caps and plugs
curing applies light energy to the part surface by direct transmission from
an emitter. Some of the energy emitted will be reflected off
the surface, some is absorbed into the polymer and some is transmitted into
the substrate. This direct transfer of energy creates an
immediate reaction in the polymer and crosslinking begins quickly
once the surface is exposed to the emitter.
Although IR ovens can cure a coating much faster than convection,
differences in the part structure and mass will affect the results.
Light energy is dependent on a precise and consistent relationship to the product for even distribution of energy and uniform curing. Areas of a
part that are obscured from the emitter by their geometry will not heat
up at the same pace as areas that are flush to the emitter. The distance
of the part surface to the emitter will also have a profound influence on the curing of the coating.
For these reasons, IR is seldom ideal as a 100% cure option for powder
coating but it can be very useful when used with convection to
accelerate the cure. Ovens that use some IR in combination with convection can provide some of the benefits of both technologies.
There are certain situations where IR is the best cure method. When the part is very heavy and consistent in shape, IR emitters can be
arranged in a pattern that can raise the surface temperature much
faster than convection and cure in much less time. IR may be the only
practical way to cure certain types of parts by providing a much shorter
For systems that run only one product IR can work very well. This is
especially true of parts with even surfaces that can be run through the
oven with a consistent relationship to the emitter.
In cases where a variety of product with different shapes and masses
is coated, IR alone will require frequent adjustments to the position
and intensity of the emitters. Powder cure ovens are usually all or part convection type to provide the necessary flexibility for part size, mass and shape.
Infrared energy is a form of electromagnetic radiation, located on the
electromagnetic spectrum between visible light and microwaves and
measured in microns.
The three wave lengths of infrared, low, medium and high, will emit
energy that will be absorbed by the organic coating, reflected from the
surface, or transmitted into the substrate.
The actual behavior of IR energy depends on the wavelength, the
distance between the substrate and the emitter, the mass of the part
and the surface area. The surface of a part with a low mass/high
surface area will heat up very quickly when exposed to IR, while a part
with a lot of mass and not much surface area will react more slowly.
Short wave IR provides high electromagnetic energy levels. This intense
band of electromagnetic energy does not provide much flexibility
in the cure cycle.
Long wave IR has low electromagnetic energy levels, providing less surface heating than short wave or medium wave.
The most efficient use of IR energy is absorption by the coating. Higher
absorption rates are usually found in the medium wavelength, 2.3 to
3.3 microns. Typically, testing should be done to determine the best
setting, measured in microns on the electromagnetic scale.
A radiant preheat zone at the entrance end of the cure oven can be an
excellent enhancement. Curing will begin immediately and can be
accomplished faster than convection heating alone. Radiant cure can
be energy efficient if properly designed because it does not waste
Btu's to heat the entire substrate or the surrounding air.
In most cases where there is a lot of product variety, curing with radiant
emitters alone is not practical. The variety of shapes and sizes
require convection heating to ensure that all areas reach the cure window temperature and no area is overheated.
High purity (quartz lamp emitter) IR ovens are very effective for this
purpose and the bring up, or response time, is instant. The emitters
can be arranged in zones for different part geometries. Turning on
different numbers of emitters within a zone can vary the energy levels.
The zone control can be turned on by a remote controller or operated
Gas fired radiant emitters work very well also. Like quartz lamps, they
can be purchased in modular sections, they can be zoned, and they
have a fast response time. The Btu's required to operate a gas-fired IR
emitter will typically cost less than the kilowatt-hours required for electric infrared emitters.
Infrared preheat zones for powder cure ovens are typically around 30
to 60 seconds. For exact process requirements, testing must be performed.
September 15, 2012