-resistant coating on faces of
buildings and window frames made of
aluminum needs to be cured 5 minutes
longer than usual coatings. It was not
possible to extend the existing oven. Finally
the company decided to install an infrared
booster with two emitters positioned opposite
to each other. The system could be installed
in front of the entrance of the oven. The
modules inside can be moved electromotively according to different frame
thicknesses. The distance between the
emitters can be adjusted to different lengths
form 0,2 meters to 0,5 meters. The oven is
0,7 meters long and 7,5 meters high. In total
it consists of 36 carbon emitters. They are
positioned in 12 movable zones, each side
with 6 zones aligned vertically. So the upper
emitters can run at lower power. Furthermore
the emitters can be adjusted to different wall
thicknesses of the frames. In case of line
stoppage or breaks during transportation the
emitters can be shut off. The power of the
emitters can be changed very quickly which
ensures a save heating process and a great
deal of energy savings. Four pyrometers
measure the temperature of the object
without getting in contact with the material. For the
company this is an easy way of process controlling.
3.2 Steel cylinders
In a plant for coating gas bottles,
replacing a gas convection oven with an
infrared oven allowed the oven footprint
to be reduced by 80% and the gelling
and curing time to be cut by up to 80%.
The existing convection oven was 30 m
long and took around 60 minutes to cure
the powder coating onto metal gas
bottles. Powder turbulence also resulted
in quality problems. The move to an
infrared drying unit produced significant
process improvements. The new infrared
oven is only 6 m long and, depending on
the bottle size, the powder coating is
gelled and cured on at a maximum
temperature of 200°C in just 12 to 18
minutes. In order to ensure even heat
distribution, the heaters have been
installed in the infrared oven in such a
way that the surface coverage is adapted
to the bottle shape and wall thickness.
More heat is generated in thicker areas,
such as the bottle base and neck, than in
the thinner walls.
3.3 Curing powder coatings on compressors using infrared heaters.
A range of compressors used in small
refrigeration units are around 600 mm high
and have a circumference of up to 280 mm.
They weigh up to 250 kg and are coated with
a black powder coating. The powder coating
is melted and cured in a 10-metre long
infrared oven with long-wave infrared
heaters. The problem for the company using
the oven was that the curing temperature
was too low and the temperature distribution
was also uneven. In order to resolve these
problems and to increase the processing
speed, the compressor manufacturer
replaced the existing long-wave heaters with
medium-wave quartz heaters with gold
reflectors. This simple solution allowed the
processing speed to be increased by a third
and the curing temperature to be raised
significantly. However, the problem of
uneven temperature distribution was not
resolved to the compressor
manufacturer‘s satisfaction. The next
step towards further increasing
productivity was to install a one-meter
long infrared booster at the entrance to
the infrared oven. The booster has
vertical, short-wave, twin-tube heaters
(with gold reflectors) which make it
suitable for use with the angled chain
conveyor. It has an electric power input
of 108 kW. The heaters can be turned on
and off quickly. When no compressors
are approaching the oven, the control
system turns the heaters off. Fitting the
short-wave heaters resulted in the production
speed doubling, when compared with original
system. In addition, no parts of the product are
left at too low a temperature and the oven
reaches the required curing temperature of 220 °C.